October 27, 2000
The female hormone estrogen offers significant protection against Simian Immunodeficiency Virus, the primate version of HIV, according to recently published research by scientists at the Tulane Regional Primate Research Center. Researchers predict it will be demonstrated to offer women the same protection.
On the other hand, elevated levels of progesterone can make women more vulnerable to HIV infection, which is a concern for those using progestin-only contraceptives like Depo-Provera and Norplant.
"Estrogen and progesterone are the yin and yang of female hormones," explained Preston A. Marx, director of the AIDS Research Program at the primate center and senior researcher on the study published last month in the Journal of Infectious Diseases.
"Estrogen has a positive effect. Progesterone blocks estrogen. When there is an excess of progesterone, there is an estrogen deficiency."
Marx was working on developing a model of heterosexual HIV transmission when he noticed that the physiology of the vagina changed dramatically over the course of the menstrual cycle. Infection was more likely to occur post-ovulation, when progesterone levels are high. He and his co-researchers, Gary Baskin of Tulane and Stephen Smith of St. Michael's Medical Center in New Jersey, developed a study in which monkeys were inoculated with high levels of SIV.
Some of the monkeys were treated with estrogen or progesterone implants. All of the monkeys untreated with hormones and 83 percent of those treated with progesterone became infected, but none of those treated with estrogen contracted the disease.
"It was really a bit of a surprise to us that estrogen gave us 100 percent protection," Marx said. Their results complement those of researchers who have found that users of the progestin-based Depo-Provera have increased vulnerability to HIV infection.
Estrogen seems to prevent infection by creating a natural barrier that prevents the virus from crossing over into tissues and working its way into the bloodstream. Estrogen causes the vaginal lining to thicken and also creates a slight acidity in the vagina that makes it less hospitable to a virus. However, researchers are not proposing that women take estrogen orally for HIV protection because of the risks and side effects involved.
Instead, they are planning trials to see if topically applied estrogen creams can afford the same protection as the estrogen implants used in the earlier study. The effect of the cream is confined to the vagina, so there is less worry about pre-cancerous conditions arising as a side effect.
"Estrogen contained in a vaginal cream is one of the most commonly prescribed medications today for postmenopausal women," Marx said. "So we have a drug that's already approved, it's already been safety tested, and we think we can find that this vaginal cream will promote resistance."
Although Marx will test the cream on monkeys, he says current evidence of estrogen's effectiveness is strong enough to justify testing on women. If estrogen cream is found to provide protection against HIV, women will have a way to protect themselves that doesn't depend on the cooperation of their partner. Marx and his co-researchers have been awarded $2 million from the National Institutes of Health for their study of topical estrogen and plan to begin work on the project soon.
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