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New Vaccine to Block AIDS Virus Shows Promising Results in Monkeys

By Robert Cooke

The first animal tests of a new drug aimed at blocking the AIDS virus are showing surprisingly good results, perhaps offering a way to protect health workers and newborn infants from infection, researchers reported Thursday.

The drug, called PMPA, was given in large doses to 25 monkeys close to the time when they were infected with the simian AIDS virus, SIV. All 25 were protected; none has shown signs of infection since being inoculated about 18 months ago. In contrast, all 10 of the monkeys given the virus, but no drug, became infected.

The drug injections were also well-tolerated, the researchers said, with no indications of toxicity, even though PMPA was injected daily for four weeks.

"It was definitely a big surprise," said veterinarian/pathologist Che-Chung Tsai, whose research team at the University of Washington Primate Center, in Spokane, is running the experiment. At first, Tsai said, it seemed "too good to believe."

PMPA works by the same mechanism as AZT, the most widely used AIDS treatment. Both block the action of a critical enzyme, reverse transcriptase, that the virus needs to reproduce itself. Fortunately, PMPA seems to be far less toxic than AZT, and it tends to remain in the body much longer, finding its way into infected cells and uninfected cells.

The researchers, who reported their results in the journal Science, also suspect that the virus is less likely to outmaneuver PMPA and develop resistance, since PMPA acts faster and stays in cells longer than AZT does.

"It's a very impressive result," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "It demonstrates that you can clearly block infection if this compound is given at the time of exposure to infection."

Fauci cautioned, however, that "what we don't know yet is how effective it would be in people who are chronically infected for a long time."