"It's not a matter of if, but when," said gerontologist David Sinclair of a drug that promises a long and healthy life – not quite a fountain of youth, but perhaps a fountain of fitness.

Best of all, predicted Sinclair, you'll be able to afford it.

Speaking yesterday at a World Science Festival discussion on the science of longevity, Sinclair predicted that the drugs "could have as big an impact as antibiotics in the 20th century, and it's just around the corner."

Five years ago, Sinclair, a Harvard University professor and co-founder of Sirtris Pharmaceuticals, discovered that the molecule resveratrol targets a gene activated by calorically restricted diets, which have extended the lifespans of laboratory animals from yeast to monkeys.

Despite the paucity of human testing, some people already practice caloric restriction. Most, however, are discouraged by the spartan dietary discipline required of adherents. Questions also remain about long-term side effects. Instead, scientists are shooting for pharmaceutical shortcuts that do the same thing – namely, reinvigorating our mitochondria, fixing a lifetime of cellular wear-and-tear.

Many gerontologists believe that the so-called diseases of aging – cancer, diabetes, heart disease, dementia and any other condition whose primary risk factor is age – originate in damage caused to our mitochondria by free oxygen radicals. These are an inevitable byproduct of turning chemical energy into our body's fuel, but corrode mitochondrial DNA, eventually causing organs and systems to malfunction and shut down.

Fix the mitochondria, say scientists, and the rest of you won't break down. What's offered isn't a protraction of physical decrepitude, but a slowing of the biological clock.

"The biggest myth," said University of Wisconsin caloric restriction pioneer Richard

Weindruch of the new anti-aging drugs, "is that if we extend lifespan, that would involve more unhealthy years at the end. But we'll add years of healthy life."

Every major pharmaceutical company is conducting research on the handful of genes activated by caloric restriction. GlaxoSmithKline today finished its $720 million purchase of Sirtris, a business-page declaration that anti-aging drugs are serious business.

"This is happening now. They're in clinical trials – and the general public has no idea what's coming," said Sinclair.

I've written before about these drugs, which I believe have a shot at fulfilling their promise. What intrigues me now is as much social as scientific: if they work, how will they affect our experience of life?

Could they change the nature of death? And who will be able to afford them? Will the fountain of youth be affordable to only the wealthy?

I posed the question to Sinclair, who explained that Sirtris'

resveratrol formulation is now in Phase II clinical trials for diabetes. When it hits the market in four or five hears, he said,

"It'll be on the market as a diabetes drug. It'll have to sell for $3

or $4 a pill, in order to stay competitive."

He continued, "And once it goes off-patent, companies will be able to make it for pennies. It'll be like aspirin." **

Image: Richard Weindruch compares pictures of a calorically-restricted monkey, below, and a monkey on a regular diet, above.

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