• Abstract 84
  Phenotypic resistance testing significantly improves response to therapy: final analysis of a randomized trial (VIRA 3001).
  Cohen C, Kessler H, Hunt S et al.
  

Coverage provided by Cal Cohen, M.D.

Each test methodology has increasingly been used in a variety of studies. Phenotyping is, simply put, looking at the ability of HIV to grow in a test tube environment containing a specific concentration of antiviral drug. If it can grow despite the concentration we can get inside the body, it is resistant, but if it is stopped, it is still sensitive to the drug. Genotyping on the other hand is a simpler test to do technically, looking only at the genetic changes that HIV creates in response to our drugs. Certain genetic changes occur very commonly as a result of specific drugs, although the complexity of different patterns and on certain drugs continues to challenge us and provide new insights into how we best will interpret results of such testing.

At this meeting, more data were presented to validate testing. One study, the first one to look at the impact of phenotyping on clinical outcomes, was updated at this meeting. (Abstract 84: Phenotypic resistance testing significantly improves response to therapy: final analysis of a randomized trial [VIRA 3001]; Cohen C, Kessler H, Hunt S et al.) In this study, patients who had previous use of only one PI, in combination with nucleoside antivirals, were enrolled if they were experiencing viral load rebound on this regimen. All patients were randomly assigned to either receive a phenotype (the antivirogram from Virco) to guide the choice of new antivirals, or assigned to use clinicians best understanding of what should be done. The study showed that the test had an impact on medication choice, as there were additional changes in the choice of medications selected after clinicians were provided the phenotype. Significantly more patients who had a phenotype result were put on active medications from both the nucleoside class and the PI class, while there was an interesting trend in favor of using less nonnucleosides in the phenotype arm. Nonetheless, there was a significant difference favoring the use of phenotyping, with a larger viral load drop in the phenotype arm (-1.23 vs -0.87 log drop). In addition, there was a greater percent of those on the phenotype arm who had a viral load below 400 copies at week 16. These results will be further updated at the Durban meeting, but continue to demonstrate the clinical benefit of using phenotypic resistance testing.