PrEP, shorthand for pre-exposure prophylaxis, is the next great hope for HIV prevention. Given recent failures of vaccine and microbicide trials, participants at the XVII International AIDS Conference in Mexico City were eager to give PrEP a whirl.

The concept involves giving HIV drugs to people to prevent them from becoming infected. Several studies are getting under way at sites throughout the world, with most involving tenofovir or Truvada, which combines that first drug with another.

There are biological reasons to believe that PrEP can work, and trials in monkeys are encouraging. However, preventing infection may not be quite the same thing as treating but not curing the disease; a single virus may be enough to establish lifelong infection.

As Kevin De Cock, the head of HIV programs at the World Health Organization put it, "There is biological plausibility but not proven efficacy" in the concept.

Dr. Kenneth Mayer is a physician involved with a PrEP trial at Fenway Community Health in Boston where participants will be randomized to receive either the drug or placebo. He says one major question is whether the gay men in the trial will think they are protected and let their guard down, engaging in riskier sexual activity. Another question is w

Tom Coates, Ph.D. Photo: Bob Roehr

Tom Coates, Ph.D., literally gushes with enthusiasm for PrEP. "I really believe it will work. If it is as effective as we think it is going to be, it should revolutionize the way that HIV prevention dollars are spent."

"We in the United States have an industry of AIDS services organizations that are built upon a traditional model of HIV prevention. That may have to change," added Coates, who has long helped lead U.S. HIV prevention research, first at the University of California San Francisco and now the University of California Los Angeles.

Coates argues, "It is not just giving out the pill, it becomes another opportunity to have a conversation about sexual behavior." He says male circumcision in Africa has revitalized prevention on that continent by offering something new. "We really need something new to offer gay men in the U.S. to re-engage them in a discussion on HIV prevention."

In a subsequent conversation Coates backed off from earlier talk of massive shifts in HIV prevention dollars. He acknowledged that if PrEP does work, community-based organizations are best positioned to bring the intervention to people most at risk and engage in the prevention dialog with them. PrEP is likely to require new money.

Gilead Sciences makes the drugs being used in most of the PrEP trials and has been supportive, though not enthusiastic, about them. The studies are being funded by the National Institutes of Health and the Bill and Melinda Gates Foundation. The company is wary of liability issues should a customer become infected with HIV while using, or claiming to use, its product.

An unspoken concern is the gap between what societies are willing to pay for treatment and prevention. Truvada is one of the most popular HIV drugs in the U.S. and costs about $10,000 a year. While many people and insurers are willing to pay that to save someone's life, they are not likely to pay that for prevention when a condom would do the same job for a lot less money.

If the PrEP trials prove to have the blockbuster results that some advocates are hoping for, there is likely to be tremendous pressure on Gilead to slash the price to something closer to the special "at cost" treatment price that it charges the world's poorest nations, about a dollar a day. Without such a reduction, even the most effective PrEP is unlikely to be used widely.