Hopes for a vaccine to prevent HIV infection have taken a blow as Merck has stopped all trials on its experimental product. The vaccine, V520, was canned because preliminary results from a proof-of-concept study in 3000 subjects showed it didn’t work. Enrolment of patients in a second large Phase II study due to be run in South Africa has also been stopped, along with another two Phase I trials that were also under way.
V520 was a mixture of three synthetic HIV genes, each of which was carried by a weakened adenovirus, the type of virus that causes the common cold. The idea was that the HIV genes in the vaccine would stimulate an immune response in the body that would programme its CD8 T-cells to kill HIV infected cells. Initial trials showed it could induce these HIV-specific immune responses.
The international trial was carried out in healthy volunteers at high risk of contracting HIV. An interim analysis of the results in half of the subjects was carried out, and 24 of the half of these who had received the vaccine had become infected with HIV, compared to 21 in those who had been given placebo. It had no effect on the amount of virus circulating in the bloodstream of those who became infected, either.
This was the first vaccine to make it into such large scale trials, and finding a vaccine with sufficiently broad coverage to prevent all strains of the virus will be extremely challenging. Numerous other potential HIV vaccines are under development under the auspices of the HIV Vaccine Trials Network. For example, results from one of these, from GeoVax, were reported at the AIDS Vaccine Conference last month, and 80% of those given the vaccine developed anti-HIV T-cells. A large-scale Phase II trial is tentatively scheduled for 2008.