AN EXPERIMENTAL gel can prevent herpes infection in mice. If it works as well in people, one application would protect women for several days.

Genital herpes is caused by the herpes simplex virus. In the US, 1 in 5 people are infected. The virus can lie dormant in nerve cells, periodically reactivating and causing a recurrence of symptoms. There are treatments but no cure – people remain infected for life and can infect partners. While vaccines against herpes are in development, none has proved fully effective.

Now Judy Lieberman’s team at Harvard Medical School has harnessed the power of RNA interference to block herpes infection. A gel containing small interfering RNAs (siRNAs) that trigger the destruction of viral genes is applied to the vagina, where they are absorbed and remain active for at least 10 days. The gel completely blocked infection in mice, Lieberman told a meeting in Boston recently.

If it works in people too, the long-lasting protection would be a great advantage, she says. "The problem with microbicides is that people don’t remember to use them before they have sex."

An antimicrobial gel like this would have an enormous value in the developing world, for women whose partners refuse to use a condom, says Victor Ambros of Dartmouth College in New Hampshire. "An antimicrobial cream is something women could use and control." The hope is that the siRNA approach might work for preventing HIV infection too.

Initial studies suggest that besides prevention, the gel might also be useful for treating herpes infections. Other teams are developing siRNA gels for treating the persistent viral infections that cause cervical cancer (New Scientist, 27 November 2004, p 15), and siRNA inhalers for treating the respiratory infection RSV.

Applying siRNAs directly to accessible tissues such as the lungs avoids the as yet unsolved problem of getting them to organs within the body. If this can be overcome, RNA interference has great potential for treating a huge range of diseases.