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Kosan's Lead Hsp90 Inhibitor, Tanespimycin, Shows High Level of Activity in Trastuzumab Resistant/Refractory HER2-Positive Breast Cancer in a Phase 2 Trial

Wednesday, June 4, 2008 General News J E 4
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HAYWARD, Calif., June 3 Kosan BiosciencesIncorporated (Nasdaq: KOSN) today announced that its lead Hsp90 inhibitor,tanespimycin, demonstrated a high level of antitumor activity in a Phase 2trial when administered in combination with trastuzumab (Herceptin(R)) topatients with HER2-positive metastatic breast cancer whose disease had eitherprogressed after, or did not respond to, treatment with trastuzumabimmediately prior to entering the trial. Of 27 evaluable patients, 17 patientsshowed clinical benefit including 7 patients with partial response (PR), 5patients with tumor regression (minor response or MR) and 5 patients withextended stable disease (SD, greater than 4 cycles), yielding an overallclinical benefit incidence of 63%. Common toxicities were mainly mild tomoderate and included fatigue and gastrointestinal symptoms.

Updated data from the Phase 2 trial were presented in a poster titled,"Phase 2 trial of the Hsp90 inhibitor tanespimycin (Tan) + trastuzumab (T) inpatients (pts) with HER2-positive metastatic breast cancer (MBC)," by ShanuModi, M.D., of Memorial Sloan-Kettering Cancer Center, at the 44th AmericanSociety of Clinical Oncology (ASCO) Annual Meeting being held in Chicago, IL.

"The response data from this Phase 2 trial of tanespimycin plustrastuzumab have grown stronger as more patients have been treated,underscoring our observation that tanespimycin is a highly active andtolerable agent in patients whose disease is refractory to trastuzumab," saidClifford A. Hudis, M.D., Chief, Breast Cancer Medicine Service, MemorialSloan-Kettering Cancer Center, and senior author on the poster. "We lookforward to seeing tanespimycin advance into its next set of clinical trialswith the goal of elucidating its potential role in the metastatic breastcancer treatment paradigm."

"Tanespimycin is the most advanced Hsp90 inhibitor in development forHER2-positive breast cancer and has generated what we believe is highlypromising data in this indication," said Pamela Cohen, M.D., Kosan's ChiefMedical Officer. "We believe that Hsp90 inhibition is among the most novel andbroadly applicable anticancer targets being explored today, and that it mayhave broad applicability in breast cancer and other cancer indications."

Phase 2 Tanespimycin Data

Tanespimycin is an Hsp90 inhibitor that has demonstrated the potential todisrupt the activity of multiple oncogenes and cell signaling pathwaysimplicated in tumor growth, including HER2, a key pathway in breast cancer.

The objective of the Phase 2 trial is to determine the objective responserate by RECIST in patients with HER2-positive metastatic breast cancer. To beeligible for the trial, patients must have had either progressive diseasewithin 3 months following last dose of adjuvant treatment with trastuzumab orhave progressive disease following initial therapy for metastatic disease withtrastuzumab (trastuzumab may have been administered with cytotoxicchemotherapy or as a single agent). Tanespimycin was administered at a dose of450 mg/m2 following administration of the standard dose of trastuzumab.

Of 31 patients enrolled in the trial, 27 were evaluable for efficacy(as of May 15, 2008).

Antitumor activity and toxicity were similar between the Injection andInjectable Suspension products.

Tanespimycin plus trastuzumab was highly tolerable at the recommendedPhase 2 dose of 450 mg/m2 weekly. Common toxicities were mainlymild-to-moderate diarrhea, fatigue, nausea, headache and vomiting (limitedduration and amenable to supportive care). The few drug-related Grade 3 and 4toxicities (including fatigue, increased AST and headache, two patients each)were manageable and only one patient discontinued treatment for an adverseevent. Noticeably absent were toxicities common to cytotoxic chemotherapyincluding alopecia and myelosuppression. The Injectable Suspension productprov
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