Kosan's Hsp90 Inhibitor, Tanespimycin, Shows 55% Clinical Benefit in Patients With Trastuzumab-Refractory Metastatic Bre...( Five of 20 Evaluable Patients Had...)

Five of 20 Evaluable Patients Had Confirmed Partial Responses (25%);

Updated Data Presented at San Antonio Breast Cancer Symposium

HAYWARD, Calif., Dec. 17 /PRNewswire-FirstCall/ -- Kosan Biosciences Incorporated (Nasdaq: KOSN) today announced that its Hsp90 inhibitor, tanespimycin, demonstrated meaningful antitumor activity and tolerability in a Phase 2 trial in patients with HER2-positive metastatic breast cancer, when administered in combination with trastuzumab (Herceptin(R)) in patients whose disease progressed following treatment with trastuzumab immediately prior to entering the trial. Of 20 evaluable patients, 11 patients benefited from activity, yielding an overall clinical benefit incidence of 55%, including 5 patients with confirmed partial responses, 2 patients with minor responses and 4 patients with extended stable disease. Common toxicities were mainly mild to moderate and included fatigue and gastrointestinal symptoms. Data from the Phase 2 trial were presented at the 2007 San Antonio Breast Cancer Symposium on Sunday, December 16, 2007, in a poster titled, "Tanespimycin (an Hsp90 Inhibitor) and Trastuzumab is an Active Combination in Patients (Pts) with HER2-Positive Trastuzumab-Refractory Metastatic Breast Cancer (MBC): Phase 2 Trial," by Shanu Modi, M.D., of Memorial Sloan-Kettering Cancer Center.

"Tanespimycin is clearly active in patients who are refractory to trastuzumab and whose disease progressed prior to entering the study," said Clifford A. Hudis, M.D., Chief, Breast Cancer Medicine Service, Memorial Sloan-Kettering Cancer Center, and senior author on the poster. "The tolerability of tanespimycin combined with the ability to generate meaningful and durable responses,
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