Monkeys bulk up as Myostatin blocking increases leg muscle size.

Monkeys that had a gene injected into their legs developed bigger, stronger thighs in an experiment that may pave the way for human trials testing the therapy in people with muscle-wasting diseases.

The therapy works by blocking a protein, myostatin, that degrades muscle. Reversing muscle loss in the thigh may help patients who struggle to stand or walk, said R. Rodney Howell, chairman of the board of the Muscular Dystrophy Association, a nonprofit group based in Tucson, Arizona. The results may also interest a group of people who aren’t the intended beneficiaries -- athletes who want to improve performance, Howell said.

Amgen Inc., based in Thousand Oaks, California, and closely held Acceleron Pharma Inc. of Cambridge, Massachusetts, are experimenting with spurring muscle growth by suppressing myostatin. The idea is to reverse atrophy caused by wasting illnesses and aging. It may also create a hard-to-detect, non- steroid shortcut for increasing the size of healthy tissues.

“This threat is one of several that sports has to consider and be prepared for,” said Travis Tygart, chief executive officer for the U.S. Anti-Doping Agency, a nonprofit organization based in Colorado Springs, Colorado, charged with preventing the use of performance-enhancing drugs in Olympic sports.

Use of myostatin-blocking therapies by athletes has been banned by the World Anti-Doping Agency even though they aren’t yet available, Tygart said in a telephone interview today.

“We wanted to improve strength in one muscle group in a way that would benefit patients and that’s the thigh muscle,” Mendell said yesterday in a telephone interview. “The muscle weakens, patients are prone to fall frequently and they may lose the ability to ambulate.”

The researchers tried to use one protein called follistatin to impede the action of another, myostatin, that’s known to inhibit muscle growth. They injected the gene for follistatin into the right legs of six healthy monkeys and after eight weeks, their right legs had grown and were larger than their left legs.

“We created a stronger muscle,” said Brian Kaspar, the principal investigator for Nationwide’s research institute. “We also showed that the muscle generated more force.”

To deliver the gene, the researchers loaded it onto a so- called adeno-associated virus and injected it into the monkeys. This type of virus is designed to be harmless and is commonly used as a delivery vehicle in gene therapy procedures. The Ohio researchers followed the monkeys for 15 months after the treatment and found no evidence of any unwanted side effects.

“I think it’s a lot less of a threat than easily obtained designer steroids,” said Tygart of the Anti-Doping Agency. “I’m more concerned about designer steroids that I can get off the Internet.”