As filed with the U.S. Securities and Exchange Commission on January 30, 2026

Registration No. 333-293019​

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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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AMENDMENT NO. 1
TO

FORM S-4
REGISTRATION STATEMENT

UNDER
THE SECURITIES ACT OF 1933

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ENZON PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)​

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20 Commerce Drive, Suite 135
Cranford, New Jersey 07016
(732) 980-4500

(Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)​

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Richard L. Feinstein
Chief Executive Officer, Chief
Financial Officer and Secretary
Enzon Pharmaceuticals, Inc.
20 Commerce Drive, Suite 135 Cranford, New Jersey 07016
(732) 980-4500

(Name, address, including zip code, and telephone number, including area code, of agent for service)​

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Copies of all communications, including communications sent to agent for service, should be sent to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after this Registration Statement is declared effective and upon the satisfaction or waiver of all other conditions to consummation of the transactions described herein.

If the securities being registered on this form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.  ☐

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933, as amended (the "Securities Act"), check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company" and "emerging growth company" in Rule 12b-2 of the Securities Exchange Act of 1934, as amended (the "Exchange Act").

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 7(a)(2)(B) of the Securities Act.  ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer)  ☐

Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)  ☐

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE SECURITIES ACT OF 1933, AS AMENDED, OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A), MAY DETERMINE.

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EXPLANATORY NOTE

Enzon Pharmaceuticals, Inc. is filing this Amendment No. 1 to its Registration Statement on Form S-4 (File No. 333-293019) (the "Registration Statement") as an exhibits-only filing. Accordingly, this amendment consists only of the facing page, this explanatory note, Item 21 of Part II of the Registration Statement, the signature page to the Registration Statement and the filed exhibits. The remainder of the Registration Statement is unchanged and has therefore been omitted.

Item 21. Exhibits and Financial Statement Schedules

EXHIBIT INDEX

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*

Filed herewith.

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**

Previously filed.

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^

Portions of this exhibit have been redacted and filed separately with the SEC pursuant to a confidential treatment request.

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#

Management contracts or compensatory plans and arrangements required to be filed pursuant to Item 601(b)(10)(ii)(A) or (iii) of Regulation S-K.

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†

Schedules, exhibits and/or annexes have been omitted pursuant to Item 601(b)(2) of Regulation S-K. A copy of any such omitted item will be furnished supplementally to the Securities and Exchange Commission upon request

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Cranford, State of New Jersey, on January 30, 2026.

ENZON PHARMACEUTICALS, INC.

By:

/s/ Richard L. Feinstein

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Richard L. Feinstein
Chief Executive Officer, Chief Financial
Officer and Secretary

Pursuant to the requirements of the Securities Act, this registration statement has been signed by the following persons in the capacities and on the dates indicated, on January 30, 2026.