BOSTON — Researchers are reporting that injections of long-lasting AIDS drugs protected monkeys for weeks against infection, a finding that could lead to a major breakthrough in preventing the disease in humans.
Two studies by different laboratory groups each found 100 percent protection in monkeys that got monthly injections of antiretroviral drugs, and there was evidence that a single shot every three months might work just as well.
If the findings can be replicated in humans, they have the potential to overcome a major problem in AIDS prevention: that many people fail to take their antiretroviral pills regularly.
A preliminary human trial is to start late this year, said Dr. Wafaa El-Sadr, an AIDS expert at Columbia University’s Mailman School of Public Health, but a larger trial that could lead to a treatment in humans may still be some years away.
It has been known since 2010 that healthy people taking a small daily dose of antiretroviral drugs — a procedure known as pre-exposure prophylaxis, or PreP (pronounced prep) — can achieve better than 90 percent protection against infection.
Dr. Ho’s team tested 16 monkeys with rectal washes of the virus that causes AIDS. All the injected monkeys were protected. Credit Ozier Muhammad/The New York Times
But in several clinical trials since then in gay men, in intravenous drug users and in couples where one partner is infected, it has been shown that the only participants protected were those who took their pills every day without fail. Many did not.
The failure rate was particularly acute among women in Africa. Although some participants in one PreP study told researchers that they were scared by rumors about side effects, many also said they were afraid to keep the pills in their home because their sexual partner or a neighbor might see them and mistakenly assume they already had the disease.
An intramuscular injection that a woman could get every three months could change all that, several AIDS experts said.
In Africa and elsewhere in the developing world, many women already receive shots of long-lasting birth control hormones like Depo-Provera, preferring them to daily pills, which might anger spouses or boyfriends who find them.
About the injection protocol tested in monkeys, Dr. David Ho, director of the Aaron Diamond AIDS Research Center at Rockefeller University and an author of one of the studies, said the popularity of Depo-Provera was “a good analogy for how it might work in developing countries.”
In the other study, conducted by the Centers for Disease Control and Prevention in Atlanta, six female monkeys were given monthly injections of GSK744, an experimental drug that is a long-lasting form of an antiretroviral drug already approved for H.I.V. treatment by the Food and Drug Administration.
Six other monkeys got a placebo.
Twice a week, liquid containing human-simian immunodeficiency virus, a hybrid human-monkey version of the virus that causes AIDS, was pumped into their vaginas, simulating sex with an infected monkey.
None of the monkeys protected by GSK744 became infected. All six who got the placebo were infected quickly.
The Rockefeller researchers did a similar experiment with 16 monkeys using the same drug. They got rectal washes of the virus, imitating anal sex.
The results were the same: All the monkeys that got the drug were protected, compared with none of the monkeys that did not get it.
Dr. Ho’s team also tested to see how much of the drug had to be in a monkey’s blood and tissue to be protective. They found that an amount large enough to protect was “eminently achievable in humans with a quarterly injection,” Dr. Ho said.
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