0001628280-22-014095 | 10-Q | XBRL Viewer

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Commitment and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Indemnification Agreements
Contingencies And Commitments [Line Items]
Accrued liabilities	$ 0.0	$ 0.0

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2022

Accounting Policies [Abstract]

Schedule of Restricted Cash

Restricted Cash


(in thousands)	As of March 31, 2022		As of December 31, 2021

Letter of credit security: Waltham lease	250		250
Letter of credit security: Vienna Austria lease	212		216
Letter of credit security: Boston lease	855		855


Restricted cash included in other assets	$	1,317	$	1,321

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

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


(in thousands)	March 31, 2022		December 31, 2021
Cash and cash equivalents	$	66,427	$	81,787

Restricted cash, non-current	1,317		1,321
Total cash, cash equivalents and restricted cash	$	67,744	$	83,108

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 3,784	$ 5,417
Accrued external research and development expenses	2,013	1,507
Accrued professional fees	762	632
Other	318	314
Total accrued expenses	$ 6,877	$ 7,870

Net Loss per Share - Additional Information (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Nov. 29, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average common shares outstanding—basic and diluted (in shares)	33,737,000	20,751,000
Pre Funded Warrant
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Exercise Price (usd per share)			$ 11.999
Pre Funded Warrant \| Maximum
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Exercise Price (usd per share)	$ 0.01

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Detail) - USD ($)	Mar. 31, 2022	Dec. 31, 2021
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 2,626,000	$ 47,793,000
Fair value of derivative liability	645,000	821,000
Fair Value, Measurements, Recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	0	20,000,000
Fair value of derivative liability	0	0
Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	2,626,000	27,793,000
Fair value of derivative liability	0	0
Fair Value, Measurements, Recurring \| Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	0	0
Fair value of derivative liability	645,000	$ 821,000
Measurement Input, Discount Rate
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	$ 0.14

Leases - Additional Information (Detail) $ in Thousands		3 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Nov. 11, 2019 USD ($) ft²	Mar. 31, 2022 USD ($) m² ft²	Dec. 31, 2021 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, liability		$ 5,591
Right-of-use assets		$ 8,344	$ 8,710
Waltham Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement \| ft²		6,000
Operating lease expiration (years)		5 years
Current base rent		$ 300
Lease, term of contract (years)		5 years
Vienna Lease
Lessee, Lease, Description [Line Items]
Office space \| m²		1,200
Lease not yet commenced, term of contract		7 years
Annual base rent		$ 300
Allston Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement \| ft²	28,000
Lease not yet commenced	$ 1,000
Lease, renewal term of contract (years)	5 years

Leases

3 Months Ended

Mar. 31, 2022

Leases [Abstract]

Leases

LEASES

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

•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 7 years. The annual base rent for the Vienna Lease is approximately $300 thousand.

•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 $1.0 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 $0.9 million for the benefit of the landlord.

•Waltham Lease— The Company leases approximately 6,000 square feet of office space in Waltham, Massachusetts (“Waltham Lease”). The Waltham Lease, as amended, commenced on January 1, 2019, and expires approximately five years from the commencement date. The base rent is approximately $0.3 million annually. In addition to the base rent, the Company is also responsible for its share of operating expenses, electricity and real estate taxes, which costs are not included in the determination of the leases’ right-of-use assets or lease liabilities. The Company is subleasing the space to a third party for the duration of the lease. The right-of-use asset is being amortized to rent expense over the five-year term of the lease.

As the Company’s leases do not provide an implicit rate, the Company estimated the incremental borrowing rate in calculating the present value of the lease payments. The Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term and amount equal to the lease payments in a similar economic environment.

The components of lease expense for the three months ended March 31, 2022 and 2021 were as follows:


(dollars in thousands)	For the Three Months Ended March 31,
Lease Cost	2022			2021
Fixed operating lease cost	$	530		$	476
Short-term lease costs	—			42
Total lease expense	$	530		$	518
Other information
Right-of-use asset obtained in exchange for operating lease liabilities	$	—		$	1,343
Operating cash flows from operating leases	$	342		$	323
Sublease income	$	49		$	49
Weighted-average remaining lease term—operating leases	4.7 years
Weighted-average discount rate—operating leases	11.3		%

Maturities of lease liabilities due under lease agreements that have commenced as of March 31, 2022 are as follows (in thousands)


Maturity of lease liabilities	Operating Leases
2022 (remainder of the year)	$	1,194
2023	1,619
2024	1,384
2025	1,412
2026	1,318
Thereafter	363
Total lease payments	7,290
Less: interest	(1,699)
Total operating lease liabilities as of March 31, 2022	$	5,591

Stock-Based Compensation

3 Months Ended

Mar. 31, 2022

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation

STOCK-BASED COMPENSATION

Summary of Plans— The Company has the following equity incentive plans:

•The X4 Pharmaceuticals Inc. 2015 Employee, Director and Consultant Equity Incentive Plan, as amended (the “2015 Plan”);

•The X4 Pharmaceuticals Inc. 2017 Equity Incentive Plan (the “2017 Plan”); and

•The X4 Pharmaceuticals Inc. 2019 Inducement Equity Incentive Plan (the “2019 Plan”)

The Company also has the following employee stock purchase plan:

•The X4 Pharmaceutical Inc. 2017 Employee Stock Purchase Plan (the “2017 ESPP”)

These plans are administered by the Board of Directors or by a committee of the Board of Directors. 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.

As of March 31, 2022, there are an aggregate of approximately 550,000 shares of common stock available for issuance under the Company’s equity incentive plans. Approximately 270,000 shares of common stock remain available for issuance under the 2017 ESPP.

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 to employees, directors and non-employees.


	Three Months Ended March 31,
	2022		2021
Risk-free interest rate	2.0	%	0.9	%
Expected term (in years)	6.1		6.0
Expected volatility	94.5	%	101.4	%
Expected dividend yield	0	%	0	%

Stock Options

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2022:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2021	1,916,051	$	10.01	7.8	$	—
Granted	232,850	1.85

Forfeited and Expired	(99,743)	23.52
Outstanding as of March 31, 2022	2,049,158	$	8.43	7.9	$	11
Exercisable as of March 31, 2022	1,029,146	$	10.44	6.8	$	—
Vested and expected to vest as of March 31, 2022	1,798,855	$	8.84	7.7	$	8

There were no stock options exercised in the three months ended March 31, 2022. The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2021 was $13 thousand. The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2022 and 2021 was $1.42 and $6.52, respectively.

Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the three months ended March 31, 2022:


			Number of Shares
Unvested as of December 31, 2021			925,101
Granted			1,242,756
Vested			(192,255)
Forfeited			(34,814)
Unvested as of March 31, 2022			1,940,788

During the three months ended March 31, 2022, the Company granted time-based restricted stock units to employees. These restricted stock units vest annually over a two or three year period. Stock-based compensation expense is being recognized for these time-based awards ratably based on the intrinsic value of the awards as of the date of grant, less estimated forfeitures.

Stock-Based Compensation— As of March 31, 2022, total unrecognized compensation expense related to unvested stock options and restricted stock units was $8.5 million, which is expected to be recognized over a weighted average period of 2.0 years.

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


	Three Months Ended March 31,
(in thousands)	2022		2021
Research and development expense	$	704	$	577
Selling, general and administrative expense	755		681
Total stock-based compensation	$	1,459	$	1,258

Common Stock Warrants

3 Months Ended

Mar. 31, 2022

Equity [Abstract]

Common Stock Warrants

COMMON STOCK AND COMMON STOCK WARRANTS

As of March 31, 2022 and December 31, 2021, the Company’s Restated Certificate of Incorporation authorized the Company to issue 125,000,000 shares of common stock, par value $0.001 per share. 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 any preferred stock that may be issued. 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.

On March 3, 2022, the Company entered into a securities purchase agreement pursuant to which it agreed to issue and sell to an investor (the “Investor”), in a private placement (the “Q1 2022 Private Placement”), 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 Private Placement closed on March 7, 2022 and the Company received gross proceeds of 3.0 million, before deducting offering expenses payable by the Company.

Also, on March 3, 2022, the Company entered into a registration rights agreement (the “Registration Rights Agreement”) with the Investor, pursuant to which the Company agreed to register for resale the shares of common stock purchased by the Investor and the shares of common stock underlying the pre-funded warrants held by the Investor (the “Registrable Securities”). Pursuant to the Registration Rights Agreement, the Company filed a registration statement covering the resale of the Registrable Securities on April 22, 2022.

In connection with its issuance of common stock in a public offering 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 pre-funded 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 and expire on a date that is the earlier of (a) the date that is 30 calendar days from the date on which the Company issues a press release announcing top-line data from its Phase 3 clinical trial of mavorixafor for the treatment of patients with WHIM syndrome (or, if such date is not a business day, the next business day) and (b) November 28, 2024. The Class B warrants have a contingent price adjustment feature pursuant to which the exercise price of the Class B warrants is adjusted to the lowest weighted average offering price at which the Company sells its common stock or certain securities convertible into or exercisable for the Company's common stock in one or more subsequent offerings, if the weighted

average offering price for such offering is below $15.00. On March 3, 2022, the exercise price of the Class B warrants was adjusted to $1.80 as a result of the Q1 2022 Private Placement.

In connection with previous equity offerings, the Company has issued pre-funded warrants, which are exercisable into one share of the Company's common stock, are immediately exercisable upon issuance, and for which the remaining exercise price is equal to or less than $0.01 per share.

The following table provides a roll forward of outstanding warrants for the three month period ended March 31, 2022:


	Number of warrants	Weighted Average Exercise Price	Weighted Average Contractual Term (Years)
Outstanding and exercisable warrants to purchase common shares as of December 31, 2021	13,257,160	$7.96	2.72
Issued	766,666	$1.80
Exercised	(100)

Outstanding and exercisable warrants to purchase common shares as of March 31, 2022	14,023,726	$6.91	2.47

As of March 31, 2022, the Company’s outstanding 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	5,416,567	$	1.80		November 28, 2024
November 29, 2019	1,750,000	$	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
	14,023,726

(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 $3.0 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant.

License, Collaboration, and Funding Agreements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Collaboration License And Funding Arrangements [Line Items]
Other income, net	$ 60	$ 186
Gain (Loss) on Disposition of Other Assets	500
Research and Development Incentive
Collaboration License And Funding Arrangements [Line Items]
Grant receivable	900
Other income, net	$ 100	$ 300

Long-Term Debt (Tables)

3 Months Ended

Mar. 31, 2022

Debt Disclosure [Abstract]

Summary of Long Term Debt

Long-term debt consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Principal amount of long-term debt	$	32,500	$	32,500
Debt discount, net of accretion	127		277
Cumulative accretion of end of term payments	597		1,157
Long-term debt	$	33,224	$	33,934
Less: current portion of long-term debt	$	(3,415)	$	(795)
Long-term debt, net of current portion	$	29,809	$	33,139

Interest Income and Interest Expense Disclosure

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


(in thousands)	For the Three Months ended March 31,
	2022		2021
Total interest expense	$	909	$	890
Non-cash interest expense	$	198	$	178

Schedule of Future Principal Payments and the Final Payments Due

As of March 31, 2022, future principal and end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31,	Total
2022	$	—
2023	19,802
2024	13,295
Long-term debt	$	33,097

Net Loss per Share - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	13,438,974	12,467,976
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	2,049,158	1,858,154
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	1,940,788	1,135,419
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	9,449,028	9,474,403

Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Cash and Cash Equivalents [Line Items]
Total restricted cash	$ 1,317	$ 1,321
Letter of Credit \| Waltham Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	250	250
Letter of Credit \| Vienna Austria Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	212	216
Letter of Credit \| Allston Lease Agreement
Cash and Cash Equivalents [Line Items]
Restricted Cash	$ 855	$ 855

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2022	Dec. 31, 2021
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000
Common stock, shares issued	30,809,476	28,127,657
Common stock, shares outstanding	30,809,476	28,127,657

Common Stock Warrants - Schedule of Outstanding Warrants (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Warrants [Roll Forward]
Outstanding and exercisable warrants to purchase common shares as of December 31, 2021	13,257,160
Issued	766,666
Exercised	(100)
Number of warrants, Outstanding and exercisable warrants, Ending Balance (in shares)	14,023,726	13,257,160
Class Of Warrant Or Right Issued, Weighted Average Exercise Price	$ 1.80
Warrants, Weighted Average Exercise Price [Roll Forward]
Weighted Average Exercise Price, Outstanding and exercisable warrants, Beginning Balance (in usd per share)	7.96
Weighted Average Exercise Price, Outstanding and exercisable warrants, Ending Balance (in usd per share)	$ 6.91	$ 7.96
Weighted Average Contractual Term (Years)	2 years 5 months 19 days	2 years 8 months 19 days

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows from operating activities:
Net loss	$ (21,965)	$ (18,676)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,459	1,258
Depreciation and amortization expense	133	106
Non-cash lease expense	366	314
Accretion of debt discount	198	178
Other	(120)	99
Changes in operating assets and liabilities:
Prepaid expenses, other current assets and research and development incentive receivable	1,633	(711)
Accounts payable	(713)	(23)
Accrued expenses	(984)	(786)
Lease liabilities	(236)	(160)
Operating lease right-of-use asset, net of non-cash portion	0	43
Net cash used in operating activities	(20,229)	(18,444)
Cash flows from investing activities:
Acquisition of property, equipment and intangible assets	(22)	(496)
Net cash used in investing activities	(22)	(496)
Cash flows from financing activities:
Proceeds from exercise of stock options and pre-funded warrants and issuance of shares of common stock under employee stock purchase plan	0	41
Employee taxes paid related to net share settlement of vested restricted stock units	(12)	0
Proceeds from borrowings under loan and security agreements, net of issuance costs	(114)	0
Repayments of Secured Debt	(795)	0
Proceeds from sale of shares of common stock, redeemable common stock and pre-funded warrants, net of issuance costs	5,877	55,000
Net cash provided by financing activities	4,956	55,041
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(69)	(143)
Net (decrease) increase in cash, cash equivalents and restricted cash	(15,364)	35,958
Cash, cash equivalents and restricted cash at beginning of period	83,108	80,702
Cash, cash equivalents and restricted cash at end of period	67,744	116,660
Supplemental disclosure of non-cash investing and financing activities:
Acquisition of property, equipment and right-of-use assets included in accounts payable and accrued expenses	0	17
Acquisition of right-of-use asset financed by lease liabilities	0	1,343
Issuance costs not yet paid related to sale of shares of common stock, redeemable common stock and pre-funded warrants	$ 0	$ 3,485

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2022

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Accrued employee compensation and benefits	$	3,784	5,417
Accrued external research and development expenses	2,013		1,507
Accrued professional fees	762		632

Other	318		314
	$	6,877	$	7,870

Leases (Tables)

3 Months Ended

Mar. 31, 2022

Leases [Abstract]

Schedule of Components of Lease Expense

The components of lease expense for the three months ended March 31, 2022 and 2021 were as follows:


(dollars in thousands)	For the Three Months Ended March 31,
Lease Cost	2022			2021
Fixed operating lease cost	$	530		$	476
Short-term lease costs	—			42
Total lease expense	$	530		$	518
Other information
Right-of-use asset obtained in exchange for operating lease liabilities	$	—		$	1,343
Operating cash flows from operating leases	$	342		$	323
Sublease income	$	49		$	49
Weighted-average remaining lease term—operating leases	4.7 years
Weighted-average discount rate—operating leases	11.3		%

Schedule of Maturities of Lease Liabilities

Maturities of lease liabilities due under lease agreements that have commenced as of March 31, 2022 are as follows (in thousands)


Maturity of lease liabilities	Operating Leases
2022 (remainder of the year)	$	1,194
2023	1,619
2024	1,384
2025	1,412
2026	1,318
Thereafter	363
Total lease payments	7,290
Less: interest	(1,699)
Total operating lease liabilities as of March 31, 2022	$	5,591

Accrued Expenses

3 Months Ended

Mar. 31, 2022

Payables and Accruals [Abstract]

Accrued Expenses

ACCRUED EXPENSESAccrued expenses consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Accrued employee compensation and benefits	$	3,784	5,417
Accrued external research and development expenses	2,013		1,507
Accrued professional fees	762		632

Other	318		314
	$	6,877	$	7,870

Leases - Schedule of Components of Lease Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Leases [Abstract]
Fixed operating lease cost	$ 530	$ 476
Short-term lease costs	0	42
Total lease expense	530	518
Right-of-use asset obtained in exchange for operating lease liabilities	0	1,343
Operating cash flows from operating leases	342	323
Sublease income	$ 49	$ 49
Weighted-average remaining lease term-operating leases (in years)	4 years 8 months 12 days
Weighted-average discount rate-operating leases	11.30%

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 66,427	$ 81,787
Research and development incentive receivable	868	747
Prepaid expenses and other current assets	3,586	5,344
Total current assets	70,881	87,878
Property and equipment, net	1,403	1,514
Goodwill	17,351	17,351
Right-of-use assets	8,344	8,710
Other assets	1,646	1,723
Total assets	99,625	117,176
Current liabilities:
Accounts payable	3,569	4,283
Accrued expenses	6,877	7,870
Current portion of lease liability	1,109	1,075
Current portion of long-term debt	3,415	795
Total current liabilities	14,970	14,023
Long-term debt, net of discount and current portion	29,809	33,139
Lease liabilities	4,482	4,776
Other liabilities	650	826
Total liabilities	49,911	52,764
Commitments and contingencies (Note 9)
Stockholders’ equity:
Common stock, $0.001 par value, 125,000,000 shares authorized as of each of March 31, 2022 and December 31, 2021; 30,809,476 and 28,127,657 shares issued and outstanding as of March 31, 2022 and December 31, 2021, respectively	31	28
Additional paid-in capital	354,638	347,374
Accumulated other comprehensive loss	(119)	(119)
Accumulated deficit	(304,836)	(282,871)
Total stockholders’ equity	49,714	64,412
Total liabilities and stockholders’ equity	$ 99,625	$ 117,176

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Cash and cash equivalents	$ 66,427	$ 81,787
Accumulated deficit	$ (304,836)	$ (282,871)

- Definition

The cumulative amount of the reporting entity's undistributed earnings or deficit.

+ References + Details

Stock-Based Compensation (Details)	3 Months Ended
Stock-Based Compensation (Details)	Mar. 31, 2022 shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)	1,242,756
Vested (in shares)	(192,255)
Forfeited (in shares)	(34,814)
Nonvested ending balance (in shares)	1,940,788
Restricted Stock Units (RSUs)
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)	925,101

Net Loss per Share

3 Months Ended

Mar. 31, 2022

Earnings Per Share [Abstract]

Net Loss per Share

NET LOSS PER SHAREBasic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Three Months Ended March 31,
(in thousands, except per share data)	2022		2021
Numerator:
Net loss	$	(21,965)	$	(18,676)
Deemed dividend as a result of Class B warrant price reset (Note 10)	(2,259)		(8,239)
Net loss attributable to common stockholders	$	(24,224)	$	(26,915)
Denominator:
Weighted average shares of common stock outstanding—basic and diluted	33,737		20,751
Net loss per share attributable to common stockholders— basic and diluted	$	(0.72)	$	(1.30)

Basic and diluted weighted average shares of common stock outstanding for the three months ended March 31, 2022 and March 31, 2021 include the weighted average effect of outstanding pre-funded warrants for the purchase of shares of common stock for which the remaining unfunded exercise price is $0.01 or less per share. During the quarters ended March 31, 2022 and March 31, 2021, in accordance with the Class B Warrant agreement, the exercise price of each outstanding Class B Warrant was adjusted to the price of shares of the Company’s common stock sold in public or private offerings to the extent such price is lower than the previous Class B warrant price. These price 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 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.

The Company’s potentially dilutive securities include outstanding stock options, unvested restricted stock units and warrants to purchase shares of common stock for the three months ended March 31, 2022 and 2021. All 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 shares of common stock 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, presented based on amounts outstanding at each period end, 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:


	Three Months Ended March 31,
	2022	2021
Options to purchase shares of common stock	2,049,158	1,858,154
Unvested restricted stock units	1,940,788	1,135,419
Warrants to purchase shares of common stock (excluding prefunded warrants, which are included in basic shares outstanding)	9,449,028	9,474,403
	13,438,974	12,467,976

License, Collaboration, and Funding Agreements	3 Months Ended
License, Collaboration, and Funding Agreements	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration, and Funding Agreements	LICENSE, COLLABORATION AND FUNDING AGREEMENTS 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 and capital expenditures incurred by the Company’s subsidiary in Austria. As of March 31, 2022, the amount due under the program is $0.9 million, which amount was included in research and development incentive receivable in the condensed consolidated balance sheet. During the three months ended March 31, 2022 and 2021, the Company recorded $0.1 million and $0.3 million, respectively, of income related to the program within the condensed consolidated statements of operations as other income. License and Collaboration Agreements During the three months ended March 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 three months ended March 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 as part of gain on sale of non-financial asset for the period ended March 31, 2022. There were no material modifications of the Company’s license or collaboration agreements during the three months ended March 31, 2022.

Long-Term Debt - Summary of Long Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Disclosure [Abstract]
Principal amount of long-term debt	$ 32,500	$ 32,500
Debt discount, net of accretion	127	277
Cumulative accretion of end of term payments	597	1,157
Long-term debt	33,224	33,934
Current portion of long-term debt	(3,415)	(795)
Long-term debt, net of current portion	$ 29,809	$ 33,139

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References + Details

Property and Equipment, Net

3 Months Ended

Mar. 31, 2022

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

PROPERTY AND EQUIPMENT, NET

Property and equipment, net consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,254		1,251
Computer equipment	169		150
Software	33		33
Lab equipment	576		576
	2,260		2,238
Less: Accumulated depreciation and amortization	(857)		(724)
	$	1,403	$	1,514

Depreciation and amortization expense related to property and equipment was $133 thousand and $106 thousand for the three months ended March 31, 2022 and 2021, respectively.

Common Stock Warrants - Additional Information (Detail) - USD ($)				3 Months Ended
	Mar. 23, 2021	Nov. 29, 2019	Nov. 26, 2019	Mar. 31, 2022	Mar. 03, 2022	Dec. 31, 2021	Nov. 09, 2021	Nov. 02, 2020	Apr. 16, 2019
Class of Stock [Line Items]
Issued				766,666
Class of warrants or rights expiration period (in years)			30 days
Common stock, par value (in usd per share)				$ 0.001		$ 0.001
Number of Shares of Common Stock Issuable (in shares)				14,023,726		13,257,160
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights	$ 0.01	$ 0.001						$ 0.01
Class Of Warrant Or Right. Aggregate Proceeds From Warrants Issued	$ 435,000	$ 21,000,000			$ 3,000,000		$ 10,000,000
Common stock, shares authorized				125,000,000
Dividends on common stock declared or paid				$ 0
Stock Issued During Period, Shares, New Issues				900,000
Sale of stock, price per share (in usd per share)	$ 1.80
Class of warrant or right, warrants to purchase of common stock (in shares)				766,666
Proceeds from issuance of common stock before underwriting discounts commissions and other expenses	$ 3,000,000
Funded
Class of Stock [Line Items]
Warrant exercise price (usd per share)	$ 8.69				$ 1.79		$ 4.97
Class B Warrants
Class of Stock [Line Items]
Issued		5,416,667
Warrant exercise price (usd per share)		$ 15.00
Pre Funded Warrant
Class of Stock [Line Items]
Warrant exercise price (usd per share)		$ 11.999
Number of warrants for purchase of convertible preferred shares (in shares)									1

Fair Value of Financial Assets and Liabilities (Tables)

3 Months Ended

Mar. 31, 2022

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value

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 March 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	—	$	2,626	$	—	$	2,626
	$	—	$	2,626	$	—	$	2,626
Liabilities:
Embedded derivative liability	$	—	$	—	$	645	$	645
	$	—	$	—	$	645	$	645


	Fair Value Measurements as of December 31, 2021 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	20,000	$	27,793	$	—	$	47,793
	$	20,000	$	27,793	$	—	$	47,793
Liabilities:
Embedded derivative liability	$	—	$	—	$	821	$	821
	$	—	$	—	$	821	$	821

Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability

The following table provides a roll-forward of the aggregate fair values financial instruments for which fair values are determined using Level 3 inputs:


(in thousands)	Embedded Derivative Liability
Balance as of December 31, 2021	$	821
Change in fair value	(176)
Balance as of March 31, 2022	$	645

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical)	3 Months Ended
	Dec. 31, 2021 USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance costs

Long-Term Debt

3 Months Ended

Mar. 31, 2022

Debt Disclosure [Abstract]

Long-Term Debt

Long-term debt consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Principal amount of long-term debt	$	32,500	$	32,500
Debt discount, net of accretion	127		277
Cumulative accretion of end of term payments	597		1,157
Long-term debt	$	33,224	$	33,934
Less: current portion of long-term debt	$	(3,415)	$	(795)
Long-term debt, net of current portion	$	29,809	$	33,139

Hercules Loan Agreement, As Amended

In October 2018, the Company entered into a Loan and Security Agreement (the “Hercules Loan Agreement”), as amended in December 2018, June 2019, March 2020, December 2020 and February 2022, with Hercules, under which the Company has borrowed an aggregate of $32.5 million of term loans to date. The Hercules Loan Agreement provides for maximum borrowings of up to $50.0 million, which include, subject to Hercules investment committee’s sole discretion, a right of the Company to request that Hercules make additional term loan advances in an aggregate amount of up to $17.5 million through December 31, 2022. Borrowings under the Hercules Loan Agreement accrues interest at a variable rate equal to the greater of (i) 8.75% or (ii) 8.75% plus The Wall Street Journal prime rate minus 6.0%. 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 under the Hercules Loan Agreement are repayable in monthly interest-only payments through January 1, 2023, and in equal monthly payments of principal and accrued interest from February 1, 2023 until the maturity date of the loan, which is July 1, 2024. The Company may prepay all, but not less than all, of the outstanding borrowings, subject to a prepayment premium of up to 1.0% of the principal amount outstanding as of the date of repayment. In addition, the Hercules Loan Agreement provides for payments of $1.3 million and $0.8 million payable on July 1, 2023 and July 1, 2024, respectively, which payments are accelerated upon the prepayment of the borrowings upon the Company’s election on upon default of the loan. Borrowings under the Hercules Loan Agreement are collateralized by substantially all of the Company’s personal property and other assets except for their intellectual property (but including rights to payment and proceeds from the sale, licensing or disposition of the intellectual property).

As noted above, on February 9, 2022, the Company entered into Amendment No. 3 the Hercules Loan Agreement. The amendment primarily added a financial milestone that if met by June 30, 2022 would modify the minimum cash covenant contained in the Hercules Loan Agreement, which is effective beginning September 1, 2022, as further described below. The Company concluded that the amendment represented a modification to the debt. Accordingly, fees paid to third parties directly related to the amended debt were expensed as incurred and fees paid to Hercules in conjunction with the amendment were deferred and will be amortized to interest expense over the life of the debt arrangement using the effective interest method.

Pursuant to the Hercules Loan Agreement, effective as of the earlier of (a) certain specified events impacting the Company’s Phase III trial of mavorixafor for the treatment of WHIM syndrome and (b) September 1, 2022, the Company at all times thereafter must maintain cash in an account or accounts in which Hercules has a first priority security interest, in an aggregate amount greater than or equal to the greater of (i) $30.0 million or (ii) 6 multiplied by a metric based on prior months’ cash expenditures; provided, however, that from and after the Company’s achievement of certain financing milestones through June 30, 2022, the Company must maintain cash in an account or accounts in which Hercules has a first priority security interest in an aggregate amount of $30.0 million. Following the achievement of certain operational performance milestones, the required level shall be reduced to the greater of $20.0 million, or, if the Company had not met the financing milestones noted above, 3 multiplied by the current cash expenditures metric; and provided further, that subject to the achievement of certain milestones, this covenant will be extinguished. 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 Three Months ended March 31,
	2022		2021
Total interest expense	$	909	$	890
Non-cash interest expense	$	198	$	178

The annual effective interest rate of the Hercules Loan Agreement as of March 31, 2022 is 10.7%. There were no principal payments due or paid under the Hercules Loan Agreement during the three months ended March 31, 2022. An end-of-term payment of $0.8 million was paid during the three months ended March 31, 2022 in accordance with the Hercules Loan Agreement.

As of March 31, 2022, future principal and end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31,	Total
2022	$	—
2023	19,802
2024	13,295
Long-term debt	$	33,097

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, options exercised	$ 13
Options granted, weighted average grant date fair value (in usd per share)	$ 1.42	$ 6.52
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost of stock based awards	$ 8,500
Unrecognized compensation cost of stock based awards, recognition period	2 years
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for issuance	550,000
Two Thousand Seventeen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for issuance	270,000

Stock-Based Compensation Summary of Stock-Based Compensation Expense Classification (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,459	$ 1,258
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	704	577
Selling, general and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 755	$ 681

Commitment and Contingencies	3 Months Ended
Commitment and Contingencies	Mar. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Commitment and Contingencies	COMMITMENTS AND CONTINGENCIES The Company has agreements with Contract Research Organizations (“CROs”) pursuant to which the Company and the CROs are conducting clinical trials of mavorixafor for the treatment of WHIM syndrome, Waldenström’s and SCN 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. 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 condensed consolidated financial statements as of March 31, 2022 or December 31, 2021. 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 any legal proceedings.

Fair Value of Financial Assets and Liabilities

3 Months Ended

Mar. 31, 2022

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 March 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	—	$	2,626	$	—	$	2,626
	$	—	$	2,626	$	—	$	2,626
Liabilities:
Embedded derivative liability	$	—	$	—	$	645	$	645
	$	—	$	—	$	645	$	645


	Fair Value Measurements as of December 31, 2021 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	20,000	$	27,793	$	—	$	47,793
	$	20,000	$	27,793	$	—	$	47,793
Liabilities:
Embedded derivative liability	$	—	$	—	$	821	$	821
	$	—	$	—	$	821	$	821

The Company’s cash equivalents consisted of money market funds invested in U.S. Treasury securities. The money market funds were valued based on reported market pricing for the identical assets, which represents a Level 1 measurement, or by using inputs observable in active markets for similar securities, which represents a Level 2 measurement.

The following table provides a roll-forward of the aggregate fair values financial instruments for which fair values are determined using Level 3 inputs:


(in thousands)	Embedded Derivative Liability
Balance as of December 31, 2021	$	821
Change in fair value	(176)
Balance as of March 31, 2022	$	645

Embedded Derivative Liability— The fair value of the embedded derivative liability recognized in connection with the Company’s loan agreement with Hercules (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 condensed consolidated balance sheets, is estimated by the Company at each reporting date based, in part, on the results of third party valuations, which are prepared based on a discounted cash flow model that considers 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 14%.

- 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

Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 31, 2022	Mar. 03, 2022	Dec. 31, 2021	Nov. 09, 2021	Mar. 23, 2021	Nov. 02, 2020	Nov. 29, 2019
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	14,023,726		13,257,160
Class Of Warrant Or Right. Aggregate Proceeds From Warrants Issued		$ 3,000		$ 10,000	$ 435		$ 21,000
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights					$ 0.01	$ 0.01	$ 0.001
Funded
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)		$ 1.79		$ 4.97	$ 8.69
Pre Funded Warrant
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)							11.999
Class B Warrants
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)							$ 15.00
Issuance On October 25, 2016
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On October 25, 2016 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	5,155
Issuance On December 28, 2017 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On December 28, 2017 One \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	115,916
Issuance On September 12, 2018 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On September 12, 2018 One \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	20,220
Issuance On October 19, 2018
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On October 19, 2018 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	20,016
Issuance On March 13, 2019
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On March 13, 2019 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	5,000
Issuance On April 16, 2019
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 13.20
Issuance On April 16, 2019 \| Class A Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	3,866,154
Issuance On November 29, 2019
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 1.80
Issuance On November 29, 2019 \| Class B Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	5,416,567
Issuance On November 29, 2019 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 12.00
Issuance On November 29, 2019 One \| Pre Funded Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	1,750,000
Issuance On March 23, 2021
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 8.70
Issuance On March 23, 2021 \| Pre Funded Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	50,000
Issuance on November Nine Two Thousand Twenty One
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	2,008,032
Exercise Price (usd per share)	$ 4.98
Issuance on March 3 2022
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	766,666
Exercise Price (usd per share)	$ 1.80

Leases - Schedule of Maturities of Lease Liabilities (Detail) $ in Thousands	Mar. 31, 2022 USD ($)
Leases [Abstract]
2020	$ 1,194
2021	1,619
2022	1,384
2023	1,412
2024	1,318
2025	363
Total lease payments	7,290
Less: interest	(1,699)
Operating lease, liability	$ 5,591

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2022

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net consisted of the following:


(in thousands)	March 31, 2022		December 31, 2021
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,254		1,251
Computer equipment	169		150
Software	33		33
Lab equipment	576		576
	2,260		2,238
Less: Accumulated depreciation and amortization	(857)		(724)
	$	1,403	$	1,514

Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net loss	$ (21,965)	$ (18,676)
Dividends, Paid-in-kind	(2,259)	(8,239)
Net loss attributable to common stockholders	$ (24,224)	$ (26,915)
Denominator:
Weighted average common shares outstanding—basic and diluted (in shares)	33,737,000	20,751,000
Net loss per share attributable to common stockholders—basic and diluted	$ (0.72)	$ (1.30)

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
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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.

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+ Details

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2022
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXESThe Company did not record a U.S. federal or state income tax benefit for its losses for the three months ended March 31, 2022 and 2021, due to the conclusion that a full valuation allowance is required against the Company’s U.S. federal and state deferred tax assets. For the three months ended March 31, 2022 and March 31, 2021, the Company recorded an immaterial income tax provision related to its Austrian subsidiary.

- 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.

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+ Details

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2022

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

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


	Three Months Ended March 31,
(in thousands, except per share data)	2022		2021
Numerator:
Net loss	$	(21,965)	$	(18,676)
Deemed dividend as a result of Class B warrant price reset (Note 10)	(2,259)		(8,239)
Net loss attributable to common stockholders	$	(24,224)	$	(26,915)
Denominator:
Weighted average shares of common stock outstanding—basic and diluted	33,737		20,751
Net loss per share attributable to common stockholders— basic and diluted	$	(0.72)	$	(1.30)

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, presented based on amounts outstanding at each period end, 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:


	Three Months Ended March 31,
	2022	2021
Options to purchase shares of common stock	2,049,158	1,858,154
Unvested restricted stock units	1,940,788	1,135,419
Warrants to purchase shares of common stock (excluding prefunded warrants, which are included in basic shares outstanding)	9,449,028	9,474,403
	13,438,974	12,467,976

Long Term Debt - Hercules Loan Agreement - Additional Information (Detail) - USD ($)			3 Months Ended		30 Months Ended
	Jul. 01, 2024	Jul. 01, 2022	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2021
Schedule Of Long Term Debt [Line Items]
Principal payments			$ 800,000
Hercules Loan Agreement
Schedule Of Long Term Debt [Line Items]
Proceeds from lines of credit					$ 32,500,000
Line of credit facility, maximum borrowing capacity			$ 50,000,000
Debt instrument variable percentage			8.75%
Interest rate increase percentage			4.00%
Contingent additional term loan advances, tranche three			$ 17,500,000
Interest expense, debt			909,000	$ 890,000
Amortization of debt discount			$ 198,000	$ 178,000
Effective interest rate of loan			10.70%
Principal payments			$ 0
Hercules Loan Agreement \| Subsequent Event
Schedule Of Long Term Debt [Line Items]
Line of credit facility periodic payment	$ 800,000	$ 1,300,000
Hercules Loan Agreement \| Prime Rate
Schedule Of Long Term Debt [Line Items]
Debt instrument variable percentage			6.00%
Hercules Loan Agreement \| Maximum \| Period Two
Schedule Of Long Term Debt [Line Items]
Debt instrument prepayment premium, percentage			1.00%
Hercules Second Amended Loan Agreement \| Min Cash Test Date 2
Schedule Of Long Term Debt [Line Items]
Line of Credit, Covenant, Minimum Cash			$ 20,000,000
Hercules Second Amended Loan Agreement \| Min Cash Test Date 1
Schedule Of Long Term Debt [Line Items]
Line of Credit, Covenant, Minimum Cash			$ 30,000,000

Stock-Based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-date Fair Value of Stock Options Granted (Detail)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Risk-free interest rate (in percentage)	2.00%	0.90%
Expected term (in years)	6 years 1 month 6 days	6 years
Expected volatility (in percentage)	94.50%	101.40%
Expected dividend yield (in percentage)	0.00%	0.00%

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Federal income tax benefit	$ 0	$ 0
State income tax benefit	0	0
Provision for income taxes	$ 23	$ 6

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2022

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIESSignificant Accounting Policies—The Company’s significant accounting policies are disclosed in the audited consolidated financial statements and the notes thereto in the Company’s Annual Report on Form 10-K for the year ended December 31, 2021 filed with the Securities and Exchange Commission (“SEC”) on March 17, 2022 (the “2021 Annual Report”). Since the

date of those consolidated financial statements, there have been no material changes to the Company’s significant accounting policies.

Risks and Uncertainties—The impact of the ongoing COVID-19 pandemic has been and, notwithstanding vaccination efforts, is expected to continue to be extensive in many aspects of society, which has resulted in and will likely continue to result in significant disruptions to the global economy, as well as businesses and capital markets around the world. Impacts to the Company’s business have included temporary closures or postponements of activation of its clinical trial sites or facilities, disruptions or restrictions on its employees’ ability to travel, disruptions to or delays in ongoing clinical trials, including patient enrollment at a slower pace than initially projected and the diversion of healthcare resources away from the conduct of the Company’s clinical trials as a result of the ongoing COVID-19 pandemic, including the diversion of hospitals serving as the Company’s clinical trial sites and hospital staff supporting the conduct of the Company’s clinical trials.

In addition, the Company is subject to other challenges and risks specific to its business and its ability to execute on its business plan and strategy, as well as risks and uncertainties common to companies in the biotechnology industry with research and development operations, including, without limitation, risks and uncertainties associated with: obtaining regulatory approval of its product candidates; delays or problems in obtaining clinical supply, loss of single source suppliers or failure to comply with manufacturing regulations; identifying, acquiring or in-licensing additional products or product candidates; product development and the inherent uncertainty of clinical success; and the challenges of protecting and enhancing its intellectual property rights; and the challenges of complying with applicable regulatory requirements. In addition, to the extent the ongoing COVID-19 pandemic adversely affects the Company’s business and results of operations, it is expected also to have the effect of heightening many of the other risks and uncertainties discussed above.

Principles of Consolidation—The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 Austria”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated.

Unaudited Interim Condensed Consolidated Financial Statements— The condensed consolidated balance sheet at December 31, 2021 that is presented in these interim condensed consolidated financial statements was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim condensed consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the SEC for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2021 included in the 2021 Annual Report. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three months ended March 31, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022.

Use of Estimates— The preparation of the Company’s condensed consolidated financial statements in conformity with 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 condensed consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and the constraint of variable consideration from contracts with customers. 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. The COVID-19 pandemic has impacted and is expected to continue to impact the clinical development timelines for certain of the Company's clinical programs. As of the date of issuance of these condensed 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 condensed consolidated financial statements.

Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2022 and December 31, 2021.

Restricted Cash


(in thousands)	As of March 31, 2022		As of December 31, 2021

Letter of credit security: Waltham lease	250		250
Letter of credit security: Vienna Austria lease	212		216
Letter of credit security: Boston lease	855		855


Restricted cash included in other assets	$	1,317	$	1,321

In connection with the Company’s lease agreements 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.


(in thousands)	March 31, 2022		December 31, 2021
Cash and cash equivalents	$	66,427	$	81,787

Restricted cash, non-current	1,317		1,321
Total cash, cash equivalents and restricted cash	$	67,744	$	83,108

Goodwill— Goodwill is tested 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 or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit.

The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby 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 its 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. There were no triggering events during the three months ended March 31, 2022 that necessitated an interim impairment test of goodwill.

Recently Adopted Accounting Standards

In May 2021, the Financial Accounting Standards Board (“FASB”) issued ASU 2021-04, Issuer’s Accounting for Certain Modifications or Exchanges of Freestanding Equity-Classified Written Call Options. ASU 2021-04 was issued to clarify and reduce diversity in an issuer’s accounting for modifications or exchanges of freestanding equity-classified written call options (such as warrants for the purchase of common shares) that remain classified as equity following the modification or exchange. ASU 2021-04 is effective January 1, 2022 for the Company. The adoption of this standard did not have an impact on the Company’s consolidated financial statements.

Recently Issued Accounting Standards Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13"), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. In accordance with ASU 2019-10, Financial Instruments-Credit Losses (Topic 326), Derivative and Hedging (Topic 815), and Leases (Topic 842)- Effective

Dates, as the Company meets the definition of a “smaller reporting company”, the Company has elected to defer the adoption of ASU 2016-13 until January 1, 2023. The Company expects that the adoption of ASU 2016-13 may accelerate the timing and could increase the level of credit loss expense in the consolidated statement of operations and will likely require an increased level of disclosure in the notes to the consolidated financial statements.

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Operating expenses:
Research and development	$ 14,113	$ 12,104
Selling, general and administrative	7,664	5,832
Gain (Loss) on Disposition of Assets	(509)	0
Total operating expenses	21,268	17,936
Loss from operations	(21,268)	(17,936)
Other income (expense):
Interest income	3	2
Interest expense	(913)	(891)
Change in fair value of derivative liability	176	(31)
Other income, net	60	186
Total other expense, net	(674)	(734)
Loss before provision for income taxes	(21,942)	(18,670)
Provision for income taxes	23	6
Net loss and comprehensive loss	(21,965)	(18,676)
Deemed dividend on Class B Warrant price reset	(2,259)	(8,239)
Net loss attributable to common stockholders	$ (24,224)	$ (26,915)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.72)	$ (1.30)
Weighted average common shares outstanding—basic and diluted (in shares)	33,737,000	20,751,000

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2022
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation—The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 Austria”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated.
Unaudited Interim Financial Statements	Unaudited Interim Condensed Consolidated Financial Statements— The condensed consolidated balance sheet at December 31, 2021 that is presented in these interim condensed consolidated financial statements was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim condensed consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the SEC for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2021 included in the 2021 Annual Report. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three months ended March 31, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022.
Use of Estimates	Use of Estimates— The preparation of the Company’s condensed consolidated financial statements in conformity with 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 condensed consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and the constraint of variable consideration from contracts with customers. 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. The COVID-19 pandemic has impacted and is expected to continue to impact the clinical development timelines for certain of the Company's clinical programs. As of the date of issuance of these condensed 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 condensed consolidated financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2022 and December 31, 2021.
Goodwill	Goodwill— Goodwill is tested 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 or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit.The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby 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 its 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. There were no triggering events during the three months ended March 31, 2022 that necessitated an interim impairment test of goodwill.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Standards In May 2021, the Financial Accounting Standards Board (“FASB”) issued ASU 2021-04, Issuer’s Accounting for Certain Modifications or Exchanges of Freestanding Equity-Classified Written Call Options. ASU 2021-04 was issued to clarify and reduce diversity in an issuer’s accounting for modifications or exchanges of freestanding equity-classified written call options (such as warrants for the purchase of common shares) that remain classified as equity following the modification or exchange. ASU 2021-04 is effective January 1, 2022 for the Company. The adoption of this standard did not have an impact on the Company’s consolidated financial statements. Recently Issued Accounting Standards Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13"), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. In accordance with ASU 2019-10, Financial Instruments-Credit Losses (Topic 326), Derivative and Hedging (Topic 815), and Leases (Topic 842)- Effective Dates, as the Company meets the definition of a “smaller reporting company”, the Company has elected to defer the adoption of ASU 2016-13 until January 1, 2023. The Company expects that the adoption of ASU 2016-13 may accelerate the timing and could increase the level of credit loss expense in the consolidated statement of operations and will likely require an increased level of disclosure in the notes to the consolidated financial statements.
Research and Development Expense, Policy

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Cash and cash equivalents	$ 66,427	$ 81,787
Restricted cash, non-current	1,317	1,321
Total cash, cash equivalents and restricted cash	$ 67,744	$ 83,108	$ 116,660	$ 80,702

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,260	$ 2,238
Less: Accumulated depreciation and amortization	(857)	(724)
Property and equipment, net	1,403	1,514
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	1,254	1,251
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	169	150
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	33	33
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 576	$ 576

Long-Term Debt - Schedule of Future Principal Payments and the Final Payments Due (Detail) $ in Thousands	Mar. 31, 2022 USD ($)
Debt Disclosure [Abstract]
2022	$ 0
2023	19,802
2024	13,295
Principal amount of long-term debt	$ 33,097

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2022	Jun. 30, 2021
Number of Shares
Beginning balance (in shares)	1,916,051
Granted (in shares)	232,850
Forfeited (in shares)	(99,743)
Ending balance (in shares)	2,049,158
Number of shares Options, Exercisable (in shares)	1,029,146
Number of shares Options, Vested and expected to vest (in shares)	1,798,855
Weighted Average Exercise Price
Beginning balance (in usd per share)	$ 10.01
Granted (in usd per share)	1.85
Forfeited (in usd per share)	23.52
Ending balance (in usd per share)	8.43
Weighted average exercise price, Exercisable (in usd per share)	10.44
Weighted average exercise price, Vested and expected to vest (in usd per share)	$ 8.84
Weighted average contractual term outstanding (in years)	7 years 10 months 24 days	7 years 9 months 18 days
Weighted average contractual term outstanding, Exercisable (in years)	6 years 9 months 18 days
Weighted average contractual term outstanding, Vested and expected to vest (in years)	7 years 8 months 12 days
Aggregate intrinsic value, Beginning balance	$ 0
Aggregate intrinsic value, Ending balance	11
Aggregate intrinsic value, exercisable	0
Aggregate intrinsic value, vested and expected to vest	$ 8

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 133	$ 106

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [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 focused on the research, development and commercialization of novel therapeutics for the treatment of rare diseases. The Company’s lead product candidate, mavorixafor, is a potential first-in-class, once-daily, oral inhibitor of CXCR4 and is currently in a Phase 3 clinical trial for the treatment of Warts, Hypogammaglobulinemia, Infections, and Myelokathexis (“WHIM”) syndrome, a rare, inherited, primary immunodeficiency disease caused by genetic mutations in the CXCR4 receptor gene. The Company is also conducting a 14-day, proof-of-concept Phase 1b clinical trial of mavorixafor in patients with Severe Congenital Neutropenia (“SCN”) and chronic neutropenia disorders, and a Phase 1b clinical trial of mavorixafor in combination with ibrutinib in Waldenström’s macroglobulinemia (“Waldenström’s”). The Company is headquartered in Boston, Massachusetts and has an additional facility in Vienna, Austria. Going Concern Assessment—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 these condensed consolidated financial statements are issued. As of March 31, 2022, the Company had $66.4 million of cash and cash equivalents. Based on its current operating plan, the Company believes that its existing cash and cash equivalents will be sufficient to fund its operating expense and capital expenditure requirements into the fourth quarter of 2022. However, as further discussed in Note 7, the Company has a covenant under its loan agreement with Hercules Capital Inc. (“Hercules”) that requires that the Company maintain a minimum level of cash, as defined, beginning on September 1, 2022. Based on its current financial projections, the Company believes it would be in violation of this covenant in the third quarter of 2022. If the Company is in violation of this covenant, Hercules could require the repayment of all outstanding debt. As a result, the Company believes that, in the aggregate, these conditions raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that these condensed consolidated financial statements are issued. Nevertheless, the accompanying condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. In order to fund its operations beyond the fourth quarter of 2022, the Company is seeking to raise funds potentially 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 future funding when needed, the Company may be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or pre-commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. There is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. Impact of the COVID-19 Pandemic— Beginning in late 2019 and continuing into 2022, the outbreak of COVID-19 has resulted in the declaration of a global pandemic and adversely affected economic activity across virtually all sectors and industries on a local, national and global scale. Impacts to the Company’s business have included temporary closures or postponements of activation of its clinical trial sites or facilities, disruptions or restrictions on its employees’ ability to travel, disruptions to or delays in ongoing clinical trials, including patient enrollment at a slower pace than initially projected and the diversion of healthcare resources away from the conduct of the Company’s clinical trials as a result of the ongoing COVID-19 pandemic, including the diversion of hospitals serving as the Company’s clinical trial sites and hospital staff supporting the conduct of the Company’s clinical trials. While global vaccination efforts are underway and certain jurisdictions, including Massachusetts, where the Company is headquartered, have reopened businesses and governmental agencies, there remain limitations on the physical operations of businesses and prohibitions on certain non-essential gatherings, and the Company is unable to accurately predict the full impact that COVID-19 will have due to numerous uncertainties, including the duration of the outbreak, the result of vaccination efforts, resurgence of the virus, actions that may be taken by governmental authorities, the impact on the Company’s business including its clinical programs and timelines, and the impact to the business of its service providers and partners.

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Redeemable Common Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Increase (Decrease) in Temporary Equity [Roll Forward]
Redeemable Common Shares, Value		$ 0
Redeemable common shares, beginning balance, shares at Dec. 31, 2020		0
Convertible preferred shares, ending balance, shares at Mar. 31, 2021		229,885
Beginning balance, shares at Dec. 31, 2020			16,305,731
Beginning balance at Dec. 31, 2020	$ 72,799		$ 16	$ 267,077	$ (119)	$ (194,175)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues		229,885	6,041,951
Stock Issued During Period, Value, New Issues	49,640	$ 1,875	$ 7	49,633
Stock Issued During Period Shares Warrants Exercised			1,072,887
Stock Issued During Period Value Warrants Exercised	1		$ 1
Exercise of stock options, shares			5,860
Vesting of restricted stock units, less shares withheld and retired to satisfy tax obligations	40			40
Stock-based compensation expense	1,258			1,258
Net loss	(18,676)					(18,676)
Ending balance, shares at Mar. 31, 2021			23,426,429
Ending balance at Mar. 31, 2021	105,062		$ 24	318,008	(119)	(212,851)
Increase (Decrease) in Temporary Equity [Roll Forward]
Redeemable Common Shares, Value		$ 1,875
Beginning balance, shares at Dec. 31, 2021			28,127,657
Beginning balance at Dec. 31, 2021	$ 64,412		$ 28	347,374	(119)	(282,871)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	900,000		2,512,902
Stock Issued During Period, Value, New Issues	$ 5,820		$ 3	5,817
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings			168,817
Restricted Stock, Value, Shares Issued Net of Tax Withholdings	(12)			(12)
Stock Issued During Period Shares Warrants Exercised			100
Stock-based compensation expense	1,459			1,459
Net loss	(21,965)					(21,965)
Ending balance, shares at Mar. 31, 2022			30,809,476
Ending balance at Mar. 31, 2022	$ 49,714		$ 31	$ 354,638	$ (119)	$ (304,836)

Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	$ 645	$ 821
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	$ (176)

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

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 to employees, directors and non-employees.


	Three Months Ended March 31,
	2022		2021
Risk-free interest rate	2.0	%	0.9	%
Expected term (in years)	6.1		6.0
Expected volatility	94.5	%	101.4	%
Expected dividend yield	0	%	0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2022:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2021	1,916,051	$	10.01	7.8	$	—
Granted	232,850	1.85

Forfeited and Expired	(99,743)	23.52
Outstanding as of March 31, 2022	2,049,158	$	8.43	7.9	$	11
Exercisable as of March 31, 2022	1,029,146	$	10.44	6.8	$	—
Vested and expected to vest as of March 31, 2022	1,798,855	$	8.84	7.7	$	8

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

Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the three months ended March 31, 2022:


			Number of Shares
Unvested as of December 31, 2021			925,101
Granted			1,242,756
Vested			(192,255)
Forfeited			(34,814)
Unvested as of March 31, 2022			1,940,788

Summary of Stock-Based Compensation Expense Classification

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


	Three Months Ended March 31,
(in thousands)	2022		2021
Research and development expense	$	704	$	577
Selling, general and administrative expense	755		681
Total stock-based compensation	$	1,459	$	1,258

Common Stock Warrants (Tables)

3 Months Ended

Mar. 31, 2022

Equity [Abstract]

Schedule of Warrants

The following table provides a roll forward of outstanding warrants for the three month period ended March 31, 2022:


	Number of warrants	Weighted Average Exercise Price	Weighted Average Contractual Term (Years)
Outstanding and exercisable warrants to purchase common shares as of December 31, 2021	13,257,160	$7.96	2.72
Issued	766,666	$1.80
Exercised	(100)

Outstanding and exercisable warrants to purchase common shares as of March 31, 2022	14,023,726	$6.91	2.47

As of March 31, 2022, the Company’s outstanding 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	5,416,567	$	1.80		November 28, 2024
November 29, 2019	1,750,000	$	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
	14,023,726

(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.

(d) In March 2022, the Company received $1.79 per pre-funded warrant, or $3.0 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant.