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Session 77 Poster Session Resistance Testing in Drug Selection Session Time: 4:30-6:30 pm Room 4E-F

580-T. Incidence and Nature of Phenotype-Genotype Discordance: Maximizing the Utility of Resistance Testing N. T. Parkin*1, C. Chappey1, L. Maroldo1, J. Arocha2, T. Fralich3, S. Schrader4, D. Ward5, M. Wohlfeiler6 , and M. Bates1 1ViroLogic, Inc., South San Francisco, CA; 2Miami, FL; 3Ft. Lauderdale, FL; 4Southampton Med. Group, Houston, TX; 5Dupont Circle Physicians Group, Washington, DC; and 6Wohlfeiler, Piperato and King, Miami, FL Background: Resistance testing, using either genotype or phenotype, has become standard of care in the management of HIV treatment failure. Genotype relies on accurate interpretation algorithms, while phenotype provides quantitative data but requires a clinical cut-off value for maximum utility. Methods: Genotype and phenotype tests were performed on approximately 200 patients participating in a pilot program to evaluate the impact of combining both resistance assays in a single report (PhenoSenseGT). Genotypic interpretations were based on an updated algorithm reflecting state of the art knowledge. Discordance was defined for drugs with a fold-change (FC) in IC50 over the PhenoSenseHIV assay cut-off but scored as genotype sensitive (PR/GS), or vice versa (PS/GR). Results: Most patients were highly treatment-experienced (85% had 2 or more treatment failures). Phenotype/genotype discordance was commonly observed: in 75, 54, 33, and 22% of samples at least 1, 2, 3, or 4 drugs, respectively, were discordant. The drugs with discordance in over 10% of samples were ddI (37%), ddC (25%), ABC (18%), 3TC (16%), SQV (15%), APV (12%), LPV (12%), IDV (12%), d4T (11%), and RTV (10%). Two-thirds of the PS/GR results were associated with mixtures at resistance-associated positions. After accounting for this, only ddI (29%), ddC (20%), 3TC (14%), ABC (14%), and APV (11%) had discordance rates over 10%. For ddI and ddC, most of the discordance was related to the presence of the M184V mutation, which causes genotype to be called resistant, although not all have FC > 1.7. For SQV, many samples with the L90M mutation, which is interpreted as genotype resistance when present with at least two secondary mutations, retained susceptibility to SQV (FC < 2.5). PR/GS was observed most commonly for 3TC, APV, and LPV. In the case of 3TC, phenotype loss of susceptibility (FC > 2.5) was frequently observed in samples with many ZDV mutations but lacking M184V, K65R, or E44D/V118I. For APV and LPV, complex patterns of PI mutations that did not meet the resistance criteria in the genotype algorithm led to phenotype FC values > 2.5- or 10-fold, respectively. Conclusions: In this pilot program of combined PT/genotype, the majority of samples showed phenotype/genotype discordance in at least 2 of 15 drugs. genotype results added valuable information when mixtures helped to explain the PS/GR result, while phenotype was important for interpretation of PR/GS results and for providing quantitative assessment of the degree of resistance.

©2002 9th Conference on Retroviruses and Opportunistic Infections