JOHANNESBURG, 20 August 2010 (PlusNews) - One of the cheapest and most commonly used drugs for treating HIV in Africa - nevirapine - has been associated with an increased risk of treatment failure in a retrospective South African study.

The study, published in the August 15 issue of the Journal of AIDS, looked at adult patients given antiretroviral (ARV) treatment at public sector clinics in South Africa's Western Cape Province between 2001 and 2006. It found that the use of single-dose nevirapine for the prevention of mother-to-child transmission (PMTCT) increased a patient's chances of treatment failing by nine-fold.

Taking the drug as part of a first-line treatment regimen doubled a patient's risk of treatment failure and having to be switched to much more expensive second-line ARVs.

Nevirapine is often prescribed for HIV-positive women of child-bearing age because the alternative - efavirenz - has been linked to birth defects.

The study found that a CD4 count of less than 150 at the time of starting ARV treatment, and interruptions in treatment, were also associated with poorer treatment outcomes.

Experts have long known that patients who interrupt ARV treatment or miss doses are more likely to develop drug resistance and stop responding to treatment. A number of studies have also found that patients who start taking ARVs late, when their CD4 count is low, are less likely to do well on treatment.

"[This] paper again reinforces the fact that we really need to identify HIV-positive people eligible for ARV treatment earlier, and minimize treatment interruptions," commented Dr Francois Venter, president of the Southern African HIV Clinicians Society.

South Africa recently raised the threshold at which HIV-positive pregnant women, and those co-infected with tuberculosis (TB), can start ARV treatment, but the majority of patients have to wait until their CD4 count drops to below 200.

Venter also described the data on the role of nevirapine in treatment failure as "worrying", and noted that the alternatives to nevirapine were either "toxic, expensive or potentially dangerous [for pregnant women]". If further research confirmed the study's findings, he said, "it puts us in a difficult position."

The study gives strong support to the new guidelines, which increased the CD4 count threshold for the provision of ARV therapy to pregnant women, and provided for two additional ARVs to be given after delivery to reduce the risk of nevirapine resistance.

According to a recently released review of the country's national strategic HIV/AIDS plan however, funding shortfalls have meant that some districts have yet to fully implement revised guidelines introduced in 2008 which replaced single dose nevirapine with dual-ARV therapy for pregnant women not eligible to start ARV therapy.

As South Africa continues expanding its national ARV treatment programme - already the largest in the world - keeping the number of patients who experience treatment failure to a minimum is vital to both the programme's success and its affordability.

But the study's lead author, Ishaaq Datay, of Oxford University and the University of Cape Town, worried that the percentage of patients experiencing treatment failure might increase as South Africa's ARV programme grows.

"The clinics started out with quite robust adherence systems," he told IRIN/PlusNews. "It may be that as they become overburdened, the percentage of [treatment] interrupters might increase."

He said this could be countered by better ARV treatment coverage, which would reduce the number of patients who defaulted on treatment due to the difficulty of reaching distant health facilities.