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): February 28, 2026

PULMATRIX, INC.

(Exact name of registrant as specified in its charter)

945 Concord Street, Suite 1217

Framingham, MA 01701

(Address of principal executive offices) (Zip Code)

(888) 355-4440

(Registrant’s telephone number, including area code)

N/A

(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:

Securities registered pursuant to Section 12(b) of the Act:

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 1.02 Termination of a Material Definitive Agreement.

On February 28, 2026, Pulmatrix, Inc. (the “Company” or “Pulmatrix”) received a termination notice (the “Notice”) from Cullgen Inc. (“Cullgen”) pursuant to which Cullgen informed the Company that in accordance with Sections 11.7 and 10.1(b) of that certain Agreement and Plan of Merger and Reorganization, dated as of November 13, 2024, as amended by Amendment No. 1 thereto dated as of April 7, 2025 (the “Merger Agreement”), by and among the Company, Pulmatrix and PCL Merger Sub, Inc. and PCL Merger Sub Inc. II, LLC, Cullgen was terminating the Merger Agreement and all related transactions thereunder.

Neither the Company nor Cullgen will be required to pay any termination fee as a result of the termination of the Merger Agreement. The Company and Cullgen will each bear their respective costs and expenses related to the Merger Agreement and the transactions contemplated thereby in accordance with the terms of the Merger Agreement.

Item 7.01 Regulation FD Disclosure.

On March 2, 2026, the Company issued a press release announcing the receipt from Cullgen of the Notice in connection with the Merger Agreement. A copy of the press release is furnished as Exhibit 99.1 hereto and is incorporated herein by reference.

The information in this Current Report on Form 8-K (including Exhibit 99.1 attached hereto) is being furnished pursuant to Item 7.01 and shall not be deemed to be filed for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise be subject to the liabilities of that section, nor shall it be deemed to be incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof and regardless of any general incorporation language in such filing.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

SIGNATURES

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