Despite promising early trial results, another microbicide for the prevention of HIV transmission was deemed ineffective, scientists at the UK Medical Research Council's Microbicides Development Programme (MDP) announced Monday (December 14). The failure of the PRO 2000 gel is part of a decade-long history of unsuccessful attempts to develop vaginal microbicides, which many researchers see as the most effective way to protect women from HIV infection in the developing world.

HIV budding from infected host cell Image: NIH

PRO 2000 was the largest international clinical microbicide trial to date, involving 9,385 women at six research centers in South Africa, Tanzania, Uganda, and Zambia. The placebo-controlled trial was carried out between September 2005 and September 2009. The polymer gel, provided by Endo Pharmaceuticals, was intended to disrupt HIV's interactions with its target cells. But scientists found that the risk of HIV infection for women using PRO 2000 was not significantly different than those using the placebo gel -- 130 of the 3,156 women given PRO 2000 developed HIV infections; 123 of the 3,112 women given the placebo gel developed HIV infections.

After the failures of trials for Ushercell, which was halted when initial data suggested it increased risk of infection, and Carraguard, which was shown to be ineffective in preventing HIV transmission, PRO 2000 was seen by many scientists as microbicide development's last hope. There are currently no microbicides approved for the prevention of HIV infection.