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): March 31, 2025

ENZON PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

(732) 980-4500

(Registrant’s telephone number, including area code)

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:

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.  ¨

On March 31, 2025, Enzon Pharmaceuticals, Inc. (the “Company”) entered into the Third Amendment to the Section 382 Rights Agreement (the “Third Amendment”), which amends the Section 382 Rights Agreement, dated as of August 14, 2020 (the “Rights Agreement”), by and between the Company and Continental Stock Transfer & Trust Company, as rights agent. Effective June 2, 2021, the Company amended the Rights Agreement (the “First Amendment”) to extend the Final Expiration Date (as defined in the Rights Agreement) of the rights issued pursuant to the Rights Agreement from the close of business on August 13, 2021 to the close of business on June 2, 2024. Effective, May 16, 2024, the Company amended the Rights Agreement (the “Second Amendment”) to extend the Final Expiration Date of the rights issued pursuant to the Rights Agreement from the close of business on June 2, 2024 to the close of business on March 31, 2025. On March 31, 2025, the Company entered into the Third Amendment, which extends the Final Expiration Date of the rights issued pursuant to the Rights Agreement from the close of business on March 31, 2025 to the close of business on June 30, 2026. Except for the extension of the Final Expiration Date, the Rights Agreement otherwise remains unmodified.

The Third Amendment has been adopted because the Company’s management believes that it is in the best interests of the Company and its stockholders to extend the Final Expiration Date of March 31, 2025, as established in the Second Amendment.

The foregoing description does not purport to be complete and is qualified in its entirety by reference to the complete text of the Rights Agreement, which was filed with the U.S. Securities and Exchange Commission (the “SEC”) as Exhibit 4.1 to the Current Report on Form 8-K filed on August 14, 2020, the First Amendment, which was filed with the SEC as Exhibit 4.1 to the Current Report on Form 8-K filed on June 8, 2021, the Second Amendment, which was filed with the SEC as Exhibit 4.1 to the Current Report on Form 8-K filed on May 22, 2024, and the Third Amendment, a copy of which is attached as Exhibit 4.1 hereto and incorporated herein by reference.

The information set forth in Item 1.01 of this Current Report on Form 8-K is incorporated by reference into this Item 3.03.

(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.