Enzon Announces Amendment to ONCASPAR Agreement with Members of the Sanofi-Aventis Group; Reduced Royalty Obligation Provides Enzon with Increased Profit Ownership

Enzon Pharmaceuticals, Inc. (NASDAQ: ENZN) todayannounced that it has amended its license agreement with members ofthe sanofi-aventis Group (NYSE: SNY; EURONEXT: SAN) for the Company'soncology product, ONCASPAR(R) (pegaspargase). The amendment, whichwill become effective in January 2006, includes a significantreduction in the royalty rate, with a single digit royalty percentagenow payable by Enzon only on those annual sales of ONCASPAR thatexceed a certain predetermined level. Previously, Enzon was obligatedto pay a 25% royalty on all sales of ONCASPAR in the U.S. and Canada.

"Today's announcement further underscores our commitment to drivefuture growth by investing in our marketed brands," said Jeffrey H.Buchalter, Enzon's chairman and chief executive officer. "Since itsapproval, ONCASPAR has offered a significant clinical advantage toleukemia patients who are hypersensitive to native L-asparaginase. Webelieve there is a strong growth opportunity for ONCASPAR beyond itscurrently approved indication and we are committed to making theappropriate investments to optimize that potential."

While specific financial terms are not being disclosed, under theamended agreement, Enzon will pay a single digit royalty percentage onannual sales of ONCASPAR that are in excess of $25 million. Enzon willmake an upfront cash payment of $35 million. Enzon is obligated tomake royalty payments through June 30, 2014, at which time all of itsroyalty obligations will cease.

About ONCASPAR

ONCASPAR is a PEG-enhanced version of the naturally occurringenzyme L-asparaginase. L-asparaginase is an enzyme that depletes theamino acid asparagine, which certain leukemic cells are dependent uponfor survival. Enzon developed ONCASPAR and obtained a marketinglicense by the U.S. Food and Drug Administration in February 1994 forthe treatment of patients with acute lymphoblastic leukemia whorequire L-asparaginase in their treatment regimen, but have developedhypersensitivity to native forms of L-asparaginase. Through itsproprietary PEGylation technology, Enzon designed ONCASPAR to offertherapeutic advantages over unmodified L-asparaginase. In addition toreduced immunogenicity, ONCASPAR provides a more convenient,patient-friendly dosing regimen that allows for administration every14 days, versus twice weekly for unmodified L-asparaginase. Enzon'sspecialized oncology sales force currently markets ONCASPAR in NorthAmerica. Enzon reported $21.2 million in net sales of ONCASPAR for theyear ended June 30, 2005, as compared to $18.1 million for the yearended June 30, 2004.

About 35,000 new cases of leukemia will be diagnosed in the UnitedStates during 2005. Of these about 4,000 will be acute lymphoblasticleukemia (also called acute lymphocytic leukemia or ALL). Although ALLis mainly a childhood leukemia, about one-third of new cases willoccur in adults.

About Enzon

Enzon Pharmaceuticals, Inc. is a biopharmaceutical companydedicated to the development and commercialization of therapeutics totreat patients with cancer and other life-threatening diseases.Enzon's specialized sales force markets ABELCET(R), ONCASPAR(R),ADAGEN(R), and DEPOCYT(R) in North America. In addition, Enzon alsoreceives royalties on sales of PEG-INTRON(R), marketed bySchering-Plough Corporation, and MACUGEN(R), marketed by EyetechPharmaceuticals and Pfizer Inc. Enzon's product-driven strategyincludes an extensive drug development program that leverages itsproprietary technologies, including a next-generation PEGylationplatform that utilizes linkers designed to release compounds at acontrolled rate. Enzon complements its internal research anddevelopment efforts with strategic initiatives, such as partnershipsdesigned to broaden its revenue base or provide access to promisingnew technologies or product development opportunities. Furtherinformation about Enzon can be found on the Company's web site atwww.enzon.com.

There are forward-looking statements contained herein that are notbased on historical fact, including without limitation statementscontaining the words "believes," "may," "plans," "will," "estimate,""continue," "anticipates," "intends," "expects," and similarexpressions. Such forward-looking statements involve known and unknownrisks, uncertainties and other factors, including those discussed inour most recent annual report on Form 10-K on file with the U.S.Securities and Exchange Commission, that may cause actual results,events or developments to be materially different from the futureresults, events or developments discussed above. All information inthis press release is as of October 31, 2005 and the Companyundertakes no duty to update this information.