The renewed interest in PrEP comes after two high-profile trials of a vaccine candidate produced by Merck were stopped early in 2007. At least 2.7 million people around the world are HIV-positive, according to the United Nations. Public health experts believe an arsenal of effective prevention tools against HIV is needed to curb its spread, because not everyone will practice existing methods known to work, such as condom use, monogamy between uninfected partners, and abstinence. Trials of potential microbicides, a woman-controlled prevention method that would work similarly to spermicides and other topical birth control methods by blocking or killing HIV during sex, have also been disappointing so far and may not be practical for all women to use. PrEP would offer an alternative.
The trials are investigating whether a daily dose of one of two antiretroviral drugs—either tenofovir disoproxil fumarate (TDF), commercially known as Viread, or Truvada (TDF combined with emtricitabine) can protect people at high risk of HIV from becoming infected.
Antiretroviral drugs, which stop retroviruses like HIV from replicating, are used to treat HIV-positive people. In certain cases they are also employed as a prophylactic to prevent the transmission of the virus from a mother to her newborn or to reduce the chance that someone who has been exposed to the virus, such as through rape or a needle prick, becomes infected.
TDF and emtricitabine are being tested for use as PrEP because they are known to cause low levels of resistance and fewer side effects, and because they remain in the bloodstream for a long time.
Results from studies in gay American men and injecting drug users in Thailand may be available next year. But the first full study testing whether PrEP stops the transmission of HIV through heterosexual sex (the main driver of the epidemic in Africa), which is being carried out in Botswana with plans to expand into South Africa, is not expected to yield results until 2011. And the three biggest trials, involving a total of 12,000 people across Africa, will not be complete until at least 2012.
Researchers are already looking forward to the potential issues that will arise if PrEP is found to be effective, such as whether the use of antiretroviral drugs as preventives will lead to increased resistance or how it could affect the future treatment options of people who later become infected.
If it does work, the public health community will also have to grapple with the tough question of when and how to use PrEP as well as how to balance the need to keep drug-resistant strains of HIV from thriving in the population while saving lives in the short term.
"PrEP has to be implemented as part of a formal program with guidelines and a funding stream. We have to start planning for that," says Lynn Paxton, a PrEP researcher at the U.S. Centers for Disease Control and Prevention.