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Kosan Presents Preliminary Phase 2 Data Showing Antitumor Activity,of Hsp90 Inhibitor Tanespimycin in Metastatic Melanoma at ASCO

HAYWARD, Calif., June 04, 2007 /PRNewswire-FirstCall/ -- Kosan Biosciences Incorporated presented preliminary data from a Phase 2 monotherapy trial showing that tanespimycin demonstrated antitumor activity and tolerability in patients with metastatic melanoma, including one patient with a BRAF mutation (V600E) who had failed prior chemotherapy (within 6 weeks of initiation) and while on tanespimycin demonstrated progression-free survival for more than six months and 20% lesion reduction by RECIST as well as reduction in the size and number of extensive subcutaneous nodules. Data from the ongoing trial were presented on Sunday, June 3, 2007, by Richard Kefford, M.D., Ph.D., Westmead Hospital, University of Sydney, Australia, in a poster titled, "Phase 2 Trial of Tanespimycin (KOS-953), a Heat Shock Protein-90 (Hsp90) Inhibitor in Patients with Metastatic Melanoma," at the 2007 Annual Meeting of the American Society of Clinical Oncology (ASCO).

"Hsp90 inhibition is of significant interest for exploration as an antitumor approach in melanoma, as members of the BRAF signaling pathway are client proteins and are implicated in the majority of melanomas," said Dr. Kefford. "These very early data with tanespimycin demonstrate tolerability in this advanced metastatic melanoma patient population. The conditions have been fulfilled for expanded patient accrual in this Phase 2 trial to further explore efficacy."

"Metastatic melanoma is the fourth cancer indication in which a Kosan Hsp90 inhibitor has demonstrated antitumor activity and a high degree of tolerability, providing additional proof-of-concept that Hsp90 inhibition has applicability in a broad range of tumor types and that our lead Hsp90 inhibitor, tanespimycin, has significant therapeutic potential," said Robert G. Johnson, Jr., M.D., Ph.D., Kosan's President and Chief Executive Officer. "Tanespimycin will be the first Hsp90 inhibitor to enter regi stration trials and has the potential to be the first Hsp90 inhibitor to reach the market. Our TIME registration program for tanespimycin in patients with multiple myeloma is on track to be initiated in the third quarter of 2007, which will be a major milestone for our company."

Phase 2 Tanespimycin Melanoma Trial

Tanespimycin binds to and inhibits the activity of Hsp90. Hsp90 inhibition results in the degradation of a variety of RAF family proteins, including mutant BRAF. As the majority of melanomas have activation of the BRAF-MAP kinase pathway, it is postulated that tanespimycin interrupts the MAP kinase pathway and may lead to clinically significant anti-melanoma effects. The Phase 2 trial of tanespimycin was designed as a multi-center two-stage Simon design study; continuation to the second stage required at least 1 of the first 9 patients to have a progression-free survival (PFS) time of at least 24 weeks, the primary endpoint of the trial. Eligible patients had stage M1 melanoma, measurable/unresectable disease and had progressed within 3 months of study entry with no more than 1 prior chemotherapy treatment for metastatic melanoma. Tanespimycin was administered at a dose of 275 mg/m2 by 1-2 hour intravenous infusion 2 times per week every 2 out of 3 weeks until disease progression or toxicity. 20 patients have been enrolled, and efficacy data are available on 14 evaluable patients. One of the 6 evaluable patients with a BRAF mutation (V600E) showed evidence of anti-melanoma activity. An additional patient with BRAF mutation continues on study but is too early to assess for efficacy at present. The safety profile has been manageable and included predominantly Grade 1 and 2 toxicities (constitutional and gastrointestinal symptoms). Grade 3 or 4 toxicity, which was generally manageable and reversible, resulted in treatment discontinuation in 2 patients. Accrual is continuing toward the goal of enrolling a total of 30 evaluable patients (with cont inued BRAF mutation analysis).

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. Kosan is leveraging its proprietary discovery platform to generate a pipeline of potentially significant product candidates, primarily in the area of oncology.

Hsp90 inhibitors have a novel mechanism of action targeting multiple pathways involved in cancer cell growth and survival. Kosan's proprietary formulation of tanespimycin (KOS-953) is currently in Phase 1 and 2 clinical trials, primarily for multiple myeloma in combination with Velcade(R) (bortezomib) and HER2-positive metastatic breast cancer in combination with Herceptin(R) (trastuzumab). In addition, intravenous and oral formulations of Kosan's second-generation Hsp90 inhibitor, alvespimycin (KOS-1022), are being evaluated in Phase 1 clinical trials.

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 Phase 1 clinical trials in patients with solid tumors. Kosan's epothilone program is partnered with Roche through a global development and commercialization agreement.

For additional information on Kosan Biosciences, please visit the company's website at

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 the further development and potential safety, efficacy, commercialization and other characteristics of Kosan's Hsp90 inhibitors; and initiations of clinical trials and the timing thereof. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. There are a number of important factors that could cause the results of Kosan to differ materially from those indicated by these forward-looking statements, including, among others, risks related to the development of Kosan's Hsp90 inhibitors, including the risk that studies may not demonstrate safety and efficacy sufficient to initiate clinical trials, continue clinical development, obtain the requisite regulatory approvals or to result in a marketable product; and other risks detailed from time to time in the Company's SEC reports, including its Quarterly Report on Form 10-Q for the quarter ended March 31, 2007, and other periodic filings with the SEC. Kosan does not undertake any obligation to update forward-looking statements.

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

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

CONTACT: Jane Green, VP of Corporate Communications for Kosan BiosciencesIncorporated, +1-510-731-5335, mobile, +1-415-652-4819,

Web site:

Ticker Symbol: (NASDAQ-NMS:KOSN)

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