Plexxikon Announces First Patient Dosed in First of Two Pivotal Trials of PLX4032 for Metastatic Melanoma.

BERKELEY, Calif. -- Plexxikon Inc. announces that enrollment has been initiated and the first patient has been treated in the first of two pivotal trials of PLX4032 (RG7204) in patients with metastatic melanoma. PLX4032 is a novel, oral and highly selective drug that targets the BRAF(V600E)cancer-causing mutation occurring in about 50 percent of melanomas and about eight percent of all solid tumors. This single arm Phase 2 B-Raf Inhibitor in Melanoma (BRIM2) trial for previously-treated metastatic melanoma patients, along with a randomized, controlled Phase 3 trial (BRIM3) expected to start by the end of 2009 in first-line patients, are part of the planned registration program for PLX4032. The initiation of the Phase 2 trial has triggered a significant milestone payment to Plexxikon from Roche. Plexxikon is entitled to receive additional payments for milestone achievements as well as royalties on sales of PLX4032. Plexxikon and Roche are co-developing PLX4032 under their 2006 license and collaboration agreement.

The Phase 2 trial (Study #NP22657) is an open label, single arm study in which approximately 100 metastatic melanoma patients who test positive for the BRAF(V600E)cancer-causing mutation will be treated with PLX4032 at a dose of 960 mg twice-daily (BID). Eligible patients for this trial must have progressed after at least one prior treatment. Patients will be monitored throughout the study for safety and efficacy endpoints. The primary endpoint of this trial is Best Overall Response Rate (BORR) as indicated by tumor regression measured by RECIST (Response Evaluation Criteria in Solid Tumors) criteria. Secondary endpoints include progression-free survival (PFS), symptom improvement and overall survival.

The Phase 2 trial will be conducted at 13 sites in the United States and Australia, and enrollment is expected to be complete by the end of 2009. Patients interested in enrolling in the Phase 2 trial may find additional information at the Roche Clinical Trials Registry (http://www.roche-trials.com/), by visiting www.clinicaltrials.gov, or by contacting the Roche Call Center at 973-235-5000. Roche is conducting all future clinical trials of PLX4032, including these Phase 2 and 3 clinical trials.

Plexxikon has conducted the Phase 1 dose escalation study, and has completed an extension study in exclusively mutation-positive metastatic melanoma patients. Enrollment for a second extension study of mutation-positive colorectal cancer patients is expected to complete by year end.

"Since presenting our positive clinical data at both ASCO and ECCO this year, we have had overwhelming interest in PLX4032 from physicians and patients worldwide. We are looking forward to advancing PLX4032 with the hope of registering this drug as soon as possible in order to make this medicine available to the many metastatic melanoma patients who may benefit," stated K. Peter Hirth, Ph.D., chief executive officer of Plexxikon. "The data generated from the melanoma extension study clearly demonstrate single agent activity with PLX4032, which is unprecedented for melanoma. Additionally, the high degree of selectivity of PLX4032 has enabled us to dose to near complete target inhibition while such treatment has been well tolerated by most patients."

Melanoma Extension Study Confirms Safety & Tumor Regression Seen in Phase 1 Dose Escalation

In an extension study of melanoma patients with the BRAF(V600E) mutation, 31 patients have been enrolled, most with advanced metastatic disease (Stage M1c), and treated with PLX4032 at 960 mg BID. Among the 27 evaluable patients to date, results confirm the preliminary safety and efficacy seen in the previous Phase 1 dose escalation study:

* PLX4032 was well tolerated at 960 mg BID, now confirmed as the maximum tolerated dose

* Complete response in 1 patient treated for 3 cycles

* Partial responses of greater than 30% tumor regression by RECIST criteria have been observed in 18 patients, with 15 patients showing responses of greater than 50%

* Minor responses in 6 patients showed tumor regression between 10% and 30%

A median PFS has not been achieved since it is too early in the study to report. These data were presented earlier this month at the ECCO 15 / ESMO 34 2009 Conference in Berlin, Germany.

Drug-related adverse events were predominantly mild in severity and included rash, joint pain, photosensitivity and fatigue. Serious adverse events were observed in some patients after chronic treatment, including seven patients with cutaneous squamous cell carcinoma (keratoacanthoma subtype) that were treated by excision, while treatment with PLX4032 was continued. A risk management plan is in place for baseline evaluation of the skin and monitoring of all patients while on study.

About PLX4032 (RG7204)--A Personalized Medicine for Cancer Treatment

PLX4032 is a novel, oral small molecule for the treatment of melanoma and other cancers harboring the V600E mutation of the BRAF kinase gene. This defect is present in approximately 50 percent of melanoma skin cancers, 10 percent of colorectal cancers, and overall, about eight percent of all solid tumors. Plexxikon and Roche are co-developing PLX4032 under their 2006 license and collaboration agreement. A companion diagnostic is being co-developed by Plexxikon and Roche Molecular Systems alongside of PLX4032 to determine the BRAF mutation status of patients. Melanoma is the most serious type of skin cancer. More than 50,000 people in the U.S. and 160,000 people worldwide are diagnosed with melanoma each year. The median progression-free survival for a patient with metastatic melanoma is less than 60 days, and the median overall survival for these patients is less than 12 months.

About Plexxikon

Plexxikon is a leader in the structure-guided discovery and development of novel small molecule pharmaceuticals to treat human disease. The company's clinical stage programs include PLX4032 for the treatment of melanoma and colorectal cancer, PLX5568 for the treatment of polycystic kidney disease, PLX204 for the treatment of diabetes and PLX3397 for the treatment of metastatic disease and rheumatoid arthritis. Among the company's preclinical development programs, candidates are being developed for the treatment of rheumatoid arthritis, multiple sclerosis and other autoimmune diseases as well as for the treatment of other cancers.

Plexxikon's proprietary Scaffold-Based Drug Discovery[TM] platform integrates multiple state-of-the-art technologies, including structural screening as one key component that provides a significant competitive advantage over other drug discovery approaches. To date, the company has discovered a portfolio of clinical and preclinical stage compounds being developed to address significant unmet medical needs in cardio-renal disease, CNS disorders, inflammatory and neuro-inflammatory diseases and oncology. For more information: www.plexxikon.com.