It doesn't seem possible—that a pill being tested on humans in Food and Drug Administration–approved trials might keep people young longer and increase life span.

Even more astonishing is that pharmaceutical giant GlaxoSmithKline has agreed to buy the company developing the "fountain of youth" pill, Boston-based Sirtris Pharmaceuticals, for $720 million in cash.

Also on Portfolio.com: So You Want to Live Forever? Sirtris Pharmaceuticals is testing a "fountain of youth" pill on humans. You won't live forever, but it may slow aging and increase life span. So far, it's working. Search for an Anti-Aging Pill Reaches a Milestone Researchers report that three compounds invented by Sirtris have succeeded in activating cellular defenses that slow diseases of aging.

Normally, a four-year-old biotech that went public less than a year ago and has a product pipeline still in early- to mid-stage human trials, would not sell for such a heady sum. But the science at work is so compelling that Sirtris C.E.O. Christoph Westphal says he talked to several potential buyers before settling on Glaxo.

The offer was 84 percent over the stock price before the sale was announced; Sirtris stock promptly soared upward, from $12.23 to a closing price today of $22.25—an 82 percent increase.

"We made the sale because we think GSK can speed up our efforts to get our drugs approved," Westphal said in a telephone interview with Portfolio.com. "We are a small company, and we need deep pockets to get through the expensive and difficult process of late-stage human trials and marketing."

Sirtris was founded to develop drugs based on a new category of enzymes called sirtuins that seem to regulate a number of functions in cells that slow aging. One naturally occurring version is resveratrol, a molecule found in red wine.

Sirtris' lead compound mimics and greatly amplifies the activity of resveratrol and is being tested as a treatment for type 2 diabetes, a disease that is often associated with aging.

The drug has performed well in mice in treating diabetes and other age-related diseases. In humans, early tests have proven it to be safe, and it has reduced glucose levels in a small group of diabetics.

Moncef Slaoui, the chairman of Glaxo's research and development division, said Sirtris' work on metabolism and aging is a "potentially transformative science" that could address a number of diseases, including diabetes, muscle wasting, and some degenerative nerve disorders.

The acquisition has all of the risk that most pharmaceuticals deals have in an industry in which 89 percent of all drugs that are tested on humans fail, plus the added risk that this is a novel mechanism that has had limited testing in humans.