People who start antiretroviral treatment for the first time with a viral load over 100,000 do equally well on a regimen containing abacavir (found in Ziagen, Epzicom and Trizivir) as on a regimen containing tenofovir (found in Viread, Truvada and Atripla), according to a study published in the September 1 issue of The Journal of Infectious Diseases. These results counter a previously reported study that suggested abacavir—notably its Epzicom co-formulation with lamivudine—is less likely to keep viral load undetectable among people initiating HIV treatment with high viral loads.

Early results from the AIDS Clinical Trial Group (ACTG) study 5202, reported in August 2008, indicated that patients with viral loads above 100,000 upon starting a regimen containing Epzicom failed treatment faster than those taking Truvada (tenofovir plus emtricitabine). A subsequent analysis of several studies by the company that makes abacavir, GlaxoSmithKline (GSK), found no greater risk of early failure in people on their drug who’d started treatment with high viral loads, than in people on tenofovir.

To examine the potential for early virologic failure with abacavir, Loveleen Bansi, MSc, from the Royal Free & University College Medical School in London, and her colleagues looked at the medical records of 1,548 HIV-positive patients enrolled in the U.K. Collaborative HIV Cohort Study (UK-CHIC). In all, 1,136 of the participants started HIV treatment with an abacavir-containing regimen and 412 started therapy with a tenofovir-containing regimen. About 45 percent of the people taking abacavir started treatment with a viral load over 100,000.

When Bansi’s team accounted for variants such as CD4 count, age, sex and other drugs in the regimen, they found that people who took an abacavir-containing regimen had a similar drop in virus as people taking tenofovir—roughly 2 logs (99 percent). People who started treatment with very high viral loads—greater than 300,000—were less likely to have a virologic response to treatment, but this was true whether they took abacavir or tenofovir. In fact, people with high viral loads on abacavir had the same drops in viral load and the same degree of viral success at 24 and 48 weeks after starting treatment.

The authors acknowledge that their study, which simply looks at what happened in a group of HIV-positive people (called a cohort study), is not as rigorous as studies planned and conducted solely to compare the two drugs. However, the authors suggest that the new data, when viewed along with the results of controlled studies, add weight to the argument that people with high viral loads who start abacavir are likely to do well.