February 28, 2011
New research presented at the 18th Conference on Retroviruses and Opportunistic Infections (CROI) in Boston, Feb. 27-March 2, shows that a gel containing tenofovir may help prevent HIV when used rectally.
The gel containing Gilead Sciences Inc.'s AIDS drug has been previously shown to sharply reduce HIV infection in women when used vaginally. The HIV transmission risk from unprotected anal sex may be more than 20 times greater than unprotected vaginal sex, partly because the rectal lining is just one-cell thick compared to the vagina's multiple cell layers. Thus, HIV can more easily reach cells to infect, said researchers from the University of California-Los Angeles (UCLA) and the University of Pittsburgh.
Dr. Peter Anton, director of the Center for Prevention Research at UCLA, and colleagues studied rectal tissue biopsies from HIV-negative men and women who used the tenofovir gel or a placebo gel daily for one week. Tissue samples were sent to a laboratory where they were exposed to HIV. The samples showed HIV was significantly blocked in participants using the tenofovir gel compared to those using the placebo. An oral dose of tenofovir did not appear to provide protection against HIV.
While the tenofovir gel was found to be generally safe, two participants reported severe gastrointestinal side effects, including diarrhea and lower abdominal cramping. In addition, the researchers are reformulating the gel to make it less harmful to the rectum. It is hoped that a formulation that uses less glycerin, an additive common in many gel-like products, will be better tolerated.
Results from a separate, mid-stage study comparing oral and gel forms of tenofovir presented at CROI found daily use of tenofovir gel resulted in a more than 100-times higher concentration of active drug in vaginal tissue compared with the oral drug. However, the daily tablet was associated with a 20-times higher active drug concentration in blood.
That study, led by Dr. Craig Hendrix, a professor of medicine and pharmacology at Johns Hopkins School of Medicine, included U.S. and African women. Most U.S. women in the trial preferred the oral tablet, while African women favored the gel and tablet equally; many reported the gel enhanced sexual pleasure, according to the researchers.
"How the differences between the gel and the tablet will translate in terms of protective effect, we can't say just yet," said Hendrix.