An experimental gel containing a prescription HIV drug has been shown for the first time to protect rectal tissue against the virus that causes AIDS, according to new research.

The gel, containing Gilead Sciences Inc's AIDS drug tenofovir, has previously been shown to sharply reduce HIV infections in women when applied inside the vagina.

The latest study, which involved rectal tissue biopsies taken from HIV-negative men and women who used the product daily for one week, provides the first evidence that tenofovir gel could help reduce the risk of HIV from anal sex.

The researchers, who presented the findings at a medical meeting in Boston, are reformulating the gel so that it is less harmful to the rectum than the original vaginal microbicide.

The risk of becoming infected with HIV from unprotected anal sex may be at least 20 times greater than unprotected vaginal sex, in part because the rectal lining is only one-cell thick compared to the vagina's multiple layers, making it easier for the virus to reach cells to infect, the researchers said in a statement.

Tissue samples obtained in the study were sent to a laboratory where they were exposed to HIV. The study found that the virus was significantly blocked in samples from participants who used tenofovir gel compared with samples from participants who used a placebo gel. An oral dose of tenofovir was not shown to provide protection from HIV.

Although the study found use of the gel generally safe, two participants had severe gastrointestinal side effects, including diarrhea and lower abdominal cramps.

Another study is under way using a formulation of tenofovir gel with less glycerin, an additive found in many gel-like products, in the hope that this will make it better tolerated.

A separate mid-stage study presented at the meeting compared the oral and gel forms of the drug in American and African women. It found that daily use of tenofovir gel resulted in a more than 100-times higher concentration of active drug in vaginal tissue compared with use of an oral tablet.

Compared with the gel, a daily tablet was associated with a 20-times higher active drug concentration in blood.

"How the differences between the gel and the tablet will translate in terms of protective effect, we can't say just yet," Dr. Craig Hendrix, a professor of medicine and pharmacology at Johns Hopkins School of Medicine in Baltimore and the study's leader, said in a statement.

Most American women studied in the trial favored the oral tablet. The African women favored the gel and tablet equally, although many of them said they liked the gel because it enhanced sexual pleasure, the researchers reported.