Onyx Pharmaceuticals Initiates Phase 1 Study of ONX 0801 in Advanced Solid Tumors

Emeryville, CA. — Sep. 28, 2009

Onyx Pharmaceuticals, Inc. (Nasdaq: ONXX) today announced that it has begun enrolling patients in a Phase 1 study of ONX 0801, a novel alpha-folate receptor-mediated thymidylate synthase (TS) inhibitor, as a potential treatment for advanced solid tumors. This open-label, dose-finding study will evaluate the safety and pharmacokinetics of ONX 0801 in patients with advanced solid tumors.

“We are pleased to advance this product candidate so quickly into the clinic, following our acquisition of the compound late last year. ONX 0801 has a high selectivity for cancer cells that we believe will distinguish it from other agents in this proven class of drugs,” said Tony Coles, M.D., president and chief executive officer of Onyx. “We expect this forward momentum in building our portfolio to continue, as we strategically maximize opportunities that will drive long-term sustainable growth.”

Trial Design
The open-label, dose-finding Phase 1 study is evaluating the safety and pharmacokinetics of ONX 0801 in approximately 60 cancer patients with advanced solid tumors. Cohorts of 3 to 6 patients will receive ONX 0801 at escalating doses until a maximum tolerated dose is determined. Each patient will receive a 3-hour intravenous infusion of ONX 0801 weekly (i.e., on days 1, 8, and 15) of repeated 21-day treatment cycles.

About ONX 0801
ONX 0801 is designed to work by combining two proven approaches to improving outcomes for cancer patients. These include receptor-mediated targeting of tumor cells and inhibition of thymidylate synthase (TS), a key enzyme involved in cell growth and division. In pre-clinical studies, ONX 0801 targeted malignant cells that overexpress the alpha-folate receptor, which is located on the cell’s surface. ONX 0801 is distinct from currently marketed TS inhibitors due to its selective tumor cell-specific uptake by the alpha-folate receptor, which is overexpressed in a number of tumor types with significant unmet needs, including ovarian cancer, lung cancer, breast cancer, and colorectal cancer. ONX 0801 was discovered at the Institute of Cancer Research in the UK and is licensed to Onyx by BTG International.

About Onyx Pharmaceuticals, Inc.
Onyx Pharmaceuticals, Inc. is a biopharmaceutical company committed to improving the lives of people with cancer. The company, in collaboration with Bayer HealthCare Pharmaceuticals, Inc., is developing and marketing Nexavar®, a small molecule drug. Nexavar is currently approved for the treatment of liver cancer and advanced kidney cancer. Additionally, Nexavar is being investigated in several ongoing trials in a variety of tumor types. For more information about Onyx, visit the company’s website at www.onyx-pharm.com.

Forward-Looking Statements
This news release may contain “forward-looking statements” of Onyx within the meaning of the federal securities laws. These forward-looking statements include without limitation, statements regarding timing, progress and results of the clinical development, safety, regulatory processes, commercialization efforts or commercial potential of ONX 0801. These statements are subject to risks and uncertainties that could cause actual results and events to differ materially from those anticipated. Reference should be made to Onyx’s Annual Report on Form 10-K for the year ended December 31, 2008, filed with the Securities and Exchange Commission under the heading “Risk Factors” and Onyx’s Quarterly Reports on Form 10-Q for a more detailed description of such factors. Readers are cautioned not to place undue reliance on these forward-looking statements that speak only as of the date of this release. Onyx undertakes no obligation to update publicly any forward-looking statements to reflect new information, events, or circumstances after the date of this release except as required by law.

Nexavar® (sorafenib) tablets is a registered trademark of Bayer HealthCare Pharmaceuticals, Inc.

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