Onyx Pharmaceuticals Achieves Milestone in Cancer Program With Pfizer

Onyx Pharmaceuticals Achieves Milestone in Cancer Program With
Pfizer
PD332991 Enters Phase I Human Clinical Trials 


RICHMOND, Calif., Sept. 20  /PRNewswire-FirstCall/ -- Onyx Pharmaceuticals,
Inc. (NASDAQ:ONXX) announced today that its collaborator, Pfizer Inc, has
initiated Phase I clinical trials for an anti-cancer compound discovered during
the two companies' research collaboration.  The initiation of human clinical
trials triggers a $500,000 milestone payment to Onyx.

Pfizer is managing and funding all clinical development and commercialization
activities for this compound, known as PD332991.  In exchange, Onyx will
receive a high single-digit royalty if PD332991 is commercialized.  Onyx will
also receive milestone payments associated with the achievement of certain
clinical and regulatory events.

This novel compound is intended to intervene in the misregulated cell cycle of
tumor cells by inhibiting a key enzyme, cyclin-dependent kinase 4 (cdk4).  The
normal cell cycle consists of distinct phases during which the cell's DNA is
copied and the cell divides.  Cdks are enzymes that operate as switches to move
the cell through these stages by controlling protein phosphorylation.  PD332991
is a cdk4 inhibitor active in the initial stage (G1) of the cell cycle, a phase
where most human tumors have abnormal regulation.  It is anticipated that
blocking cdk4 activity may provide an approach to inhibiting proliferation of
tumor cells at a key step in the cell cycle, one where cancer cells become
distinct from normal cells.

Earlier this year, at the annual meeting of the American Association for Cancer
Research (AACR), Pfizer researchers reported that PD332991 is a highly
selective and potent cdk4 inhibitor.  It was shown to arrest the cell in the G1
phase of the cell cycle, preventing it from entering the S phase, the step
where the cell's DNA is replicated.  Pfizer researchers demonstrated that oral
administration of PD332991 resulted in marked tumor regression in in vivo
models.  Thus it appears that PD332991 has the potential to shrink tumors, as
well as to prevent tumor growth.

Onyx Pharmaceuticals is engaged in the development of novel cancer therapies
that target the molecular basis of cancer.  With its collaborators, the company
is developing small molecule drugs, including BAY 43-9006 with Bayer
Pharmaceuticals Corporation.  For more information about Onyx's pipeline and
activities, visit the company's website at http://www.onyx-pharm.com/.

This news release contains forward-looking statements regarding expectations
about the development of a novel Cdk inhibitor, including further clinical
testing and the timing of such clinical trials; and the company's expectations
or beliefs of the compound's commercial potential.  These forward-looking
statements involve a number of risks and uncertainties that could cause actual
events to differ from the company's expectations.  These risks are addressed in
the company's periodic reports filed with the Securities and Exchange
Commission, including but not limited to its Annual Report on Form 10-K filed
on March 15, 2004 and its Quarterly Reports on Form 10-Q. 

DATASOURCE: Onyx Pharmaceuticals, Inc.



CONTACT:  Julie Wood of Onyx Pharmaceuticals, Inc., [phone]



Web site:  http://www.onyx-pharm.com/ 

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