Through the acquisition of Sirtris, GSK will significantly enhance its metabolic, neurology, immunology and inflammation research efforts by establishing a presence in the field of sirtuins, a recently-discovered class of enzymes that are believed to be involved in the ageing process. Sirtris Pharmaceuticals has established a drug discovery capability to exploit sirtuin modulation for the treatment of human disease, an approach that has the potential to generate multiple clinically and commercially important products. Their focus to date has been on the development of SIRT1 activators for the treatment of Type 2 Diabetes Mellitus (T2DM).

"Modulation of this family of enzymes is a potentially transformative science that could address diseases associated with metabolism and ageing such as diabetes, muscle wasting, and neurodegeneration," commented Moncef Slaoui, Chairman GSK R&D. "This acquisition continues GSK’s strategy of pursuing the best new science, externally or internally, to bring new medicines to patients and value to the GSK pipeline. Our intent is to retain all Sirtris employees and continue the entrepreneurial and innovative culture they created."

Sirtris will become part of GSK’s Drug Discovery organisation, while continuing to operate from laboratories in Cambridge, Massachusetts as an autonomous drug discovery unit. Christoph Westphal, CEO and Vice Chair of Sirtris and the management team will continue to lead this autonomous unit.

Dr Westphal commented, "We have built a dynamic and scientifically-driven organisation. We expect this transaction will accelerate our vision to target sirtuins to treat diseases of metabolism and ageing and deliver tremendous value to patients, our shareholders and our employees. We look forward to working with GlaxoSmithKline and their world-class research, development and commercialisation organisation."

Under the agreement, a subsidiary of GSK will commence a cash tender offer to purchase all of the outstanding shares of Sirtris, at USD22.50 (or approx. GBP11.33) per share followed by a second step merger in which any untendered Sirtris shares would be acquired at the same price per share. All outstanding stock options will be cancelled with holders receiving the excess of the transactions price over the exercise price. The acquisition has been approved by the board of directors of each company and is subject to customary closing conditions, including the tender of at least a majority of Sirtris's shares and clearance under the Hart-Scott-Rodino Antitrust Improvements Act. The parties anticipate that the tender offer will be commenced in early May and close in the second quarter of 2008.

About GSK
GSK - 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 company information including a copy of this announcement and details of the company's updated product development pipeline, visit GSK at www.gsk.com.

About Sirtris Pharmaceuticals
Sirtris Pharmaceuticals is a biopharmaceutical company focused on discovering and developing proprietary, orally available, small molecule drugs with the potential to treat diseases associated with ageing, including metabolic diseases such as Type 2 Diabetes. Our drug candidates are designed to mimic certain beneficial health effects of calorie restriction by activation of sirtuins, a recently discovered class of enzymes that the Company believes control the ageing process. The company's headquarters are in Cambridge, Massachusetts.