PRESS RELEASE: Exelixis Retains Right to Develop and Commercialize XL647

SOUTH SAN FRANCISCO, Calif., July 26 -- Exelixis, Inc. today announced that GlaxoSmithKline (GSK) has decided not to exercise its option to license XL647 for further development and commercialization. As a result, Exelixis retains the right to develop and commercialize XL647 either independently or in collaboration with third parties. The company intends to move forward aggressively with the full development of this promising compound in patients with non small cell lung cancer (NSCLC) and potentially other indications as well.

"We are very pleased to retain the right to develop and commercialize XL647. We view the data generated with XL647 to date to be compelling and to warrant the aggressive advancement of the XL647 clinical program," said George A. Scangos, Ph.D., president and chief executive officer of Exelixis. "We expect XL647 to advance into pivotal registration trials for NSCLC in early 2008 and view this event as a key transformational step for Exelixis as we mature into a commercially focused organization." Dr. Scangos added, "The evidence of XL647 activity in our phase 2 trial to date has created a high level of excitement in the oncology community, which has helped to drive rapid recruitment and set the stage for moving this compound into late stage development for a variety of oncology indications.

Two phase 2 trials of XL647 in NSCLC indications are ongoing, and data from a trial of XL647 in previously untreated patients with Stage IIIB or IV NSCLC have been accepted for presentation at the 12th International Association for the Study of Lung Cancer (IASLC) World Conference on Lung Cancer, which will be held September 2-6, 2007, in Seoul, South Korea.

"The data to date for XL647 have shown clear anti-tumor activity in non- small cell lung cancer. There have been several durable responses in patients with and without EGFR activating mutations which could give this compound broad utility in a diverse patient population," said Dr. Thomas Lynch, Chief Hematology and Oncology, Director Center for Thoracic Cancers at Massachusetts General Hospital. "Small molecule EGFR inhibitors have been a great addition to the treatment of lung cancer; however, there is room for improvement in efficacy and tolerability. XL647 retains activity against the T790M mutation which confers erlotinib resistance which could potentially provide a necessary step forward in the treatment for lung cancer and potentially other cancers."

The proof-of-concept data for XL647 were generated in the ongoing phase 2 trial in previously untreated patients with stage IIIB or IV NSCLC. The trial is designed to select a proportion of patients with activating mutations in the EGF receptor (EGFR), which have been associated with improved sensitivity to other EGFR inhibitors. This selection is achieved by requiring patients to have adenocarcinoma histology and meet one of the following three criteria: Asian, female, no or minimal smoking history. Based on these criteria, Exelixis anticipates that approximately 30 percent of subjects in the trial will have activating EGFR mutations. This design should allow the accumulation of data for patients with and without EGFR mutations, helping to identify those patients most likely to benefit from treatment with XL647.

Additionally a phase 2 trial is ongoing in NSCLC patients who have initially responded, but later failed treatment with EGFR inhibitors erlotinib or gefitinib. The rationale for this study is XL647's activity against the T790M mutation in the EGFR which develops in approximately 50% of patients as a mechanism of resistance to erlotinib or gefitinib.

The Exelixis collaboration with GSK covers 10 compounds and their back-up & follow-up compounds in Exelixis' development pipeline, of which GSK is entitled to select up to three upon achieving clinical proof-of-concept. XL647 was the first collaboration compound submitted to GSK for development election. Exelixis expects to submit two additional compounds to GSK later this year and several others, if necessary, in 2008. Under the terms of the agreement, if XL647 is successfully commercialized, Exelixis will pay GSK a royalty of 3% on net sales of XL647.

XL647 is also part of the company's clinical development financing arrangement with Symphony Evolution, Inc. (SEI). In 2005, Exelixis licensed three of its compounds, XL784, XL647, and XL999, to SEI in return for $80 million for the clinical development of these compounds and an exclusive option to reacquire the compounds from SEI's investors at a specified purchase price. Exelixis is primarily responsible for the development of these compounds in accordance with specified development plans and related development budgets. As of June 30, 2007, SEI had $44.7 million of cash and cash equivalents, which Exelixis anticipates will be used to advance the clinical trial programs for XL647 in addition to XL784 and XL999 until Exelixis repurchases the compounds from SEI's investors.

Conference Call and Webcast

Exelixis will hold a live webcast on Thursday, July 26, at 5:00 a.m. PDT/8:00 a.m. EDT, in which Exelixis management will provide an update on developments under its collaboration with GlaxoSmithKline. The webcast may be accessed in the Event Calendar page under Investors at

About XL647

XL647 is a potent inhibitor of receptor tyrosine kinases (RTKs) that are implicated in driving tumor proliferation and vascularization (blood vessel formation). XL647 inhibits EGFR, HER2 and VEGFR2. The compound has been optimized for high potency and oral bioavailability (using in vivo systems), demonstrates excellent activity in target-specific cellular functional assays and has shown sustained inhibition of target RTKs preclinically in vivo following a single oral dose. Positive data from a phase 1 trial of XL647 in patients with advanced solid tumors were reported most recently at the 18th EORTC-NCI-AACR Symposium on Molecular Targets and Cancer Therapeutics in November 2006. Two phase 2 trials in patients with NSCLC are ongoing.

About Exelixis

Exelixis, Inc. is a development-stage biotechnology company dedicated to the discovery and development of novel small molecule therapeutics for the treatment of cancer and other serious diseases. The company is leveraging its fully integrated drug discovery platform to fuel the growth of its development pipeline, which is primarily focused on cancer. Currently, Exelixis' broad product pipeline includes investigational compounds in phase 2 and phase 1 clinical development for cancer and renal disease. Exelixis has established strategic corporate alliances with major pharmaceutical and biotechnology companies, including GlaxoSmithKline, Bristol-Myers Squibb Company, Genentech, Wyeth Pharmaceuticals and Daiichi-Sankyo. For more information, please visit the company's web site at

This press release contains forward-looking statements, including, without limitation, statements related to the future development and potential efficacy of XL647, the submission of further compounds to GSK and the timing thereof and the future development of XL784 and XL999. Words such as "intends," "believes," "promising," "anticipates," "plans," "view," "expects," "could" and similar expressions are intended to identify forward-looking statements. These forward-looking statements are based upon Exelixis' current expectations. Forward-looking statements involve risks and uncertainties. Exelixis' actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, the potential failure of XL647, XL784 and/or XL999 to demonstrate safety and efficacy in clinical testing, risks related to Exelixis' dependence on and relationship with GSK and SEI and risks related to Exelixis' need for additional financing. These and other risk factors are discussed under "Risk Factors" and elsewhere in Exelixis' quarterly report on Form 10-Q for the fiscal quarter ended March 30, 2007 and other filings with the Securities and Exchange Commission. Exelixis expressly disclaims any obligation or undertaking to release publicly any updates or revisions to any forward-looking statements contained herein to reflect any change in the company's expectations with regard thereto or any change in events, conditions or circumstances on which any such statements are based.