Gene therapy won’t replace Viagra—yet

Older men who want to revive flagging sexual function have turned in large numbers to Viagra. For a few hours, the pills restore the vascular system’s sensitivity to nitric oxide, a compound that facilitates erections. However, Viagra doesn’t work in all men, and it also can aggravate heart conditions in some people. That’s why several university teams have been investigating gene therapy as a different way to cure impotence.

With age comes a natural waning in the activity of the genes for making nitric oxide and many other chemicals critical to vascular health. By inserting healthy copies of these genes directly into the penises of affected individuals, researchers hope to offer long-lasting but localized vascular improvements that are unlikely to pose side effects in the heart or other tissues. Last week, urology researchers reported promising preliminary data. By injecting a gene into the penises of aging rats, they restored the organs’ vascular responsiveness and erectile function.

Trinity Bivalacqua of Tulane University School of Medicine in New Orleans and his colleagues focused on the gene that produces CGRP, or calcitonin-gene-related peptide. A CGRP shortfall can make the penis’ blood conduits ignore nerve signals that tell them to relax. That relaxation normally enables these conduits to expand, allowing the flow of blood for erections.

Bivalacqua’s team inserted the CGRP gene into a virus, which they injected into the animals. As long as the carrier virus survived in the rats’ penises–about 1 month–the smooth muscle there produced CGRP. After that, CGRP diminished and so did the old rats’ erections.

Because the diminished activity of different genes may underlie impotence, Bivalacqua expects that gene combos delivered via a longer-lasting virus are the way to go. “The best-case scenario,” he says, “would be something you’d need to inject just once or twice a year. But we’re nowhere near that.”