Tropism Assays

What Is Tropism?

Coreceptor tropism is defined as the ability of a particular HIV-1 virus to infect a target cell using a specific coreceptor. HIV requires two binding events to enter into a cell. It must first bind CD4 and then, secondly, a chemokine receptor. Tropism is a label given to the virus that describes which chemokine receptor the virus is using.

The HIV-1 viruses can be characterized into four broad classifications based on their tropism status.

R5-tropic: Viruses or virus populations that can use only the CCR5 chemokine coreceptor to infect CD4+ cells. X4-tropic: Viruses or virus populations that can use only the CXCR4 chemokine coreceptor to infect CD4+ cells.
Dual (D)-tropic: Viruses or virus populations that can use either the CCR5 or CXCR4 coreceptors to infect CD4+ cells. Mixed (M)-tropic: Virus populations that may contain various combinations of R5 virus, X4 virus, and/or dual-tropic viruses.

Trofile® Assays Determine Viral Tropism

Monogram currently offers two tests for tropism determination for both suppressed and nonsuppressed patients:

For patients with detectable viral loads ≥ 1000 copies/mL For patients with undetectable viral loads

More than 23,000 samples have been tested using Monogram’s Trofile® assays. All trials of coreceptor antagonists have used Trofile in their clinical development. Trofile has been shown to be accurate, precise, sensitive, reproducible, and robust in the measurement of HIV-1 coreceptor tropism.

The current US Department of Health and Human Services (DHHS) and the Infectious Diseases Society of America (IDSA) guidelines recommend that a coreceptor tropism test be performed whenever the use of a CCR5 inhibitor is being considered.1,2 Coreceptor tropism testing might also be considered for patients who exhibit virologic failure on a CCR5 inhibitor.1