U.S. drug firm Endo Pharmaceuticals' gel designed to prevent infection with the AIDS virus has proved ineffective in trials in Africa, Britain's Medical Research Council (MRC) said on Monday.

A large international trial of vaginal microbicide Pro 2000 in more than 9,000 women in four African countries found no evidence that it reduces the risk of HIV infection.

The result is a setback for the specialty drugmaker, whose shares were hit earlier in the month when U.S. health regulators declined to approve its Aveed drug for low testosterone.

To date, no gel microbicide has been shown to work against HIV infection and this trial "showed conclusively that Pro 2000 gel was of no added benefit", the MRC said in a statement.

It said this would end scientific speculation about its clinical importance.

"This result is disheartening, particularly in light of the results of a smaller trial sponsored by the U.S. National Institutes of Health which suggested that Pro 2000 could reduce the risk of HIV infection by 30 per cent," said the MRC's Sheena McCormack, who led the trial.

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

Studies presented at an AIDS conference in Canada in February suggested the Pro 200o gel, which ENDO acquired through its purchase of Indevus Pharmaceuticals earlier this year, could cut transmission rates by a third.

The MRC's trial was the largest international clinical trial to date into a preventative HIV gel.

Almost 60 million people have been infected with HIV and 25 million people have died of HIV-related causes since the beginning of the HIV/AIDS epidemic.

United Nations data have shown that globally some 33.4 million people were living with HIV in 2008 and there were 2 million AIDS-related deaths in the same year.

The MRC trial, which took place between September 2005 and September 2009, involved 9,385 women and was carried out by the Microbicides Development Programme (MDP), a not-for-profit partnership of 16 African and European research institutions.

It found that the risk of HIV infection in women who were given PRO 2000 gel was not significantly different than in women supplied with a placebo gel.

Jonathan Weber of the MDP said the result was disappointing but added: "The trial itself was very well designed and undertaken, so we know that the results are definitive.

"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," Weber said in a statement.