In a randomized controlled trial, a gel containing the drug tenofovir (Viread) reduced the risk of women acquiring HIV by 39 percent compared with a placebo gel, according to Quarraisha Abdool Karim of the Centre for the AIDS Program of Research in South Africa (CAPRISA) in Durban, South Africa.
Among women who used the gel more than 80 percent of the times they had sex, however, the risk was reduced by 54 percent, Abdool Karim said at the International AIDS Conference here and in a paper published simultaneously online in the journal Science.
That figure is comparable to the 57 percent reduction in risk seen in three major clinical trials that tested the effect of male circumcision. Moreover, the overall benefit of 39 percent is even higher than the 31 percent risk reduction reported last year to be associated with an HIV vaccine candidate.
The finding comes after years of failed efforts to create a vaginal gel that can block HIV -- something that's regarded as an important preventive tool in places where women do not have the social power to insist on condom use.
It is not the end of the search, but it is the "end of the beginning," said Mitchell Warren, executive director of the New York-based AIDS Vaccine Advocacy Coalition.
Warren told MedPage Today the overall size of the protective effect is probably not enough to put a product forward for licensing and general use, but it is enough to show that a microbicide gel can be made to work.
It's a "thrilling moment," he said.
The protective effect, Abdool Karim and colleagues found, was evident regardless of sexual behavior, condom use, herpes simplex type 2 virus infection, or differences between urban and rural women.
At the same time, they found the topical gel was safe, with the overall rate and type of adverse events comparable to placebo -- although those using the tenofovir gel had an increased incidence of mild diarrhea.
The trial enrolled 889 South African women, ages 18 to 40, who were HIV-negative, sexually active, and at high risk of becoming infected with the virus. About half were given vaginal applicators containing a 1 percent concentration of tenofovir gel, and the remaining half got an identical-appearing placebo gel.
The researchers assessed the participants' HIV status, sexual behavior, and gel and condom use every month for 30 months, and also recorded adverse events. In addition to the apparent protective effects of the gel, they found that the drug did not appear to cause kidney damage, which had been thought to be a possible complication of the drug.
The researchers cautioned that the study is small and intended to provide a proof of concept, so that the results are not widely generalizable. As well, the study "needed to attain higher and sustained levels of adherence."
Indeed, Warren told MedPage Today, that's one of the questions that will have to be answered as researchers try to build on the study -- why did some women use the gel regularly, while others did not?
Equally, he said, it will be important to find out how well women will adhere to the gel outside of the confines of a clinical trial, where they were monitored every month.
Other questions include whether different delivery methods will work as well or better and whether combinations of drugs might give more powerful protection, Warren said.