Nature Medicine 15, 829 (2009)

by Cassandra Willyard

Despite some disappointing results from recent trials, microbicides have been touted as a potential tool to stop HIV infection through vaginal sex. Last month, however, the International AIDS Society conference in Cape Town, South Africa dedicated an entire session to rectal microbicides for the first time. “The field is picking up momentum now,” says Ian McGowan, an infectious disease expert at the University of Pittsburgh School of Medicine who organized the session. “We’re beginning a new phase of clinical trials.”

The current focus is on testing vaginal microbicides in the rectum. Researchers have already completed a phase 1 clinical trial to test the rectal safety of UC-781, an antiretroviral being developed for vaginal use. The results have yet to be published, but McGowan, who was a co-investigator on the study, says UC-781 seemed to be safe and acceptable. Other forthcoming trials will test the rectal safety of a vaginal microbicide that contains the anti-HIV drug tenofovir.

The rectum and the vagina are “two very different environments,” McGowan says, so there is no guarantee that one product will be safe and effective both places. The vaginal lining is composed of “layer upon layer of cells,” he says. The lining of the rectum, in contrast, is just one cell thick, and thus more fragile. Moreover, because the rectum feeds into the colon, a larger surface area needs to be protected. Researchers are already thinking about how to design microbicides specifically for rectal use.

Money will probably be a limiting factor, however. “It’s still not clear whether there’s enough will from funders to do a full-scale efficacy trial for any rectal microbicide,” says Robin Shattock, an immunologist at St. George’s, University of London. “That’s the biggest hurdle.”

[A number of our presenters’ slide sets are available. Click the Community Presentations link on the IRMA website to access presentations such as “Epidemiology of HIV among MSM in Africa: Implications for rectal microbicides and “Go tell it on Table Mountain – rectal microbicide advocacy from here to there.”]