UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): April 11, 2019

X4 Pharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (857) 529-8300

Not applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company  ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ☒

Item 2.02 Results of Operations and Financial Condition.

On April 12, 2019, X4 Pharmaceuticals, Inc. (the “Company”) filed with the Securities and Exchange Commission a preliminary prospectus supplement (the “Preliminary Prospectus Supplement”) to its effective shelf registration statement on Form S-3 (File No. 333-229377) pursuant to Rule 424(b)(5) under the Securities Act of 1933, as amended (the “Securities Act”), relating to an underwritten public offering of its common stock (or pre-funded warrants to purchase common stock in lieu thereof) and Class A warrants to purchase its common stock (the “Offering”). The Company included the following disclosure in the Preliminary Prospectus Supplement:

“We estimate that we had cash and cash equivalents of $23.3 million as of March 31, 2019, which excludes restricted cash of $0.7 million as of that date that is restricted as to use. As of April 30, 2019, $4.6 million of our available cash and cash equivalents will become restricted as to use in connection with our outstanding long-term debt and will become restricted cash.

Our actual consolidated financial results as of and for the three months ended March 31, 2019 are not yet available. Our financial closing procedures for the three months ended March 31, 2019 are not yet completed and, as a result, our final results upon completion of those procedures may differ materially from our preliminary estimates. The preliminary consolidated financial data presented above as of March 31, 2019 is not a comprehensive statement of our financial position or operating results; reflects our preliminary estimates based on information available as of the date of this prospectus supplement; and is subject to change, and those changes may be material. Accordingly, you should not place undue reliance upon these preliminary estimates.”

The information in this Item 2.02 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act or the Exchange Act, except as expressly set forth by specific reference in such a filing.

Item 8.01 Other Events.

On April 11, 2019, the Company issued a press release announcing the commencement of the Offering. The Offering press release is attached hereto as Exhibit 99.1 and incorporated by reference in this Item 8.01.

This Current Report on Form 8-K, including the exhibits hereto, shall not constitute an offer to sell or the solicitation of an offer to buy any securities of the Company, which is being made only by means of a written prospectus meeting the requirements of Section 10 of the Securities Act, nor shall there be any sale of the Company’s securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such jurisdiction.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Exhibit 99.1

X4 Pharmaceuticals Announces Proposed Public Offering of Common Stock and Class A Warrants

Cambridge, MA — April 11, 2019 — X4 Pharmaceuticals, Inc. (Nasdaq: XFOR), a clinical-stage biopharmaceutical company focused on the development of novel therapeutics for the treatment of rare diseases, announced today that it has commenced an underwritten public offering of shares of its common stock (or pre-funded warrants to purchase its common stock in lieu thereof) and Class A warrants to purchase its common stock. All of the securities in the offering will be sold by X4. The offering is subject to market and other conditions, and there can be no assurance as to whether or when the offering may be completed or as to the actual size or terms of the offering.

Cowen and Stifel are acting as joint book-running managers and representatives of the underwriters for the offering. Canaccord Genuity is acting as lead manager of the offering.

The offering will be made only by means of a written prospectus and related prospectus supplement forming part of a shelf registration statement on Form S-3 that was filed with the Securities and Exchange Commission (SEC) on January 25, 2019 and declared effective by the SEC on February 19, 2019. A preliminary prospectus supplement and accompanying prospectus relating to and describing the terms of the offering will be filed with the SEC and will be available at the SEC’s website located at www.sec.gov, copies of which may be obtained, when available, from Cowen and Company, LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717, Attn: Prospectus Department, or by telephone at 631-274-2806, and from Stifel, Nicolaus & Company, Incorporated, Attention: Syndicate, One Montgomery Street, Suite 3700, San Francisco, California 94104, or by telephone at 415-364-2720 or by email at syndprospectus@stifel.com.

This press release shall not constitute an offer to sell or a solicitation of an offer to buy, nor shall there be any sale of these securities in any state or other jurisdiction in which such an offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or other jurisdiction.

About X4 Pharmaceuticals

X4 Pharmaceuticals is developing novel therapeutics designed to improve immune cell trafficking to treat rare diseases, including primary immunodeficiencies and cancer. X4’s oral small molecule drug candidates antagonize the CXCR4 pathway, which plays a central role in immune surveillance. X4’s most advanced product candidate, mavorixafor (X4P-001), will be commencing a global Phase 3 pivotal trial in patients with WHIM syndrome, a rare genetic, primary immunodeficiency disease, in the second quarter of 2019 and is currently also under investigation in a Phase 2a clinical trial in clear cell renal cell carcinoma. X4 was founded and is led by a team with extensive product development and commercialization expertise, including several former members of the Genzyme leadership team, and is located in Cambridge, Massachusetts.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, as amended. The words “may,” “will,” “could,”

“would,” “should,” “expect,” “plan,” “anticipate,” “intend,” “believe,” “estimate,” “predict,” “project,” “potential,” “continue,” “target” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Forward-looking statements include statements regarding the completion of the proposed public offering. These statements are subject to various risks and uncertainties, actual results could differ materially from those projected and X4 cautions investors not to place undue reliance on the forward-looking statements in this press release. These risks and uncertainties include, without limitation, risks and uncertainties related to whether or not X4 will be able to raise capital through the sale of shares of common stock (or pre-funded warrants to purchase common stock in lieu thereof) or the exercise of the Class A warrants to purchase common stock, the final terms of the proposed offering, market conditions and satisfaction of customary closing conditions related to the public offering. There can be no assurance that X4 will be able to complete the public offering on the anticipated terms, or at all. Any forward-looking statements in this press release are based on management’s current expectations and beliefs and are subject to a number of risks, uncertainties and important factors that may cause actual events or results to differ materially from those expressed or implied by any forward-looking statements contained in this press release, including, without limitation, the risks and uncertainties described in X4’s most recent Annual Report on Form 10-K filed with the SEC and in other filings X4 makes with the SEC from time to time. X4 undertakes no obligation to update the information contained in this press release to reflect subsequently occurring events or circumstances.

Investors:

Stephanie Carrington

Westwicke, an ICR company

646-277-1282

Stephanie.Carrington@icrinc.com

Media:

Darcie Robinson

Westwicke, an ICR company

203-919-7905

Darcie.robinson@icrinc.com