Material Agreement, Corporate Action (Form 8-K)

On December 23, 2025, Enzon Pharmaceuticals, Inc. (the "Company") entered into the Sixth Amendment to the Section 382 Rights Agreement (the "Sixth 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 as of 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 as of 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. Effective as of March 31, 2025, the Company entered into the Third Amendment (the "Third Amendment") to extend 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. On August 13, 2025, the Company entered into the Fourth Amendment (the "Fourth Amendment"), to amend the Final Expiration Date of the rights issued pursuant to the Rights Agreement from the close of business on June 30, 2026 to the close of business on September 30, 2025. On September 30, 2025, the Company entered into the Fifth Amendment (the "Fifth Amendment") to amend the Final Expiration Date of the rights issued pursuant to the Rights Agreement from the close of business on September 30, 2025 to the close of business on December 31, 2025.

On December 23, 2025, the Company entered into the Sixth Amendment (the "Sixth Amendment") to amend the Final Expiration Date of the rights issued pursuant to the Rights Agreement from the close of business on December 31, 2025 to the close of business on January 31, 2026. Except for the adjustment to the Final Expiration Date, the Rights Agreement otherwise remains unmodified.

The Sixth Amendment has been adopted because the Company's management believes that it is in the best interests of the Company and its stockholders to provide for a Final Expiration Date of January 31, 2026, as established in the Sixth Amendment.

The foregoing description does not purport to be complete and is qualified in its entirety by reference to the complete text of (i) the Rights Agreement, which was filed with the U.S. Securities and Exchange Commission (the "SEC") as Exhibit 4.1 to the Company's Registration Statement on Form 8-A filed with the SEC on August 14, 2020, (ii) 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, (iii) 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, (iv) the Third Amendment, which was filed with the SEC as Exhibit 4.1 to the Current Report on Form 8-K filed on April 1, 2025, (v) the Fourth Amendment, which was filed with the SEC as Exhibit 4.1 to the Quarterly Report on Form 10-Q filed on August 14, 2025, (vi) the Fifth Amendment, a copy of which was filed with the SEC as Exhibit 4.1 to the Current Report on Form 8-K on September 30, 2025, and (vii) the Sixth Amendment, a copy of which is attached as Exhibit 4.1 hereto and incorporated herein by reference. The Rights Agreement, the First Amendment, the Second Amendment, the Third Amendment, the Fourth Amendment, the Fifth Amendment, and Sixth are 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.