XenoPort to Expand Clinical Development of XP19986

XenoPort, Inc. (Nasdaq:XNPT) announced plans to initiate an exploratory Phase 2 clinical trial of XP19986 in patients with acute back spasms, which is a debilitating condition affecting approximately two-thirds of patients experiencing lower back pain. XenoPort intends to initiate this two-week, multi-center, double-blind, placebo controlled trial in the fourth quarter of this year. The primary objective of the study will be to evaluate the safety and tolerability of XP19986 in subjects with acute back spasms, and the secondary objectives will include the evaluation of efficacy measures and pharmacokinetics.

Ronald W. Barrett, Ph.D., chief executive officer of XenoPort, stated, “Following a thorough review of the current treatment options available for patients suffering from acute back spasms, we believe that XP19986 could provide an effective alternative for patients who suffer from lower back pain that is musculoskeletal in origin. We look forward to investigating whether XP19986 will demonstrate acceptable efficacy, safety and tolerability in this patient population where rapid relief of discomfort and return of function are critical.”

About Acute Back Spasms

Acute back spasms are involuntary and, frequently, painful contractions of the muscles of the lower back. It is estimated that the prevalence of lower back pain is approximately 25% in the adult population of the United States. Lower back pain is the fifth leading cause of physician visits in the United States, accounting for more than 15 million outpatient visits every year. Muscle relaxants can help alleviate the pain that is often associated with acute back spasms. According to data from Wolters Kluwer Health, Source(R) Pharmaceutical Audit Suite, there were 47.5 million prescriptions for muscle relaxants in the United States in 2007.

About XP19986

XP19986 is designed to overcome certain deficiencies of baclofen, a currently marketed generic drug approved for the treatment of spasticity. Baclofen is a racemic drug (a 50:50 mixture of R- and S-isomers). Studies have shown that the beneficial therapeutic properties of baclofen are attributable to the R-isomer of baclofen only. R-baclofen is a selective agonist of GABA-B receptors. Baclofen has a short half-life in blood after oral dosing, which necessitates frequent daily dosing and is associated with unwanted side effects. Absorption of baclofen in the colon is limited, which has prevented the development of a sustained-release formulation that could address these deficiencies.

XP19986 is a new chemical entity that is a Transported Prodrug of R-baclofen. XP19986 is designed to engage natural nutrient transport mechanisms found on intestinal cell membranes, thereby gaining efficient entrance into the bloodstream. XP19986 is then rapidly converted to R-baclofen and metabolic breakdown products that are natural substances with favorable safety characteristics by high-capacity enzymes. In addition to the forthcoming Phase 2 study for the potential treatment of acute back spasms, XP19986 is currently being evaluated in Phase 2 studies for the potential treatment of gastroesophageal reflux disease, or GERD, and spasticity related to spinal cord injury.

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. XenoPort’s most advanced product candidate, XP13512, which is known as Solzira(TM) in the United States, has successfully completed three pivotal trials in its Phase 3 clinical program for the treatment of moderate-to-severe primary restless legs syndrome, or RLS. It has also successfully completed a Phase 2a clinical trial for the management of post-herpetic neuralgia and is currently being evaluated by XenoPort’s partners, Astellas Pharma Inc. and GlaxoSmithKline, in Phase 2 clinical trials as a potential treatment for neuropathic pain, by GlaxoSmithKline in a Phase 2/3 clinical trial for migraine prophylaxis and by Astellas in a Phase 2 clinical trial as a potential treatment for RLS. XenoPort has reported positive results from a Phase 2a clinical trial of its second product candidate, XP19986, in patients with GERD and is currently conducting a second Phase 2 clinical trial in GERD patients. 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.

Forward-Looking Statements

This press release contains “forward-looking” statements, including, without limitation, all statements related to our future clinical development of XP19986 and the timing thereof; the therapeutic and commercial potential of XP13512, XP19986 and XP21279; the suitability of XP19986 as a treatment for GERD, spasticity and acute back spasms; and the suitability of XP21279 as a treatment for Parkinson’s disease. 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,”"expects,”"will,”"intends,”"potential” and similar expressions are intended to identify forward-looking statements. These forward-looking statements are based upon our current expectations. Forward-looking statements involve risks and uncertainties. Our 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; our ability to successfully conduct the clinical trials for XP13512, XP19986 and XP21279 on the anticipated timeframes, or at all; our dependence on our current and additional collaborative partners; the uncertainty of the FDA approval process and other regulatory requirements and the uncertain therapeutic and commercial value of our compounds. These and other risk factors are discussed under the heading “Risk Factors” in our Quarterly Report on Form 10-Q for the quarter ended June 30, 2008, filed with the Securities and Exchange Commission on August 7, 2008. We expressly disclaim any obligation or undertaking to release publicly any updates or revisions to any forward-looking statements contained herein to reflect any change in our expectations with regard thereto or any change in events, conditions or circumstances on which any such statements are based.

XenoPort and Transported Prodrug are a trademarks of XenoPort, Inc. Solzira is a U.S. trademark of GlaxoSmithKline.