A pièce de resistance: how HIV-1 escapes small molecule CCR5 inhibitors

Abstract

Purpose of review: Small molecule inhibitors targeting the CCR5 coreceptor represent a new class of drugs for treating HIV-1 infection. Maraviroc has received regulatory approvals, and vicriviroc is in phase 3 trials. Understanding how resistance to these drugs develops and is diagnosed is essential to guide clinical practice. We review what has been learned from in-vitro resistance studies, and how this relates to what is being seen, or can be anticipated, in clinical studies.

Recent findings: The principal resistance pathway in vitro involves continued use of CCR5 in an inhibitor-insensitive manner; the resistant viruses recognize the inhibitor-CCR5 complex, as well as free CCR5. Switching to use the CXCR4 coreceptor is rare. The principal genetic pathway involves accumulating 2-4 sequence changes in the gp120 V3 region, but a non-V3 pathway is also known. The limited information available from clinical studies suggests that a similar escape process is followed in vivo. However, the most common change associated with virologic failure involves expansion of pre-existing, CXCR4-using viruses that are insensitive to CCR5 inhibitors.

Summary: HIV-1 escapes small molecule CCR5 inhibitors by continuing to use CCR5 in an inhibitor-insensitive manner, or evades them by expanding naturally insensitive, CXCR4-using variants.

Figure 1. A commonly used assay for HIV-1 coreceptor usage

Plasmids expressing a HIV-1 Env glycoprotein and an Env-deleted HIV-1 genome with a linked luciferase reporter gene are cotransfected into a producer cell line (typically 293T cells). The resulting Env-pseudotyped viruses are collected and used to infect a target cell line that expresses CD4 and either CCR5 or CXCR4 (typically U87-CD4-CCR5 or U87-CD4-CXCR4 cells). Infection is quantified by the amount of luciferase activity present in the target cells. The commercially available Trofile and PhenoSense assays from Monogram Inc are based on this method [39]. This figure is adapted from [39].

Figure 2. Competitive and noncompetitive inhibition

With competitive inhibition (left), resistance is manifested by a rightwards shift in the dose response curve (dotted line) such that the IC₅₀ value for the inhibitor increases. If enough inhibitor is present, the extent of inhibition can still reach 100%. With noncompetitive inhibition (right), resistance is manifested by a reduction in the extent of inhibition (dotted line) but without a change in IC₅₀ value. Increasing the inhibitor concentration beyond the point at which inhibition is maximal (the maximum percentage inhibition, MPI, value) has no additional effect. Resistance to the small molecule CCR5 inhibitors is noncompetitive, sometimes referred to as allosteric [12••, 16••].