Supplemental Prospectus (Form 424B3)

Filed Pursuant to Rule 424(b)(3)

Registration No. 333-297342

PROXY STATEMENT/PROSPECTUS SUPPLEMENT NO. 1

(to Proxy Statement/Prospectus dated September 25, 2026)

EXPLANATORY NOTE

SUPPLEMENT NO. 1 TO THE PROXY STATEMENT/PROSPECTUS

The following information relates to and supplements the proxy statement/prospectus of Pulmatrix, Inc., a Delaware corporation ("Pulmatrix"), dated September 25, 2026 and first mailed to Pulmatrix stockholders on or about September 25, 2026 (the "Proxy Statement/Prospectus"), in connection with the special meeting in lieu of the annual meeting of stockholders of Pulmatrix, Inc. (the "Pulmatrix Special Meeting"). This supplement (this "Supplement") is being filed to announce the postponement of the Pulmatrix Special Meeting to October 26, 2026, to update related voting and document request deadlines, to update and correct the deadlines for submission of stockholder proposals and nominations for the Pulmatrix 2027 Annual Meeting (as defined below), and to provide a revised proxy card reflecting the postponed meeting date and updated voting deadlines. Capitalized terms used but not defined in this Supplement have the meanings ascribed to them in the Proxy Statement/Prospectus.

Except as described in this Supplement, no other changes have been made to the Proxy Statement/Prospectus, the record date and the proposals to be voted on at the Pulmatrix Special Meeting remain unchanged, and the Pulmatrix board of directors continues to recommend that Pulmatrix stockholders vote as set forth in the Proxy Statement/Prospectus.

To the extent that information in this Supplement differs from or updates information contained in the Proxy Statement/Prospectus, the information in this Supplement supersedes such information. The Proxy Statement/Prospectus contains additional information. This Supplement should be read in conjunction with the Proxy Statement/Prospectus.

YOU SHOULD CAREFULLY CONSIDER THE MATTERS DISCUSSED UNDER "RISK FACTORS" BEGINNING ON PAGE 24 OF THE PROXY STATEMENT/PROSPECTUS.

Neither the Securities and Exchange Commission (the "SEC") nor any state securities commission has approved or disapproved of the securities described in the Proxy Statement/Prospectus or passed upon the adequacy or accuracy of the Proxy Statement/Prospectus or this Supplement. Any representation to the contrary is a criminal offense.

This Supplement is dated October 7, 2026, and is first being mailed to Pulmatrix stockholders, together with a revised proxy card, on or about October 7, 2026.

NOTICE OF POSTPONEMENT OF STOCKHOLDER SPECIAL MEETING

945 Concord Street, Suite 1217

Framingham, MA 01701

(888) 355-4440

To the stockholders of Pulmatrix, Inc.:

NOTICE IS HEREBY GIVEN that the Pulmatrix Special Meeting, originally scheduled for October 16, 2026, at 8:30 a.m., Eastern Time, has been postponed. The Pulmatrix Special Meeting will now be held on Monday, October 26, 2026, at 8:30 a.m., Eastern Time. The Pulmatrix Special Meeting will be held exclusively online. You will be able to attend and participate in the Pulmatrix Special Meeting online by visiting www.virtualshareholdermeeting.com/PULM2026SM, where you will be able to listen to the meeting live, submit questions and vote.

The Pulmatrix Special Meeting will be held for the purposes set forth in the Notice of Special Meeting of Stockholders in Lieu of Annual Meeting of Stockholders included in the Proxy Statement/Prospectus, including to consider and vote upon Proposal Nos. 1 through 9 described therein, and to transact such other business as may properly come before the Pulmatrix Special Meeting or any adjournment or postponement thereof. The proposals have not changed.

Pulmatrix has postponed the Pulmatrix Special Meeting to provide stockholders with additional time to receive this Supplement and the annual report to stockholders, to review this Proxy Statement/Prospectus, this Supplement and the annual report to stockholders, and to submit their votes.

The record date for determining stockholders entitled to receive notice of and to vote at the Pulmatrix Special Meeting remains the close of business on August 28, 2026. Your signed proxy card, telephonic proxy instructions or internet proxy instructions must be received by 11:59 p.m., Eastern Time, on October 25, 2026, to be counted. If you hold your shares through a broker, bank or other agent, please follow the voting instructions provided by your broker, bank or other agent, which may set an earlier deadline.

A revised proxy card reflecting the postponed date of the Pulmatrix Special Meeting and the updated voting deadlines accompanies this Supplement. Proxies previously submitted, whether on the original proxy card or by telephone or internet, will remain valid and will be voted at the postponed Pulmatrix Special Meeting, and at any further adjournment or postponement thereof, unless properly revoked. If you have already submitted a proxy and do not wish to change your vote, you do not need to take any further action, and your shares will be voted as originally directed. Stockholders who have not yet voted, or who wish to change their vote, are encouraged to vote promptly using the revised proxy card or the telephone or internet voting instructions provided therein. If you submit a revised proxy card or vote again by telephone or internet, your later-dated proxy will revoke any proxy previously submitted, and only your latest-dated proxy will be counted.

By Order of the Pulmatrix Board of Directors,

Peter Ludlum

Interim Chief Executive Officer

October 7, 2026

REFERENCES TO ADDITIONAL INFORMATION

The Proxy Statement/Prospectus incorporates important business and financial information about Pulmatrix that is not included in or delivered with this document. You may obtain this information without charge through the SEC's website (www.sec.gov) or upon your written or oral request by contacting Pulmatrix's Corporate Secretary at 945 Concord Street, Suite 1217, Framingham, MA 01701 or by calling (888) 355-4440.

To ensure timely delivery of these documents, any request should be made no later than October 19, 2026, to receive them before the Pulmatrix Special Meeting.

If you have any questions or need assistance voting your shares, please contact Pulmatrix's proxy solicitor, Campaign Management, toll-free in North America at +1 (855) 434-5243, by calling collect outside North America at +1 (212) 632-8422, or by email at [email protected].

OTHER MATTERS

Submission of Future Stockholder Proposals

Pursuant to Rule 14a-8 under the Exchange Act ("Rule 14a-8"), a Pulmatrix stockholder who intends to present a proposal at Pulmatrix's next annual meeting of stockholders (the "Pulmatrix 2027 Annual Meeting") and who wishes the proposal to be included in the proxy statement and form of proxy for that meeting must submit the proposal in writing no later than the close of business on May 28, 2027, after which date such stockholder proposal will be considered untimely. Such proposal must be submitted to the attention of Pulmatrix's corporate secretary at 945 Concord Street, Suite 1217, Framingham, MA 01701. This deadline corrects the May 31, 2027 deadline previously disclosed in the Proxy Statement/Prospectus.

Pulmatrix stockholders wishing to nominate a director or submit proposals to be presented directly at the Pulmatrix 2027 Annual Meeting instead of by inclusion in the proxy statement for the Pulmatrix 2027 Annual Meeting must follow the submission criteria and deadlines set forth in the Pulmatrix Bylaws concerning stockholder nominations and proposals. Pulmatrix stockholder nominations for director and other proposals that are not to be included in such materials must be received by Pulmatrix's corporate secretary in writing at its corporate offices, as listed above, no earlier than June 28, 2027, and no later than the close of business on July 28, 2027; provided, however, that if such meeting is held more than 30 days before or more than 30 days after the anniversary of the Pulmatrix Special Meeting, notice by the stockholder to be timely must be received no earlier than the close of business on the 120th day prior to such annual meeting and not later than the close of business on the later of the 90th day prior to such annual meeting or the 10th day following the day on which public announcement of the date of such meeting is first made. Any such Pulmatrix stockholder proposals or nominations for director must also satisfy the requirements set forth in the Pulmatrix Bylaws. To be eligible for inclusion in Pulmatrix's proxy materials, Pulmatrix stockholder proposals must also comply with the requirements of Rule 14a-8. Stockholders are also advised to review the Pulmatrix Bylaws, which contain additional advance notice requirements, including requirements with respect to advance notice of stockholder proposals and director nominations. A proxy granted by a Pulmatrix stockholder will give discretionary authority to the proxies to vote on any matters introduced pursuant to the above advance notice provisions in the Pulmatrix Bylaws, subject to applicable rules of the SEC.

In addition, stockholders who intend to solicit proxies in support of director nominees other than Pulmatrix's nominees must comply with the requirements of Rule 14a-19.

If a stockholder who has notified Pulmatrix of the stockholder's intention to present a proposal at an annual meeting of stockholders does not appear to present the stockholder's proposal at such annual meeting, Pulmatrix is not required to present the proposal for a vote at such annual meeting.