XenoPort Reports Preliminary Results From a Safety and Tolerability Trial of Arbaclofen Placarbil in Patients With Acute Back Spasms

XenoPort, Inc. (Nasdaq:XNPT) announced today preliminary results from a Phase 2 clinical trial that evaluated the safety and tolerability of arbaclofen placarbil (AP), also known as XP19986, when administered without titration to patients with acute back spasms. All doses of AP were safe and generally well tolerated.

This randomized, double-blind, placebo-controlled, Phase 2 clinical trial in patients with acute moderate to severe muscle spasms in the lumbar region was conducted at 17 sites in the United States. Subjects were randomized into one of four treatment arms (placebo, 20 mg BID, 30 mg BID or 40 mg BID administered without titration; 1:1:1:1 ratio) and were treated for ten days, followed by a dose-dependent taper schedule of up to four days. Evaluations of safety, tolerability and efficacy were conducted in those subjects (n=151) who received at least one dose of study medication.

The primary objective of this study was to evaluate the safety and tolerability of AP administered without titration to patients with acute back spasms. AP was safe and generally well tolerated at all dose levels. There were no drug-related serious adverse events. Six subjects in the AP dose groups withdrew from the study due to adverse events (one in each of the 20 mg and 30 mg BID groups and four in the 40 mg BID group). The most common adverse events (reported more frequently in any AP group than in the placebo group) were somnolence, dizziness and nausea. The highest incidence of any adverse event was 19% for somnolence in the 40 mg BID group compared to 12% for the placebo group. Adverse event rates for the 20 mg BID and 30 mg BID groups were comparable to the placebo group, with the exception of dizziness for the 30 mg BID group (14% versus 5% for placebo). All adverse events were rated as mild to moderate in intensity.

Several secondary efficacy measures were assessed, including pain severity score. Efficacy measures in subjects in all treatment groups were highly variable, and no differences between AP doses and placebo on these secondary efficacy measures were observed.

"We are pleased with the safety and tolerability of AP observed in this trial, especially since this was the first study in which we have administered AP without titration,” said Ronald W. Barrett, Ph.D., XenoPort’s chief executive officer. "We believe that this important result could impact the dosing regimen used in future trials evaluating AP for the potential treatment of gastroesophageal reflux disease and spasticity.”

Dr. Barrett continued, "Given the high variability in efficacy measures observed in this trial, we will not be pursuing further development of AP for patients with acute back spasms at this time. We intend to focus on the two indications where we have generated encouraging efficacy and tolerability data in Phase 2 trials. We expect to commence a Phase 2b study of AP as an adjunctive therapy in GERD patients who remain symptomatic despite PPI therapy later this year. Additionally, we will continue to develop AP as a potential treatment in patients with spasticity, and we hope to begin the next study in 2010.”

About AP

AP (arbaclofen placarbil) is a Transported Prodrug of R-baclofen that is designed to engage natural nutrient transport mechanisms found on intestinal cell membranes, thereby gaining efficient entrance into the bloodstream. AP is then rapidly converted by high-capacity enzymes to R-baclofen and natural substances that have well-studied, favorable safety characteristics. The current sustained-release tablet formulation of AP could enable convenient once- or twice-daily dosing of subjects in future clinical trials.

R-baclofen is an agonist of the target known as gamma amino-butyric acid(B), or GABA(B), receptor. Racemic baclofen (a mixture of R and S isomers) has been approved for the treatment of spasticity and has been shown in clinical studies to have efficacy in a number of other therapeutic indications, including gastroesophageal reflux disease (GERD).

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 is developing its lead product candidate, XP13512, in collaboration with Astellas Pharma Inc. and GlaxoSmithKline. The Food and Drug Administration is currently reviewing the new drug application for this product candidate as a potential treatment for moderate-to-severe primary restless legs syndrome. XenoPort’s product candidates are also being studied for the potential treatment of GERD, migraine headaches, neuropathic pain, spasticity related to spinal cord injury and 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 program for AP and the timing thereof; the therapeutic and commercial potential of AP; the suitability of AP as a treatment for GERD, spasticity and acute back spasms; and our future clinical trials and the timing thereof. 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 "believe,” "could,” "expect,” "hope,” "intend,” "planning,” "potential,” ”will” 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 future clinical trials; XenoPort’s ability to successfully conduct future clinical trials for AP; the uncertainty of the FDA approval process and other regulatory requirements; and the uncertain therapeutic and commercial value of AP. These and other risk factors are discussed under the heading "Risk Factors” in XenoPort’s Quarterly Report on Form 10Q filed with the Securities and Exchange Commission on August 6, 2009. 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 and Transported Prodrug are trademarks of XenoPort, Inc.

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