By Maggie Fox

WASHINGTON (Reuters) - A cheap, generic drug long used to treat herpes may also help control the AIDS virus, U.S. researchers reported on Wednesday.

They found that acyclovir can work against HIV, but only in tissues that are also infected with herpes.

The findings, published in the journal Cell Host & Microbe, help explain why some studies have shown that people taking acyclovir have lower levels of HIV, yet others show that taking acyclovir does not prevent infection with the AIDS virus.

The herpes virus itself changes the drug into a form that can work against HIV, said Dr. Leonid Margolis of the National Institute of Child Health and Human Development, who helped lead the study.

"If you suppress herpes, HIV also goes down," Margolis said in a telephone interview.

He hopes his team can now find better ways to use acyclovir against AIDS, and perhaps design new products, like a microbicide, to prevent infection.

"The findings open up promising new avenues of investigation in the fight against the AIDS virus," NICHD director Dr. Duane Alexander said in a statement.

Acyclovir was the first drug to be designed and made synthetically, without the use of any compounds from plants or animals, Margolis said.

It is also very safe, as it does not become active until it encounters a herpes virus -- usually herpes simplex 2, the strain that causes genital herpes. The virus completes a chemical reaction called phosphorylation, turning the acyclovir into an active compound.

Researchers had noticed that patients with HIV who also took acyclovir for herpes infections tended to have less virus in their blood, a measure called viral load. The lower the viral load, the healthier an HIV patient is.

But several high-profile experiments aimed at preventing HIV infection by treating patients for herpes failed.

Margolis thinks he knows why.

"If you test acyclovir against HIV in pure cell lines, it doesn't work," he said.

And in the prevention studies, doctors were trying to completely suppress herpes. "If you suppress herpes virus completely, there is nothing to phosphorylate," he said.

So perhaps lower or more infrequent doses might be more effective, he said. This will have to be tested.

And the good news is that a patient does not have to be infected with genital herpes to benefit. Any herpes virus will do, including HHV6 and HHV7, viruses that cause a nearly universal childhood infection sometimes called roseola.

Like HIV, herpes takes up permanent residence in the body and cannot be eliminated. But drugs can suppress both to prevent symptoms. Adding acyclovir to the cocktails used to treat HIV may strengthen the mix, Margolis said.

HIV infects 33 million people globally. It has killed 25 million and there is no cure and no vaccine.

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