HBY 097 [(S)-4-isopropoxycarbonyl-6-methoxy-3-(methylthiomethyl)-3, 4-dihydroquinoxaline-2(1H)-thione] was selected from a series of quinoxalines as a nonnucleoside inhibitor of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (NNRTI). HBY 097 was shown to be a highly potent inhibitor of HIV-1 induced cell killing and HIV-1 replication in a variety of human cell lines as well as in fresh human peripheral blood lymphocytes and macrophages. The compound was also active against a variety of clinical isolates of HIV-1 including different HIV-1 subtypes and viruses resistant to 3'-deoxy-3'-azidothymidine. Mutant reverse transcriptases which arise as a consequence of treatment with other nonnucleoside inhibitors of HIV-1 reverse transcriptase were still inhibited by HBY 097 at relatively low concentrations. An HIV-1MN variant resistant to inhibition by HBY 097 displayed in the reverse transcriptase gene a mutation causing a substitution at position 190 of a glutamic acid for a glycine residue (G190 --> E), which is characteristic for quinoxaline derivatives. The drug was demonstrated to possess a favorable toxicity profile and to show good oral bioavailability in both mice and dogs. As a consequence of its outstanding properties, HBY 097 was selected for further development and is at present undergoing clinical trials.