Kosan Presents Preclinical Antitumor and Safety Data On Next-Generation Epothilone KOS-1803 at AACR

HAYWARD, Calif., April 16, 2007 /PRNewswire-FirstCall/ -- Kosan Biosciences Incorporated today presented preclinical data on KOS-1803, a next-generation epothilone, showing a high level of antitumor activity and a favorable pharmacokinetic and safety profile in a broad range of in vitro tests and in vivo cancer models. KOS-1803 has been chemically engineered to have a favorable tissue distribution pattern, which has been shown in models to result in excellent tumor penetration and a low side effect profile. In human tumor xenograft models, KOS-1803 has demonstrated antitumor activity in breast, lung, prostate and colorectal cancers, as well as in ovarian cancer and neuroblastoma where the extent and duration of tumor regression was curative. Kosan is currently advancing KOS-1803 through later-stage preclinical studies with the potential for future clinical development of the compound.

"KOS-1803 is an exciting, next-generation clinical candidate based on Kosan's proprietary epothilone D backbone that is demonstrating excellent potency and antitumor activity in multiple in vivo cancer models and in taxane-sensitive and resistant cancer cell lines," said Robert G. Johnson, Jr., M.D., Ph.D., Kosan's President and Chief Executive Officer. "KOS-1803 is particularly interesting due to its ability to induce tumor regression, activity at low doses, infrequent dosing schedule and attractive safety profile demonstrated in multiple xenograft models. Our development partner, Roche, and we expect that our lead epothilone compound, KOS-1584 (R1645), will enter Phase 2 trials in solid tumors later in 2007. KOS-1803 may represent a future opportunity to expand the therapeutic and commercial value of our companies' epothilone franchise."

Kosan's epothilone program is partnered with Roche through a global development and commercialization agreement in the area of cancer. In early 2007, Roche and Kosan announced the decision to advance KOS-1584 into Phase 2 trials in solid tumors. The Phase 2 program is anticipated to begin in the second half of 2007.

Data on KOS-1803 were presented in a poster titled, "KOS-1803: A New, Rationally Designed Epothilone Analog with an Improved Pharmacological Profile and Efficacy in Xenograft Mouse Models," at the 2007 Annual Meeting of the American Association for Cancer Research (AACR), being held in Los Angeles, CA.

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 (R1645) 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.

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 relating to the further development and potential safety and efficacy of KOS-1803 and KOS-1584; 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 product candidates, 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; uncertainties with regards to the development activities that Roche will conduct or permit to be conducted for KOS-1803; and other risks detailed from time to time in the Company's SEC reports, including its Annual Report on Form 10-K for the year ended December 31, 2006 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.

Posted: April 2007

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