The non-nucleoside reverse transcriptase (RT) inhibitor RD4-2217 is a thiadiazole derivative that has proved to be a highly potent and selective inhibitor of human immunodeficiency virus type 1 (HIV-1) replication in vitro. In this study we examined genotypic and phenotypic characteristics of RD4-2217-resistant mutants that have been obtained by serial passage of HIV-1 in MT-4 cells in the presence of increasing concentrations (0.05, 0.25, 1 and 10 µM) of the compound. The strains obtained, III

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... B/2217RE/0.05 and III B/2217RE/0.25 , were twoand 15-fold resistant to RD4-2217, respectively, whereas III B/2217RE/1 and III B/2217RE/10 displayed 161and >238-fold resistance, respectively. Both III B/2217RE/1 and III B/2217RE/10 had two amino acid substitutions, V189I and T240I, in the RT. Furthermore, RD4-2217 did not inhibit the replication of an HIV-1 molecular clone, which had the same mutation, at concentrations up to 10 µM, indicating that the V189I plus T240I mutation confers high-level resistance to RD4-2217. Interestingly, the replicability of III B/2217RE/1 and III B/2217RE/10 appeared to be lower than that of wildtype III B in MT-4 cells, suggesting that the V189I plus T240I mutation may impair the enzymatic activity of HIV-1 RT.