Co-receptor inhibitors

To enter an uninfected CD4 T-cell, HIV uses the CD4 receptor and either chemokine receptor 5 (CCR5) or chemokine receptor 4 (CXCR4). Viral chemokine use is generally related to immune status. In early stage HIV infection, almost all HIV is CCR5-tropic, sometimes referred to as R5 virus. In patients with more advanced HIV infection, about half will have X4 virus (HIV that uses the CXCR4 co-receptor) or dual-/mixed-tropic virus.^{1 2}

Dual-tropic virus can use either CCR5 or CXCR4. Some individuals carry HIV variants that use CCR5 and others that use CXCR4; this is referred to as mixed tropism.

Although some experts have expressed concern that use of a CCR5 antagonist might encourage the emergence of potentially more aggressive CXCR4-using virus or dual-/mixed-tropic virus, it is not yet clear whether this will be a major problem.

One other concern is that a mutation in gp120 might allow virus to bind to the CCR5 receptor, despite the presence of an antagonist designed to block receptor use. In vitro, this occurs by accumulating two to four sequence changes in the gp120 V3 region and a non-V3 pathway is known of as well.³ 

CXCR4-tropic strains have generally been linked with more rapid disease progression, though some studies suggest that the presence of CXCR4-tropic virus does not adversely affect the outcome of antiretroviral therapy.^{4 5}

For further information on co-receptors and viral tropism, see Receptors, co-receptors and immunity to HIV in How the immune system works.