UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

Enzon Pharmaceuticals, Inc.

(Name of Issuer)

Common Stock, $0.01 par value per share

(Title of Class of Securities)

293904108

(CUSIP Number)

June     , 2013

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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Enzon Pharmaceuticals, Inc.

20 Kingsbridge Road

Piscataway, NJ 08854

Steelhead Pathfinder Master, L.P. (“Pathfinder”)

Steelhead Partners, LLC

James Michael Johnston

Brian Katz Klein

The business address of each reporting Person other than Pathfinder is:

333 108^th Avenue NE, Suite 2010

Bellevue, WA 98004

The principal business office of Pathfinder is:

c/o Citco Fund Services (Bermuda) Limited

Mintflower Place, 4^th Floor

8 Par-La-Ville Road

Hamilton, HM 08 Bermuda

Reference is made to Item 4 of pages 2–5 of this Schedule 13G (this “Schedule”), which Items are incorporated by reference herein.

Common Stock, par value $0.01 per share

293904108

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Reference is hereby made to Items 5-9 and 11 of pages 2 - 5 of this Schedule, which Items are incorporated by reference herein.

If this statement is being filed to report the fact that as of the date hereof each of the reporting persons has ceased to be the beneficial owner of more than five percent of the class of securities, check the following x.

Not Applicable.

Not Applicable.

Not Applicable.

Not Applicable.

By signing below the undersigned certify that, to the best of their knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

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Signature

After reasonable inquiry and to the best of my knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

By signing below the undersigned certifies that, to the best of its knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Signature

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

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EXHIBIT INDEX

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EXHIBIT A

JOINT FILING UNDERTAKING

The undersigned, being authorized thereunto, hereby execute this agreement as an exhibit to this Schedule 13G to evidence the agreement of the below-named parties, in accordance with rules promulgated pursuant to the Securities Exchange Act of 1934, to file this Schedule, as it may be amended, jointly on behalf of each of such parties.

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