XenoPort and GlaxoSmithKline Report Positive Top-Line Results of Second Phase 3 Restless Legs Syndrome Trial for XP13512/GSK1838262

Study Highlights Safety and Efficacy of XP13512 in Primary Restless Legs Syndrome Patients Treated for Nine Months

SANTA CLARA, Calif. & RESEARCH TRIANGLE PARK, N.C.--(BUSINESS WIRE)--Jan 16, 2008 - XenoPort, Inc. (Nasdaq:XNPT) and GlaxoSmithKline (NYSE:GSK) today announced positive top-line results from a placebo-controlled Phase 3 clinical trial designed to evaluate the potential of XP13512 (GSK1838262) to maintain efficacy over the course of nine months in patients with moderate-to-severe primary Restless Legs Syndrome, or RLS. Data from this randomized withdrawal trial showed that XP13512 was generally well-tolerated and that there was a statistically significant difference between the percentage of patients treated with XP13512 and placebo who met a pre-specified relapse criteria during the randomized phase of the study.

"The results of this study strengthen our belief that XP13512 has the potential to be a safe and effective treatment for primary RLS. Our first placebo-controlled Phase 3 efficacy trial of XP13512, with results announced in April 2007, demonstrated statistically significant benefits of XP13512 in treating RLS symptoms over 12 weeks. We are encouraged by the results of this new placebo-controlled Phase 3 clinical trial, which demonstrated that XP13512 was effective and generally well-tolerated when administered to primary RLS patients for nine months," said Ronald W. Barrett, Ph.D., chief executive officer of XenoPort.

"This study is an important next step in moving toward the submission of the NDA for XP13512, a potential new treatment for patients with primary RLS. XenoPort and GSK are committed to continuing research in this important area where there is still significant unmet medical need," said Atul Pande, M.D., senior vice president, GlaxoSmithKline Neurosciences Medicines Development Center.

Trial Design

This multi-center, randomized, placebo-controlled clinical trial enrolled 327 patients diagnosed with moderate-to-severe primary RLS. In the first phase of the trial, all patients were administered 1200 mg of XP13512, taken at approximately 5:00 p.m., for 24 weeks. Patients were assessed to determine treatment response at the end of this single-blind phase of the clinical trial. Responders were defined as those patients who met three criteria: a decrease in total International RLS (IRLS) rating scale score of six or more points compared to baseline score, a decrease in IRLS score to less than 15 and an assessment of "much improved" or "very much improved" on the Investigator Clinical Global Impression of Improvement (CGI-I). Responders were required to have been stable on 1200 mg of XP13512 for at least the last month of the 24-week treatment period.

After completing the single-blind phase, responders entered the 12-week, randomized, double-blind phase of the clinical trial. Patients randomized to the placebo group received 600 mg of XP13512 for two weeks and then received placebo for an additional ten weeks. Patients randomized to the XP13512 treatment group continued to receive 1200 mg of XP13512 for the entire 12-week, double-blind period.

The primary endpoint of the trial was the proportion of RLS patients who "relapsed," or had their RLS symptoms worsen, during the 12-week, double-blind treatment period. Patients were considered to have "relapsed" if either or both of the following occurred: 1) the patient withdrew from the clinical trial due to lack of efficacy; and/or 2) there were two consecutive study visits in which (a) the patient's IRLS score worsened by at least six points compared to the IRLS score at the point of randomization; (b) the patient achieved an IRLS score of at least 15; and (c) the patient received an assessment of "much worse" or "very much worse" as compared to the condition of the patient at the point of randomization using an Investigator Clinical Global Impression of Change (CGI-C).

Clinical Trial Results

Three hundred twenty seven patients were enrolled in the study, and 221 patients completed the 24-week, single-blind portion of the clinical trial, of which 194 (88%) met the responder criteria and were randomized to double-blind treatment.

Analysis of the primary endpoint indicated that treatment with XP13512 resulted in a statistically significant lower proportion of relapses compared to placebo during the double-blind treatment period (23% placebo compared to 9% XP13512; p= 0.0158).

During the single-blind phase of the trial, 13% and 4% of XP13512-treated patients withdrew from the clinical trial due to adverse events or lack of efficacy, respectively. The most commonly reported adverse events during the single-blind phase of the clinical trial were somnolence (30%) and dizziness (22%), which were generally mild or moderate in intensity and transient in nature.

During the double-blind phase, 0% and 3% of XP13512- and placebo-treated patients, respectively, withdrew from the trial due to adverse events. The incidence of somnolence and dizziness in XP13512-treated patients during the double-blind portion of the trial were 3% and 2%, respectively.

During the trial, there was one death that was determined to be unrelated to XP13512 treatment. There were five other serious adverse events, only one of which was judged as possibly related to XP13512 treatment.

"We are encouraged that XP13512 treatment was associated with a statistically significant difference from placebo in this randomized withdrawal study, particularly in light of the stringent criteria for relapse used in this clinical trial," added Dr. Barrett. "We are also pleased with the tolerability profile of XP13512 in this nine-month study. We look forward to reporting the top-line results of the final 12-week Phase 3 efficacy study of XP13512 in RLS patients later this quarter and the expected filing of the NDA for RLS treatment in the third quarter of this year by GSK."

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 11:30 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 31203493.

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.

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 transport mechanisms to improve the therapeutic benefits of existing drugs. XenoPort's most advanced product candidate, XP13512, has successfully completed two 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 release of additional XP13512 clinical trial data 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" 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, the ability of the company 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 therapeutic and commercial value of the company's compounds. These and other risk factors are discussed under the heading "Risk Factors" in XenoPort's Quarterly Report on Form 10-Q for the quarter ended September 30, 2007, filed with the Securities and Exchange Commission on November 9, 2007. 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

Contact

XenoPort Analyst/Investor Inquiries:
Jackie Cossmon, 408-616-7220
or
GSK Analyst/Investor Inquiries:
Frank Murdolo, 215-751-7002
Tom Curry, 215-751-5419
or
US Media Inquiries:
Holly Russell, 919-483-2839

Posted: January 2008

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