Detectable viral loads in patients on antiretroviral therapy causes slow development of resistance

HIV-infected patients taking antiretroviral therapy but with detectable levels of HIV in the blood acquire drug resistance mutations relatively slowly, according to a prospective observational cohort study presented in the 1^st September edition of The Journal of Acquired Immune Deficiency Syndromes.

The study also showed that the likelihood of resistance developing is linked to higher levels of HIV replication and to fewer pre-existing resistance mutations.

A substantial proportion of HIV-positive patients taking antiretroviral therapy fail to achieve or maintain viral levels below the limit of detection, typically 50 copies/ml. Although it is generally recommended that these patients switch to a new regimen containing at least two drugs to which they are likely to respond, this runs the risk of the patient developing side-effects associated with the new drugs. It may also exhaust the drug options available for future treatment combinations, or it may be impossible to find two new drugs for patients with substantial treatment experience.

“Our findings demonstrate a relatively slow rate of resistance evolution in patients with HIV-1 subtype B, especially among individuals with multiple mutations, who have stable HIV RNA levels below 1000 copies/ml,” the investigators conclude. “Delaying switching of suboptimal regimens may be indicated in some patients. However, patients with HIV-1, with limited resistance, especially those with plasma HIV RNA above 1000 copies/ml, are at risk for emergence of increasingly resistant virus.”

To gain a better understanding of the risk of resistance developing, investigators from the University of North Carolina enrolled 98 adult patients from their HIV clinic who had been on a stable antiretroviral drug regimen for at least six months but had detectable viral loads. All of the patients had two genotypic resistance tests, taken at least 30 days apart.

Most of the patients had substantial treatment experience, with a median of three prior regimens and six antiretroviral drugs. At the start of the study, the median CD4 cell count was 246 cells/mm³ and viral load was 7940 copies/ml, with the 55% of the patients on a protease inhibitor-based drug regimen.

At the first genotypic test, 88% of the patients had at least one resistance mutation, with a median of three per patient. Resistance mutations were defined according to the International AIDS Society-USA guidelines.

After a median follow-up of 9.3 months, 60% of the patients had acquired at least one new mutation. CD4 cell counts had remained stable at a median of 242 cells/mm³ but viral loads had risen to 20,000 copies/ml (p = 0.02).

The investigators calculated that the development of new mutations was equivalent to an average rate of 1.61 new mutations in every patient every year (95% confidence interval: 1.36 – 1.90).

Despite the overall increase in the number of patients with at least one mutation, the proportion did not increase significantly for patients with protease inhibitor or nucleoside reverse transcriptase inhibitor (NRTI) mutations. However, there was a significant increase in the proportion of patients with non-nucleoside reverse transcriptase inhibitor (NNRTI) mutations, from 57 to 86% (p

This was paralleled by the patients’ predicted drug sensitivity over the course of the study. While the investigators found no significant differences in the predicted number of active protease inhibitors or NRTIs, among the patients receiving NNRTIs, the number of NNRTIs to which they remained susceptible decreased significantly (p = 0.046). “Because a single mutation may cause resistance to the NNRTIs, which also have a substantial degree of cross-resistance, patients were susceptible to fewer NNRTIs at follow-up, in contrast to baseline,” they explain.

Using a multivariable analysis, the investigators calculated that the risk of a new mutation was associated with the average viral load across the study (p = 0.001), the rate of change of viral load (p

Specifically, they saw that patients with average viral loads between 1000 and 10,000 copies/ml were twice as likely to develop new mutations than those with higher or lower average viral loads. “Occurrence of new mutations is a function of replication rate, and at HIV RNA levels less than 1000 copies/ml, ongoing continuous rounds of replication are likely to be low,” the authors explain. “Conversely, at higher replication rates new mutations are more likely to occur.”

Patients with viral loads rising at a rate of 0.2log₁₀ per month were also at a twofold greater risk of new mutations than those with stable or falling viral loads. This, they argue, is due to high replication rates allowing any drug resistant virus particles to increase in number more rapidly.

Finally, having no mutations at the start of the study was also an independent risk factor for the risk of mutations developing, with these patients being at more than three times the risk of developing mutations than those with one to three mutations, and around twice as likely as those with more than three mutations. This could be due to the decreased fitness of HIV with multiple mutations, leading to lower replication rates.

Limitations of this study include the inability of the genotype tests used to detect mutations present in small numbers. It was also unable to detect resistance mutations for the fusion inhibitor T-20 (enfuvirtide, Fuzeon), which was being taken by two (2%) of the 98 patients.

“Further studies monitoring resistance evolution over time are needed, and combined analyses across observational cohorts would strengthen our initial observations,” the investigators write.