CHICAGO, June 11, 2011 — Two novel drugs produced unprecedented gains in survival in separate studies of people with melanoma, the deadliest form of skin cancer, doctors reported Sunday.

In one study, an experimental drug showed so much benefit so quickly in people with advanced disease that those getting a comparison drug were allowed to switch after just a few months.

The drug, vemurafenib, targets a gene mutation found in about half of all melanomas. The drug is being developed by Genentech, part of Swiss-based Roche, and Plexxikon Inc., part of the Daiichi Sankyo Group of Japan.

Daiichi Sankyo Co. Ltd. completed its acquisition of Plexxikon Inc.    , giving shareholders $805 million upfront with the possibility of near-term milestone payments of another $130 million on Plexxikon’s potential melanoma drug.

Plexxikon will retain its name, employees and Berkeley facilities, and it will continue research and development operations as an independent unit of Tokyo-based Daiichi Sankyo.

The deal was announced in late February.

Plexxikon’s lead drug is PLX-4032, or vemurafenib, that targets a specific mutation present in about half of melanoma cancers and about 8 percent of all solid tumors. Clinical trials of the drug, which is in a Phase III trial, have garnered a lot of attention because of their strong results and a collaboration with Pleasanton’s Roche Molecular Diagnostics, which has developed a test to help identify patients with the specific mutation.

Plexxikon and its development partner, Swiss drug maker Roche, plan to file this year for market approval in the United States and Europe.

Plexxikon also has a potential Hodgkin lymphoma drug, PLX-3397, in a Phase II trial and expects to start Phase II trials of the drug this year in acute myeloid leukemia, the brain cancer glioblastoma and metastatic breast cancer. It also has started a Phase I trial of PLX-5622, a possible treatment of rheumatoid arthritis.

Plexxikon Inc. today announced it has entered into a merger agreement with Daiichi Sankyo Company, Limited, a Japan‐based global pharmaceutical company. The purchase price for Plexxikon is $805 million up‐front. Near‐term milestone payments associated with the approval of PLX4032 could total an additional $130 million. Closure of the transaction is subject to clearance under the Hart‐Scott‐Rodino (HSR) Antitrust Improvements Act and customary closing conditions.

Plexxikon Inc. today announced positive data from an interim analysis of the BRIM3 trial, a large multi‐ center Phase 3 clinical study of PLX4032 (RG7204) in patients with previously untreated metastatic melanoma with the BRAF mutation. Patients treated with PLX4032 had an improved overall survival (OS) compared to patients treated with dacarbazine, the current standard of care. In addition, these patientslived longer on average without their disease getting worse, as measured by progression‐free survival (PFS). PLX4032 is an oral, novel kinase inhibitor that targets the oncogenic BRAF mutation.

Plexxikon today announced publication of data from the Phase 1 clinical trial of PLX4032 (RG7204), confirming that treatment of metastatic melanoma patients with the BRAF V600E mutation resulted in significant tumor shrinkage in the majority of patients. Specifically, in the melanoma extension cohort of the study, nearly all patients showed some response; 81 percent of patients had tumor shrinkage of at least 30 percent. The data were published in the August 26, 2010 issue of the New England Journal of Medicine, based on an analysis as of January 31, 2010. These results further support the current PLX4032 development strategy, which includes parallel and ongoing Phase 2 (BRIM2) and Phase 3 (BRIM3) studies to support registration. PLX4032 is a novel, orally administered, targeted agent that is selective for a key oncogenic driver in melanoma and other cancers.