Shares of Sirtris Pharmaceuticals rose as much as 80.46 percent to $22.07 in after trading hours on Tuesday in the New York Stock Exchange as healthcare group GlaxoSmithKline Plc. entered into a definitive agreement to acquire Sirtris for $720 million in a cash tender offer of $22.50 per share.

Through the acquisition announced on Tuesday, Glaxo will reinforce its research in the field of enzymes involved in the aging process that could treat multiple human diseases associated with aging such as diabetes, muscle wasting and neurodegeneration, according to Glaxo Research and Development chairman Moncef Slaoui.

Both companies anticipate that the buyout will proceed in early May and close in the second quarter of 2008.

Shares of Glaxo, the biggest european drugmaker closed 0.76 cents up at $43.73 on the New York Stock Exchange on Tuesday.