GlaxoSmithKline and XenoPort receive FDA Complete Response letter for Horizant™ (GSK1838262/XP13512) for RLS

Issued: Wednesday, 17 February 2010, London, UK, Research Triangle Park, NC & Santa Clara, CA —GlaxoSmithKline (NYSE: GSK) and XenoPort, Inc. (Nasdaq: XNPT) Wednesday 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.

Conference Call and Webcast Information

XenoPort will host a conference call at 8:30 a.m. Eastern Time tomorrow. To access the conference call via the Internet, go to www.XenoPort.com. To access the live conference call via phone, dial 1-888-275-3514. International callers may access the live call by dialing 1-706-679-1417.

The replay of the conference call may be accessed after 12:30 p.m. Eastern Time Thursday, February 18, via the Internet, 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 both the call and the replay of the call is 58168197.

Editor’s Note: Horizant was formerly referred to by the proposed trade name Solzira™.

GlaxoSmithKline Enquiries:

UK Media enquiries:

Philip Thomson

(020) 8047 5502

Claire Brough

(020) 8047 5502

Stephen Rea

(020) 8047 5502

Alexandra Harrison

(020) 8047 5502

Jo Revill

(020) 8047 5502

US Media enquiries:

Nancy Pekarek

(919) 483 2839

Mary Anne Rhyne

(919) 483 2839

Kevin Colgan

(919) 483 2839

Sarah Alspach

(919) 483 2839

European Analyst/Investor enquiries:

David Mawdsley

(020) 8047 5564

Sally Ferguson

(020) 8047 5543

Gary Davies

(020) 8047 5503

US Analyst/ Investor enquiries:

Tom Curry

(215) 751 5419

Jen Hill Baxter

(215) 751 7002

XenoPort Enquiries:

Analyst/Investor enquiries:

Jackie Cossmon

(408) 616 7220

GlaxoSmithKline cautionary statement regarding forward-looking statements
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 2008.

XenoPort Forward-Looking Statement

This press release contains "forward-looking" statements, including, without limitation, all statements related to the therapeutic and commercial potential of XenoPort’s product candidates and potential future interactions with the FDA related to the NDA for Horizant for the treatment of RLS. 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 "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 ability of the company to successfully conduct clinical trials of its product candidates, 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, 2009, filed with the Securities and Exchange Commission on November 4, 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.

Horizant™ is a trademark of GSK. XenoPort is a trademark of XenoPort, Inc.

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