Some HIV medications lead to the development of drug-resistant HIV when patients take as few as two percent of their medications. For other medications, resistance occurs only when patients take most of their pills.
These differences appear to be explained by the different levels of viral "fitness" of the drug-resistant HIV, say AIDS researchers in a new study.
The research, led by David Bangsberg, MD, MPH, an AIDS specialist at the University of California, San Francisco, is reported in the January 9 issue of the journal AIDS.
Viral "fitness" refers to the inherent ability of a virus to replicate and cause disease. Incomplete pill-taking by patients causes HIV to mutate and become resistant to the effects of the medications, while the medications that were consumed, in turn, cause the newly resistant virus to become less fit.
The type of medication also factors in. Differences in viral fitness of mutated resistant virus occur between different classes of antiretroviral drugs, said Bangsberg, who is an associate professor of medicine at UCSF and director of the UCSF Epidemiology and Prevention Interventions Center at San Francisco General Hospital Medical Center.
When patients succeed in completely suppressing HIV, which requires that patients take all or almost all of their medications as directed, resistant strains either do not occur or are suppressed, he added.
Explaining the study results, Bangsberg said, "A non-nucleoside reverse transcriptase inhibitor (NNRTI), for example, can be taken one time by a pregnant woman to prevent mother-to-child transmission, and NNRTI-resistant HIV virus can develop. Yet patients taking unboosted protease inhibitors (PI) do not experience the peak risk of PI-resistant HIV developing unless they are taking most of their PIs but fall just short of full viral suppression."