Kosan Announces Data Presentations at ASCO 2008 Annual Meeting

HAYWARD, Calif., May 14, 2008 /PRNewswire-FirstCall/ -- Kosan Biosciences Incorporated today announced that updated data will be presented on tanespimycin, the company's lead Hsp90 inhibitor, from a Phase 2 trial in combination with Herceptin(R) in patients with HER2-positive metastatic breast cancer, at the 2008 Annual Meeting of the American Society of Clinical Oncology (ASCO), being held at McCormick Place, Chicago, IL, May 31 through June 3, 2008.

Abstract 1027, titled, "Phase 2 trial of the Hsp90 inhibitor tanespimycin (Tan) + trastuzumab (T) in patients (pts) with HER2-positive metastatic breast cancer (MBC)," will be presented in a poster session in room E450a on Tuesday, June 3, 2008 from 8:00 am through 12:00 noon CT. The presenter is Shanu Modi, M.D., of Memorial Sloan-Kettering Cancer Center. This abstract will be highlighted in a poster discussion session to be held from 11:00 am - 12:00 noon CT in room E450b.

In addition, data from a Phase 1 trial of Kosan's Hsp90 inhibitor, alvespimycin, will be presented in an oral session on Monday, June 2, 2008 at 8:00 am to 11 am CT. Abstract 2502, titled, "First use of an oral Hsp90 inhibitor in patients (Pts) with solid tumors: Alvespimycin (A) administered QOD or QD," will be presented at 8:00 to 8:15 am CT in room W375a by Keith Flaherty, M.D., of the .

About Kosan

Kosan Biosciences is a biotechnology company advancing two new classes of anticancer agents through clinical development: Hsp90 (heat shock protein 90) inhibitors and epothilones. Hsp90 inhibitors have a novel mechanism of action targeting multiple pathways involved in cancer cell growth and survival. Tanespimycin (KOS-953) is being tested in combination with Velcade(R) (bortezomib) in patients with multiple myeloma in a clinical program called TIME. Tanespimycin is also being studied in HER2-positive metastatic breast cancer in combination with Herceptin(R) (trastuzumab). Epothilones inhibit cell division with a mechanism of action similar to taxanes, one of the most successful classes of anti-tumor agents. KOS-1584 is in a Phase 2 clinical trial in patients with non-small cell lung cancer. Kosan's motilin agonist compound, KOS-2187, licensed to Pfizer for development in gastroesophagel reflux disease, is in a Phase 1 trial.

This press release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995 (the "Act"). Such forward-looking statements include but are not limited to statements regarding the further development and potential safety, efficacy, regulatory status, commercial potential and other characteristics of Kosan's product candidates; the continuation of current clinical trials; the initiation of additional clinical trials and the timing thereof and the use of Kosan's financial resources. Words such as "will," "expect," "believe," "may," "intend," "plan," "potential" and similar expressions are intended to identify forward-looking statements. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. These forward-looking statements are based upon Kosan's current expectations. Forward-looking statements involve risks and uncertainties. Kosan's actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks related to the uncertain progress and results of Kosan's preclinical and clinical testing, including the risks that studies and trials may not demonstrate safety and efficacy sufficient to initiate clinical trials on the timing currently anticipated, or at all, continue clinical development, obtain the requisite regulatory approvals or result in a marketable product; the conduct of clinical trials; manufacturing; regulatory approval requirements and process; the effort and expense necessary for further development of Kosan's product candidates, including the costs of bortezomib; intellectual property matters, including Kosan's ability to obtain valid and enforceable patents covering its product candidates; Kosan's dependence on its collaboration with Pfizer for development of its motilin agonist product candidate; Kosan's need for additional financing and Kosan's strategy to enter into partnering or licensing arrangements. These and other risk factors are discussed under "Risk Factors" in Kosan's Annual Report on Form 10-K for the year ended December 31, 2007 and other periodic filings with the SEC. Kosan expressly disclaims any obligation or undertaking to release publicly any updates or revisions to any forward-looking statements contained herein.

Velcade(R) (bortezomib) is a registered trademark of Millennium Pharmaceuticals, Inc.

Herceptin(R) (trastuzumab) is a registered trademark of Genentech, Inc.

Posted: May 2008

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