Page 36 - HIV/AIDS Guidelines

Coreceptor Tropism Assays (Last updated January 10, 2011; last reviewed January 10, 2011)

Panel’s Recommendations
• Coreceptor tropism assay should be performed whenever the use of a CCR5 inhibitor is being considered (AI).
• Coreceptor tropism testing might also be considered for patients who exhibit virologic failure on a CCR5 inhibitor (CIII).

Rating of Recommendations: A = Strong; B = Moderate; C = Optional

Rating of Evidence: I = data from randomized controlled trials; II = data from well-designed nonrandomized trials or observational
cohort studies with long-term clinical outcomes; III = expert opinion

HIV enters cells by a complex process that involves sequential attachment to the CD4 receptor followed by
binding to either the CCR5 or CXCR4 molecules and fusion of the viral and cellular membranes. CCR5
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inhibitors (i.e., maraviroc [MVC]), prevent HIV entry into target cells by binding to the CCR5 receptor. 2
Phenotypic and, to a lesser degree, genotypic assays have been developed that can determine the coreceptor
tropism (i.e., CCR5, CXCR4, or both) of the patient’s dominant virus population. One assay (Trofile,
Monogram Biosciences, Inc., South San Francisco, CA) was used to screen patients who were participating
in studies that formed the basis of approval for MVC, the only CCR5 inhibitor currently available. Other
assays are under development and are currently used primarily for research purposes or in clinical situations
in which the Trofile assay is not readily available.

Background
The vast majority of patients harbor a CCR5-utilizing virus (R5 virus) during acute/recent infection, which
suggests that the R5 variant is preferentially transmitted compared with the CXCR4 (X4) variant. Viruses in
many untreated patients eventually exhibit a shift in coreceptor tropism from CCR5 to either CXCR4 or both
CCR5 and CXCR4 (i.e., dual- or mixed-tropic; D/M-tropic). This shift is temporally associated with a more
rapid decline in CD4 T-cell counts, 3-4 although whether this shift is a cause or a consequence of progressive
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immunodeficiency remains undetermined. Antiretroviral (ARV)-treated patients who have extensive drug
resistance are more likely to harbor detectable X4- or D/M-tropic variants than untreated patients who have
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comparable CD4 T-cell counts. The prevalence of X4- or D/M-tropic variants increases to more than 50% in
treated patients who have CD4 counts <100 cells/mm .
3 5-6
Phenotypic Assays

There are now at least two high-throughput phenotypic assays that can quantify the coreceptor characteristics
of plasma-derived virus. Both involve the generation of laboratory viruses that express patient-derived
envelope proteins (i.e., gp120 and gp41). These pseudoviruses are either replication competent (Phenoscript
assay, VIRalliance, Paris, France) or replication defective (Trofile assay, Monogram Biosciences, Inc.). 7-8
These pseudoviruses then are used to infect target cell lines that express either CCR5 or CXCR4. In the
Trofile assay, the coreceptor tropism of the patient-derived virus is confirmed by testing the susceptibility of
the virus to specific CCR5 or CXCR4 inhibitors in vitro. The Trofile assay takes about 2 weeks to perform
and requires a plasma HIV RNA level ≥1,000 copies/mL.
The performance characteristics of these assays have evolved. Most, if not all, patients enrolled in
premarketing clinical trials of MVC and other CCR5 inhibitors were screened with an earlier, less sensitive
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version of the Trofile assay. This earlier assay failed to routinely detect low levels of CXCR4-utilizing
variants. As a consequence, some patients enrolled in these clinical trials harbored low, undetectable levels of
CXCR4-utilizing viruses at baseline and exhibited rapid virologic failure after initiation of a CCR5
inhibitor. This assay has since been revised and is now able to detect lower levels of CXCR4-utlizing
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Guidelines for the Use of Antiretroviral Agents in HIV-1-Infected Adults and Adolescents C-18

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