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Trials of a vaginal gel in South Africa have failed to prove conclusively that it prevents HIV transmission from men to women.

The microbicide, Carraguard, underwent phase III clinical trials in 6000 women between 2004–2007. But this month (14 February) The Population Council, the US-based nongovernmental organisation that ran the trials, announced that the microbicide had been found to be safe but not effective.

"With Carraguard there was not a big enough difference between the number of women who contracted HIV on the placebo and those using Carraguard to prove that the microbicide had a significant impact," Sumen Govender, clinical study manager for the Population Council in South Africa, told SciDev.Net.

Most women enrolled on the Carraguard trial reported only using the gel before sex some of the time, which could have affected the results, say the researchers. Govender said researchers were now analysing social aspects of the Carraguard data.

Researchers will report the results of the Carraguard trial at the International Microbicide 2008 Conference in New Delhi, India, this week (24–27 February).

But Carraguard will continue to be used in further trials. The Population Council will commence phase I safety trials of a new microbicide, PC-815 — which comprises Carraguard and an antiretroviral drug called MIV-150 — this year.

Carraguard contains carrageenan, a chemical derived from seaweed that is effective against HIV in the laboratory. The addition of an antiretroviral drug to the microbicide should also prevent the HIV from multiplying.

Research into PC-815 will begin with safety testing on 50 participants in June this year in the Dominican Republic, pending government approval.

The failure of Carraguard represents another setback to microbicide research, following the termination of trials of the microbicide cellulose sulphate in 2007 (see Safety concerns halt trials of HIV microbicide).

Ayesha Kharsany, project director at the Centre for the AIDS Programme of Research in South Africa (CAPRISA) and head of a team developing a microbicide called Tenofovir, says researchers are united in their efforts to find a product that works.

Govender says a microbicide could never completely prevent infection but could reduce it by between 30–40 per cent and would be most effective when used with another preventive measure.