Clinical trials of a microbicide -- a gel or cream designed to block the sexual transmission of HIV -- have been terminated early for safety reasons, but scientists insist that this must not hinder research into other microbicides.

Researchers announced last week (31 January) that two clinical trials for cellulose sulphate, or ushercell, had been stopped because one trial appeared to increase the risk of women becoming infected with HIV.

According to the International Herald Tribune, the study that led to stopping the trials involved 1,333 women in Benin, South Africa and Uganda. The other trial involved 1,700 women in Nigeria.

The Word Health Organization said the trial's failure was "disappointing" and an "unexpected setback" in the search for safe and effective microbicides.

Gita Ramjee, director of the HIV Prevention Research Unit of the South African Medical Research Council, said scientists were baffled by the trial's failure and are still trying to find the reason for it.

She said cellulose sulphate went through rigorous laboratory and clinical testing prior to large-scale trials and all data had indicated it was safe.

However, Ramjee insisted that research into anti-HIV microbicides must go on, as to do otherwise would mean "giving up for women all over the world".

"We should continue with research but ensure that data is reviewed more frequently, so that rapid action can be taken should there be a cause for concern," she told SciDev.Net.

Three other microbicide compounds are currently undergoing phase III clinical trials in Africa.

In South Africa, trials of the Carraguard microbicide are nearing completion and results are expected by the end of 2007. Another product, Pro2000, is being tested in South Africa, Tanzania and Uganda. The BufferGel microbicide is being tested in Malawi, South Africa, Zambia and Zimbabwe.

HIV specialist Jonathan Weber of Imperial College London, United Kingdom, said there was no suggestion that other microbicides currently being investigated, based on different chemical compounds, have similar problems.

"There are no silver bullets in HIV prevention research. We need a package of tools to combat the HIV epidemic, and it is hoped that a safe and effective microbicide could be part of this package," he told SciDev.Net.

Microbicides have gained both status and funding in the past few years as a way for women to protect themselves against HIV infection in situations where they have little negotiating power to persuade male partners to use condoms.