GlaxoSmithKline (NYSE: GSK) and XenoPort, Inc. (Nasdaq: XNPT) received a Complete Response letter from the U.S. Food and Drug Administration (FDA) regarding the New Drug Application (NDA) for Horizant™(gabapentin enacarbil) Extended-Release Tablets, an investigational non-dopaminergic treatment for moderate-to-severe primary Restless Legs Syndrome (RLS).

A Complete Response letter is issued by the FDA’s Center of Drug Evaluation and Research when the review of a file is completed and questions remain that preclude the approval of the NDA in its current form. GSK and XenoPort are currently evaluating the Complete Response letter, in which the FDA indicated that a preclinical finding of pancreatic acinar cell tumors in rats was of sufficient concern to preclude approval of Horizant for RLS at this time. FDA acknowledged that similar findings were known for gabapentin at the time of its approval for refractory epilepsy, but concluded that the seriousness and severity of refractory epilepsy justified the potential risks. The companies are assessing the appropriate next steps and will be communicating with FDA.

The NDA was submitted to the FDA on January 9, 2009.

GlaxoSmithKline - one of the world's leading research-based pharmaceutical and healthcare companies - is committed to improving the quality of human life by enabling people to do more, feel better and live longer. For further information please visit www.gsk.com.

XenoPort 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. Gabapentin enacarbil is also being developed in partnership with Astellas Pharma Inc. for the potential treatment of Restless Legs Syndrome in Japan. XenoPort's product candidates are being evaluated as potential treatments for patients with neuropathic pain, migraine (as prophylactic treatment), gastroesophageal reflux disease, spasticity and Parkinson's disease. To learn more about XenoPort, please visit the Web site at www.XenoPort.com.