[HIV tropism assays when first CCR5-antagonist becomes available]

Since its co-receptors were discovered, HIV tropism is defined by the type of co-receptor used to infect its host cells : R5 for viruses using only CCR5, X4 for viruses using only CXCR4, and R5/X4 or dual for viruses using either CCR5 or CXCR4. Tropism prediction actually made is via recombinant phenotypic assays, which are labour-intensive, long-lasting and expensive, available in only a few highly qualified laboratories. Prescription of CCR5-antagonists requires screening for the presence of X4 variants before starting therapy. With development of this new class of antiretrovirals there is a need for genotypic tests, based on V3 loop gp 120 env gene sequencing and its interpretation with an algorithm. As for resistance tests 10 years ago, these genotypic tests will be easier to introduce into routine clinical practice. These genotypic tropism assays will help to select patients before CCR5-antagonist prescription and to follow patients treated by this class of antiretrovirals, looking for a potential switch of tropism during therapy.