Motivated by positive results reported in this week's Nature, two drug companies have given away rights to two key compounds, so that they can be developed into gels that protect against HIV.

Such a gel could help many women to protect themselves, as they often find it difficult to get partners to use condoms — particularly in the developing world, where men may disapprove of the practice. Experts say that a microbicide applied to the vagina before sex could save 2.5 million lives in just three years.

But progress to develop such gels has been slow. Only one microbicide trial has been completed in humans, with disastrous results — the women became more susceptible to HIV because the gel, essentially a detergent that destroys the virus, damaged their vaginal tissue. Five other microbicides are in clinical trials in Africa after proving moderately successful in monkeys, but critics point out that the virus used in those animal tests infects cells in a different way from the one that causes AIDS.

John Moore from Cornell University in New York and his colleagues tried a different approach (see HIV treatment begins to gel). They combined three compounds that each uses a different mechanism to block the virus's entry into cells. Merck's compound CMPD167 competes with the virus for cell receptors inside the vagina. Bristol-Myers Squibb's BMS-378806 interacts with the virus itself, stopping it binding to cells. And a peptide developed by Moore's team inhibits the process used by the virus to enter a cell.

When the researchers tested combinations of the compounds in macaques, they found that they offered at least partial protection against a virus closely resembling HIV. But three animals that received the three compounds together were all protected against infection. These results were enough to persuade the drug firms to give away rights to the compounds, says Moore. “This is the first time there has been a joint announcement like this,” adds Mark Mitchnick, chief scientific officer of the International Partnership for Microbicides, the non-profit group that will develop the gel.

Partners including the Bill and Melinda Gates Foundation and the US National Institutes of Health are helping to fund a clinical trial, set to start in 2007. This is estimated to cost between US$150 million and $200 million and will involve about 10,000 women in Africa.