DURBAN, SOUTH AFRICA (FRONTLINE MEDICAL NEWS) – Oral antiretroviral pre-exposure prophylaxis (PrEP) against HIV also reduces the risk of acquiring herpes simplex virus type 2, according to research presented at the 21st International AIDS Conference.

“Given the limited interventions for primary prevention of HSV-2, efficacy against HSV-2 provides additional benefit to oral PrEP,” observed Connie Celum, MD, professor of global health and medicine at the University of Washington, Seattle.

HSV-2 is, however, the only non-HIV sexually transmitted infection whose incidence is reduced by PrEP with emtricitabine/tenofovir (Truvada), the sole approved agent for oral HIV PrEP, she added.

Dr. Celum was lead author in a report from the landmark Partners PrEP study of HIV serodiscordant heterosexual Kenyan and Ugandan couples, which demonstrated that oral PrEP provided a 33% reduction in the risk of HSV-2 infection in participants with a known HSV-2-positive partner ( Ann Intern Med. 2014 Jul 1;161[1]:11-9. doi: 10.7326/M13-2471 ).

Tenofovir has been shown to have in vitro activity against HSV-2, providing biologic plausibility to the Partners PrEP study finding, but the 90% effective concentration of the drug required to achieve strong anti-HSV-2 activity in the laboratory makes it likely that good adherence to daily oral PrEP is necessary to see the clinical benefit in terms of reduced HSV-2 acquisition, Dr. Celum said.

She also addressed other questions about the interaction between PrEP and non-HIV STIs she often receives from physicians who provide care for HIV-infected or at-risk patients.

Do STIs reduce the efficacy of oral PrEP? “My answer would be no,” Dr. Celum said. She noted that no difference in PrEP efficacy was seen between patients with and without STIs in Partners PrEP or the French IPERGAY study.

Does PrEP increase the rate of other STIs? The concern here has been that PrEP’s beneficial effect in reducing the risk of HIV infection could be counteracted by a compensatory increase in unsafe sexual practices among PrEP users, with a resultant increase in other STIs. That didn’t occur, however, in the recently reported UK PROUD randomized study of 544 men who have sex with men (MSM), which showed no increase in other STIs with the addition of daily oral PrEP ( Lancet. 2016 Jan 2;387[10013]:53-60. doi: 10.1016/S0140-6736[15]00056-2 ).

Are STIs useful in selecting patients for PrEP by serving as a marker of increased risk for HIV? The answer is a strong yes for MSM. The iPrEX study documented that the number of MSM and transgender women who needed to be treated with PrEP for 1 year to prevent one additional case of HIV infection dropped from 62 for the group as a whole to 36 for those self-reporting condomless receptive anal intercourse and 41 for those with another STI ( Lancet Infect Dis. 2014 Jun;14[6]:468-75. doi: 10.1016/S1473-3099[14]70025-8 ). To cast a wide net for potential beneficiaries, most PrEP programs targeting MSM offer PrEP to those with other STIs or who report engaging in condomless anal sex, Dr. Celum said.

Can PrEP programs reduce STIs through engagement in care? “I think there’s a great opportunity here,” she said. “Most programs are rolling out PrEP with quarterly visits for refills. And I think particularly in MSM and probably in young women, STI testing at those visits provides an opportunity for earlier diagnosis and treatment of STIs as well as for partner notification.”

Dr. Celum reported having no financial conflicts regarding her presentation.