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Q&A: CDC's Clinical Studies of Pre-Exposure Prophylaxis for HIV Prevention: Research Rationale

January 26, 2009

Why is CDC conducting trials of pre-exposure prophylaxis for HIV prevention?

CDC is sponsoring these trials because safe and effective new approaches to HIV prevention are urgently needed. More than 7,000 people continue to become infected around the world every day (approximately 2.7 million per year). Although behavior change programs have contributed to dramatic reductions in the number of annual infections in the United States and many other nations, far too many people remain at high risk.

With an effective vaccine years away, there is mounting evidence that antiretroviral agents may be able to play an important role in reducing the risk for transmission. Researchers believe that an HIV drug approved by the U.S. Food and Drug Administration (FDA) -- tenofovir disoproxil fumarate (tenofovir, brand name Viread) used alone or in combination with emtricitabine (together, known by the brand name Truvada) -- taken daily as an oral preventive drug, is among the most important new prevention approaches being investigated today. The approach is called pre-exposure prophylaxis, or PrEP.

If proven safe and effective, PrEP could help address the urgent need for a female-controlled prevention method for women worldwide who are unable, because of cultural and other barriers, to negotiate condom use. Furthermore, if effective, it could provide an additional safety net for all men and women at risk due to sexual or drug-using behaviors, when combined with reducing the number of sexual partners, HIV counseling and testing, condom use, use of sterile syringes, and other prevention measures.

How would HIV treatment drugs work to protect against HIV infection?

The concept of providing a preventive drug before exposure to an infectious agent is not new. For example, travelers to an area where malaria is common are advised to take medication before and during travel to prevent the disease. The medicine to prevent illness is then already in their bloodstream if they are exposed to the malaria parasite.

Researchers believe that the same concept may work to protect people from HIV infection. Theoretically, if HIV replication can be inhibited from the moment the virus enters the body, it may not be able to establish a permanent infection.

What data suggest that this approach may be safe and effective?

Several sources of data suggest that an antiretroviral drug, taken regularly, may prove effective in reducing a person's risk for HIV infection:

  • Providing a single dose of the antiretroviral drug nevirapine to HIV-infected women during labor and to their newborns immediately after birth has reduced the risk for mother-to-child transmission of HIV by about 50 percent.
  • In observational studies, the antiretroviral drug zidovudine, taken soon after exposure and continued for several weeks, has been associated with an 80 percent reduction in the risk of HIV infection among health care workers after needlesticks or other accidental exposures.
  • Animal studies have shown that tenofovir, administered before and immediately after a single retroviral exposure, can reduce the transmission of a virus similar to HIV in monkeys.
  • Finally, animal studies have also demonstrated that pre-exposure administration of tenofovir plus emtricitabine, provided significant protection to monkeys exposed repeatedly to an HIV-like virus.

The safety and efficacy of tenofovir and a tenofovir-emtricitabine combination pill for the treatment of HIV infection has been well established in clinical studies and medical settings. The FDA licensed tenofovir for use as an HIV treatment in adults in October 2001, and licensed the tenofovir-emtricitabine combination pill for use as an HIV treatment in August 2004. More than 200,000 HIV-infected patients around the world have now used these drugs. Among these patients, tenofovir has resulted in a relatively low level of side effects, compared to other HIV treatments. The most common side effects include nausea and vomiting, and loss of appetite. Tenofovir plus emtricitabine has also been associated with a relatively low level of side effects, which include diarrhea, nausea, fatigue, headache, dizziness, and rash.

One of the key objectives of these trials is to determine for certain whether the study drugs are safe and well tolerated by HIV-negative persons; and safety will be closely monitored throughout the trials. Researchers expect side effects to be even less common in the healthy, HIV-negative volunteers in these HIV prevention trials.

Why study two different drugs?

Globally, more than 7,000 new HIV infections occur daily, and additional prevention approaches are urgently needed. A combination of studies -- of both tenofovir alone and tenofovir plus emtricitabine -- will allow us to move forward as quickly and effectively as possible in the search for new solutions.

There are significant data suggesting the promise of both of these HIV treatment drugs. Because we don't yet know for sure how the animal data will correlate to human protection, we believe it is essential to move forward as quickly as possible to evaluate both of these promising interventions.

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This article was provided by U.S. Centers for Disease Control and Prevention. Visit the CDC's website to find out more about their activities, publications and services.
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