The largest trial to date of an experimental vaginal microbicide to prevent HIV infection has concluded with no evidence that the once-promising product works, researchers said on Monday. Earlier results presented in February at the Montreal AIDS conference had suggested PRO 2000 could cut HIV transmission by one-third.

The latest Phase III trial evaluating PRO 2000 involved 9,385 women in South Africa, Tanzania, Uganda, and Zambia and took place between September 2005 and September 2009. Participants were asked to apply the gel or placebo up to one hour before sexual intercourse, counseled about sexual risk behaviors, and encouraged to use condoms, which were provided free.

The risk of HIV infection in women using PRO 2000 was not significantly different than the infection risk among placebo users, researchers discovered.

"This result is disheartening, particularly in light of the results of a smaller trial sponsored by the US National Institutes of Health which suggested that PRO 2000 could reduce the risk of HIV infection by 30 percent," said Sheena McCormack of the UK's Medical Research Council, which funded the trial with Britain's Department for International Development.

"Nevertheless, we know this is an important result and it shows clearly the need to undertake trials which are large enough to provide evidence for whether or not a product works," McCormack said.

"The trial itself was very well designed and undertaken, so we know that the results are definitive," said Jonathan Weber of the Microbicides Development Program, a non-profit partnership of 16 African and European research institutions that conducted the trials. "It is unfortunate that this microbicide is ineffective at preventing HIV infection, but it's still vital for us as scientists to continue to look for new ways of preventing HIV."