A form of an antiretroviral drug to prevent HIV has been found safe, according to the Pittsburgh-based Microbicide Trials Network.

Since October 2010, researchers at the trials network, funded by the National Institutes of Health and based at the University of Pittsburgh and Magee-Women's Research Institute, have been studying a formulation of tenofovir gel, specifically for rectal use, to reduce the sexual transmission of HIV. They presented the results of an early phase of the study last week at the 19th Conference on Retroviruses and Opportunistic Infections.

"One of the most important findings in this study is the formula we used was safer and more acceptable than those in previous studies," Dr. Ian McGowan, co-principal investigator of the Microbicide Trials Network (MTN) tells City Paper.

McGowan notes that a previous study assessed the rectal use of a vaginal formulation of tenofovir, but found participants suffered from multiple "gastrointestinal issues."

"We really felt at the time it wasn't the ideal formulation for rectal use," says McGowan.

So researchers turned their attention to a rectal-specific formulation, and the results have been promising: 80 percent of participants reported only minor side effects. Adherence for the gel was also high; according to the MTN, 94 percent of participants used the gel daily as directed.

McGowan says researchers are preparing a second phase of the study, which will further study tenofovir. They will assess the effects of the rectal tenofovir gel used daily, used before and after anal sex and the daily use of an antiretroviral tablet. The study will focus on 186 men who have sex with men and transgender women — both among the most at-risk populations for HIV — in Pittsburgh, Boston, San Francisco, Thailand, Peru and South Africa.

Microbicides, or products used in the rectum or vagina to prevent or reduce the sexual transmission of HIV, "are emerging as a National Institutes of Health priority because of the [HIV] epidemic," McGowan says. "There needs to be other options out there for HIV-prevention and at the same time, we need to put a product out there for everyone."

The Phase II study of tenofovir is expected to begin later this year.