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participating institutions:
Johns Hopkins University AIDS Service, New York State DOH AIDS Institute, The CORE Center, Cook County Hospital



NEWS AND NEW DEVELOPMENTS



Phenotypic Drug Susceptibility Testing Predicts Long-Term Virologic Suppression Better than Treatment History in Patients with Human Immunodeficiency Virus Infection [Call SA, et al. JID 2001;183:401]: This is a retrospective analysis of a cohort of patients followed at the University of Alabama at Birmingham School of Medicine. Resistance testing was done with the Pheno Sense HIV-1 assay, and two separate criteria were used for susceptibility, IC 50 values of 2.5x or 4x higher than the reference values. Multivariate analysis including clinical and treatment history plus phenotypic drug susceptibility demonstrated that the susceptibility test result was a strong predictor to time of treatment failure. These results applied to both the 2.5-fold and the 4.0-fold thresholds. The median time to treatment failure based on the number of drugs that were active in vitro at the 4.0-fold threshold for 0 - 1 drugs was 107 days, for 2 drugs was 182 days, and for 3 - 4 drugs it was 242 days. Compared to clinical decisions based on history, phenotypic susceptibility proved to be an independent predictor of time to treatment failure with an OR of 0.7.
Comment: Drug susceptibility for HIV is clearly in a state of evolution in which there are several studies showing benefit, but many of these show the differences decline with prolonged periods of observation. One of the major concerns about these tests is the interpretation, and this applies to both genotypic testing and phenotypic testing. In this case, the phenotypic test results were analyzed with two thresholds, 2.5-fold above the reference strain and 4.0-fold above the reference strain. There are obvious differences between drugs, so this arbitrary selection of a threshold for all drugs is clearly inappropriate. The two companies that produce phenotypic assays now report sensitivities based on thresholds for each drug. The results of the study from Alabama seem to show benefit compared to clinical decisions based on history. This is in agreement with some studies, but others show no benefit compared to standard of care without resistance testing. At the moment, resistance testing appears to be a relatively crude science that requires expert interpretation.
posted 2/22/2001





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