Interpretation of Resistance Assay Results

Resistant viruses may emerge in patients via two different mechanisms [5]. In the first instance, resistant viruses may be transmitted [5‐7]. Alternatively, resistance mutations may be selected for during therapy. This will take place if virus replication occurs in the presence of antiviral drugs, as may be the case due to insufficient drug levels or insufficient inhibitory activity of a regimen [8]. Reasons for suboptimal drug levels include non-adherence, problems with absorption or pharmacokinetic interactions. In this instance, interventions may be more successful by focusing on improving adherence and reducing drug-related toxicities in order to prevent the exhaustion of limited therapy options [5]. The two main types of assay currently used to detect drug-associated resistance are genotypic and phenotypic assays. Genotypic testing uses DNA sequencing or hybridization techniques to identify the presence of individual mutations in the viral genome that are known to be associated with reduced susceptibility to antiretroviral drugs. Phenotypic testing measures the in vitro susceptibility of a viral isolate to one or more drugs. This is generally measured as the concentration of drug required to inhibit replication by 50% in cell culture (IC50). A quantitative measure of isolate susceptibility is derived by comparison of the isolate IC50 with that of a reference wild-type strain, and expressed as the fold-difference between the two. Thus, an isolate displaying a twofold reduction in susceptibility to a particular drug has an IC50 twice as high as the wildtype control. An additional method that combines some of the benefits of both genotyping and phenotyping is the VirtualPhenotype, which relies on a large relational database of genotypes and their corresponding phenotypes [9]. For prevalent mutational patterns this allows an isolate’s genotype to be predictive of a phenotype. For less common mutational patterns, there may be few or no matches in the database and traditional phenotyping assays are required [9]. The application of the results produced by these tests is subject to influence by host factors, pharmacokinetic issues and interactions between mutations [5]. The aim of this article is to review the recent prospective studies and discuss some of the central issues surrounding resistance testing technologies and the interpretation of assay results.