Volunteers who responded most to a cocktail of antiretroviral drugs in an HIV prevention trial among gay men had a reduced risk of 92 percent of being infected by the AIDS virus, researchers said on Wednesday. They presented the work as a last-minute, or "late-breaker", contribution to the world forum on AIDS science in Rome. The research looked at a group of men who took part in a major trial called iPrEx HIV Prevention Study. iPrEx explored the idea that an uninfected person taking daily AIDS drugs could be shielded from the human immunodeficiency virus (HIV), a novel approach called pre-exposure prophylaxis, or PrEP. The overall findings, published last November, found that use of a drug called Truvada reduced HIV infections by 44 percent compared with a dummy pill, also called a placebo. The new study looked at a sub-set of men who had the highest concentrations of the drug in their blood -- a telltale that they had been highly disciplined about taking the pill and that their system had also absorbed it better. In this smaller group, the risk of HIV transmission was 92 percent lower compared to counterparts who took the placebo. "PrEP is an important HIV prevention tool with the potential to prevent significant numbers of new HIV infections," said the study's director, Robert Grant of the Gladstone Institute of Virology and Immunology in San Francisco. "These data confirm that PrEP is safe and effective in MSM, one of the populations most affected by HIV worldwide," he said. In the terminology of AIDS research, MSM means men who have sex with men. The study was conducted among 2,499 men, including 29 transgendered women, between the ages of 18 and 67 who were sexually active with other men but were not infected with HIV. Research was carried out from July 2007 to December 2009 in Brazil, Ecuador, Peru, South Africa, Thailand and the United States. Participants were selected at random to take a daily dose of Truvada -- a combination of 200 milligrams of emtricitabine and 300 milligrams of tenofovir disoproxil fumarate -- or a placebo. A total of 100 HIV infections were recorded among the participants during the nearly three-year-long clinical study. Of those, 36 were recorded among the 1,251 Truvada group and 64 among the 1,248 in the placebo group, showing that the drug reduced the risk of infection by 43.8 percent. Tested among heterosexuals, PrEP showed a reduction in infection risk of up to 73 percent, according to studies presented in Rome.