Second Generation of Hsp90 Inhibitor

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Kosan Initiates Phase I Clinical Trial of Second-Generation Hsp90 Inhibitor

KOS-1022 in Hematologic Cancers

Monday June 6, 7:30 am ET

Company Also Announces Grant of U.S. Patent Covering KOS-1022

HAYWARD, Calif., June 6 /PRNewswire-FirstCall/ -- Kosan Biosciences

Incorporated (Nasdaq: <http://finance./q?s=kosn & d=t> KOSN -

<http://finance./q/h?s=kosn> News) announced today that it has

initiated a company-sponsored multicenter Phase I clinical trial of KOS-1022

(DMAG) in patients with advanced hematologic malignancies. The Company also

announced that the U.S. Patent & Trademark Office has granted patent No.

6,890,917 B2, entitled " Geldanamycin Derivative and Method of Treating

Cancer Using Same, " covering the KOS-1022 composition of matter and its use

for treating a range of cancers. The Company is the exclusive licensee of

the patent in these fields and others. KOS-1022 is a second-generation Hsp90

inhibitor that is highly potent and water soluble, with good

bioavailability.

" The rationale for testing KOS-1022 in these hematologic cancers is

particularly compelling. Preclinical data have demonstrated that KOS-1022

reduces the levels of several client proteins, such as Flt3, Akt, and

Bcr-Abl, which may be important targets in leukemias, " stated V.

Santi, M.D., Ph.D., Kosan Chairman and Chief Executive Officer. " This

Kosan-sponsored trial expands the clinical development program for KOS-1022,

which is currently in Phase I testing through our collaboration with the

National Cancer Institute. We are also pleased by the grant of this patent,

which will accord us important intellectual property protections as we

continue our work in this area. "

The Phase I clinical trial will assess the safety, pharmacokinetics and

pharmacodynamics of escalating doses of KOS-1022 in patients with acute

myelogenous leukemia, acute lymphocytic leukemia, or chronic myelogenous

leukemia. According to the Phase I trial design, KOS-1022 will be given by

one-hour infusion on a twice-weekly schedule every three weeks. In addition,

the pharmacodynamics of KOS-1022 in leukemic blast cells will be

investigated in order to determine the biological activity of KOS-1022

against client proteins critical to the progression of leukemia. The

clinical trial will be conducted at leading cancer centers in the United

States.

In addition to the Phase I study announced today, KOS-1022 is being

evaluated in Phase I clinical trials in advanced solid tumors sponsored by

the National Cancer Institute (NCI) under a ative Research and

Development Agreement (CRADA) between Kosan and the NCI Cancer Therapy

Evaluation Program (CTEP). 17-AAG, a first-generation Hsp90 inhibitor, as

well as KOS-953, Kosan's proprietary formulation of 17-AAG, are also

currently being investigated in multiple Phase II single-agent and Phase Ib

combination clinical trials in a variety of tumor types. In 2004, Kosan

obtained orphan drug designation for 17-AAG from both the U.S. Food and Drug

Administration (FDA) and the European Medicines Agency (EMEA) for the

treatment of multiple myeloma and chronic myelogenous leukemia.

About Hsp90 Inhibitors

KOS-1022, 17-AAG, and KOS-953 inhibit Hsp90 (heat shock protein 90), a

protein found in high levels in tumor cells. Hsp90 is a protein " chaperone "

that maintains the stability of numerous " client proteins " implicated in

tumor growth and metastasis, including protein kinases and transcription

factors. By blocking the activity of Hsp90, Kosan's geldanamycin analogs

disrupt the Hsp90-client protein complexes, leading to the degradation of

the client proteins. By targeting multiple growth-signaling pathways

involved in cancer, these compounds may have potential use in a variety of

tumor types, both as single agents and in combination with other signal

transduction inhibitors and cytotoxic drugs.

About Kosan

Kosan Biosciences has two first-in-class anticancer agents in Phase II and

Phase Ib clinical trials: KOS-862 (Epothilone D) and 17-AAG, a geldanamycin

analog and Hsp90 inhibitor. KOS-862, the Company's lead drug candidate, has

a mechanism of action similar to taxanes and is partnered with Roche in a

global development and commercialization agreement, along with a follow-on

compound, KOS-1584, currently in Phase I testing. 17-AAG targets multiple

pathways required for tumor growth and is being developed in collaboration

with the National Cancer Institute, in addition to a second-generation

geldanamycin analog, KOS-1022 (DMAG), now in Phase I trials. Kosan's

proprietary formulation of 17-AAG, KOS-953, is in Phase I and Phase Ib

trials. Kosan has generated a pipeline of potentially significant products

for cancer, infectious disease and other therapeutic areas based on its

proprietary technologies for discovering, developing and manufacturing

polyketide analogs. Polyketides are an important class of natural products

that have yielded numerous pharmaceuticals for the treatment of cancer,

infectious diseases, high cholesterol, transplant rejection and other

diseases.