GSK and Xenoport announce submission of new drug application requesting FDA approval of Solzira™ for Restless Legs Syndrome

Submission based on results from pivotal trials evaluating efficacy, safety and maintenance of effect in patients with moderate-to-severe primary RLS

Issued: September 16, 2008, London, UK

GlaxoSmithKline (NYSE: GSK) and XenoPort, Inc. (Nasdaq: XNPT) announced today that a New Drug Application (NDA) has been submitted to the U.S. Food and Drug Administration (FDA) requesting approval of Solzira™ (gabapentin enacarbil) Extended Release Tablets for the treatment of moderate-to-severe primary Restless Legs Syndrome (RLS). RLS affects an estimated 12 million people in the United States and can result in distressing symptoms that disrupt sleep and significantly impact daily activities.

Solzira is a non-dopaminergic new chemical entity that provides improvement in the symptoms of RLS with the convenience of a once-daily formulation.

“GSK is committed to bringing innovative products to patients where there is unmet medical need,” said Atul Pande, M.D., senior vice president, GlaxoSmithKlineNeurosciencesMedicineDevelopmentCenter. “We believe that Solzira may offer a new therapeutic option to treat primary Restless Legs Syndrome, a condition that includes both sensory and motor symptoms.”

The NDA submission is based on a comprehensive Phase 3 clinical development program for Solzira in patients with moderate-to-severe primary RLS, including data from two randomized, double-blind, placebo-controlled trials (PIVOT RLS I and PIVOT RLS II), which evaluated the safety and efficacy of Solzira over 12 weeks. The submission also included results from a third pivotal trial (PIVOT RLS Maintenance) evaluating the ability of Solzira to maintain efficacy in treating RLS symptoms over a nine-month period. The most common side effects of Solzira were dizziness and somnolence.

“We are very encouraged by the results that we have seen in the clinical development program for Solzira,” said Ronald W. Barrett, Ph.D., chief executive officer of XenoPort. “Solzira is the first non-dopaminergic compound to demonstrate efficacy in large, controlled clinical trials for the treatment of primary RLS, and we believe it will offer patients a beneficial alternative to currently approved therapies.”

About SOLZIRA
Solzira is a new chemical entity that is designed to improve upon the pharmacokinetics of gabapentin by taking advantage of high-capacity transport mechanisms in the gastrointestinal tract to improve absorption.

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.

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. Its development and commercialization efforts are currently focused on potential treatments of central nervous system disorders. Its most advanced product candidate, XP13512, which is known as Solzira in the United States, has been evaluated for the treatment of RLS. As announced today, GlaxoSmithKline, in collaboration with XenoPort, has filed with the FDA the NDA for Solzira™ for the treatment of RLS. XenoPort has also successfully completed a Phase 2a clinical trial of XP13512 for the management of post-herpetic neuralgia. XP13512 is currently being evaluated by XenoPort’s collaborators, Astellas Pharma Inc. and GlaxoSmithKline, in Phase 2 clinical trials as a potential treatment for neuropathic pain, by Astellas in a Phase 2 clinical trial as a potential treatment for RLS in Japan and by GlaxoSmithKline in a Phase 2b clinical trial as a potential treatment for prophylaxis of migraine headache. XenoPort has reported positive results from a Phase 2a clinical trial of its second product candidate, XP19986, in patients with gastroesophageal reflux disease, or GERD. It is currently conducting a second Phase 2 clinical trial in GERD patients and plans to initiate an exploratory Phase 2 clinical trial of XP19986 in patients with acute back spasms. It is also evaluating XP19986 as a potential treatment of patients with spasticity related to spinal cord injury. XenoPort’s third product candidate, XP21279, has been evaluated in a Phase 1 clinical trial that produced positive data regarding its use as a potential treatment for Parkinson’s disease. To learn more about XenoPort, please visit the Web site at www.XenoPort.com.

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 ’Business Review’ in the company’ s Annual Report on Form 20-F for 2007.

XenoPort Forward-Looking Statement
This press release contains “forward-looking” statements, including, without limitation, all statements related to the therapeutic and commercial potential of Solzira. 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,” ”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 Solzira, 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 June 30, 2008, filed with the Securities and Exchange Commission on August 7, 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.

XenoPort is a registered trademark.

Solzira is a U.S. trademark of GSK.

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