XenoPort and GlaxoSmithKline Report Positive Top-Line Results of Final Pivotal Trial of XP13512/GSK1838262 for Restless Legs Syndrome

XenoPort, Inc. (Nasdaq:XNPT) and GlaxoSmithKline (NYSE:GSK) today announced positive top-line results from the final pivotal Phase 3 clinical trial of XP13512 (GSK1838262) for the treatment of moderate-to-severe symptoms of primary restless legs syndrome, or RLS. XP13512 demonstrated statistically significant improvements compared to placebo on both of the co-primary endpoints of the trial and was generally well-tolerated. "We have now demonstrated in three Phase 3 trials that XP13512 treatment leads to a clear improvement in RLS symptoms. Based on data from the RLS clinical program, we believe that XP13512 could offer compelling benefits to patients with primary RLS,” said Ronald W. Barrett, Ph.D., chief executive officer of XenoPort. "This novel compound is the first non-dopaminergic agent to demonstrate efficacy in controlled clinical trials for the treatment of primary RLS and may offer patients a new treatment option,” said Atul Pande, M.D., senior vice president, GlaxoSmithKline Neurosciences Medicines Development Center. "With the completion of this third Phase 3 clinical trial, we look forward to filing the NDA for primary RLS in the third quarter of 2008.” This clinical trial (XP053) was a 12-week, double-blind, placebo-controlled Phase 3 clinical trial that enrolled 325 patients who were diagnosed with moderate-to-severe primary RLS. Patients were treated with 1200 mg or 600 mg of XP13512 or placebo, given once per day. The pre-specified primary efficacy analysis of the study was the comparison of the 1200 mg XP13512 and placebo treatment groups. The co-primary endpoints for the clinical trial were the change from baseline for the International Restless Legs Syndrome (IRLS) rating scale score at end of treatment and the percentage of patients showing significant improvement on the Investigator Clinical Global Impression of Improvement (CGI-I) scale at end of treatment. Results from the pre-specified analysis indicate that treatment with 1200 mg of XP13512 was associated with statistically significant improvements in the co-primary endpoints compared to placebo. Improvements in the IRLS scale score were significantly greater for 1200 mg XP13512 than for placebo (unadjusted mean values: -13.0 for 1200 mg XP13512; -9.8 for placebo; p=0.0015). At the end of treatment, significantly more patients treated with 1200 mg of XP13512 were reported as "much improved” or "very much improved” on the CGI-I scale compared to those treated with placebo (78% for 1200 mg XP13512; 45% for placebo; p< 0.0001). Treatment with 600 mg of XP13512 was also associated with statistically significant improvements in the IRLS and CGI-I endpoints compared to placebo. The unadjusted mean reduction in the IRLS scale score was -13.8 for 600 mg XP13512 patients (p<0.0001 compared to placebo). At the end of treatment, 73% of patients treated with 600 mg of XP13512 were reported as "much improved” or "very much improved” on the CGI-I scale (p<0.0001 compared to placebo). During the 12-week treatment period, the most commonly reported adverse events for XP13512 were dizziness (24% 1200 mg XP13512; 10% 600 mg XP13512; 5% placebo) and somnolence (18% 1200 mg XP13512; 22% 600 mg XP13512; 2% placebo). These adverse events were generally mild or moderate in intensity. Withdrawals due to adverse events were 7% in the 1200 mg XP13512 group, 6% in the 600 mg XP13512 group and 6% in the placebo group. There were three reported serious adverse events in the study (one in the placebo group, two in the 600 mg XP13512 group), none of which were considered treatment-related. Dr. Barrett stated, "This study provides additional confirmation of the efficacy and safety of the 1200 mg dose of XP13512 in RLS patients. The efficacy of 600 mg XP13512 in this study was somewhat surprising since a two-week Phase 2 clinical trial with a 600 mg dose did not show a statistical difference from placebo. Data from all doses examined in the RLS trials will be evaluated as part of the XP13512 NDA. With three positive pivotal RLS trials now completed, we hope to provide an important new treatment option to patients with primary RLS.” Conference Call and Webcast Information XenoPort will host a conference call at 8:30 a.m. Eastern Time today. To access the conference call via the Internet, go to www.XenoPort.com. To access the conference call via phone, dial 1-888-275-3514. International callers may access the call by dialing 1-706-679-1417. The replay of the conference call may be accessed via the Internet, after 11:00 a.m. Eastern Time today, at www.XenoPort.com, or via phone at 1-800-642-1687 for domestic callers or 1-706-645-9291 for international callers. The reference number to enter the call and the replay of the call is 37264684. About XP13512 XP13512 is a patented, new chemical entity that is designed to improve upon the clinical utility of gabapentin by taking advantage of high-capacity transport mechanisms in the gastrointestinal tract to improve absorption. Completed Phase 3 Clinical Trials for Moderate-to-Severe Primary RLS XP052 A double-blind, 12-week clinical trial comparing 1200 mg of XP13512 to placebo designed to evaluate safety and efficacy. Results from this trial were released on April 25, 2007. XP060 A placebo-controlled clinical trial designed to evaluate the potential of 1200 mg of XP13512 to maintain efficacy over nine months in patients. Results from this trial were released on January 16, 2008. About RLS According to the National Institutes of Health, up to 12 million people in the U.S. are afflicted with RLS across a range of severity from mild to severe. The syndrome is characterized by disturbing, unpleasant and sometimes painful sensations in the legs that result in a compelling urge to move. The discomfort is often temporarily relieved by movement. Because symptoms typically occur at night, RLS patients often suffer from sleep disruption. RLS symptoms can be debilitating -- published data suggest that RLS can have an impact on quality of life equivalent to, or worse than, major chronic medical disorders such as diabetes and osteoarthritis. About XP13512 Collaborations In December 2005, XenoPort licensed to Astellas Pharma Inc. rights to develop and commercialize XP13512 in Japan, Korea, the Philippines, Indonesia, Thailand and Taiwan. Astellas is currently conducting Phase 2 clinical trials of XP13512 in painful diabetic neuropathy and RLS in Japan. In February 2007, XenoPort entered into a collaboration with GSK for the development and commercialization of XP13512 in all countries of the world, excluding the Astellas territory. XenoPort is completing RLS clinical trials in the U.S. in support of the submission of the NDA by GSK, which is expected in the third quarter of this year. GSK is responsible for all other clinical development and commercialization of XP13512 outside the Astellas territory. In December 2007, GSK announced plans to initiate clinical trials of XP13512 in 2008 for treatment of sleep disturbance in patients with moderate-to-severe primary RLS, management of post-herpetic neuralgia and painful diabetic neuropathy and for migraine prophylaxis. About XenoPort XenoPort, Inc. is a biopharmaceutical company focused on developing a portfolio of internally discovered product candidates that utilize the body’s natural nutrient transporter mechanisms to improve the therapeutic benefits of drugs. Its development and commercialization efforts are currently focused on potential treatments of central nervous system disorders. XenoPort’s most advanced product candidate, XP13512, has successfully completed three pivotal trials in its Phase 3 clinical program for the treatment of moderate-to-severe primary RLS, and has successfully completed a Phase 2a clinical trial for the management of post-herpetic neuralgia. XenoPort has also reported positive results from a Phase 2a clinical trial of its second product candidate, XP19986, in patients with gastroesophageal reflux disease. To learn more about XenoPort, please visit the web site at www.XenoPort.com. About GlaxoSmithKline GlaxoSmithKline is one of the world’s leading research-based pharmaceutical and healthcare companies and is committed to improving the quality of human life by enabling people to do more, feel better and live longer. For more information, visit GlaxoSmithKline at www.gsk.com. Forward-Looking Statements This press release contains "forward-looking” statements, including, without limitation, all statements related to XenoPort’s and its partners’ clinical development programs for XP13512 and the timing thereof; the potential filing of an NDA for XP13512 and the timing thereof; the therapeutic and commercial potential of XP13512; and GSK’s future clinical trials. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. Words such as "believes,” "plans,” "expected,” "will,” "potential,” "may,” "could,” "hope” and similar expressions are intended to identify forward-looking statements. These forward-looking statements are based upon XenoPort’s current expectations. Forward-looking statements involve risks and uncertainties. XenoPort’s 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, risks related to the uncertain results of clinical trials; XenoPort’s or its partners’ ability to successfully conduct the clinical trials for XP13512, and the results thereof; the uncertainty of the FDA approval process and other regulatory requirements; XenoPort’s dependence on its current and additional collaborative partners; and the uncertain therapeutic and commercial value of XenoPort’s compounds. These and other risk factors are discussed under the heading "Risk Factors” in XenoPort’s Annual Report on Form 10-K for the year ended December 31, 2007, filed with the Securities and Exchange Commission on February 22, 2008. XenoPort 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. GlaxoSmithKline Forward-Looking Statement Under the safe harbor provisions of the U.S. Private Securities Litigation Reform Act of 1995, GSK cautions investors that any forward-looking statements or projections made by GSK, including those made in this announcement, are subject to risks and uncertainties that may cause actual results to differ materially from those projected. Factors that may affect GSK’s operations are described under ‘Risk Factors’ in the operating and Financial Review and Prospects in the company’s Annual Report on Form 20-F for 2006. XenoPort is a registered U.S. trademark. XNPT2C