Tenofovir (Viread) once a day can prevent HIV transmission among injection drug users, researchers reported.
In a randomized, phase III trial, daily tenofovir reduced the risk of HIV by nearly half, compared with placebo, in a cohort of injection drugs users in Bangkok, according to Michael Martin, MD, of the CDC, and colleagues.
The study is the latest to show that so-called pre-exposure prophylaxis (PrEP) can protect people at high risk for HIV from infection, Martin and colleagues reported online in The Lancet.
Previous research has shown that tenofovir PrEP -- either alone or in combination with emtricitabine (Emtriva) -- can reduce the risk of infection in men who have sex with men, in heterosexual men and women, and in heterosexual couples where one partner has HIV and the other does not.
"This study completes the picture of PrEP efficacy for all major HIV risk groups," Martin said in a statement. "We now know that pre-exposure prophylaxis can be a potentially vital option for HIV prevention in people at very high risk for infection, whether through sexual transmission or injecting drug use."
But an outside expert cautioned that the study does not show clearly that PrEP prevents HIV transmission caused by sharing needles.
There's "no biological marker" that can distinguish between HIV acquired through sex and HIV transmitted by needles, noted Salim Abdool Karim, MBChB, PhD, of the University of KwaZulu-Natal in Durban, South Africa.
It's likely that at least some of the observed efficacy was due to reduced sexual transmission, he argued in an accompanying commentary piece.
Nonetheless, Karim concluded, "the overall result is that daily tenofovir does reduce HIV transmission in injecting drug users" and PrEP should be considered as an additional prevention method, along with such things as needle exchanges, promotion of safer sex and injecting practices, and counseling.
The researchers enrolled 2,413 volunteers -- ages 20 through 60, HIV-negative, and reporting injecting drugs within the previous 12 months -- from 17 drug-treatment clinics in Bangkok.
Study participants were randomly assigned on a one-to-one basis to either tenofovir or placebo and followed for an average of 4 years. They were offered condoms and methadone treatment and got monthly HIV testing, combined with risk-reduction and adherence counseling and blood safety tests every 3 months.
Two participants had HIV at enrollment (both in the placebo group) and 50 became infected during follow-up.
Of those, 17 were in the tenofovir arm, for an incidence of 0.35 per 100 person-years, and 33 were in the placebo group, for an incidence of 0.68 per 100 person-years.
Those rates yielded a 48.9% reduction in HIV incidence among those taking the drug, which was significant at P=0.01.
The drug was safe and well tolerated, the researchers reported, with only two significant differences in adverse events: 8% of those getting tenofovir reported nausea and or vomiting, compared with 5% in the placebo arm, which was significant at P=0.002.
And 53% of tenofovir volunteers had grade 1 or 2 elevations in alanine aminotransferase, compared with 49% of those getting placebo, which was significant at P=0.003.
As in previous PrEP trials, adherence was an important factor in efficacy, Martin and colleagues noted.
For instance, in a case-control substudy among participants in the tenofovir arm, the risk of HIV infection was reduced 70% for those in whom tenofovir was found, compared with participants without detectable tenofovir in their blood, the researchers reported.
"These results underscore the importance of helping people using pre-exposure prophylaxis achieve effective levels of adherence," Martin said.
Primary source: The Lancet
Source reference: Choopanya K, et al "Antiretroviral prophylaxis for HIV infection in injecting drug users in Bangkok, Thailand (the Bangkok Tenofovir Study): a randomised, double-blind, placebo-controlled phase 3 trial" Lancet 2013; DOI: 10.1016/S0140-6736(13)61127-7.
Additional source: The Lancet
Source reference: Abdool Karim SS "HIV pre-exposure prophylaxis in injecting drug users" Lancet 2013; DOI: 10.1016/S0140-6736(13)61140-X.