Multi-step inhibition explains HIV-1 protease inhibitor pharmacodynamics and resistance

Abstract

HIV-1 protease inhibitors (PIs) are among the most effective antiretroviral drugs. They are characterized by highly cooperative dose-response curves that are not explained by current pharmacodynamic theory. An unresolved problem affecting the clinical use of PIs is that patients who fail PI-containing regimens often have virus that lacks protease mutations, in apparent violation of fundamental evolutionary theory. Here, we show that these unresolved issues can be explained through analysis of the effects of PIs on distinct steps in the viral life cycle. We found that PIs do not affect virion release from infected cells but block entry, reverse transcription, and post-reverse transcription steps. The overall dose-response curves could be reconstructed by combining the curves for each step using the Bliss independence principle, showing that independent inhibition of multiple distinct steps in the life cycle generates the highly cooperative dose-response curves that make these drugs uniquely effective. Approximately half of the inhibitory potential of PIs is manifest at the entry step, likely reflecting interactions between the uncleaved Gag and the cytoplasmic tail (CT) of the Env protein. Sequence changes in the CT alone, which are ignored in current clinical tests for PI resistance, conferred PI resistance, providing an explanation for PI failure without resistance.

Quantitative evaluation of the antiretroviral efficacy of dolutegravir

“…Antiretroviral drugs that act at multiple steps of the HIV-1 life cycle have nonlinear dose-response curves with steep slopes (12), which yield high efficacy at clinical concentrations (10)(11)(12). Both inocula A and B have DTG dose-response curves with slopes of 1.3 ± 0.1.…”

“…Although frequently overlooked, slope is an important determinant of efficacy because of its exponential relationship with inhibition (10,11). High slopes for HIV-1 protease inhibitors (12) and allosteric integrase inhibitors (ALLINIs) (13)(14)(15)(16)(17) reflect inhibition at multiple steps in the viral life cycle. The first-generation InSTI raltegravir has not shown activity at HIV-1 life cycle steps other than integration (13)(14)(15), but DTG has not been evaluated for such activity.…”

Genetic Changes in HIV-1 Gag-Protease Associated with Protease Inhibitor-Based Therapy Failure in Pediatric Patients

“…A recent study identified changes in gp41 of the envelope gene as an alternate route of PI resistance suggesting that sites outside of PR and Gag may also be involved. 35 Others have suggested that due to the short half life of PIs, there is a limited mutation selection window such that wild-type virus emerges faster than mutant virus resulting in the restricted evolution of drug resistance. 36 Thus further studies are needed to determine the reasons why patients fail PIs in the absence of detectable resistance mutations.…”

Lack of protease inhibitor resistance following treatment failure — too good to be true?

“…The observations of Rabi et al shed new light on the inhibition of the HIV life cycle and alternative mechanisms of resistance to PIs (7). First, the authors have utilized a series of clever experiments to identify the inhibitory effect of PIs on several steps in the life cycle, including virus entry, reverse transcription, and posttranscription events.…”