ONYX Pharmaceuticals Reports Phase II Clinical Trial Results At ASCO Meeting

May 18, 1998

RICHMOND, CALIF. (Monday, May 18, 1998) --- Onyx Pharmaceuticals, Inc. (Nasdaq:ONXX) today reported results from two Phase II studies of its lead anticancer viral therapeutic, ONYX-015, in patients with tumors of the head and neck. In an ongoing Phase II study combining ONYX-015 with standard chemotherapy, the company reported that nine out of ten patients with recurrent tumors have experienced tumor regressions of greater than 50 percent to date, with two patients experiencing complete responses. In addition, the company reported that 30 percent of patients treated with ONYX-015 alone had significant tumor regressions, including two complete regressions in patients with end-stage, treatment-resistant disease.

During a poster session today at the American Society of Clinical Oncology (ASCO) meeting in Los Angeles, Onyx presented interim results from a Phase II trial combining ONYX-015 with Cisplatin and 5-FU, two standard chemotherapeutic agents used in the treatment of head and neck cancers. In addition to the two patients who had complete responses, seven patients experienced between 50 and 95 percent reductions in tumor mass. The one remaining patient has experienced a 40 percent tumor regression. To date, all of the patients continue on treatment with no evidence of disease progression. The average overall response rate with Cisplatin and 5-FU alone is about 35 percent historically in patients with recurrent disease,noted David H. Kirn, M.D., director of clinical research at Onyx.

"We have seen significant tumor regression in the patients treated to date, including some complete responses, and this is a dramatic early finding," Dr. Kirn said. "But we have only evaluated ten patients at this point. While we are encouraged by these initial results, we remain cautiously optimistic."

"Of course, because this is an ongoing study, we do not yet have information regarding durability of responses. We will be able to discuss these findings with greater confidence when we have seen them repeated in a larger number of patients over a longer follow-up period," Dr. Kirn said.

Patients participating in the ONYX-015/chemotherapy study may or may not have received prior chemotherapy, but all have failed other forms of up-front therapy, i.e. surgery and radiation. Toxicities observed in the study to date are consistent with those associated with Cisplatin and 5-FU alone, Dr. Kirn noted.

"The responses we've seen with this combination therapy to date are quite striking and unlike any we've seen with chemotherapy alone," said John Nemunaitis, M.D. of Physician's Reliance Network in Dallas. "We've been amazed at how quickly and how broadly the tumors have responded to this therapy."

Dr. Kirn and Dr. Nemunaitis led the discussion at ASCO today, along with Fadlo Khuri, M.D. of M.D. Anderson Cancer Center in Houston.

In summarizing the experience to date with single agent, multi-day administration of ONYX-015, the investigators noted that antitumoral activity has now been clearly observed and a high level of safety firmly established in a larger group of patients. The fact that no toxicities were reported in cases where the virus has been injected into adjacent normal tissue is particularly noteworthy, they said.

"We are encouraged by the 30 percent overall regression rate we have seen to date in a group of patients that has failed all other available therapies," said Dr. Nemunaitis.

"Based on these encouraging results as well as on the excellent safety profile, we proceeded to the combination chemotherapy study and to a more intensive treatment regimen using the virus alone," Dr. Kirn said. "We continue to enroll patients into these studies."

Onyx initiated the Phase II, single agent intensive regimen trial in March of this year. All of the Phase II studies of ONYX-015 in head and neck cancer are expected to be completed before the end of 1998. At that time, the company will evaluate the full complement of data and determine the next step in clinical development of ONYX-015. Depending on the outcome of current and future clinical studies, it could be several years, if ever, before ONYX-015 is available to cancer patients, the company cautioned.

ONYX-015 is a genetically engineered adenovirus that, in prior preclinical and clinical studies, has been shown to replicate in and kill tumor cells deficient in p53 tumor suppressor gene activity. Mutations in p53 are the most common type of genetic abnormality in cancer, occurring in more than 50 percent of human cancer cases, including more than 70 percent of recurrent and refractory cancers of the head and neck.

Onyx Pharmaceuticals was founded in 1992 for the purpose of discovering and developing novel therapeutics based upon the genetics of human disease, with an emphasis on cancer. The company focuses on defining the function of certain mutated genes which are known to cause cancer, and on developing innovative therapies to reverse the effects of the mutation or to kill the cancer cell. Onyx pursues a strategy of establishing corporate partnerships that provide complementary skills in chemistry, drug development, and marketing and sales.

This press release contains certain forward-looking statements with regard to the development of potential human therapeutic products that involve a number of risks and uncertainties. Actual events may differ from the company's expectations.In addition to the matters described in this press release, the timeline for clinical activity, results of pending or future clinical trials, including the trials of ONYX-015, and changes in the status of the company's collaborative relationships, as well as the risk factors listed from time to time 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, may affect the actual results achieved by the company.

Kureczka/Martin Associates

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