0001628280-24-012578 | 10-K | XBRL Viewer

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Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
COMMON STOCK, REDEEMABLE COMMON STOCK AND PREFERRED STOCK	COMMON STOCK AND REDEEMABLE COMMON STOCK Common Stock— As of December 31, 2023, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 500 million shares of $0.001 par value common stock. The voting, dividend and liquidation rights of the holders of the Company’s common stock are subject to and qualified by the rights, powers and preferences of the holders of the preferred stock. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any. No cash dividends have been declared or paid to date. ATM Sales Agreement On August 7, 2020, the Company entered into a Controlled Equity OfferingSM Sales Agreement (the “ATM Sales Agreement”) with B. Riley Securities, Inc., Cantor Fitzgerald & Co., and Stifel, Nicolaus & Company, Incorporated (collectively the “Sales Agents”), pursuant to which the Company may offer and sell, at the Company’s sole discretion through one or more of the Sales Agents, shares of its common stock having an aggregate offering price of up to $75.0 million. To date, the Company has sold approximately $14.3 million, net of offering costs, of common stock under the ATM Sales Agreement. Q1 2021 Private Placement On March 18, 2021, the Company entered into a securities purchase agreement with several institutional and accredited investors pursuant to which the Company agreed to issue and sell in a private placement (the “Private Placement”) an aggregate of 6,271,836 shares of common stock, 229,885 shares of redeemable common stock and, to certain investors, in lieu of common stock, pre-funded warrants to purchase an aggregate of 50,000 shares of common stock at a price of $8.70 per share of common stock (or $8.69 per pre-funded warrant). The Private Placement closed on March 23, 2021 and the Company received gross proceeds of $55.0 million, before deducting offering expenses payable by the Company. In August 2021, the investor who purchased the redeemable common stock exercised its option to sell its 229,885 shares of redeemable common stock back to the Company at the original purchase price of $8.70 per share for an aggregate of $2.0 million. The Company adjusted the carrying amount of the redeemable common stock to its redemption value and subsequently retired these shares. Lincoln Park Capital Fund Purchase Agreement On January 14, 2022, the Company and Lincoln Park Capital Fund, LLC (“Lincoln Park”) entered into a securities purchase agreement (the “LPC Purchase Agreement”) and a registration rights agreement (the “Registration Rights Agreement”), pursuant to which the Company has the right to sell shares of common stock to Lincoln Park, having an aggregate value of up to $50.0 million, subject to certain limitations and conditions set forth in the LPC Purchase Agreement, at the Company’s request from time to time during the 36-month term of the LPC Purchase Agreement. In consideration for entering into the LPC Purchase Agreement, the Company issued 230,414 shares of common stock to Lincoln Park as an initial commitment fee. Upon execution of the LPC Purchase Agreement and the Registration Rights Agreement on January 14, 2022, the Company sold to Lincoln Park, as an initial purchase under the LPC Purchase Agreement, a total of 1,382,488 shares of common stock, at a per share price of $2.17 per share, for aggregate consideration of approximately $3.0 million. Q1 2022 PIPE On March 3, 2022, the Company entered into a securities purchase agreement pursuant to which it agreed to issue and sell to an investor, in a private placement (the “Q1 2022 PIPE”), 900,000 shares of common stock at a price of $1.80 per share, which represents the volume weighted average price per share of the Company’s common stock as quoted on the Nasdaq Stock Market for the thirty (30) consecutive-day trading day period ending on March 2, 2022, and pre-funded warrants to purchase 766,666 shares of common stock at a purchase price of $1.79 per pre-funded warrant (representing the price of $1.80 per share minus the $0.01 per share exercise price of each such prefunded warrant). The pre-funded warrants are exercisable at any time after their original issuance date and will have no expiration date. The Q1 2022 PIPE closed on March 7, 2022 and the Company received gross proceeds of $3.0 million, before deducting offering expenses payable by the Company. In accordance with an associated registration rights agreement, the Company filed a registration statement covering the resale of these securities in April 2022. Q2 2022 PIPE On June 30, 2022, the Company entered into a securities purchase agreement with several institutional and accredited investors pursuant to which the Company agreed to issue in a private placement (the “Q2 2022 PIPE”) an aggregate of 37,649,086 shares of common stock and, to certain investors, in lieu of common stock, pre-funded warrants to purchase an aggregate of 13,276,279 shares of common stock at a price of $1.095 per share of common stock (or $1.094 per pre-funded warrant) and 50,925,365 warrants (the “Warrants”) for the purchase of shares of common stock. Each Warrant has an exercise price equal to $1.095 per share and will expire on the date that is 60 months from their original issue date. The price per pre-funded warrant represents the price of $1.095 per share sold in the Q2 2022 PIPE, minus the $0.001 per share exercise price of each such pre-funded warrant. The pre-funded warrants are exercisable, subject to certain beneficial ownership restrictions, at any time after their original issuance and will not expire. The Q2 2022 PIPE closed on July 6, 2022 and the Company received gross proceeds of $55.7 million, before deducting offering expenses paid by the Company. The exercise of any Warrant was conditioned upon the Company increasing its authorized shares. Accordingly, the Company convened a special meeting of its stockholders on September 1, 2022, during which the stockholders approved an increase in the number of authorized shares of common stock from 125 million to 500 million pursuant to an amendment to the Company’s Certificate of Incorporation. As of July 6, 2022, due to the shortfall in authorized and available common shares, the Warrants did not meet the criteria required for permanent equity accounting. As a result, the Company allocated $41.2 million of the gross proceeds from the offering to the fair value of the Warrants, which was recorded as a warrant liability, and the remaining $13.5 million was allocated to the common shares and pre-funded warrants and recorded as permanent equity. The fair value of the warrant liability was calculated using the Black-Scholes option valuation model. The Company also allocated a portion of the transaction fees, including commissions and legal fees, to the warrant liability and expensed within other expense, net, approximately $2.9 million of these fees upon the closing of the Q2 2022 PIPE. Upon shareholder approval of the increase to the Company’s authorized shares, the Warrants met all criteria required for permanent equity accounting and, accordingly, the Company remeasured the fair value of the warrant liability through earnings, which resulted in approximately $2.5 million of income included within other income (expense) and reclassified the fair value of the warrant liability to additional paid-in capital. Also on June 30, 2022, the Company entered into a registration rights agreement, pursuant to which the Company agreed to register for resale the common shares issued in the Q2 2022 PIPE and the issuance of the shares of common stock underlying the Pre-Funded Warrants and the Warrants sold in the offering. Such registration statement was filed on July 29, 2022 and was declared effective by the SEC on August 5, 2022. Q4 2022 Public Offering On December 7, 2022, the Company sold 52,300,000 shares of common stock and, in lieu of common stock, prefunded warrants to purchase 6,800,000 shares of common stock, and accompanying Class C warrants to purchase 32,762,947 shares of its common stock. The common stock was issued at a price to the public of $1.10 per share and the accompanying Class C warrants and prefunded warrants were issued at a price of $1.099 per prefunded warrant and accompanying Class C warrant. The Class C warrants have an exercise price of $1.50, will expire 5 years from the date of issuance, and are immediately exercisable with certain restrictions. The gross proceeds from the offering, which closed on December 9, 2022, were $65.1 million before deducting underwriting discounts and offering expenses. The Company concluded that the Class C warrants do not meet the equity contract scope exception under ASC 815-40 as in the event of a fundamental transaction such as a merger certain provisions may require the Company to adjust the settlement value that is not consistent with a fixed-for-fixed option pricing model. As a result, as of issuance date, the Company allocated $21.5 million of the gross proceeds from the offering to the Class C Warrants based on their fair value, which was recorded as a warrant liability, and the remaining $43.6 million was allocated to the common shares and pre-funded warrants and recorded as permanent equity. The Class C warrant liability has been subsequently adjusted to fair value at December 31, 2022 and will be adjusted to fair value at each subsequent balance sheet date until the warrants are settled. Changes in fair value of the Class C warrants are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. The fair value of the Class C warrant liability is calculated using the Black-Scholes option valuation model as further described in Note 4. The Company also allocated a portion of the transaction fees, including commissions and legal fees, to the Class C Warrant liability and expensed within other expense, net, approximately $1.7 million of these fees upon the closing of the Q4 2022 Public Offering. Q2 2023 PIPE On May 15, 2023, the Company entered into a securities purchase agreement pursuant to which it agreed to issue and sell to several institutional and accredited investors (the “Investors”), in a private placement (the “Q2 2023 PIPE”), 34,521,046 shares of common stock at a price of $1.52 per share and pre-funded warrants to purchase 8,263,157 shares of common stock at a purchase price of $1.519 per pre-funded warrant (representing the price of $1.52 per share minus the $0.001 per share exercise price of each such prefunded warrant). The pre-funded warrants are exercisable, subject to certain beneficial ownership restrictions, at any time after their original issuance and will not expire. The Q2 2023 PIPE closed on May 18, 2023. The Company received gross proceeds of $65.0 million, before deducting offering expenses paid by the Company. Also on May 15, 2023, the Company entered into a registration rights agreement with the Investors, pursuant to which the Company agreed to register for resale the common shares issued in the Q2 2023 PIPE and the issuance of the shares of common stock underlying the pre-funded warrants held by the Investors. Such registration statement was filed on June 9, 2023 and was declared effective by the SEC on June 20, 2023.

- Definition

The entire disclosure for equity.

+ References

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+ Details

Income Taxes - Reconciliation of Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	(6.00%)	(5.50%)	(5.40%)
Research and development tax credits	(1.40%)	(1.00%)	(0.90%)
Other permanent differences	(0.20%)	3.00%	3.70%
Change in deferred tax asset valuation allowance	29.30%	23.20%	23.70%
Other	(0.70%)	1.30%	(0.10%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Schedule of Income before Income Tax, Domestic and Foreign

Loss before the provision for income taxes for the years ended December 31, 2023, 2022 and 2021 consisted of the following:


	Year Ended December 31,
(in thousands)	2023		2022		2021
United States	$	(102,126)	$	(94,742)	$	(89,865)
Foreign (Austria)	1,037		903		1,186
	$	(101,089)	$	(93,839)	$	(88,679)

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows:


	Year Ended December 31,
	2023		2022		2021
U.S. federal statutory income tax rate	(21.0)	%	(21.0)	%	(21.0)	%
State income taxes, net of federal benefit	(6.0)		(5.5)		(5.4)

Research and development tax credits	(1.4)		(1.0)		(0.9)

Other permanent differences	(0.2)		3.0		3.7
Change in deferred tax asset valuation allowance	29.3		23.2		23.7
Other	(0.7)		1.3		(0.1)
Effective income tax rate	—	%	—	%	—	%

Schedule of Deferred Tax Assets and Liabilities

Net deferred tax assets as of December 31, 2023 and 2022 consisted of the following:


	December 31,
(in thousands)	2023		2022
Net operating loss carryforwards	$	122,942	$	107,296
Tax credit carryforwards	8,416		6,694
Capitalized research and development expenses	28,402		16,004


Lease liabilities	755		1,016
Other	4,616		3,604
Total deferred tax assets	165,131		134,614
Valuation allowance	(163,994)		(133,112)
Deferred tax assets, net of valuation allowance	$	1,137	$	1,502
Right of use assets	1,137		1,502
Total deferred tax liabilities	$	1,137	$	1,502
Total deferred tax assets, net	$	—	$	—

Summary of Valuation Allowance

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2023, 2022 and 2021 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards and were as follows:


	Year Ended December 31,
(in thousands)	2023		2022		2021
Valuation allowance, beginning of year	$	(133,112)	$	(111,835)	$	(92,197)
Increases recorded to income tax provision	(30,882)		(21,277)		(19,638)

Valuation allowance, end of year	$	(163,994)	$	(133,112)	$	(111,835)

Schedule of Unrecognized Tax Benefits Roll Forward

The following table summarizes the Company’s reserve for uncertain tax positions for the three years ended December 31, 2023:


(in millions)	Reserve for Uncertain Tax Position
Balance as of December 31, 2020	$	0.3
Settlement of unrecognized tax benefit	(0.1)
Balance as of December 31, 2021	$	0.2

Balance as of December 31, 2022	$	0.2
Balance as of December 31, 2023	$	0.2

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Boston, Massachusetts, US
Auditor Firm ID	238

Consolidated Statements of Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Equity (Deficit) - USD ($)	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Redeemable Common Stock
Beginning balance, convertible preferred stock (in share) at Dec. 31, 2020						0
Beginning balance, convertible preferred stock at Dec. 31, 2020						$ 0
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2021						0
Ending balance, convertible preferred stock at Dec. 31, 2021						$ 0
Beginning balance (in shares) at Dec. 31, 2020		16,305,731
Beginning balance at Dec. 31, 2020	$ 72,799,000	$ 16,000	$ 267,077,000	$ (119,000)	$ (194,175,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued (in shares)		9,435,951				229,885
Stock issued	73,868,000	$ 10,000	73,858,000			$ 1,875
Exercise of stock options (in shares)		5,860
Exercise of stock options	40,000		40,000
Stock-based compensation expense	$ 6,180,000		6,180,000
Issuance of shares under employee stock purchase plan (in shares)	45,816
Issuance of shares under employee stock purchase plan	$ 219,000		219,000
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations (in shares)		204,531
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations	$ 0
Repurchase and retirement of redeemable common stock (in shares)	(229,885)
Repurchase and retirement of redeemable common stock	$ 0					$ (1,875)
Net income (loss)	(88,696,000)				(88,696,000)
Exercise of warrants (in shares)		2,129,768
Exercise of warrants and prefunded warrants	2,000	$ 2,000
Ending balance (in shares) at Dec. 31, 2021		28,127,657
Ending balance at Dec. 31, 2021	64,412,000	$ 28,000	347,374,000	(119,000)	(282,871,000)
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2022						0
Ending balance, convertible preferred stock at Dec. 31, 2022						$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued (in shares)		92,461,988
Stock issued	$ 59,362,000	$ 92,000	59,270,000
Exercise of stock options (in shares)	2,031
Stock-based compensation expense	$ 5,199,000		5,199,000
Issuance of shares under employee stock purchase plan (in shares)	204,903
Issuance of shares under employee stock purchase plan	$ 203,000	$ 1,000	202,000
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations (in shares)		372,831
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations	(13,000)	$ 0	(13,000)
Net income (loss)	(93,867,000)				(93,867,000)
Exercise of warrants (in shares)		499,871
Exercise of warrants and prefunded warrants	1,000	$ 1,000
Reclassifications of Temporary to Permanent Equity	38,754,000		38,754,000
Ending balance (in shares) at Dec. 31, 2022		121,667,250
Ending balance at Dec. 31, 2022	74,051,000	$ 122,000	450,786,000	(119,000)	(376,738,000)
Ending balance, convertible preferred stock (in shares) at Dec. 31, 2023						0
Ending balance, convertible preferred stock at Dec. 31, 2023						$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued (in shares)		34,521,046
Stock issued	60,443,000	$ 35,000	60,408,000
Stock-based compensation expense	$ 8,687,000		8,687,000
Issuance of shares under employee stock purchase plan (in shares)	257,963
Issuance of shares under employee stock purchase plan	$ 272,000		272,000
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations (in shares)		3,510,491
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations	0	$ 2,000	(2,000)
Net income (loss)	(101,167,000)				(101,167,000)
Exercise of warrants (in shares)		7,477,845
Exercise of warrants and prefunded warrants	8,813,000	$ 8,000
Reclassifications of Temporary to Permanent Equity	(101,167,000)
Ending balance (in shares) at Dec. 31, 2023		167,434,595
Ending balance at Dec. 31, 2023	$ 51,099,000	$ 167,000	$ 528,956,000	$ (119,000)	$ (477,905,000)

Fair Value of Financial Assets and Liabilities - Assumptions (Details) - $ / shares					12 Months Ended
	Jul. 01, 2023	Dec. 07, 2022	Sep. 01, 2022	Jul. 06, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate					4.10%	3.40%	1.00%
Expected term (in years)					6 years	6 years 1 month 6 days	6 years
Expected volatility					93.90%	96.00%	97.30%
Expected dividend yield					0.00%	0.00%	0.00%
PIPE Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Share Price			$ 1.04	$ 1.09
Risk-free interest rate			3.30%	3.00%
Expected term (in years)			4 years 10 months 24 days	5 years
Expected volatility			97.50%	97.30%
Expected dividend yield			0.00%	0.00%
Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Share Price		$ 0.93			$ 0.84	$ 0.99
Risk-free interest rate	3.80%	3.80%		4.00%	3.90%	4.00%
Expected term (in years)	4 years 10 months 9 days	5 years		5 years 10 months 9 days	3 years 10 months 24 days	4 years 10 months 24 days
Expected volatility	100.00%	101.80%		97.00%	96.20%	101.70%
Expected dividend yield	0.00%	0.00%		0.00%	0.00%	0.00%

Stock-Based Compensation

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock-Based Compensation

STOCK-BASED COMPENSATION

Summary of Plans— The Company issues stock awards under the following plans: (a) The 2015 Employee, Director and Consultant Equity Incentive Plan, as amended (the “2015 Plan”), (b) the Amended and Restated 2017 Equity Incentive Plan (the “2017 Plan”), (c) the Amended and Restated 2017 Employee Stock Purchase Plan (the “2017 ESPP”) and the 2019 Inducement Equity Incentive Plan (the “2019 Plan”).

These plans are administered by the Board of Directors or by a committee thereof. The exercise prices, vesting and other restrictions are determined at the discretion of the Board of Directors, or its committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of the stock option may not be greater than ten years. Incentive stock options granted to employees and restricted stock awards granted to employees, officers, members of the Board of Directors, advisors, and consultants of the Company typically vest over four years. Non-statutory options granted to employees, officers, members of the Board of Directors, advisors, and consultants of the Company typically vest over three or four years. Shares that are expired, terminated, surrendered or canceled under the Plans without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

2015 Employee, Director and Consultant Equity Incentive Plan—Under the 2015 Plan, the Company grants incentive stock options, nonqualified stock options, restricted stock awards and other stock-based awards to employees, directors and consultants of the Company. As of December 31, 2023, there were approximately 30 thousand shares available for issuance under the 2015 Plan.

2017 Equity Incentive Plan— Under the 2017 Plan, the Company may grant incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. Under an “evergreen” provision of the 2017 Plan, shares of common stock reserved for issuance under the 2017 Plan are increased annually on the first day of each year, beginning on January 1, 2021 and ending on January 1, 2027, in an amount equal to the lower of 4.0% of the number of shares of the Company’s common stock outstanding on January 1 of each year or an amount determined by the Company’s Board of Directors. As of December 31, 2023, approximately 181 thousand shares were available for future issuance under the 2017 Plan. As of January 1, 2024, an additional 6.7 million shares became available for future issuance under the 2017 Plan under the evergreen provision.

Amended and Restated 2017 Employee Stock Purchase Plan— The 2017 ESPP provides participating employees with the opportunity to purchase shares of the Company’s common stock at defined purchase prices over six-month offering periods. For the twelve months ended December 31, 2023, 257,963 shares of common stock were issued under the 2017 ESPP. As of December 31, 2023, approximately 4.9 million shares were available for future issuance under the 2017 ESPP.

2019 Inducement Equity Incentive Plan— On June 17, 2019, the Board of Directors approved the adoption of the 2019 Plan, as amended, which is used exclusively for the grant of equity awards to individuals who were not previously employees of the Company (or following a bona fide period of non-employment), as an inducement material to such individual’s entering into employment with the Company, pursuant to Nasdaq Listing Rule 5635(c)(4). The total number of shares of common stock that may be issued under the 2019 Plan, as amended, is 6.3 million shares. Shares that are expired, forfeited, canceled or otherwise terminated without having been fully exercised will be available for future grant under the 2019 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future grants. As of December 31, 2023, approximately 1.5 million shares were available for future issuance under the 2019 Plan.

Stock Option Valuation— The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted:


	Year Ended December 31,
	2023		2022		2021
Risk-free interest rate	4.1	%	3.4	%	1.0	%
Expected term (in years)	6.0		6.1		6.0
Expected volatility	93.9	%	96.0	%	97.3	%
Expected dividend yield	—	%	—	%	—	%

Stock Options

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2023:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (in Years)	Aggregate Intrinsic Value (in Thousands)
Outstanding as of December 31, 2022	2,021,480	$	6.99	6.5	$	1
Granted	4,217,650	1.24
Exercised	(2,031)	1.50
Forfeited	(228,558)	6.60
Outstanding as of December 31, 2023	6,008,541	$	2.97	8.6	$	24
Exercisable as of December 31, 2023	1,313,306	$	8.62	5.8	$	—
Vested and expected to vest as of December 31, 2023	4,563,685	$	3.49	8.4	$	16

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock to the extent the stock option had a lower exercise price. The aggregate intrinsic value of stock options exercised during the twelve months ended December 31, 2023 and 2021 was $1 thousand and $13 thousand, respectively. There were no options exercised in 2022. The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2023, 2022 and 2021 was $0.97, $1.38, and $5.17, respectively.

Restricted Stock Units— During the year ended December 31, 2023, the Company granted 5.3 million restricted stock units to employees and members of the Board of Directors at a weighted average grant date fair value of $1.76 per share. Approximately 4.9 million of these awards are performance restricted stock units (“PRSUs”) and the remainder are time-based and vest as the employee provides services to the Company. The PRSUs vest 50% based on the Company’s achievement of each of two operational milestones conditioned on the grantee’s continued employment with the Company. As of December 31, 2023, one of the two performance criteria was met. The Company believes that the achievement of the operational milestones is probable and, accordingly, stock-based compensation expense has been recognized for these awards using the accelerated attribution model, net of estimated forfeitures, based on the fair value of the awards as of the date of grant and management’s best estimate of the date each operational milestone will be achieved. The Company updates its estimates related to the probability and timing of achievement of the operational milestones each period until the award either vests or is forfeited. The Company recognizes stock-based compensation expense for time-based restricted stock units ratably, net of estimated forfeitures, over the estimated vesting period.

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2023 :


			Number of Shares
Unvested at December 31, 2022			1,680,563
Granted			5,332,051
Vested			(3,511,542)
Forfeited			(382,248)
Unvested at December 31, 2023			3,118,824

Stock-Based Compensation— As of December 31, 2023, total unrecognized compensation expense related to unvested stock options and restricted stock units was $4.3 million, which is expected to be recognized over a weighted average period of 2.5 years.

Stock-based compensation expense was classified in the consolidated statements of operations as follows:


	Year Ended December 31,
(in thousands)	2023		2022		2021
Research and development expense	$	4,357	$	2,534	$	2,723
Selling, general and administrative expense	4,330		2,665		3,457
Total stock-based compensation	$	8,687	$	5,199	$	6,180

Stock Appreciation Rights— On November 7, 2022 (the “Grant Date”), the compensation committee of the Board of Directors approved special retention and recognition grants of stock appreciation rights pursuant to the 2017 Plan to the Company’s President and Chief Executive Officer, the Company’s Chief Financial Officer and Treasurer, and certain other executive officers of the Company. The SARs have a measurement price per SAR equal to $1.80, the closing price per share of the Company’s common stock on the Grant Date, and each grant of SARs has a maximum term of ten years from the Grant Date. Unless otherwise determined by the Board of Directors, the SARs will be settled in cash upon exercise. The settlement value will be based on the difference between the closing price of the Company’s common stock on the date of settlement less $1.80 multiplied by the number of SARs exercised. The SARs will vest and become exercisable in equal annual installments on the first, second, and third anniversaries of the Grant Date, subject to the recipient remaining an employee of the Company through and including each applicable vesting date.

The calculation of the fair value of the outstanding SARs includes the closing price of the Company’s common stock of $0.84 and $0.99 as of December 31, 2023 and December 31, 2022, respectively, and the following assumptions on a weighted average basis:


	December 31, 2023		December 31, 2022
Risk-free interest rate	3.8	%	4.0	%
Expected term (in years)	4.86		5.86
Expected volatility	100.0	%	97.0	%
Expected dividend yield	—	%	—	%
Expected forfeiture rate	18.6	%	22.3	%

The SARs are accounted for as liability awards as settlement will be in the form of cash unless the Board of Directors authorizes settlement in shares of the Company’s common stock and such shares are available to be issued from the 2017 Plan. The Company currently intends to settle the SARs in cash if and when exercised. Compensation expense is recorded based the fair value of the SARs, as determined using the Black-Scholes option valuation model, using an accelerated attribution method as the SARs vest. The Company remeasures the fair value of the outstanding SARs each period until settlement and adjusts life-to-date compensation expense to the period end SARs fair value. For the years ended December 31, 2023 and 2022, the Company recognized $1.9 million and $0.4 million, respectively, of compensation expense related to the SARs.

Stock-Based Compensation - Stock Option Valuation (Details)				12 Months Ended
	Jul. 01, 2023	Dec. 07, 2022	Jul. 06, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate				4.10%	3.40%	1.00%
Expected term (in years)				6 years	6 years 1 month 6 days	6 years
Expected volatility				93.90%	96.00%	97.30%
Expected dividend yield				0.00%	0.00%	0.00%
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.80%	3.80%	4.00%	3.90%	4.00%
Expected term (in years)	4 years 10 months 9 days	5 years	5 years 10 months 9 days	3 years 10 months 24 days	4 years 10 months 24 days
Expected volatility	100.00%	101.80%	97.00%	96.20%	101.70%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%
Expected forfeiture rate	18.60%		22.30%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-Name Accounting Standards Codification
-Section 35
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Number of Shares
Outstanding, beginning balance (in shares)	2,021,480
Granted, (in shares)		4,217,650
Exercise of stock options (in shares)		(2,031)
Forfeited (in shares)		(228,558)
Outstanding, ending balance (in shares)	6,008,541	2,021,480
Exercisable, ending balance (in shares)	1,313,306
Vested and expected to vest, ending balance (in shares)	4,563,685
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.99
Granted (in dollars per share)		$ 1.24
Exercised (in dollars per share)		1.50
Forfeited (in dollars per share)		6.60
Outstanding, ending balance (in dollars per share)	2.97	$ 6.99
Exercisable (in dollars per share)	8.62
Vested and expected to vest (in dollars per share)	$ 3.49
Weighted Average Contractual Term (in Years)
Outstanding	8 years 7 months 6 days	6 years 6 months
Exercisable	5 years 9 months 18 days
Vested and expected to vest	8 years 4 months 24 days
Aggregate Intrinsic Value (in Thousands)
Outstanding beginning balance	$ 1
Outstanding ending balance	24,000	$ 1
Exercisable	0
Vested and expected to vest	$ 16,000

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Use of Estimates— The preparation of the Company’s consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. As of the date of issuance of these consolidated financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s consolidated financial statements.

Foreign Currency and Currency Translation— The functional currency of the Company’s foreign subsidiary, X4 Austria, is the U.S. dollar. However, X4 Austria maintains its books and records in Euro. As a result, monetary assets and liabilities are translated at current exchange rates as of the balance sheet date, non-monetary assets such as property and equipment and equity accounts are translated at historic rates and income and expenses are translated at the average exchanges rates for the period. Adjustments resulting from the translation of the consolidated financial statements of X4 Austria into U.S. dollars are included in the determination of net loss and are recorded in other expense, net.

Concentrations of Credit Risk and Significant Suppliers— Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash and cash equivalents, marketable securities and research and development incentive receivables. The Company generally maintains cash balances in various operating accounts at financial institutions that management believes to be of high credit quality in amounts that may exceed federally insured limits. The Company’s marketable securities and cash equivalents are invested in high quality, U.S. government obligations such as U.S. Treasury bills and U.S. government agency obligations. The Company has not experienced losses related to its cash and cash equivalents.

The Company is dependent on third-party manufacturers to supply its drug substance and clinical drug supply for research and development activities in its programs. Should the Company receive FDA approval of its NDA, these third-party manufacturers would also supply the Company’s drug product. The Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in these manufacturing services or in the supply of active pharmaceutical ingredients and formulated drugs.

Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of 90 days or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds, treasury bills and federal government agency notes as of December 31, 2023 and December 31, 2022.

Marketable Securities— Marketable securities consist of short-term debt securities classified as available-for-sale having maturities greater than 90 days, but less than 365 days from the date of acquisition (settlement). The Company determines the appropriate classification of the securities at the time they are acquired and evaluate the appropriateness of such classifications at each balance sheet date. The Company’s marketable securities, which consist of U.S. Treasury securities and federal government agency notes, are classified as available-for-sale securities whose fair value is categorized as Level 2 as their value is based on valuations using significant inputs derived from, or corroborated by, observable market data. The cost of available-for-sale securities sold is based on the specific-identification method. Unrealized gain and losses on available-for-sale are included as a component of other comprehensive loss on the consolidated balance sheet and as a component of total comprehensive loss on the consolidated statement of operations and comprehensive loss until realized. Realized gains and losses on the sale of marketable securities are determined using the specific-identification method and recorded in other (expense) income, net on the

accompanying condensed consolidated statements of operations and comprehensive loss. The Company reviews marketable securities for impairment whenever the fair value of a marketable security is less than the amortized cost and evidence indicates that a marketable security’s carrying amount is not recoverable. Unrealized losses are evaluated for impairment under ASC 326, Financial Instruments - Credit Losses, to determine if the impairment is credit-related or noncredit-related. Credit-related impairment is recognized as an allowance on the consolidated balance sheets with a corresponding adjustment to earnings, and noncredit-related impairment is recognized in other comprehensive loss, net of taxes. Evidence considered in this assessment includes reasons for the impairment, compliance with our investment policy, the severity of the impairment, collectability of the security, and any adverse conditions specifically related to the security, an industry, or geographic area.

Restricted Cash


(in thousands)	As of December 31, 2023		As of December 31, 2022
Letter of credit security: Waltham lease	$	250	$	250
Letter of credit security: Vienna Austria lease	211		205
Letter of credit security: Boston lease	571		855
Total restricted cash	$	1,032	$	1,310
Restricted cash included in prepaid expenses and other current assets	$	250	$	285
Restricted cash included in other assets	$	782	$	1,025

In connection with the Company’s lease agreement for its facilities in Massachusetts and Austria, the Company maintains letters of credit, which are secured by restricted cash, for the benefit of the respective landlord. The Company’s Waltham lease agreement expired in December 2023; however, the letter of credit will remain in place until the landlord completes its lease expiration procedures. In accordance with the Company’s Hercules Loan Agreement and as further described in Note 7, the Company at all times must maintain a minimum level of cash of $20.0 million in an account or accounts in which Hercules has a first priority security interest as further described in Note 7.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets to the sum to the total of amounts shown in the Company’s consolidated statements of cash flows as of December 31, 2023, 2022 and 2021:


(in thousands)	December 31, 2023		December 31, 2022		December 31, 2021
Cash and cash equivalents	$	99,216	$	121,718	$	81,787
Restricted cash, current (included within prepaid expenses and other current assets)	250		285		—
Restricted cash, non-current (included within other assets)	782		1,025		1,321
Total cash, cash equivalents and restricted cash	$	100,248	$	123,028	$	83,108

Property and Equipment— Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


			Estimated Useful Life
Office furniture			3 to 7 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Estimated useful lives are periodically assessed to determine if changes are appropriate. Maintenance and repairs are charged to expense as incurred. When assets are retired or otherwise disposed of, the cost of these assets and related accumulated depreciation or amortization are eliminated from the consolidated balance sheet and any resulting gains or losses are included in the consolidated statements of operations and comprehensive loss in the period of disposal. Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service.

Right-of-Use Assets and Leases— The Company accounts for leases in accordance with Accounting Standards Codification (“ASC”), Topic 842, Leases (“ASC 842”). Under ASC 842, at the inception of an arrangement, the Company determines whether the arrangement contains a lease based on the unique facts and circumstances present. Leases with a non-cancellable term greater than one year are recognized on the balance sheet as right-of-use assets with associated current and non-current lease liabilities. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew the lease. If a lease is cancellable without penalty, the Company excludes from the lease term periods following the cancellation notice period unless it is reasonably certain that the Company will not cancel the lease.

Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use operating asset may be required for items such as incentives received or accrued rent. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates it incurs to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. The Company referenced the effective rate of its Hercules Loan Agreement, as adjusted for differences terms, to determine its incremental borrowing rate for each of its operating leases at lease inception.

In accordance with the guidance in ASC 842, components of a lease are split into lease components and non-lease components. A policy election is available pursuant to which an entity may elect to not separate lease and non-lease components. Rather, each lease component and the related non-lease components are accounted for together as a single component. For new and amended office and laboratory leases beginning in 2019 and after, the Company has elected to account for the lease and non-lease components as a combined lease component.

Impairment of Long-Lived Assets— Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value. To date, the Company has not recorded any material impairment losses on long-lived assets.

Goodwill— Business combinations are accounted for under the acquisition method. The total purchase price of an acquisition is allocated to the underlying identifiable net assets, based on their respective estimated fair values as of the acquisition date. Determining the fair value of assets acquired and liabilities assumed requires management’s judgment and often involves the use of significant estimates and assumptions, including assumptions with respect to future cash inflows and outflows, probabilities of success, discount rates, and asset lives, among other items. Assets acquired and liabilities assumed are recorded at their estimated fair values. The excess of the purchase price over the estimated fair values of the net assets acquired is recorded as goodwill.

Goodwill is tested quantitatively for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action, a significant decline in the price of the Company’s common stock, or unanticipated competition.

The Company has determined that it operates in a single operating segment and has a single reporting unit. To perform its quantitative test, the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than the carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit. See Note 4 for more information on the Company’s goodwill impairment tests as of December 31, 2023, 2022 and 2021.

Fair Value Measurements— Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•Level 1—Quoted prices in active markets for identical assets or liabilities.

•Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The embedded derivative liability related to the redemption features of the Company’s debt with Hercules as described further below and the Company’s outstanding Class C warrants are carried at fair value and using a Level 3 measurements. The Company’s cash equivalents, which consist of money market funds, which are invested in U.S. Treasury securities and U.S. government agency obligations, are carried at fair value, determined based on Level 1 and Level 2 inputs in the fair value hierarchy described above. The Company’s marketable securities are carried at fair value determined based on Level 2 inputs. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding loan and security agreement with Hercules approximates its fair value at December 31, 2023 because the debt bears interest at a variable market rate and the Company’s credit risk has not materially changed since the inception of the agreement.

Segment Information— The Company has defined its Chief Operating Decision Maker (“CODM”) as its Chief Executive Officer. The CODM manages the Company’s operations as a single operating segment, which comprises its single reportable segment, for the purposes of assessing performance and making operating decisions. The Company’s focus is on the research, development and commercialization of novel therapeutics for the treatment of rare diseases.

Research and Development Programs— Proceeds under the research and development incentive program from the Austrian government are recognized as other income in an amount equal to the qualifying expenses incurred in each period multiplied by the applicable reimbursement percentage. Incentive income recognized upon incurring qualifying expenses in advance of receipt of proceeds from research and development incentives is recorded in the consolidated balance sheet as research and development incentive receivable.

Research and Development Costs— Costs associated with internal research and development and external research and development services, including drug development and preclinical studies, are expensed as incurred. Research and development expenses include costs for salaries, employee benefits, subcontractors, facility-related expenses, depreciation and amortization, stock-based compensation, third-party license fees, laboratory supplies, and external costs of outside vendors engaged to conduct discovery, preclinical and clinical development activities and clinical trials as well as to manufacture clinical trial materials, and other costs. The Company recognizes external research and development costs based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its service providers.

Nonrefundable advance payments for services to be received in the future for use in research and development activities are recorded as prepaid expenses. Such prepaid expenses are recognized as an expense when the related services have been performed, or when it is no longer expected that the goods will be delivered, or the services rendered.

Patent Costs— All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as selling, general and administrative expenses.

Debt Issuance Costs— Debt issuance costs consist of payments made to secure commitments under certain debt financing arrangements. These amounts are recognized as interest expense over the period of the financing arrangement using the effective interest method. If the financing arrangement is canceled or forfeited, or if the utility of the arrangement to the Company is otherwise compromised, these costs are recognized as interest expense immediately. The Company’s consolidated financial statements present debt issuance costs related to a recognized debt liability as a direct reduction from the carrying amount of that debt liability.

Stock-Based Compensation— The Company measures all stock-based awards granted to employees, nonemployees and directors based on the fair value on the date of the grant and recognizes compensation expense for those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The Company issues stock-based awards with service-based vesting conditions and records the expense for these awards using the straight-line method. The Company has also issued stock-based awards with performance-based vesting conditions that vest in part upon the Company’s achievement of operational milestones and over time thereafter for the subsequent two years as the employee continues to provide services. The Company assesses the probability of achievement of these operational milestones and recognizes stock-based compensation for these awards using the accelerated attribution model based on the fair value of the awards as of the date of grant and its best estimate of the date each operational milestone will be achieved.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment is recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to March 13, 2019, the Company was a private company and lacked company-specific historical and implied volatility information for its common stock. Therefore, the Company estimates its expected common stock price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employee consultants is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield considers the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future.

Derivative Liabilities: Hercules Loan Redemption Feature— The Company’s loan agreement with Hercules (see Note 7) contains a redemption feature that, upon an event of default, provides Hercules the option to accelerate and demand repayment of the debt, including a prepayment premium, or, at its election, charge additional contingent interest fees on any overdue interest or principal payments. The redemption feature meets the definition of a derivative instrument as the repayment of the debt contains a substantial premium, resulting in the redemption feature not being clearly and closely related to its host instrument. Accordingly, the Company classifies this derivative as a liability within other liabilities (non-current) on its consolidated balance sheets. The derivative liability was initially recorded at fair value on the date of the Hercules Loan Agreement and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of this derivative liability, which is included in other liabilities, are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of this derivative liability will continue to be recognized until all amounts outstanding under the Hercules Loan Agreement are repaid or until the Hercules Loan Agreement is terminated.

Comprehensive Loss— For the year ended December 31, 2023, 2022, and 2021 all foreign currency remeasurement gains and losses were included in net loss as the Company has deemed the functional currency of its foreign subsidiary to be the U.S. Dollar. Accumulated other comprehensive loss includes foreign currency translation adjustments of $119 thousand for the year ended December 31, 2019 that were included in other comprehensive loss prior to the designation of the U.S. Dollar as the functional currency of X4 Austria. As of December 31, 2023, unrealized gains on the Company’s marketable debt security portfolio were nominal in included within interest income on the consolidated statement of operations and comprehensive loss.

Income Taxes— The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Net Loss per Share—Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. In addition, during the years ended December 31, 2022 and 2021, in accordance with the provisions of the Company’s Class B Warrants, the exercise price of each outstanding Class B Warrant was adjusted to the price of subsequent sales of common stock. Such adjustment is presented as a deemed dividend that adjusts net loss available to common shareholders for purposes of basic earnings per share. The deemed dividend is calculated using the Black-Scholes pricing model, taking into account historical volatility of the Company’s common stock and the estimated remaining life of the outstanding Class B Warrants.

Basic shares outstanding includes the weighted average effect of the Company’s outstanding prefunded warrants, the exercise of which requires little or no consideration for the delivery of shares of common stock. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive shares of common stock. For purpose of this calculation, outstanding stock options, unvested restricted stock units and warrants to purchase shares of common stock are considered potential dilutive shares of common stock.

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Other Income and Expenses [Abstract]
Accrued employee compensation and benefits	$ 8,195	$ 6,592
Accrued external research and development expenses	2,804	3,906
Accrued professional fees	1,195	571
Accrued deferred financing fees	0	591
Other	622	374
Accrued expenses	$ 12,816	$ 12,034

Long-Term Debt - Future Principal Payments (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Debt Disclosure [Abstract]
2024	$ 0
2025	24,720
2026	30,767
Long-term debt, including end-of-term payments	$ 55,487

Summary of Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Cash and cash equivalents	$ 99,216	$ 121,718	$ 81,787
Restricted cash, current (included within prepaid expenses and other current assets)	250	285	0
Other assets	782	1,025	1,321
Total cash, cash equivalents and restricted cash	$ 100,248	$ 123,028	$ 83,108	$ 80,702

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2023

Other Income and Expenses [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Accrued employee compensation and benefits	$	8,195	$	6,592
Accrued external research and development expenses	2,804		3,906
Accrued professional fees	1,195		571

Accrued deferred financing fees	—		591
Other	622		374
	$	12,816	$	12,034

License, Collaboration, and Funding Agreements (Details) $ in Millions	Dec. 31, 2023 USD ($)
Abbisko Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments receivable	$ 214.0

Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income (loss)	$ (101,167)	$ (93,867)	$ (88,696)
Deemed dividend as a result of Class B Warrant price reset	0	2,546	13,943
Net loss attributable to common stockholders	$ (101,167)	$ (96,413)	$ (102,639)
Weighted average shares of common stock outstanding - basic (in shares)	177,812,480	63,525,845	25,748,797
Weighted average shares of common stock outstanding - diluted (in shares)	177,812,480	63,525,845	25,748,797
Net loss per share attributable to common stockholders - basic (in dollars per share)	$ (0.57)	$ (1.52)	$ (3.99)
Net loss per share attributable to common stockholders - diluted (in dollars per share)	$ (0.57)	$ (1.52)	$ (3.99)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

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+ Details

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Schedule of Restricted Cash

Restricted Cash


(in thousands)	As of December 31, 2023		As of December 31, 2022
Letter of credit security: Waltham lease	$	250	$	250
Letter of credit security: Vienna Austria lease	211		205
Letter of credit security: Boston lease	571		855
Total restricted cash	$	1,032	$	1,310
Restricted cash included in prepaid expenses and other current assets	$	250	$	285
Restricted cash included in other assets	$	782	$	1,025

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash to Cash Flows


(in thousands)	December 31, 2023		December 31, 2022		December 31, 2021
Cash and cash equivalents	$	99,216	$	121,718	$	81,787
Restricted cash, current (included within prepaid expenses and other current assets)	250		285		—
Restricted cash, non-current (included within other assets)	782		1,025		1,321
Total cash, cash equivalents and restricted cash	$	100,248	$	123,028	$	83,108

Property and Equipment Useful Life

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


			Estimated Useful Life
Office furniture			3 to 7 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Net Loss per Share - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	89,372,324	91,422,816	12,290,280
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	80,244,959	87,720,773	9,449,128
Equity Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	6,008,541	2,021,480	1,916,051
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded form computation of diluted net loss per share (in shares)	3,118,824	1,680,563	925,101

License, Collaboration, and Funding Agreements	12 Months Ended
License, Collaboration, and Funding Agreements	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration, and Funding Agreements	LICENSE, COLLABORATION AND FUNDING ARRANGEMENTS Genzyme Agreement In July 2014, the Company entered into a license agreement with Genzyme (the “Genzyme Agreement”) pursuant to which the Company was granted an exclusive license to certain patents and intellectual property owned or controlled by Genzyme related to the CXCR4 receptor to develop and commercialize products containing licensed compounds (including but not limited to mavorixafor) for all therapeutic, prophylactic and diagnostic uses, with the exception of autologous and allogenic human stem cell therapy. Under the terms of the Genzyme Agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize licensed products for use in the field in the United States and at least one other major market country. The Company has the right to grant sublicenses of the licensed rights that cover mavorixafor to third parties. During 2023, a $5.0 million milestone was incurred upon the FDA’s acceptance of the Company’s NDA seeking approval of oral, once-daily mavorixafor in the treatment of people aged 12 years and older with WHIM syndrome. Such amount has been included within research and development expense and was paid in the fourth quarter of 2023. As of December 31, 2023, the Company is obligated to make future milestone payments in the aggregate amount of up to $20.0 million contingent upon the achievement by the Company of certain clinical-stage regulatory and sales milestones with respect to licensed products. A $7.0 million regulatory milestone payment, which would be payable upon the FDA’s approval of the Company’s NDA, is reasonably possible of becoming payable with the next five to six months under the Genzyme Agreement. The remaining regulatory milestones include (i) $3.0 million for the acceptance by the EMA of the Company’s first drug application and (ii) $5.0 million upon the notification by the EMA of regulatory approval of our first drug application. The Company must also make one-time sales milestone payments of $0.5 million, $1.5 million and $3.0 million on cumulative net sales of $50.0 million, $150.0 million and $300.0 million, respectively. The Company is also obligated to pay Genzyme tiered royalties based on net sales of licensed products that the Company commercializes under the agreement. Upon the first sale of the Company’s drug candidate in the U.S., the Company will incur a royalty on annual net sales at a rate of 6% up to $150 million, 10% on the portion of annual net sales between $150 million and $300 million, and 12% thereafter on annual sale over $300 million. The obligation to pay royalties for each licensed product expires on a country-by-country basis on the latest of (i) the expiration of licensed patent rights that cover that licensed product in that country, (ii) the expiration of regulatory exclusivity in that country and (iii) ten years after the first commercial sale of such licensed product in that country. Royalty rates are subject to reduction under the agreement in specified circumstances, including in any country if the Company is required to obtain a license from any third party to the extent the Company’s patent rights might infringe the third party’s patent rights, if a licensed product is not covered by a valid claim in that country or if sales of generic products reach certain thresholds in that country. If the Company enters into a sublicense under the Genzyme Agreement, the Company will be obligated to pay Genzyme a percentage of certain upfront fees, maintenance fees, milestone payments and royalty payments paid to the Company by the sublicensee. Under the Genzyme Agreement, the Company will itself manufacture and supply, or enter into manufacturing or supply agreements with Genzyme or third parties to manufacture and supply, clinical and commercial supplies of licensed compounds and each licensed product. The Company is also responsible for all costs related to the filing, prosecution and maintenance of the licensed patent rights. The Genzyme Agreement will remain in effect until the expiration of the royalty term in all countries for all licensed products. The Genzyme Agreement may be terminated by either party with at least 90 days’ notice in the event of material breach by the other party that remains uncured for 90 days, by either party for insolvency or bankruptcy of the other party, immediately by Genzyme if the Company challenges the licensed patents, or immediately by the Company if a material safety issue arises. For the years ended December 31, 2022 and 2021, the Company did not incur any payment obligations to Genzyme under the Genzyme Agreement. Georgetown Agreement In December 2016, the Company entered into a license agreement (the “Georgetown Agreement”) with Georgetown University (“Georgetown”) pursuant to which the Company obtained an exclusive, worldwide license to make, have made, use, sell, offer for sale and import of products covered by patent rights co-owned by Georgetown. The rights licensed to the Company are for all therapeutic, prophylactic and diagnostic uses in all disease indications in humans and animals. Under the terms of the Georgetown Agreement, the Company paid a one-time, upfront fee of $50 thousand, and the Company may be required to make milestone payments of up to an aggregate of $800 thousand related to commercial sales of its product. Under the Georgetown Agreement, the Company is solely responsible for all development and commercialization activities and costs in its respective territories. The Company is also responsible for all costs related to the filing, prosecution, and maintenance of the licensed patent rights. The term of the Georgetown Agreement will continue until the expiration of the last valid claim within the patent rights covering the product. Georgetown may terminate the agreement in the event (i) the Company fails to pay any amount and fails to cure such failure within 30 days after receipt of notice, (ii) the Company defaults in its obligation to obtain and maintain insurance and fails to remedy such breach within 45 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. The Company may terminate the Georgetown Agreement at any time upon at least 60 days’ written notice. During the years ended December 31, 2023, 2022 and 2021, the Company did not incur any payment obligations to Georgetown under the Georgetown Agreement and no milestone payments were made or due under the Georgetown Agreement. Beth Israel Deaconess Medical Center Agreement In December 2016, the Company entered into a license agreement (the “BIDMC Agreement”) with Beth Israel Deaconess Medical Center (“BIDMC”), pursuant to which the Company obtained an exclusive, worldwide license to make, have made, use, sell, offer for sale and import products covered by patent rights co-owned by BIDMC. The rights licensed to the Company are for all fields of use. Under the terms of the BIDMC Agreement, the Company paid a one-time, upfront fee of $20 thousand, and the Company is responsible for all future patent prosecution costs. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations because the acquired technology represented in-process research and development and had no alternative future use. The term of the BIDMC Agreement will continue until the expiration of the last valid claim within the patent rights covering the licensed products. BIDMC may terminate the agreement in the event (i) the Company fails to pay any amount and fails to cure such failure within 15 days after receipt of notice, (ii) the Company is in material breach of any material provision of the BIDMC Agreement and fails to remedy such breach within 60 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. The Company may terminate the BIDMC Agreement at any time upon at least 90 days’ written notice. The Company did not incur any payment obligations under the BIDMC Agreement during the years ended December 31, 2023, 2022 and 2021. Dana Farber Cancer Institute Agreement In November 2020, the Company entered into a license agreement (the “DFCI Agreement”) with the Dana Farber Cancer Institute (“DFCI”) pursuant to which the Company obtained a non-exclusive, royalty-bearing license to use, make, have made, develop, market, import, distribute, sell and have sold products covered by patent rights owned by DFCI. Under the terms of the DFCI Agreement, the Company paid a one-time, upfront fee of $25 thousand and approximately $35 thousand for reimbursement of DFCI’s past patent expenses relating to the patent rights. The Company will pay 25% of DFCI’s ongoing patent prosecution expenses and an annual license maintenance fee of $10 thousand in each of the first three years, $40 thousand in each of the subsequent three years and $50 thousand every year after that until commercialization. The Company may be required to make milestone payments of up to an aggregate of approximately $32.3 million related to development, regulatory and commercial sales events. No such milestone payments have been incurred to date. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in-process research and development and had no alternative future use. The term of the DCFI Agreement will continue until the expiration of the last valid claim within the patent rights covering the product. DFCI may terminate the agreement if certain events occur. Research and Development Incentive Program The Company participates in a research and development incentive program provided by the Austrian government whereby the Company is entitled to reimbursement by the Austrian government for a percentage of qualifying research and development expenses incurred by the Company’s subsidiary in Austria. Under the program, the reimbursement rate for qualifying research and development expenses incurred by the Company through X4 Austria is 14% for the current year. X4 Austria also participated in a COVID-19 incentive program, which provides reimbursement for qualified capital spending during a defined time period. The Company recognizes incentive income from Austrian research and development incentives when qualifying expenses have been incurred, there is reasonable assurance that the payment will be received, and the consideration can be reliably measured. Management has assessed the Company’s research and development activities and expenditures to determine which activities and expenditures are likely to be eligible under the research and development incentive program described above. At each reporting date, management estimates the reimbursable incentive income available to the Company based on available information at the time. As of the years ended December 31, 2023 and 2022, the amounts due under these programs were $0.6 million and $1.2 million, respectively, which is included in research and development incentive receivable on the consolidated balance sheets. During the years ended December 31, 2023, 2022 and 2021, the Company recorded $0.6 million, $0.5 million and $0.8 million, respectively, of income related to the program on the consolidated statement of operations and comprehensive loss within “other income”. Abbisko Agreement In July 2019, the Company entered into a license agreement with Abbisko (the “Abbisko Agreement”). Under the terms of the Abbisko Agreement, the Company granted Abbisko the exclusive right to develop, manufacture and commercialize mavorixafor in mainland China, Taiwan, Hong Kong and Macau, the (“Abbisko Territory”). The agreement provides Abbisko with the exclusive rights in the Abbisko Territory to develop and commercialize mavorixafor in combination with checkpoint inhibitors or other agents in multiple oncology indications. The Company retains the full rest-of-world rights to develop and commercialize mavorixafor outside of Greater China for all indications and the ability to utilize data generated pursuant to the Abbisko collaboration for rest-of-world development. Assuming mavorixafor is developed by Abbisko in six indications, the Company would be entitled to milestone payments of up to $214.0 million, which will vary based on the ultimate sales, if any, of the approved licensed products. In addition, upon commercialization of mavorixafor in the Abbisko Territory, the Company is eligible to receive a tiered royalty, with a percentage range in the low double-digits, on net sales of approved licensed products. Abbisko is obligated to use commercially reasonable efforts to develop and commercialize mavorixafor in the Abbisko Territory. Abbisko has responsibility for all activities and costs associated with the further development, manufacture and commercialization of mavorixafor in the Abbisko Territory. The Company determined that the future sale of clinical and commercial supply are optional goods that will be subject to the customer's future purchasing decisions and do not represent performance obligations in the Abbisko Agreement. The Company concluded that the amount to be charged for the clinical supply will be reflective of market value and, therefore, the Abbisko Agreement does not provide a discount on such supply that would be accounted for as material right at the outset of the contract. In arriving at these conclusions, the Company considered the complexity of the manufacturing process for the licensed compound and the potential ability for Abbisko to obtain the compound directly from other manufactures in the future. The Company expects that it will recognize revenue at a point in time when such clinical supply (and commercial supply, if applicable) is delivered to Abbisko in the future. The Company re-evaluates the transaction price, including its estimated variable consideration for milestones included in the transaction price and all constrained amounts, in each reporting period and as uncertain events are resolved or other changes in circumstances occur.

Net Loss per Share - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Mar. 03, 2022
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average shares of common stock outstanding - basic (in shares)	177,812,480	63,525,845	25,748,797
Weighted average shares of common stock outstanding - diluted (in shares)	177,812,480	63,525,845	25,748,797
Convertible preferred stock exercise price (in dollars per share)				$ 1.79
Minimum \| Pre Funded Warrant
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Convertible preferred stock exercise price (in dollars per share)	$ 0.01

License, Collaboration, and Funding Agreements - Georgetown Agreement (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payment on obligation under license agreement			$ 5,000,000
Georgetown
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
One time upfront payment	$ 50,000
Payments made	$ 800,000
Termination period	60 days
Payment on obligation under license agreement		$ 0	0	$ 0
Payment obligation		$ 0	$ 0	$ 0

Long-Term Debt - Hercules Loan Agreements (Details) - Hercules Loan Agreement - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Line of Credit Facility [Line Items]
Interest expense	$ 5,777	$ 3,989	$ 3,642
Debt discount amortization	$ 929	$ 918	$ 756

Fair Value of Financial Assets and Liabilities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment	$ 0	$ 0	$ 9,758
Goodwill	17,351	17,351	$ 17,351	$ 27,109
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability	15,693	23,141
Embedded derivative liability \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability	10	10
Level 3 \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability	15,693	23,141
Level 3 \| Embedded derivative liability \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability	$ 10	$ 10

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(4)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Subparagraph (b)(2)
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-URI https://asc.fasb.org//1943274/2147480524/946-210-50-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
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-Paragraph 6
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480524/946-210-50-6

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(9)(e))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
-Section S99
-Paragraph 5C
-Subparagraph (SX 210.12-13C(Column H)(Footnote 7))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480032/946-320-S99-5C

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(9)(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Paragraph 1
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Paragraph 5
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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Reference 22: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-10

Reference 24: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482907/825-10-50-10

+ Details

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References + Details

Long-Term Debt - Narrative (Details) - USD ($)		3 Months Ended	12 Months Ended		30 Months Ended
Long-Term Debt - Narrative (Details) - USD ($)	Jul. 01, 2024	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Mar. 31, 2021
Line of Credit Facility [Line Items]
Principal payments			$ 2,100,000	$ 800,000
Debt Instrument, Debt Covenant, Minimum Cash Required			450,000,000
Hercules Loan Agreement
Line of Credit Facility [Line Items]
Proceeds from line of credit					$ 55,000,000
Maximum borrowing capacity		$ 115,000,000	115,000,000
Credit outstanding		$ 32,500,000	$ 32,500,000
Variable interest rate			10.15%
Increase in interest rate		4.00%
Hercules Loan Agreement \| Prime Rate
Line of Credit Facility [Line Items]
Variable interest rate			3.15%
Hercules Loan Agreement \| Forecast
Line of Credit Facility [Line Items]
Periodic payment	$ 2,100,000
Hercules First Amended Loan Agreement
Line of Credit Facility [Line Items]
Debt Instrument, Debt Covenant, Minimum Cash Required			$ 20,000,000
Hercules Second Amended Loan Agreement
Line of Credit Facility [Line Items]
Effective interest rate		13.60%	13.60%
Principal payments			$ 0
Hercules Second Amended Loan Agreement \| Min Cash Test Date 1 [Member]
Line of Credit Facility [Line Items]
Line of Credit, Covenant, Minimum Cash		$ 20,000,000	$ 20,000,000

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Use of Estimates

Foreign Currency and Currency Translation

Concentrations of Credit Risk and Significant Suppliers

Cash and Cash Equivalents

Marketable Securities

Property and Equipment


			Estimated Useful Life
Office furniture			3 to 7 years
Computer equipment			3 years
Laboratory equipment			3 to 10 years
Leasehold improvements			Shorter of lease term or 10 years

Right-of-Use Assets and Leases

Impairment of Long-Lived Assets

Goodwill

Goodwill is tested quantitatively for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action, a significant decline in the price of the Company’s common stock, or unanticipated competition.

Fair Value Measurement

•Level 1—Quoted prices in active markets for identical assets or liabilities.

Segment Information

Research and Development Programs

Research and Development Cost

Patent Cost

Debt Issuance Costs

Stock-Based Compensation

Derivative Liabilities

Comprehensive Loss

Income Taxes

Net Loss per Share

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock	100.00%
Exercises in period, intrinsic value		$ 1	$ 13
Options granted, weighted average grant date fair value (in dollars per share)	$ 0.97	$ 1.38	$ 5.17
Total stock-based compensation	$ 8,687	$ 5,199	$ 6,180
Two Thousand Fifteen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, reserved for future issuance (in shares)	30,000
Two Thousand Seventeen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, reserved for future issuance (in shares)	181,000			6,700,000
Aggregate intrinsic value, options exercised	4.00%
Two Thousand Seventeen Equity Incentive Plan \| Employee Stock Purchase Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares issued in period (in shares)	257,963
Share-based compensation arrangement by share-based payment award, number of shares available for grant (in shares)	4,900,000
Two Thousand Nineteen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, reserved for future issuance (in shares)	1,500,000
Common stock, authorized (in shares)	6,300,000
Incentive Stock Options and Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	4 years
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, restricted stock units granted (in shares)	5,332,051
Share based compensation, restricted stock units granted (in dollars per share)	$ 1.76
Unrecognized compensation cost of stock based awards	$ 4,300
Unrecognized compensation cost of stock based awards, recognition period	2 years 6 months
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,900	$ 400
Maximum \| Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period (in years)	10 years
Maximum \| Non-Statutory Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	4 years
Minimum \| Non-Statutory Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares based compensation, vested period (in years)	3 years

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net loss	$ (101,167)	$ (93,867)	$ (88,696)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,687	5,199	6,180
Depreciation and amortization expense	419	513	499
Goodwill impairment	0	0	9,758
Non-cash lease expense	1,577	1,481	1,393
Accretion of debt discount	929	918	756
Change in fair value of warrant and derivative liability	(7,074)	2,881	366
Other	(227)	37	337
Changes in operating assets and liabilities:
Prepaid expenses, other current assets and research and development incentive receivable	(1,370)	(610)	(1,755)
Accounts payable	1,234	3,425	1,166
Accrued expenses and other long-term liabilities	1,608	3,803	(152)
Operating lease liabilities	(1,128)	(882)	(698)
Operating lease right-of-use asset, net of non-cash portion	0	0	(59)
Net cash used in operating activities	(96,512)	(77,102)	(70,905)
Cash flows from investing activities:
Purchase of property, equipment and intangible assets	(60)	(103)	(615)
Purchase of marketable securities	(16,823)	0	0
Sales and maturities of marketable securities	2,000	0	0
Net cash used in investing activities	(14,883)	(103)	(615)
Cash flows from financing activities:
Proceeds from exercise of stock options, warrants and pre-funded warrants and issuance of shares of common stock under employee stock purchase plans	8,712	208	260
Issuance costs for amendments to loan and security agreement and for the sale of warrants accounted for as a liability (Note 10)	(631)	(4,802)	0
Employee taxes paid related to net share settlement of vested restricted stock units	0	(12)	0
Proceeds from borrowings under loan and security agreements	22,500	0	0
Repayments of borrowings under loan and security agreement	(2,064)	(795)	0
Proceeds from sale of shares of common stock, redeemable common stock, warrants and pre-funded warrants, net of issuance costs	59,999	122,631	75,985
Payments for Repurchase of Common Stock	0	0	(2,000)
Net cash provided by financing activities	88,516	117,230	74,245
Effect of exchange rate changes on cash, cash equivalents and restricted cash	99	(105)	(319)
Net (decrease) increase in cash, cash equivalents and restricted cash	(22,780)	39,920	2,406
Cash, cash equivalents and restricted cash at beginning of period	123,028	83,108	80,702
Cash, cash equivalents and restricted cash at end of period	100,248	123,028	83,108
Supplemental disclosure of cash flow information
Cash paid for interest	4,604	3,006	2,906
Supplemental disclosure of non-cash investing and financing activities:
Lease Obligation Incurred	0	0	1,343
Issuance costs not yet paid	$ 0	$ 661	$ 24

Fair Value of Financial Assets and Liabilities - Debt Securities Available-for-Sale (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale, Amortized Cost	$ 14,995
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax	10
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax	5
Debt Securities, Available-for-Sale	15,000
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale, Amortized Cost	2,927
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax	3
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax	0
Debt Securities, Available-for-Sale	2,930
Federal Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale, Amortized Cost	12,068
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax	7
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax	5
Debt Securities, Available-for-Sale	$ 12,070

Leases - Schedule of Maturities of Lease Liabilities (Detail) $ in Thousands	Dec. 31, 2023 USD ($)
Leases [Abstract]
2024	$ 1,382
2025	1,411
2026	1,341
2027	288
2028 and thereafter	47
Total lease payments	4,469
Less: interest	(758)
Operating Lease, Liability	$ 3,711

Leases (Details) $ in Thousands				12 Months Ended
Leases (Details) $ in Thousands	Sep. 01, 2020 USD ($) ft²	Nov. 11, 2019 USD ($) ft²	Mar. 13, 2019 ft²	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Lessee, Lease, Description [Line Items]
Right-of-use assets				$ 5,650	$ 7,229
Lessee, Operating Lease, Security Deposit, Line Of Credit				600
Operating lease, liability				3,711
Lease liabilities				2,612	3,603
Current portion of lease liability				1,099	1,198
Right-of-use asset, amortization term			5 years
Fixed operating lease cost				2,084	2,080	$ 2,087
Short-term lease costs				0	0	42
Total lease expense				2,084	2,080	2,129
Operating cash outflows from operating leases				1,385	1,354	1,257
Leased assets obtained in exchange for new operating lease liabilities				$ 0	$ 0	$ 1,343
Weighted-average remaining lease term—operating leases				3 years 2 months 12 days	4 years	5 years
Weighted-average discount rate—operating leases				11.50%	11.40%	11.30%
Sublease income				$ 195	$ 196	$ 196
Waltham Lease
Lessee, Lease, Description [Line Items]
Office space area \| ft²			6,000
Boston Lease
Lessee, Lease, Description [Line Items]
Current Office Space Under Lease Agreement \| ft²		28,000
Renewal term		5 years
Lease not yet commenced, amount		$ 1,100
New Vienna Lease
Lessee, Lease, Description [Line Items]
Office space area \| ft²	1,200
Base rent	$ 288
Term of contract	7 years

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

FAIR VALUE OF FINANCIAL ASSETS AND LIABILITIES

The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values:


	Fair Value Measurements as of December 31, 2023 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents--money market funds and U.S. Treasury bills	$	76,856	$	4,985	$	—	$	81,841
Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes	—		15,000		—		15,000
	$	76,856	$	19,985	$	—	$	96,841
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10

Class C Warrant Liability (Note 10)					15,683		15,683
	$	—	$	—	$	15,693	$	15,693


	Fair Value Measurements as of December 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,170	$	2,858	$	—	$	73,028
	$	70,170	$	2,858	$	—	$	73,028
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability					23,131		23,131
	$	—	$	—	$	23,141	$	23,141

All marketable securities are classified as short-term investments as all are due within one year and include investments in U.S. Treasury notes, U.S. Treasury bills and federal government agency notes. The amortized cost of each investment, individually and in aggregate, approximates fair value. The Company evaluated each marketable security for impairment that is other-than-temporary and concluded that no marketable security was impaired as of December 31, 2023.

The Company’s cash equivalents consisted of money market funds invested in U.S. Treasury securities and direct investments in U.S. Treasury securities. The money market funds were valued based on quoted prices in active markets for identical assets, which represents a Level 1 measurement. U.S. Treasury securities were valued by using inputs observable in active markets for similar securities, which represents a Level 2 measurement in the fair value hierarchy.



(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
U.S. Treasury securities	$	2,927	$	3	$	—	$	2,930
Federal Government Agency Securities	12,068		7		5		12,070
Total available-For-sale debt securities	$	14,995	$	10	$	5	$	15,000

The following table provides a roll-forward of the aggregate fair values of the Company’s Class C and PIPE warrant liabilities and derivative liability, for which fair values are determined using Level 3 inputs:


(in thousands)	Embedded Derivative Liability		PIPE Warrant Liability		Class C Warrant Liability


Balance at December 31, 2020	$	455	$	—	$	—
Change in fair value	366		—		—
Balance at December 31, 2021	821		—		—
Issuance of Class C Warrants	—		41,249		21,526
Change in fair value	(811)		(2,495)		1,605
Reclassification to permanent equity	—		(38,754)		—
Balance at December 31, 2022	10		—		23,131
Reclassification to permanent equity upon exercise					(374)
Change in fair value	—		—		(7,074)
Balance at December 31, 2023	$	10	$	—	$	15,683

Valuation of Embedded Derivative Liability— The fair value of the embedded derivative liability recognized in connection with the Company’s loan agreement with Hercules (see Note 7), which is associated with additional fees due to Hercules upon events of default, was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of this embedded derivative liability, which is reported within other non-current liabilities on the consolidated balance sheets, is estimated by the Company at each reporting date based, in part, on the results of third-party valuations, which were prepared based on a discounted cash flow model that considered the timing and probability of occurrence of a redemption upon an event of default, the potential amount of prepayment fees or contingent interest upon an event of default and the Company’s risk-adjusted discount rate of 17%. As of December 31, 2023 and December 31, 2022, the fair value of this derivative liability was $10 thousand.

Warrant Liabilities—

PIPE Warrant Liability— On July 6, 2022, the Company issued warrants for the purchase of its common stock in a private placement (the “ PIPE Warrants”). Upon issuance, the holder’s exercise of the PIPE Warrants was conditioned on the Company increasing it authorized shares. As there were insufficient authorized shares available at the time of issuance, the PIPE Warrants were classified as a liability and measured at fair value. On September 1, 2022, upon shareholder approval of the increase to the Company’s authorized shares, the PIPE Warrants met all criteria required for permanent equity accounting and, accordingly, the Company remeasured the fair value of the warrant liability through “other income (expense)” and reclassified the fair value of the warrant liability to additional paid-in capital.

Class C Warrant Liability— In December 2022, the Company issued Class C Warrants for the purchase of shares of its common stock in a public offering. The Class C Warrants are accounted for as a liability on the consolidated balance sheet and are adjusted to fair value at period end through “other income (expense).”

The Company calculated the fair value of the PIPE Warrants and the Class C Warrants using the Black-Scholes option pricing model with the following inputs:


	PIPE Warrants				Class C Warrants
	7/6/2022 (issuance)		9/1/2022 (reclassification to permanent equity)		12/9/2022 (issuance)		December 31, 2022		December 31, 2023
Common stock price	$1.09		$1.04		$0.93		$0.99		$0.84
Risk-free interest rate	3.0	%	3.3	%	3.8	%	4.0	%	3.9	%
Expected term (in years)	5.0		4.9		5.0		4.9		3.9
Expected volatility	97.3	%	97.5	%	101.8	%	101.7	%	96.2	%
Expected dividend yield	—	%	—	%	—	%	—	%	—	%

Impairment of Goodwill

Goodwill is tested quantitatively for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. During the fourth quarter of 2021, the Company’s market capitalization, measured as the price of the Company’s common stock multiplied by common shares outstanding, was below the value of the Company’s net assets, including goodwill. As a result of the sustained decline in the market price of the Company’s common stock, the fair value of the Company’s single reporting unit, determined based on Company’s market capitalization on December 31, 2021, was lower than its carrying value and the Company concluded that goodwill was impaired. Accordingly, the Company recorded an impairment charge of $9.8 million to reduce the carrying amount of goodwill to $17.4 million as of December 31, 2021. The Company tested goodwill for impairment as of December 31, 2023 and 2022 concluded that goodwill was not further impaired. Should the market value of the Company’s common stock decline, additional impairment charges may be recorded in the future.

The following table provides a rollforward of the Company’s goodwill and accumulated impairment losses.


(in thousands)	Goodwill, Gross		Accumulated Impairment Loss		Goodwill
Goodwill at December 31, 2020	$	27,109	$	—	$	27,109
Impairment losses	—		(9,758)		(9,758)
Goodwill at December 31, 2021	27,109		(9,758)		17,351

Goodwill at December 31, 2022	27,109		(9,758)		17,351
Goodwill at December 31, 2023	$	27,109	$	(9,758)	$	17,351

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Cash, Cash Equivalents, and Short-Term Investments	$ 114,200
Accumulated deficit	477,905	$ 376,738
Net income (loss)	(101,167)	(93,867)	$ (88,696)
Net Cash Provided by (Used in) Operating Activities	(96,512)	$ (77,102)	$ (70,905)
Min Cash Test Date 1 [Member] \| Hercules Second Amended Loan Agreement
Restructuring Cost and Reserve [Line Items]
Line of Credit, Covenant, Minimum Cash	$ 20,000

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

- Definition

Amount of accumulated undistributed earnings (deficit).

+ References + Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted:


	Year Ended December 31,
	2023		2022		2021
Risk-free interest rate	4.1	%	3.4	%	1.0	%
Expected term (in years)	6.0		6.1		6.0
Expected volatility	93.9	%	96.0	%	97.3	%
Expected dividend yield	—	%	—	%	—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2023:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (in Years)	Aggregate Intrinsic Value (in Thousands)
Outstanding as of December 31, 2022	2,021,480	$	6.99	6.5	$	1
Granted	4,217,650	1.24
Exercised	(2,031)	1.50
Forfeited	(228,558)	6.60
Outstanding as of December 31, 2023	6,008,541	$	2.97	8.6	$	24
Exercisable as of December 31, 2023	1,313,306	$	8.62	5.8	$	—
Vested and expected to vest as of December 31, 2023	4,563,685	$	3.49	8.4	$	16

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2023 :


			Number of Shares
Unvested at December 31, 2022			1,680,563
Granted			5,332,051
Vested			(3,511,542)
Forfeited			(382,248)
Unvested at December 31, 2023			3,118,824

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the consolidated statements of operations as follows:


	Year Ended December 31,
(in thousands)	2023		2022		2021
Research and development expense	$	4,357	$	2,534	$	2,723
Selling, general and administrative expense	4,330		2,665		3,457
Total stock-based compensation	$	8,687	$	5,199	$	6,180

Fair Value of Financial Assets and Liabilities -Aggregate Fair Value of Warrant and Derivative Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reclassifications of Temporary to Permanent Equity	$ 101,167	$ (38,754)
Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reclassification to permanent equity upon exercise	(374)
Level 3 \| Embedded derivative liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	10	821	$ 455
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	0	(811)	366
Reclassifications of Temporary to Permanent Equity		0
Ending balance	10	10	821
Level 3 \| Warrant \| PIPE Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances		41,249
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	0	(2,495)	0
Reclassifications of Temporary to Permanent Equity		(38,754)
Ending balance	0	0	0
Level 3 \| Warrant \| Class C Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	23,131	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances		21,526
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	(7,074)	1,605	0
Reclassifications of Temporary to Permanent Equity		0
Ending balance	$ 15,683	$ 23,131	$ 0

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Year Ended December 31,
(in thousands, except share and per share data)	2023		2022		2021
Numerator:
Net loss	$	(101,167)	$	(93,867)	$	(88,696)
Deemed dividend as a result of Class B Warrant price reset	—		(2,546)		(13,943)
Net loss attributable to common stockholders	$	(101,167)	$	(96,413)	$	(102,639)
Denominator:
Weighted average shares of common stock—basic and diluted	177,812,480		63,525,845		25,748,797
Net loss per share attributable to common stockholders— basic and diluted	$	(0.57)	$	(1.52)	$	(3.99)

Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders

The Company excluded the following potential shares of common stock from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Year Ended December 31,
	2023	2022	2021
Options to purchase common stock	6,008,541	2,021,480	1,916,051
Unvested restricted stock units	3,118,824	1,680,563	925,101
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)	80,244,959	87,720,773	9,449,128
	89,372,324	91,422,816	12,290,280

Preferred and Common Stock Warrants

12 Months Ended

Dec. 31, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Preferred and Common Stock Warrants

COMMON STOCK WARRANTS

Class A Warrants

In connection with its issuance of common stock in public offerings that closed on April 16, 2019, the Company issued 3,900,000 Class A warrants, which are exercisable for shares of the Company's common stock or prefunded warrants to purchase shares of the Company's common stock. The Class A warrants have an exercise price of $13.20 per warrant, expire on April 15, 2024 and were immediately exercisable upon issuance.

Class B Warrants

In connection with its issuance of common stock in public offerings that closed on November 29, 2019, the Company issued 5,416,667 Class B warrants, which are exercisable for shares of the Company's common stock or prefunded warrants to purchase shares of the Company's common stock.

The Class B warrants were immediately exercisable upon issuance, had an initial exercise price of $15.00 per share, which in accordance with the Class B warrant agreement was subsequently reduced to the price at which the Company sold its common shares in public offering or private placements. The Class B warrants expired in accordance with the Class B warrant agreement on December 29, 2022, which was 30 calendar days from the date on which the Company issued a press release announcing top-line data from its Phase 3 clinical trial of mavorixafor for the treatment of patients with WHIM syndrome.

Q2 2022 PIPE Warrants

In connection with its issuance of common stock and prefunded warrants in a private placement that closed on July 6, 2022, the Company issued warrants to purchase an aggregate of 50,925,365 warrants (the “Warrants”) for the purchase of shares of common stock. Each Warrant has an exercise price equal to $1.095 per share and will expire on the date that is 60 months from their original issue date.

Class C Warrants

In connection with its issuance of common stock in public offerings that closed on December 9, 2022, the Company issued 65,525,894 Class C warrants, which are exercisable at two Class C warrants for one share of the Company's common stock or prefunded warrants to purchase shares of the Company's common stock. The Class C warrants have an exercise price of $1.50 per set of two Class C warrants, expire on December 9, 2027 and were immediately exercisable upon issuance.

Pre-funded Warrants

In connection with the sale of its common stock in public offerings and private placements, the Company has issued pre-funded warrants to purchase shares of its common stock. The price per pre-funded warrant represents the price per share sold in the public offering or private placement, minus a nominal exercise price of either $0.001 or $0.01 per share, in accordance with the applicable pre-funded warrant agreement. The pre-funded warrants are exercisable, subject to certain beneficial ownership restrictions, at any time after their original issuance and will not expire.

The following table provides a roll forward of outstanding warrants and pre-funded warrants for the purchase of shares of the Company’s common stock for the three years ended December 31, 2023:


	Number of warrants	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)


Outstanding and exercisable as of December 31, 2020	13,354,403	$	13.52	3.72
Issued	2,058,032
Exercised	(2,130,000)
Expired	(25,275)
Outstanding and exercisable as of December 31, 2021	13,257,160	$	7.96	2.72
Issued	104,531,257
Exercised	(500,100)
Expired	(5,416,567)
Outstanding and exercisable as of December 31, 2022	111,871,750	$	1.86	4.53
Issued	8,263,157
Exercised	(7,475,814)

Outstanding and exercisable as of December 31, 2023	112,659,093	$	1.88	3.53

As of December 31, 2023, the Company’s outstanding warrants and pre-funded warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price			Expiration Date
October 25, 2016	5,155	$	19.78		October 24, 2026
December 28, 2017	115,916	$	19.78		December 28, 2027
September 12, 2018	20,220	$	19.78		September 12, 2028
October 19, 2018	20,016	$	19.78		October 19, 2028
March 13, 2019	5,000	$	19.78		March 12, 2029
April 16, 2019	3,866,154	$	13.20		April 15, 2024
November 29, 2019	1,250,000	$	12.00	(a)	n/a
March 23, 2021	50,000	$	8.70	(b)	n/a
November 9, 2021	2,008,032	$	4.98	(c)	n/a
March 3, 2022	766,666	$	1.80	(d)	n/a
July 6, 2022	13,276,279	$	1.095	(e)	n/a
July 6, 2022	44,075,050	$	1.095		July 6, 2027
December 9, 2022	32,137,448	$	1.50		December 9, 2027
December 9, 2022	6,800,000	$	1.10	(f)	n/a
May 18, 2023	8,263,157	$	1.52	(g)	n/a
	112,659,093

(a) In November 2019, the Company received $11.999 per pre-funded warrant, or $21.0 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant; (b) In March 2021, the Company received $8.69 per pre-funded warrant, or $435 thousand in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant; (c) In November 2021, the Company received $4.97 per pre-funded warrant, or $10.0 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant; (d) In March 2022, the Company received $1.79 per pre-funded warrant, or $1.40 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant; (e) In July 2022, the Company received $1.094 per pre-funded warrant, or $14.5 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant; and (f) In December 2022, the Company received $1.099 per pre-funded warrant, or $7.5 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant; and (g) In May 2023, the Company received $1.519 per prefunded warrant, or $12.6 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant.

Preferred and Common Stock Warrants - Schedule of Outstanding Warrants (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Mar. 03, 2022
Number of warrants
Outstanding and exercisable as of December 31, 2020	111,871,750	13,257,160	13,354,403
Class Of Warrant Or Right Issued	8,263,157	104,531,257	2,058,032
Exercised (in shares)	(7,475,814)	(500,100)	(2,130,000)
Outstanding and exerciseable, Number of warrants, ending balance (in shares)	112,659,093	111,871,750	13,257,160	13,354,403
Issued		5,416,567	25,275
Weighted Average Exercise Price
Outstanding and exercisable warrants to purchase preferred shares, beginning balance (in dollars per share)	$ 1.86	$ 7.96	$ 13.52
Outstanding and exercisable, ending balance (in dollars per share)	$ 1.88	$ 1.86	$ 7.96	$ 13.52
Weighted Average Contractual Term (Years)	3 years 6 months 10 days	4 years 6 months 10 days	2 years 8 months 19 days	3 years 8 months 19 days
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	112,659,093	111,871,750	13,257,160	13,354,403
Convertible preferred stock exercise price (in dollars per share)					$ 1.79
Issuance on November 29, 2019
Number of warrants
Outstanding and exerciseable, Number of warrants, ending balance (in shares)	1,250,000
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	1,250,000
Convertible preferred stock exercise price (in dollars per share)	$ 12.00

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating Expenses [Abstract]
Research and development	$ 72,017	$ 61,058	$ 50,647
Selling, general and administrative	35,505	27,020	24,702
Goodwill impairment	0	0	9,758
Gain on sale of nonfinancial assets	0	(509)	0
Total operating expenses	107,522	87,569	85,107
Loss from operations	(107,522)	(87,569)	(85,107)
Nonoperating Income (Expense) [Abstract]
Interest income	4,582	219	10
Interest expense	(5,777)	(3,993)	(3,642)
Change in fair value of warrant and derivative liabilities	7,074	1,701	(366)
Other income (expense)	554	(4,197)	426
Total other income (expense), net	6,433	(6,270)	(3,572)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(101,089)	(93,839)	(88,679)
Income Tax Expense (Benefit)	78	28	17
Net loss and comprehensive loss	(101,167)	(93,867)	(88,696)
Temporary Equity Repurchase Of Convertible Preferred Stock	0	(2,546)	(13,943)
Net loss attributable to common stockholders	$ (101,167)	$ (96,413)	$ (102,639)
Net loss per share attributable to common stockholders - basic (in dollars per share)	$ (0.57)	$ (1.52)	$ (3.99)
Net loss per share attributable to common stockholders - diluted (in dollars per share)	$ (0.57)	$ (1.52)	$ (3.99)
Weighted average shares of common stock outstanding - basic (in shares)	177,812,480	63,525,845	25,748,797
Weighted average shares of common stock outstanding - diluted (in shares)	177,812,480	63,525,845	25,748,797

Preferred and Common Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Nov. 26, 2019	Apr. 16, 2019	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 09, 2022	Jul. 06, 2022	Mar. 03, 2022	Nov. 30, 2021	Nov. 09, 2021	Mar. 23, 2021	Dec. 31, 2020	Nov. 29, 2019
Class of Stock [Line Items]
Class Of Warrant Or Right Issued			8,263,157	104,531,257	2,058,032
Convertible preferred stock exercise price (in dollars per share)								$ 1.79
Warrant expiration period	30 days
Common stock par value (in dollars per share)			$ 0.001	$ 0.001
Class Of Warrant Or Right. Aggregate Proceeds From Warrants Issued						$ 7,500	$ 14,500	$ 1,400	$ 10,000		$ 435		$ 21,000
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights						$ 0.001	$ 0.001	$ 0.01		$ 0.01	$ 0.01		$ 0.001
Class of warrant or right, outstanding (in shares)			112,659,093	111,871,750	13,257,160							13,354,403
Minimum
Class of Stock [Line Items]
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights											0.001
Maximum
Class of Stock [Line Items]
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights											0.01
Funded
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)						1,099.000	$ 1.094	$ 1,790		$ 4.97	$ 8.69
Pre Funded Warrant
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)													11.999
Issuance On March 23, 2021
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)			$ 8.70
Class of warrant or right, outstanding (in shares)			50,000
Issuance on November Nine Two Thousand Twenty One
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)			$ 4.98
Class of warrant or right, outstanding (in shares)			2,008,032
Class A Warrant
Class of Stock [Line Items]
Class Of Warrant Or Right Issued		3,900,000
Convertible preferred stock exercise price (in dollars per share)		$ 13.20
Class B Warrants
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)													$ 15.00
Number of warrants issued (in shares)	5,416,667
Class C Warrants
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)						$ 1.50
Number of warrants issued (in shares)						65,525,894

License, Collaboration, and Funding Agreements - Research and Development Incentive Program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Other income (expense)	$ 554	$ (4,197)	$ 426
Research and Development Incentive Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount due under incentive receivable plan	600	1,200
Other income (expense)	600	$ 500	$ 800
Abbisko Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments receivable	$ 214,000
X4 Austria
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Reimbursement rate for research and development	0.14

Preferred and Common Stock Warrants (Tables)

12 Months Ended

Dec. 31, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of Outstanding Warrants Table

The following table provides a roll forward of outstanding warrants and pre-funded warrants for the purchase of shares of the Company’s common stock for the three years ended December 31, 2023:


	Number of warrants	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)


Outstanding and exercisable as of December 31, 2020	13,354,403	$	13.52	3.72
Issued	2,058,032
Exercised	(2,130,000)
Expired	(25,275)
Outstanding and exercisable as of December 31, 2021	13,257,160	$	7.96	2.72
Issued	104,531,257
Exercised	(500,100)
Expired	(5,416,567)
Outstanding and exercisable as of December 31, 2022	111,871,750	$	1.86	4.53
Issued	8,263,157
Exercised	(7,475,814)

Outstanding and exercisable as of December 31, 2023	112,659,093	$	1.88	3.53

Schedule of Stockholders' Equity Note, Warrants or Rights

As of December 31, 2023, the Company’s outstanding warrants and pre-funded warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price			Expiration Date
October 25, 2016	5,155	$	19.78		October 24, 2026
December 28, 2017	115,916	$	19.78		December 28, 2027
September 12, 2018	20,220	$	19.78		September 12, 2028
October 19, 2018	20,016	$	19.78		October 19, 2028
March 13, 2019	5,000	$	19.78		March 12, 2029
April 16, 2019	3,866,154	$	13.20		April 15, 2024
November 29, 2019	1,250,000	$	12.00	(a)	n/a
March 23, 2021	50,000	$	8.70	(b)	n/a
November 9, 2021	2,008,032	$	4.98	(c)	n/a
March 3, 2022	766,666	$	1.80	(d)	n/a
July 6, 2022	13,276,279	$	1.095	(e)	n/a
July 6, 2022	44,075,050	$	1.095		July 6, 2027
December 9, 2022	32,137,448	$	1.50		December 9, 2027
December 9, 2022	6,800,000	$	1.10	(f)	n/a
May 18, 2023	8,263,157	$	1.52	(g)	n/a
	112,659,093

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of Debt

Long-term debt consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Principal amount of long-term debt	$	55,000	$	32,500
Debt discount, net of accretion	(917)		(196)
Cumulative accrual of end of term payments	487		1,315
Long-term debt	54,570		33,619
Less: current portion of long-term debt	—		(1,315)
Long-term debt, net of current portion	$	54,570	$	32,304

Interest Income and Interest Expense Disclosure

The Company recognized interest expense under the Hercules Loan Agreement as follows:


(in thousands)	For the years ended
	2023		2022		2021
Total interest expense	$	5,777	$	3,989	$	3,642
Non-cash interest expense	$	929	$	918	$	756

Schedule of Maturities of Long-term Debt

As of December 31, 2023, future principal payments and accrued end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31	Total

2024	—
2025	24,720
2026	30,767
Long-term debt, including end-of-term payments	$	55,487

As of December 31, 2022, the Company had the intent and ability to refinance the short-term principal obligations of the Hercules Loan Agreement to long-term, as demonstrated by entering into the Second A&R Hercules Loan Agreement on January 6, 2023. Therefore, short-term debt on the December 31, 2022 consolidated balance sheet includes only the accrued portion of certain end-of-term payments due within one year of the consolidated balance sheet date that remained due within one year of the

consolidated balance sheet date following the January 6, 2023 refinancing.

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2023
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	NATURE OF THE BUSINESS AND BASIS OF PRESENTATION X4 Pharmaceuticals, Inc. (together with its subsidiaries, the “Company”) is a late-stage clinical biopharmaceutical company discovering and developing novel therapeutics for the treatment of rare diseases and those with limited treatment options, with a focus on conditions resulting from dysfunction of the immune system. The Company’s lead clinical candidate is mavorixafor, a small-molecule antagonist of the chemokine receptor CXCR4 that is being developed as an oral, once-daily therapy. Due to its ability to increase the mobilization of mature, functional white blood cells from the bone marrow into the bloodstream, the Company believes that mavorixafor has the potential to provide therapeutic benefit across a variety of chronic neutropenic disorders, including WHIM (Warts, Hypogammaglobulinemia, Infections, and Myelokathexis) syndrome (“WHIM”), a rare, primary immunodeficiency. Following announcement of positive top-line data from the Company’s pivotal, global, Phase 3 clinical trial in November 2022, the Company submitted a New Drug Application (“NDA”) to the U.S. Food and Drug Administration (“FDA”) in August 2023, seeking approval of oral, once-daily mavorixafor in the treatment of people aged 12 years and older with WHIM syndrome. The FDA accepted the NDA on October 30, 2023 for Priority Review, establishing a Prescription Drug User Fee Act (“PDUFA”) action date of April 30, 2024. As the Company prepares for a potential launch of mavorixafor for WHIM in the U.S. in the second quarter of 2024, the Company is also enrolling participants in a Phase 2 clinical trial evaluating the safety and efficacy of mavorixafor in people with certain chronic neutropenic disorders. The Company also expects to initiate a pivotal, global, Phase 3 clinical trial in the first half of 2024 in certain chronic neutropenic disorders. The Company is headquartered in Boston, Massachusetts and has a research facility in Vienna, Austria. Going Concern Assessment—In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40) (“ASU 2014-15”), the Company has evaluated whether there are certain conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company does not yet have an approved drug product. Since inception, the Company has incurred significant operating losses and negative cash flows from operations. As of December 31, 2023, the Company had $114.2 million of cash, cash equivalents and short-term marketable securities and an accumulated deficit of $477.9 million. For the year ended December 31, 2023, the Company’s net losses were $101.2 million and net cash used in operating activities of $96.5 million. The Company has a covenant under its Second Amended and Restated Loan and Security Agreement (the “Hercules Loan Agreement”) with Hercules Capital Inc. (“Hercules”), as most recently amended in August 2023, that requires that the Company currently maintain a minimum level of cash of $20 million, subject to adjustments beginning January 31, 2025. Based on its cash flow projections, as discussed below, the Company believes it would not maintain the minimum cash required to satisfy this covenant noted above beginning in the first quarter of 2025. In such event, the lender could require the repayment of all outstanding debt. Accordingly, management has concluded that the Company’s accumulated deficit, history of losses, future expected losses and negative cash flows met the ASC 205-40 standard for raising substantial doubt about the Company’s ability to continue as a going concern. The Company does not have adequate financial resources to fund its forecasted operating costs for at least one year after the date that these consolidated financial statements are issued. The accompanying consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. Accordingly, the consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. The Company’s cash flow projections exclude any gross profits related to the potential sale of the Company’s drug upon potential FDA approval of the Company’s NDA and the sale of any priority review voucher that may be received upon such approval of the Company’s NDA. Projected cash flows also exclude additional borrowings under the Hercules Loan Agreement, which would become available upon FDA approval of the Company’s NDA and exclude all other sources of external financing. To finance its operations in 2025 and beyond, the Company will need to raise additional capital, which cannot be assured. Unless and until the Company reaches profitability in the future, it will require additional capital to fund its operations, which could be raised through a combination of equity offerings, debt financings, other third-party funding, marketing and distribution arrangements and other collaborations and strategic alliances. If the Company is unable to obtain funding, it could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which would adversely affect its business prospects, or it may be unable to continue operations. Principles of Consolidation— The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH (“X4 Austria”), which is incorporated in Vienna, Austria, and X4 Therapeutics, Inc. All intercompany accounts and transactions have been eliminated.

Preferred and Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2023	May 18, 2023	Dec. 31, 2022	Dec. 09, 2022	Jul. 06, 2022	Mar. 03, 2022	Dec. 31, 2021	Nov. 30, 2021	Nov. 09, 2021	Mar. 23, 2021	Dec. 31, 2020	Nov. 29, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	112,659,093		111,871,750				13,257,160				13,354,403
Exercise Price (in dollars per share)						$ 1.79
Class Of Warrant Or Right. Aggregate Proceeds From Warrants Issued				$ 7,500	$ 14,500	$ 1,400		$ 10,000		$ 435		$ 21,000
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights				$ 0.001	$ 0.001	$ 0.01			$ 0.01	$ 0.01		$ 0.001
Funded
Class of Warrant or Right [Line Items]
Exercise Price (in dollars per share)				$ 1,099.000	$ 1.094	$ 1,790			$ 4.97	$ 8.69
Issuance On October 25, 2016
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,155
Exercise Price (in dollars per share)	$ 19.78
Issuance On December 28, 2017
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	115,916
Exercise Price (in dollars per share)	$ 19.78
Issuance On September 12, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	20,220
Exercise Price (in dollars per share)	$ 19.78
Issuance On October 19, 2018
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	20,016
Exercise Price (in dollars per share)	$ 19.78
Issuance On March 13, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	5,000
Exercise Price (in dollars per share)	$ 19.78
Issuance On April 16, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	3,866,154
Exercise Price (in dollars per share)	$ 13.20
Issuance on November 29, 2019
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	1,250,000
Exercise Price (in dollars per share)	$ 12.00
Issuance On March 23, 2021
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	50,000
Exercise Price (in dollars per share)	$ 8.70
Issuance on November Nine Two Thousand Twenty One
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	2,008,032
Exercise Price (in dollars per share)	$ 4.98
Issuance on March 3 2022
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	766,666
Exercise Price (in dollars per share)	$ 1.80
Issuance on July 2022 1
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	13,276,279
Exercise Price (in dollars per share)	$ 1.095
Issuance on July 6 2022 2
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	44,075,050
Exercise Price (in dollars per share)	$ 1.095
Issuance on December 9 2022
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)				32,137,448
Exercise Price (in dollars per share)	1.50
Issuance on December 9 2022 2
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)				6,800,000
Exercise Price (in dollars per share)	1.10
Issuance on May 18 2023
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)		8,263,157
Exercise Price (in dollars per share)	$ 1.52
Pre Funded Warrant
Class of Warrant or Right [Line Items]
Exercise Price (in dollars per share)												$ 11.999

Leases (Tables)

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

Lease, Cost

The components of lease expense for the three years ended December 31, 2023, 2022 and 2021 were as follows (dollar amounts in thousands):


	For the Year Ended December 31,
Lease Cost	2023			2022			2021
Fixed operating lease cost	$	2,084		$	2,080		$	2,087
Short-term lease costs	—			—			42
Total lease expense	$	2,084		$	2,080		$	2,129
Other information
Operating cash outflows from operating leases	$	1,385		$	1,354		$	1,257
Leased assets obtained in exchange for new operating lease liabilities	$	—		$	—		$	1,343
Weighted-average remaining lease term—operating leases	3.2 years			4.0 years			5.0 years
Weighted-average discount rate—operating leases	11.5		%	11.4		%	11.3		%
Sublease income	$	195		$	196		$	196

Lessee, Operating Lease, Liability, Maturity

Maturities of lease liabilities due under lease agreements that have commenced as of December 31, 2023 are as follows (in thousands):


Maturity of lease liabilities	Operating Leases
2024	$	1,382
2025	1,411
2026	1,341
2027	288
2028 and thereafter	47
Total lease payments	4,469
Less: interest	(758)
Total operating lease liabilities as of December 31, 2023	$	3,711

Net Loss per Share

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Net Loss Per Share

NET LOSS PER SHARE

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Year Ended December 31,
(in thousands, except share and per share data)	2023		2022		2021
Numerator:
Net loss	$	(101,167)	$	(93,867)	$	(88,696)
Deemed dividend as a result of Class B Warrant price reset	—		(2,546)		(13,943)
Net loss attributable to common stockholders	$	(101,167)	$	(96,413)	$	(102,639)
Denominator:
Weighted average shares of common stock—basic and diluted	177,812,480		63,525,845		25,748,797
Net loss per share attributable to common stockholders— basic and diluted	$	(0.57)	$	(1.52)	$	(3.99)

Basic and diluted weighted average shares of common stock outstanding for the years ended December 31, 2023, 2022 and 2021 includes the weighted average effect of 32.4 million, 24.2 million and 3.8 million pre-funded warrants, for the purchase of shares of common stock, for which the remaining unfunded exercise price is less than or equal to $0.01 per share. During the years ended December 31, 2022 and 2021, in accordance with the Class B Warrant agreement, the exercise price of each outstanding Class B Warrant was adjusted to the price of subsequent sales of common stock. Such adjustments are accounted for as a deemed dividend that adjusts net loss available to common shareholders for purposes of basic earnings per share. The deemed dividend was calculated using the Black-Scholes pricing model, taking into account historical volatility of the Company’s common stock and the estimated remaining life of the outstanding Class B Warrants.

The Company’s potentially dilutive securities included outstanding stock options, unvested restricted stock units and warrants to purchase shares of common stock for the three years ended December 31, 2023. These potentially dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share, and thus they are considered “anti-dilutive.” Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential shares of common stock from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Year Ended December 31,
	2023	2022	2021
Options to purchase common stock	6,008,541	2,021,480	1,916,051
Unvested restricted stock units	3,118,824	1,680,563	925,101
Warrants to purchase common stock (excluding prefunded warrants, which are included in basic shares outstanding)	80,244,959	87,720,773	9,449,128
	89,372,324	91,422,816	12,290,280

Income Taxes

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Income Taxes

INCOME TAXES

During the years ended December 31, 2023, 2022, and 2021, the Company recorded no income tax benefits for the net operating losses incurred and research and development credits generated due to the uncertainty of realizing a benefit from those items. The Company’s overall tax provision for each of the three years ended December 31, 2023 primarily related to its Austrian subsidiary and Security Corp subsidiary.

Loss before the provision for income taxes for the years ended December 31, 2023, 2022 and 2021 consisted of the following:


	Year Ended December 31,
(in thousands)	2023		2022		2021
United States	$	(102,126)	$	(94,742)	$	(89,865)
Foreign (Austria)	1,037		903		1,186
	$	(101,089)	$	(93,839)	$	(88,679)

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows:


	Year Ended December 31,
	2023		2022		2021
U.S. federal statutory income tax rate	(21.0)	%	(21.0)	%	(21.0)	%
State income taxes, net of federal benefit	(6.0)		(5.5)		(5.4)

Research and development tax credits	(1.4)		(1.0)		(0.9)

Other permanent differences	(0.2)		3.0		3.7
Change in deferred tax asset valuation allowance	29.3		23.2		23.7
Other	(0.7)		1.3		(0.1)
Effective income tax rate	—	%	—	%	—	%

Net deferred tax assets as of December 31, 2023 and 2022 consisted of the following:


	December 31,
(in thousands)	2023		2022
Net operating loss carryforwards	$	122,942	$	107,296
Tax credit carryforwards	8,416		6,694
Capitalized research and development expenses	28,402		16,004


Lease liabilities	755		1,016
Other	4,616		3,604
Total deferred tax assets	165,131		134,614
Valuation allowance	(163,994)		(133,112)
Deferred tax assets, net of valuation allowance	$	1,137	$	1,502
Right of use assets	1,137		1,502
Total deferred tax liabilities	$	1,137	$	1,502
Total deferred tax assets, net	$	—	$	—

As of December 31, 2023, the Company had U.S. federal and state net operating loss carryforwards of $400.0 million and $389.0 million, respectively, which may be available to offset future taxable income and begin to expire in 2031 and 2035, respectively. The Company has federal net operating losses $345.9 million, which do not expire, and $54.1 million of federal net operating losses generated prior to 2018 that will expire at various dates through 2037. In addition, as of December 31, 2023, the Company had foreign net operating loss carryforward of $62.7 million, which do not expire but are generally limited in their usage to an annual deduction equal to 75% of taxable income. As of December 31, 2023, the Company also had U.S. federal and state research and development tax credit carryforwards of $6.5 million and $2.5 million, respectively, which may be available to offset future tax liabilities and each begin to expire in 2032 and 2030, respectively.

The Tax Cuts and Jobs Act (the “Act”) was enacted on December 22, 2017. Under the Act, research and experimental expenditures incurred for tax years beginning after December 31, 2021 must be capitalized and amortized ratably over five or

fifteen years for tax purposes, depending on where the research activities are conducted. If the requirement to capitalize Section 174 expenditures is not modified, it may also impact our effective tax rate and our cash tax liability in future years.

As of December 31, 2023, uncertain tax position reserves recorded were $0.2 million for U.S. federal and state research and development tax credits.

The following table summarizes the Company’s reserve for uncertain tax positions for the three years ended December 31, 2023:


(in millions)	Reserve for Uncertain Tax Position
Balance as of December 31, 2020	$	0.3
Settlement of unrecognized tax benefit	(0.1)
Balance as of December 31, 2021	$	0.2

Balance as of December 31, 2022	$	0.2
Balance as of December 31, 2023	$	0.2

Utilization of the U.S. net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the U.S. net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization.

Each period, the Company evaluates the positive and negative evidence bearing upon its ability to realize its federal, state and foreign deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2023, 2022 and 2021.


	Year Ended December 31,
(in thousands)	2023		2022		2021
Valuation allowance, beginning of year	$	(133,112)	$	(111,835)	$	(92,197)
Increases recorded to income tax provision	(30,882)		(21,277)		(19,638)

Valuation allowance, end of year	$	(163,994)	$	(133,112)	$	(111,835)

The Company’s U.S. federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2020 through December 31, 2022. There are currently no pending income tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service and state tax authorities to the extent utilized in a future period. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482526/740-270-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-2

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

Income Taxes - Deferred Tax Valuation Allowance (Details) - SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance, beginning of year	$ (133,112)	$ (111,835)	$ (92,197)
Increases recorded to income tax provision	(30,882)	(21,277)	(19,638)
Valuation allowance, end of year	$ (163,994)	$ (133,112)	$ (111,835)

Income Taxes - Deferred Income Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Components of Deferred Tax Assets [Abstract]
Net operating loss carryforwards	$ 122,942	$ 107,296
Tax credit carryforwards	8,416	6,694
Capitalized research and development expenses	28,402	16,004
Lease liabilities	755	1,016
Other	4,616	3,604
Total deferred tax assets	165,131	134,614
Valuation allowance	(163,994)	(133,112)
Deferred tax assets, net of valuation allowance	1,137	1,502
Right of use assets	1,137	1,502
Total deferred tax liabilities	1,137	1,502
Total deferred tax assets, net	$ 0	$ 0

Income Taxes - Schedule of Income Loss Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
United States	$ (102,126)	$ (94,742)	$ (89,865)
Foreign (Austria)	1,037	903	1,186
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ (101,089)	$ (93,839)	$ (88,679)

- Definition

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(11))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-30

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 31
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-31

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05(b)(2))
-Publisher FASB
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 25
-Paragraph 1
-SubTopic 20
-Topic 940
-Publisher FASB
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Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(10))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(15))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

+ Details

Cover - USD ($) $ in Millions	12 Months Ended
Cover - USD ($) $ in Millions	Dec. 31, 2023	Mar. 18, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-38295
Entity Registrant Name	X4 PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-3181608
Entity Address, Address Line One	61 North Beacon Street
Entity Address, Address Line Two	4th Floor
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02134
City Area Code	857
Local Phone Number	529-8300
Title of 12(b) Security	Common Stock
Trading Symbol	XFOR
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 318.0
Entity Common Stock, Shares Outstanding		167,937,781
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement, (the “2024 Proxy Statement”) for its 2024 Annual Meeting of Stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant's fiscal year ended December 31, 2023, are incorporated by reference into Part III of this Annual Report on Form 10-K.
Entity Central Index Key	0001501697
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Document Financial Statement Error Correction [Flag]	false

License, Collaboration, and Funding Agreements - Genzyme Agreement (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payment on obligation under license agreement		$ 5,000,000
Collaborative Arrangement Transaction Statement Of Income Or Comprehensive Income Extensible Enumeration Not Disclosed Flag	false
Genzyme Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments	$ 20,000,000
Payment on obligation under license agreement	0		$ 0
Collaborative Arrangement, Rights and Obligations, Maximum Aggregate Milestone Payments, Per Product	7,000,000
Genzyme Agreement \| 6% Net Sales
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer	$ 150,000,000
Revenue, Performance Obligation, Percentage of Net Sale	0.06
Genzyme Agreement \| 10% Net Sales
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue, Performance Obligation, Percentage of Net Sale	0.10
Genzyme Agreement \| 10% Net Sales \| Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer	$ 150,000,000
Genzyme Agreement \| 10% Net Sales \| Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer	300,000,000
Genzyme Agreement \| 12% Net Sales
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer	$ 300,000,000
Revenue, Performance Obligation, Percentage of Net Sale	0.12

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 99,216	$ 121,718
Marketable securities	15,000	0
Research and development incentive receivable	562	1,152
Prepaid expenses and other current assets	7,298	5,807
Total current assets	122,076	128,677
Property and equipment, net	745	1,104
Goodwill	17,351	17,351
Right-of-use assets	5,650	7,229
Other assets	1,436	1,225
Assets, Total	147,258	155,586
Current liabilities:
Accounts payable	8,947	7,777
Accrued expenses	12,816	12,034
Current portion of lease liability	1,099	1,198
Long-term Debt, Current Maturities	0	1,315
Liabilities, Current, Total	22,862	22,324
Long-term debt, including accretion, net of discount	54,570	32,304
Lease liabilities	2,612	3,603
Warrant liability (Note 4)	15,683	23,131
Other liabilities	432	173
Total liabilities	96,159	81,535
Commitments and contingencies (Note 9)
Stockholders’ equity:
Common Stock	167	122
Additional paid-in capital	528,956	450,786
Accumulated other comprehensive loss	(119)	(119)
Accumulated deficit	(477,905)	(376,738)
Total stockholders’ equity	51,099	74,051
Total liabilities and stockholders’ equity	$ 147,258	$ 155,586

Fair Value of Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other liabilities	Other liabilities
Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	$ 96,841	$ 73,028
Fair value of derivative liability	15,693	23,141
Fair Value, Recurring \| Cash equivalents--money market funds and U.S. Treasury bills
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	81,841	73,028
Fair Value, Recurring \| Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	15,000
Fair Value, Recurring \| Embedded derivative liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	10	10
Fair Value, Recurring \| Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	15,683	23,131
Level 1 \| Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	76,856	70,170
Fair value of derivative liability	0	0
Level 1 \| Fair Value, Recurring \| Cash equivalents--money market funds and U.S. Treasury bills
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	76,856	70,170
Level 1 \| Fair Value, Recurring \| Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	0
Level 1 \| Fair Value, Recurring \| Embedded derivative liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	0	0
Level 1 \| Fair Value, Recurring \| Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability
Level 2 \| Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	19,985	2,858
Fair value of derivative liability	0	0
Level 2 \| Fair Value, Recurring \| Cash equivalents--money market funds and U.S. Treasury bills
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	4,985	2,858
Level 2 \| Fair Value, Recurring \| Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	15,000
Level 2 \| Fair Value, Recurring \| Embedded derivative liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	0	0
Level 2 \| Fair Value, Recurring \| Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability
Level 3 \| Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	0	0
Fair value of derivative liability	15,693	23,141
Level 3 \| Fair Value, Recurring \| Cash equivalents--money market funds and U.S. Treasury bills
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	0	0
Level 3 \| Fair Value, Recurring \| Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents--money market funds and U.S. Treasury bills	0
Level 3 \| Fair Value, Recurring \| Embedded derivative liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	10	10
Level 3 \| Fair Value, Recurring \| Warrant
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability	$ 15,683	$ 23,131

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Common stock par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized (in shares)	500,000,000	125,000,000
Common stock, outstanding (in shares)	167,434,595	121,667,250
Common stock, issued (in shares)	167,434,595	121,667,250

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Unrecognized tax benefits	$ 200	$ 200	$ 200	$ 300
Unrecognized Tax Benefits, Increase Resulting from Acquisition	$ 100

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,364	$ 2,332
Less: Accumulated depreciation and amortization	(1,619)	(1,228)
Property, plant and equipment, net	745	1,104
Depreciation and amortization expense	419	513	$ 499
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	228	228
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,301	1,268
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	160	173
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	24	24
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 651	$ 639

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Tax Contingency [Line Items]
Unrecognized tax benefits	$ 200	$ 200	$ 200	$ 300
Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	400,000
Operating loss carryforwards, not subject to expiration	345,900
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration	54,100
Unrecognized tax benefits	200
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	389,000
Foreign Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	62,700
Research Tax Credit Carryforward \| Domestic Tax Authority
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	6,500
Research Tax Credit Carryforward \| State and Local Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	$ 2,500

Property and Equipment, Net

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

PROPERTY AND EQUIPMENT

Property and equipment, net consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,301		1,268
Computer equipment	160		173
Software	24		24
Lab equipment	651		639
	2,364		2,332
Less: Accumulated depreciation and amortization	(1,619)		(1,228)
	$	745	$	1,104

Depreciation and amortization expense related to property and equipment was approximately $419 thousand, $513 thousand, and $499 thousand for the years ended December 31, 2023, 2022 and 2021, respectively.

Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Disclosure [Abstract]
Principal amount of long-term debt	$ 55,000	$ 32,500
Debt Instrument, Unamortized Discount (Premium), Net	(917)	(196)
Cumulative accrual of end of term payments	487	1,315
Long-Term Debt, Including Accretion	54,570	33,619
Long-term Debt, Current Maturities	0	(1,315)
Long-term debt, net of current portion	$ 54,570	$ 32,304

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Classification (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 8,687	$ 5,199	$ 6,180
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	4,357	2,534	2,723
Selling, general and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 4,330	$ 2,665	$ 3,457

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Commitment and Contingencies	12 Months Ended
Commitment and Contingencies	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES The Company has agreements with clinical research organizations (“CROs”) pursuant to which the Company and the CROs are conducting clinical trials of mavorixafor for the treatment of WHIM syndrome and chronic neutropenia disorders. The Company may terminate these agreements by providing notice pursuant to the contractual provisions of such agreements and would incur early termination fees. The Company also has agreements with contract manufacturing organizations (“CMOs”) for the production of mavorixafor for use in clinical trials and to support the validation of the CMO’s manufacturing process as part of the NDA process. The Company’s agreement with the CMO who produces batches of API for use in the Company’s clinical drug supply contains cancellation provisions that would require the Company to pay up to the full contract value upon cancellation. As of December 31, 2023, the Company has approximately $1.5 million of such commitments in place subject to cancellation provisions. License Agreements— See Note 3 for a summary of the Company’s license agreements, which commit the Company to contingent milestone and royalty fees based on future operational events. Indemnification Agreements— In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company to, among other things, indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnification obligations. The Company is not currently aware of any indemnification claims and has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2023 or December 31, 2022. Legal Proceedings— The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Leases - Schedule of Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Lease Cost
Fixed operating lease cost	$ 2,084	$ 2,080	$ 2,087
Short-term lease costs	0	0	42
Total lease expense	2,084	2,080	2,129
Other information
Operating cash outflows from operating leases	1,385	1,354	1,257
Leased assets obtained in exchange for new operating lease liabilities	$ 0	$ 0	$ 1,343
Weighted-average remaining lease term—operating leases	3 years 2 months 12 days	4 years	5 years
Weighted-average discount rate—operating leases	11.50%	11.40%	11.30%
Sublease income	$ 195	$ 196	$ 196

Common Stock, Redeemable Common Stock and Convertible Preferred Stock (converted to Common Stock) - Additional Information (Details) - USD ($)									3 Months Ended	12 Months Ended
	May 15, 2023	Dec. 07, 2022	Mar. 03, 2022	Jan. 14, 2022	Mar. 23, 2021	Mar. 18, 2021	Oct. 14, 2020	Aug. 07, 2020	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2021	Dec. 31, 2022	Dec. 09, 2022	Sep. 01, 2022	Jul. 06, 2022	Nov. 09, 2021	Dec. 31, 2020	Nov. 29, 2019	Nov. 26, 2019
Class of Stock [Line Items]
Common stock, authorized (in shares)										500,000,000		125,000,000		500,000,000
Common stock par value (in dollars per share)										$ 0.001		$ 0.001
Cash dividends										$ 0
Class of warrant or right, outstanding (in shares)										112,659,093	13,257,160	111,871,750					13,354,403
Convertible preferred stock exercise price (in dollars per share)			$ 1.79
Proceeds from common stock issuance						$ 55,000,000
Stock Repurchased and Retired During Period, Value											$ 0
Sale of Stock, Purchase Agreement, Shares Issued				230,414
Purchase Agreement, Shares Purchased				1,382,488
Accelerated Share Repurchases, Initial Price Paid Per Share				$ 2.17
Payments for Securities Purchased under Agreements to Resell				$ 3,000,000
Warrants and Rights Outstanding, Term		5 years
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights			0.01		$ 0.01								$ 0.001		$ 0.001	$ 0.01		$ 0.001
Warrants and Rights Outstanding, Transaction Fees									$ 2,900,000
Warrant
Class of Stock [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings										$ (2,500,000)
Warrant
Class of Stock [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value															$ 41,200,000
Pre Funded Warrant
Class of Stock [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value															$ 13,500,000
Equity Unit Purchase Agreements
Class of Stock [Line Items]
Purchase agreement, term							36 months
Lincoln Park Capital Fund, LLC
Class of Stock [Line Items]
Long-term Purchase Commitment, Amount				$ 50,000,000
Funded
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)			$ 1,790		$ 8.69								$ 1,099.000		$ 1.094	$ 4.97
Issuance On March 23, 2021
Class of Stock [Line Items]
Class of warrant or right, outstanding (in shares)										50,000
Convertible preferred stock exercise price (in dollars per share)										$ 8.70
Pre Funded Warrant
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)																		$ 11.999
Redeemable Common Stock
Class of Stock [Line Items]
Stock issued (in shares)											229,885
Stock Repurchased and Retired During Period, Value						$ 2,000,000					$ 1,875
Issuance On March 23, 2021
Class of Stock [Line Items]
Stock issued (in shares)						6,271,836
Issuance On March 23, 2021 \| Redeemable Preferred Stock
Class of Stock [Line Items]
Stock issued (in shares)						229,885
Pre Funded Warrant
Class of Stock [Line Items]
Class of warrant or right, outstanding (in shares)			766,666
Pre Funded Warrant \| Issuance On March 23, 2021
Class of Stock [Line Items]
Class of warrant or right, outstanding (in shares)										50,000
Q1 2022 PIPE
Class of Stock [Line Items]
Stock issued (in shares)			900,000
Sale price (in dollars per share)			$ 1.80
Proceeds from Issuance of Warrants			$ 3,000,000
Q2 2022 Private Placement
Class of Stock [Line Items]
Stock issued (in shares)										37,649,086
Proceeds from common stock issuance					$ 55,700,000
Sale price (in dollars per share)					$ 1.095
Number of warrants issued (in shares)										50,925,365
Warrants and Rights Outstanding, Term										60 months
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights					$ 0.001
Q2 2022 Private Placement \| Pre Funded Warrant
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)																			$ 1.094
Number of warrants issued (in shares)										13,276,279
Q4 2022 Public Offering
Class of Stock [Line Items]
Stock issued (in shares)		52,300,000
Proceeds from common stock issuance		$ 65,100,000
Sale price (in dollars per share)		$ 1.50
Number of warrants issued (in shares)		32,762,947
Q4 2022 Public Offering \| Class C Warrants
Class of Stock [Line Items]
Sale price (in dollars per share)		$ 1.10
Q4 2022 Public Offering \| Pre Funded Warrant
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)		$ 1.099
Number of warrants issued (in shares)		6,800,000
Q2 2023 PIPE
Class of Stock [Line Items]
Stock issued (in shares)	34,521,046
Proceeds from common stock issuance	$ 65,000,000
Sale price (in dollars per share)	$ 1.52
Number of warrants issued (in shares)	8,263,157
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights	$ 0.001
Q2 2023 PIPE \| Pre Funded Warrant
Class of Stock [Line Items]
Convertible preferred stock exercise price (in dollars per share)	$ 1.519
Sales Agents [Member] \| Common Stock
Class of Stock [Line Items]
Number of shares authorized in transaction, amount								$ 75,000,000
Common Stocks, Including Additional Paid in Capital										$ 14,300,000

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment

Property and equipment, net consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,301		1,268
Computer equipment	160		173
Software	24		24
Lab equipment	651		639
	2,364		2,332
Less: Accumulated depreciation and amortization	(1,619)		(1,228)
	$	745	$	1,104

Summary of Significant Accounting Policies - Schedule of Property, Plant and Equipment (Details)	Dec. 31, 2023
Office furniture \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Office furniture \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	7 years
Computer equipment
Property, Plant and Equipment [Line Items]
Useful life	3 years
Laboratory equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Laboratory equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	10 years
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	10 years

Accrued Expenses

12 Months Ended

Dec. 31, 2023

Other Income and Expenses [Abstract]

Accrued Expenses

ACCRUED EXPENSES

Accrued expenses consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Accrued employee compensation and benefits	$	8,195	$	6,592
Accrued external research and development expenses	2,804		3,906
Accrued professional fees	1,195		571

Accrued deferred financing fees	—		591
Other	622		374
	$	12,816	$	12,034

Long-Term Debt

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Long-Term Debt

LONG-TERM DEBT

Long-term debt consisted of the following:


(in thousands)	December 31, 2023		December 31, 2022
Principal amount of long-term debt	$	55,000	$	32,500
Debt discount, net of accretion	(917)		(196)
Cumulative accrual of end of term payments	487		1,315
Long-term debt	54,570		33,619
Less: current portion of long-term debt	—		(1,315)
Long-term debt, net of current portion	$	54,570	$	32,304

Hercules Loan Agreement

In October 2018, the Company entered into a Loan and Security Agreement, which has been subsequently amendment from time to time, with Hercules Capital Inc. In January 2023, the Company entered into a Second Amended and Restated Loan and Security Agreement (the “Second A&R Hercules Loan Agreement”) with Hercules Capital, Inc., as agent and lender, and Hercules Capital Funding IV LLC and Hercules Capital Funding Trust 2022-1, as lenders (collectively, “Hercules”). On August 2, 2023, Hercules and the Company entered into an amendment (the “Amendment”) to the Second A&R Hercules Loan Agreement, (as amended by the Amendment, the “Hercules Loan Agreement”) with Hercules. The Hercules Loan Agreement provides for a term loan facility of up to $115.0 million, under which the Company has borrowed an aggregate of $55.0 million of term loans to date representing the maximum borrowings as of December 31, 2023. The term loan facility includes:

(i) $32.5 million outstanding (the “Conversion Balance”) prior to effectiveness of the most recent amendment in August 2023;

(ii) a $22.5 million term loan tranche drawn upon the closing of the Amendment;

(iii) an additional tranche of up to $20.0 million, which will be available in either one or two drawings following potential U.S. approval of mavorixafor in individuals with WHIM syndrome (“Approval”) until the earlier of (A) 45 days following Approval and (B) September 30, 2024 in the case of the first drawing, and until December 15, 2024 in the case of a second drawing;

(iv) an additional tranche of $7.5 million, which will be available following achievement of a certain clinical development-related milestone through the earlier of (A) 45 days following achievement of such milestone and (B) December 15, 2024; and

(v) an additional tranche of up to $32.5 million, which will be available subject to approval by Hercules in its sole discretion.

Borrowings under the Hercules Loan Agreement accrue interest at a variable rate equal to the greater of (i) 10.15% or (ii) The Wall Street Journal prime rate plus 3.15%. In an event of default and until such event is no longer continuing, the interest rate applicable to borrowings would be increased by 4.0%. Borrowings are repayable in monthly interest-only payments through March 1, 2025, and in equal monthly payments of principal and accrued interest from April 1, 2025 (the “Amortization Date”) until the maturity date of the loans. The Amortization Date may be extended to October 1, 2026, if Approval occurs on or prior to

September 30, 2026 and no event of default occurs. The loans mature on October 1, 2026; provided, however, that such maturity date will be extended to July 1, 2027 if the Amortization Date is extended pursuant to the foregoing sentence. At the Company’s option, the Company may prepay all, but not less than all, of the outstanding borrowings, subject to a prepayment premium of 2% during the one-year period from January 6, 2024 to January 5, 2025 and 1% thereafter. In addition, the Hercules Loan Agreement provides for payment of end-of-term fees of $2.1 million plus 3.5% of the aggregate principal amount of loans drawn, if any, subsequent to the Amendment, payable upon the earlier of maturity or the repayment in full of all obligations under the Hercules Loan Agreement. Borrowings under the Hercules Loan Agreement are collateralized by substantially all of the Company’s personal property and other assets except for its intellectual property (but including rights to payment and proceeds from the sale, licensing or disposition of the intellectual property).

Under the Hercules Loan Agreement, the Company has agreed to affirmative and negative covenants. Prior to January 31, 2025, the Company must maintain cash in an account or accounts in which Hercules has a first priority security interest (“Qualified Cash”) in an aggregate amount equal to at least $20.0 million.

•On and after January 31, 2025, such amount must equal at least 20% of the aggregate principal amount of loans outstanding under the Hercules Loan Agreement.

•From and after January 31, 2025, the Company must maintain trailing six-month net product revenue of at least 55% of its forecast as approved by the Company’s Board of Directors (the “Performance Covenant”). However, the Performance Covenant will be waived during any period in which:

(i) the Company maintains Qualified Cash in an aggregate amount equal to at least 75% of loans outstanding under the Amended Loan Agreement or

(ii) both (x) the Company maintains a Market Capitalization (as defined in the Hercules Loan Agreement) of at least $450.0 million and (y) the Company maintains Qualified Cash in an aggregate amount equal to at least 45% of loans outstanding.

The Hercules Loan Agreement also restricts the Company’s ability to incur additional indebtedness, pay dividends, encumber its intellectual property, or engage in certain fundamental business transactions, such as mergers or acquisitions of other businesses, with certain exceptions.

The Company recognized interest expense under the Hercules Loan Agreement as follows:


(in thousands)	For the years ended
	2023		2022		2021
Total interest expense	$	5,777	$	3,989	$	3,642
Non-cash interest expense	$	929	$	918	$	756

The annual effective interest rate on the Hercules Loan Agreement as of December 31, 2023 was 13.6%. There were no principal payments due or paid under the Hercules Loan Agreement during the year ended December 31, 2023. End-of-term payments of $2.1 million and $0.8 million were paid during the years ended December 31, 2023 and 2022, respectively, in accordance with the Hercules Loan Agreement.

As of December 31, 2023, future principal payments and accrued end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31	Total

2024	—
2025	24,720
2026	30,767
Long-term debt, including end-of-term payments	$	55,487

consolidated balance sheet date following the January 6, 2023 refinancing.

Loss on Transfer of Nonfinancial Assets	12 Months Ended
Loss on Transfer of Nonfinancial Assets	Dec. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Loss on Transfer of Nonfinancial Assets	ON SALE OF NONFINANCIAL ASSETS During the year ended December 31, 2022, a third party, who had previously acquired rights to certain intellectual property from the Company, terminated the arrangement and transferred these rights back to the Company. Also during the year ended December 31, 2022, the Company transferred these rights to another third party in return for $0.5 million. The Company has no continuing involvement in any ongoing research and development activities associated with the intellectual property. The Company concluded that these third parties are "non-customers" as the underlying intellectual property transferred to and from these third parties supports potential drug candidates that are not aligned with the Company's strategic focus and, therefore, are not an output of the Company's ordinary activities. Accordingly, the Company accounted for the sale of the intellectual property as the sale of a non-financial asset under ASC Topic 610-20, Gains and Losses from the Derecognition of Nonfinancial Assets ("ASC 610-20"), and included the gain in gain on sale of non-financial asset for the year ended December 31, 2022.

Fair Value of Financial Assets and Liabilities - Goodwill Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Goodwill, Gross	$ 27,109	$ 27,109		$ 27,109
Goodwill, Impaired, Accumulated Impairment Loss	(9,758)	(9,758)	$ (9,758)	0
Goodwill	17,351	17,351	17,351	$ 27,109
Goodwill [Roll Forward]
Goodwill, beginning balance	17,351	17,351	27,109
Goodwill impairment	0	0	(9,758)
Goodwill, ending balance	$ 17,351	$ 17,351	$ 17,351

- Definition

+ References + Details

Stock-Based Compensation - Restricted Stock Units (Details) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2023 shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance (in shares)	1,680,563
Granted (in shares)	5,332,051
Vested (in shares)	(3,511,542)
Forfeited (in shares)	(382,248)
Nonvested ending balance (in shares)	3,118,824

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring Basis


	Fair Value Measurements as of December 31, 2023 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents--money market funds and U.S. Treasury bills	$	76,856	$	4,985	$	—	$	81,841
Marketable securities-U.S. Treasury notes, U.S. Treasury bills, and federal government agency notes	—		15,000		—		15,000
	$	76,856	$	19,985	$	—	$	96,841
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10

Class C Warrant Liability (Note 10)					15,683		15,683
	$	—	$	—	$	15,693	$	15,693


	Fair Value Measurements as of December 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,170	$	2,858	$	—	$	73,028
	$	70,170	$	2,858	$	—	$	73,028
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability					23,131		23,131
	$	—	$	—	$	23,141	$	23,141

Debt Securities, Available-for-Sale



(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
U.S. Treasury securities	$	2,927	$	3	$	—	$	2,930
Federal Government Agency Securities	12,068		7		5		12,070
Total available-For-sale debt securities	$	14,995	$	10	$	5	$	15,000

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation


(in thousands)	Embedded Derivative Liability		PIPE Warrant Liability		Class C Warrant Liability


Balance at December 31, 2020	$	455	$	—	$	—
Change in fair value	366		—		—
Balance at December 31, 2021	821		—		—
Issuance of Class C Warrants	—		41,249		21,526
Change in fair value	(811)		(2,495)		1,605
Reclassification to permanent equity	—		(38,754)		—
Balance at December 31, 2022	10		—		23,131
Reclassification to permanent equity upon exercise					(374)
Change in fair value	—		—		(7,074)
Balance at December 31, 2023	$	10	$	—	$	15,683

Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions

The Company calculated the fair value of the PIPE Warrants and the Class C Warrants using the Black-Scholes option pricing model with the following inputs:


	PIPE Warrants				Class C Warrants
	7/6/2022 (issuance)		9/1/2022 (reclassification to permanent equity)		12/9/2022 (issuance)		December 31, 2022		December 31, 2023
Common stock price	$1.09		$1.04		$0.93		$0.99		$0.84
Risk-free interest rate	3.0	%	3.3	%	3.8	%	4.0	%	3.9	%
Expected term (in years)	5.0		4.9		5.0		4.9		3.9
Expected volatility	97.3	%	97.5	%	101.8	%	101.7	%	96.2	%
Expected dividend yield	—	%	—	%	—	%	—	%	—	%

Schedule of Goodwill

The following table provides a rollforward of the Company’s goodwill and accumulated impairment losses.


(in thousands)	Goodwill, Gross		Accumulated Impairment Loss		Goodwill
Goodwill at December 31, 2020	$	27,109	$	—	$	27,109
Impairment losses	—		(9,758)		(9,758)
Goodwill at December 31, 2021	27,109		(9,758)		17,351

Goodwill at December 31, 2022	27,109		(9,758)		17,351
Goodwill at December 31, 2023	$	27,109	$	(9,758)	$	17,351

- Definition

Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-9

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
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-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
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-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
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-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
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-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

- Definition

Tabular disclosure of goodwill by reportable segment and in total which includes a rollforward schedule.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482548/350-20-55-24

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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+ Details

License, Collaboration, and Funding Agreements - Beth Israel Deaconess Medical Center Agreements (Details) - USD ($)	1 Months Ended		12 Months Ended
	Nov. 30, 2020	Dec. 31, 2016	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payment on obligation under license agreement				$ 5,000,000
Bidmc Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
One time upfront payment		$ 20,000
Payment on obligation under license agreement			$ 0	$ 0	$ 0
DFCI
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
One time upfront payment	$ 25,000
Payment on obligation under license agreement			$ 32,300,000
Collaborative Agreement , Expense Reimbursement	$ 35,000
Collaborative Arrangement, Obligation, Percentage			25.00%
Collaborative agreement, future expected annual fees, year one through three			$ 10,000
Collaborative agreement, future expected annual fees, year four through six			40,000
Collaborative agreement, future expected annual fees, thereafter			$ 50,000

Summary of Significant Accounting Policies - Schedule of Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 782	$ 1,025	$ 1,321
Restricted Cash	1,032	1,310
Restricted cash, current (included within prepaid expenses and other current assets)	$ 250	$ 285	$ 0
Restricted Cash, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
Letter of credit security: Waltham lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 250	$ 250
Letter of credit security: Vienna Austria lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	211	205
Letter of credit security: Boston lease \| Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 571	$ 855

Summary of Significant Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) reporting_unit	Dec. 31, 2021 USD ($)	Dec. 31, 2022 USD ($)
Property, Plant and Equipment [Line Items]
Other assets	$ 782	$ 1,321	$ 1,025
Number of reporting units \| reporting_unit	1
Currency translation adjustments		$ (119)
Debt Instrument, Debt Covenant, Minimum Cash Required	$ 450,000
Hercules First Amended Loan Agreement
Property, Plant and Equipment [Line Items]
Debt Instrument, Debt Covenant, Minimum Cash Required	$ 20,000

Leases

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

Leases

LEASES

The Company has lease agreements for its facilities in Boston, Massachusetts, which is the Company’s principal executive offices, and in Vienna, Austria, which is the Company’s research and development center. The Company’s Waltham, Massachusetts lease, which the Company had sublet to a third party, expired in December 2023. There are no restrictions or financial covenants associated with any of the lease agreements.

Boston Lease— The Company leases approximately 28,000 square feet of office space in Boston, Massachusetts (“Boston Lease”), which serves as the Company’s headquarters. Base rental payments are approximately $1.1 million annually, plus certain operating expenses. The term of the Boston Lease will continue until November 2026, unless earlier terminated. The Company has the right to sublease the premises, subject to landlord consent and also has the right to renew the Boston Lease for an additional five years at the then prevailing effective market rental rate. The Company is required to maintain a security deposit in the form of a letter of credit for $0.6 million for the benefit of the landlord.

Waltham Lease— The Company leased approximately 6,000 square feet of office space in Waltham, Massachusetts (“Waltham Lease”). The Waltham Lease, as amended, commenced on January 1, 2019, and expired on December 31, 2023. The Company was subleasing the space to a third party for the duration of the lease and such sublease income is reflected on the table below. The right-of-use asset was amortized to rent expense over the five year term of the lease and was retired upon expiration of the lease.

Vienna Austria Leases— The Company has an operating lease for approximately 1,200 square meters of laboratory and office space in Vienna, Austria (“Vienna Lease”), which commenced in February 2021 for a term of seven years. The annual base rent for the Vienna Lease is approximately $288 thousand.

The components of lease expense for the three years ended December 31, 2023, 2022 and 2021 were as follows (dollar amounts in thousands):


	For the Year Ended December 31,
Lease Cost	2023			2022			2021
Fixed operating lease cost	$	2,084		$	2,080		$	2,087
Short-term lease costs	—			—			42
Total lease expense	$	2,084		$	2,080		$	2,129
Other information
Operating cash outflows from operating leases	$	1,385		$	1,354		$	1,257
Leased assets obtained in exchange for new operating lease liabilities	$	—		$	—		$	1,343
Weighted-average remaining lease term—operating leases	3.2 years			4.0 years			5.0 years
Weighted-average discount rate—operating leases	11.5		%	11.4		%	11.3		%
Sublease income	$	195		$	196		$	196

Maturities of lease liabilities due under lease agreements that have commenced as of December 31, 2023 are as follows (in thousands):


Maturity of lease liabilities	Operating Leases
2024	$	1,382
2025	1,411
2026	1,341
2027	288
2028 and thereafter	47
Total lease payments	4,469
Less: interest	(758)
Total operating lease liabilities as of December 31, 2023	$	3,711