Clinical trials of a new gel designed protect women from HIV infection have been halted prematurely, the World Health Organisation has announced, after it emerged that women using the gel were more likely to become infected than those receiving placebo.

The study of the cellulose sulphate vaginal gel, made by Canadian company Polydex Pharmaceuticals, was being conducted in over 1,300 women in Benin, India, South Africa and Uganda. A second study of the product, sponsored by Family Health International conducted in Nigeria, has also been stopped as a precautionary measure.

In a statement, the WHO said: “This is a disappointing and unexpected setback in the search for a safe and effective microbicide that can be used by women to protect themselves against HIV infection.” The cotton-based gel was supposed to release an active ingredient designed to kill HIV during sexual intercourse.

Cellulose sulfate was one of four compounds being evaluated in large-scale studies among women at high risk of HIV infection. The WHO statement said that it was important for the researchers to understand why cellulose sulfate was associated with a higher risk of HIV infection than the placebo product. But added it that “at present there is no explanation for this higher rate of transmission of HIV”.

Dr Lut Van Damme, a principal investigator for Conrad, the Virginia-based reproductive health research group that was running the trial, said: "It was our hope that this product would have helped women in protecting themselves from HIV. While the findings are unexpected and disappointing, we will learn scientifically important information from this trial that will inform future HIV prevention research.''

Currently, there are three other Phase III microbicide studies under way. The Carraguard study – conducted in three sites in South Africa – is nearing completion and results are expected by the end of 2007.

Another product, PRO 2000, is being tested in one study in five sites in South Africa, Tanzania and Uganda as well as in a second study in seven sites in Malawi, South Africa, Zambia and Zimbabwe. Results from these studies are expected by the end of 2009. In this second study another compound, BufferGel, (a vaginal defence enhancer) is being tested. All the other compounds mentioned are products that block HIV infection – known as HIV entry inhibitors – and have a similar presumed mechanism of action.

HIV experts had hoped that microbicides could have a major impact in the fight against Aids, especially in Africa, where women bear the brunt of the disease. “Despite the effectiveness and availability of condoms, the HIV epidemic continues to spread and the search for a safe and effective microbicide is a vital part of the effort to stem the spread of the HIV epidemic,” the WHO said. Michael Day