THURSDAY, July 10 ( HealthDay News) -- A new study questions whether vaginal microbicides being developed to help protect women against HIV infection could lead to new drug resistance from the virus that causes AIDS.

The study, published July 7 in the online issue of Proceedings of the National Academy of Sciences, also shows that, under certain circumstances, men may actually benefit a bit more than women from the microbicides, which are compounds that can be applied inside the vagina.

Drug companies are running clinical trials on some second-generation microbicides based on antiretroviral, or ARV, medicines.

The study by the UCLA AIDS Institute also questions the designs of these trials. The researchers made the conclusions by using mathematical models that simulate clinical trials and population-level transmission of HIV.

Under the scenarios the researchers developed, men were slightly more protected than woman in certain situations. This occurred when the in-trial microbicide, an ARV drug called dapivirine, was only effective about half the time in women -- a situation that could occur if HIV-positive women on microbicides developed drug-resistant strains of HIV that were then less likely to be transmitted to men.

"The antiretroviral drugs within these microbicides are the same as those used to treat people who are infected with HIV, so there is great expectation that these microbicides will be very effective," said first author Dr. David Wilson, of the National Centre in HIV Epidemiology and Clinical Research at Australia's University of New South Wales, in a prepared statement.

"But the concern is that these microbicides are going to lead to drug resistance," he said.

Concerns about drug resistance come from the fact that the current clinical trial drops women from the study if they turn up HIV-positive during monthly screening, the researchers said.

"Since monthly testing will take place in the dapivirine trial, we predict that few, if any, cases of acquired resistance will arise during the trial, even if the drug is readily absorbed (i.e., the microbicide is high risk)," the researchers wrote. "Therefore, our analyses have shown that high-risk microbicides could pass Phase III trials, as their potential to cause resistance will be masked by frequent testing."

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