The steering committee for a Phase III clinical trial evaluating a vaginal microbicide to prevent the spread of HIV has agreed to halt one arm of the trial as the higher-concentration gel used is unlikely to benefit participants.

The study will continue with a lower-strength vaginal gel formulation of the microbicide, PRO 2000/5. The recommendation to discontinue the 2% PRO 2000/5 arm of the MDP301 trial while continuing with the 0.5% PRO 2000/5 and placebo gel arms was made by the Independent Data Monitoring Committee (IDMC) for the Microbicides Development Programme (MDP) at a meeting earlier this month.

The Committee concluded there was “no more than a small chance” of showing protection against HIV infection with 2% PRO 2000/5 compared with the placebo gel, the MDP noted.

Accepting these recommendations, the MDP Trial Steering Committee stressed the importance of pushing ahead with recruitment for the 0.5% PRO 2000/5 and placebo arms, “as it is possible that 0.5% PRO 2000/5 will prove to be effective in protecting women against HIV infection”.

The IDMC will continue to monitor the trial carefully until its planned completion in late 2009.

The trial is being run by the Microbicides Development Programme, an international partnership to develop vaginal microbicides for the prevention of HIV transmission. The programme is co-ordinated jointly by Imperial College London and the Clinical Trials Unit of the UK’s Medical Research Council.

The MDP’s European partners include the London School of Hygiene and Tropical Medicine, St. George’s Hospital in London and the Universities of York, Southampton and Barcelona. The programme has clinical sites in South Africa, Tanzania, Uganda, Zambia and Mozambique.

According to the MDP, it is “biologically plausible” that a microbicide such as PRO 2000/5 could show no protective effect against HIV infection at a higher concentration while retaining the possibility of efficacy at a lower strength. “There are a number of possible explanations for this, one being that any beneficial effect against HIV could be partly outweighed by a local effect on the vaginal lining related to the higher-concentration product,” it commented.

Earlier this month, clinical trials of Canadian company Polydex Pharmaceuticals’ cellulose sulphate vaginal gel for protecting against HIV infection were stopped prematurely after it emerged that women using the gel were more likely to become infected than those on placebo.