“The Microbicide Trials Network (MTN), which is funded by the U.S. National Institutes of Health, [on Wednesday] announced that it decided to stop one arm of a study involving more than 5,000 women in South Africa, Zimbabwe, and Uganda” after “an interim review of the ongoing trial by an independent monitoring board … found that the drug tenofovir when used as pre-exposure prophylaxis (PrEP) had less effect in protecting women than anticipated,” Science Magazine’s “Science Insider” blog reports. “Although the board did not offer any specifics on how many women became infected on the drug versus placebo, they said continuing with the tenofovir arm was ‘futile’ as it would not yield meaningful results,” the blog writes.

“The new results particularly baffled people who follow this promising prevention strategy because there were mixed but encouraging findings in two similar studies reported earlier this year,” the blog writes, adding that Sharon Hillier, who heads the MTN, “emphasizes that these conflicting findings underscore that much remains to be learned about PrEP.” Additional research analyzing why the drug would work to decrease HIV infection in one population and not another needs to be conducted, the blog notes. The Vaginal and Oral Interventions to Control the Epidemic (VOICE) study, which began in September 2009, includes two other arms testing a tenofovir vaginal gel and the combination antiretroviral drug Truvada and “is not scheduled to end until about one year from now,” according to the blog (Cohen, 9/28).