Protease inhibitors are highly potent drugs which have been widely used as part of highly active antiretroviral therapy. They are recognized for their high antiviral activity which results from the steep dose-response curves. Resistance to protease inhibitors occurs through mutations in the protease gene; however, many patients who fail protease inhibitor-based therapy do so without mutations in protease. This raises the question: do other parts of the HIV-1 genome contain mutations which confer resistance to protease inhibitors in patients failing therapy?
Figure 1. Contribution of the inhibitory effect of protease inhibitors on each step of the viral life cycle.
To understand the molecular mechanisms responsible for the high antiviral activity of protease inhibitors, a group of researchers from John's Hopkins University experimentally tested each relevant step of the life cycle and measured the dose-response curves of three protease inhibitors (atazanavir, darunavir and lopinavir). Here they found that protease inhibitors do not affect the process of budding, but they do inhibit entry, reverse transcription and post-reverse transcription steps. Inhibition at the entry step was the most prominent thereby demonstrating the importance of entry inhibition in the overall effect of protease inhibitors. The researchers then cloned full-length envelope genes from patients who failed protease inhibitor-based therapy with no resistance associated mutations in protease. Pseudoviruses generated using these envelope genes cloned into a wild type backbone showed significant protease inhibitor resistance.
In summary, by experimentally testing each relevant step of the viral life cycle, this study provided a mechanistic explanation for the exceptional efficacy of protease inhibitors. It also identified the envelope gene as a possible site of protease inhibitor resistance which suggests an additional mechanism for protease inhibitor failure. Commercially available phenotypic assays do not detect the change in inhibition of entry because it uses pseudoviruses with MLV-E instead of HIV-1 envelope. Currently, patient-specific changes in the envelope gene are not considered in standard genotypic or phenotypic resistance assays.
Blog by Jennifer Giandhari, PhD student, HIV Virology, Centre for HIV and STIs, National Institute of Communicable Diseases, Johannesburg, South Africa.
Manuscript title: Multi-step inhibition explains HIV-1 protease inhibitor pharmacodynamics and resistance
Authors: S. Alireza Rabi, Gregory M. Laird, Christine M. Durand, Sarah Laskey, Liang Shan, Justin R. Bailey, Stanley Chioma, Richard D. Moore, and Robert F. Siliciano
Journal: J Clin Invest. 2013;123(9):3848-3860. doi:10.1172/JCI67399