The same drug that helps men achieve erections might also help boys with a rare and debilitating muscular condition improve their strength.

A recent study showed that the drug Cialis, which is widely used for erectile dysfunction in men, might be effective in treating Becker muscular dystrophy in boys.

Repurposing drugs -- in other words, using them for conditions for which they weren't originally intended -- is becoming increasingly common in medicine. That's especially true for rare, genetic conditions with limited therapies.

Dr. Barry Byrne is a professor of pediatrics and molecular genetics and microbiology at the University of Florida and one of the authors of the study.

“In many ways in genetic diseases, we've kind of imagined one treatment that is curative,” he said. “Practitioners are starting to recognize in oncology, for example, that there are many different strategies.”

Cialis, also known as tadalafil, works by increasing blood flow to muscles -- to the penis in the case of erectile dysfunction -- where it blocks a chemical responsible for reversing an erection. In patients with BMD, in which there is a progressive slackening of the body's muscles, Cialis opens up blood vessels by inhibiting the destruction of the intracellular messenger that activates a protein affected by the disease.

“The drug offsets the inability of that protein to modulate blood flow and muscle,” said Byrne, who is also director of the UF Powell Gene Therapy Center. Byrne added that Cialis specifically targets the defective protein that causes muscular dystrophy -- so using Cialis simply to boost muscle strength, for example, wouldn't work.

“On the same day the paper came out, there was a story of drug abuse by NFL players … on the notion that (Cialis) would improve blood flow and athletic performance … it wouldn't help them at all,” Byrne said.

The study Byrne worked on, which was published in the Nov. 28 issue of Science Translational Medicine, found that the people taking the drug -- all teenagers or men in their early 20s -- had greater use of their forearms than those who did not take it. Although the researchers were looking only at forearm function, the drug works systemically -- in other words, throughout the whole body, Byrne said.

Byrne now will study the drug in Duchenne muscular dystrophy, the more severe form of muscular dystrophy that is usually diagnosed by the time a boy turns 5.

“By the end of elementary school, they'll have a lot of difficulty walking. By middle school, they may be non-ambulatory, and by the end of high school, they are not walking and have problems with heart and lung function,” Byrne said. “By their 20s, they have significant disability and they often don't live beyond their 30s.”

There are currently about 2,000 boys living with the two forms of muscular dystrophy in Florida -- most with DMD. They are typically treated with a combination of steroids and drugs to prevent and treat cardiac dysfunction, so Cialis would be used in addition to these treatments.

“It would certainly be an important adjunctive therapy,” Byrne said, adding that the drug has few side effects.

Krista Vandenborne, chair of the department of physical therapy at UF, agreed that Cialis could have an important role in treating patients.

“This is going to be potentially an important mechanism. It can set the stage for new therapeutic strategies,” Vandenborne said.

Both Byrne and Vandenborne are collaborating in the UF contingent of a nationwide study of cialis in DMD with a grant from the National Institutes of Health. Vandenborne said the idea is to “slow down the progression of the disease so the muscle is maintained much longer.”

“Until we've done the clinical trial, it's hard to predict, but we're hopeful.” she said.