ISOLATION AND CHARACTERIZATION OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MUTANTS RESISTANT TO THE NON-NUCLEOTIDE REVERSE-TRANSCRIPTASE INHIBITOR MKC-442

MKC-442, 6-benzyl-1-ethoxymethyl-5-isopropyluracil (I-EBU), is a potent and selective non-nucleoside inhibitor of human immunodeficiency virus type-1 (HIV-1) reverse transcriptase (RT). Nevirapine, another nonnucleoside RT inhibitor (NNRTI), is associated with rapid emergence of drug-resistant variants during in vitro passages of HIV-1. The emergence of resistant viruses to MKC-442 or nevirapine was examined in vitro. MT-4 cells infected with a clinical isolate (HE) of HIV-1 were cultivated in medium containing excess concentrations of these drugs, and the drug susceptibilities of the breakthrough viruses recovered from the medium were measured. Although nevirapine lost its antiviral activity after six passages, a delay in the emergence of fully resistant viruses was observed for MKC-442. Two resistant clones for each drug were isolated and nucleotide sequences within the RT region were analysed. An amino acid substitution at position 181 (Tyr to Cys) was found, with additional substitutions at positions 103 (Lys to Arg) and 108 (Val to Ile) in the MKC-442-resistant viruses. These clones showed various susceptibilities to MKC-442, and cross-resistance to other NNRTIs but not to AZT. These results suggest that the major binding site of MKC-442 on the HIV-1 RT is the tyrosine residue common to these NNRTIs, and that drug resistance to NNRTIs is dependent on both the quality and the quantity of mutations within the HIV-1 FIT gene,