Thjazolo-iso-indolinone derivatives with high specificity toward the reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) were identified. The most potent compound, BM +51.0836, inhibited HIV-1 RT at a 50%o inhibitory concentration of 90 nM in vitro. In cell culture assays, similar 50%o inhibitory concentrations were obtained with high specificity for HIV-1. These substances were equally active against a zidovudine-resistant isolate. No antiviral effect was observed with an HIV-2 isolate. HIV-1 isolates resistant to the thiazolo-iso-indolinones were generated in cell culture, and the nucleotide sequences of the respective RT genes were analyzed subsequently. Comparison Fig. lb], a substance with 10-fold lower 50% inhibitory concentrations (IC50s) than the lead compound BM 21.1298 and higher antiviral activity than 0-TIBO and nevirapine (BI-RG-587). MATERIALS AND METHODSCompounds. The thiazolo-iso-indolinones were synthesized by Boehringer Mannheim GmbH. A detailed description of the synthesis will be published elsewhere. Zidovudine (AZT; Wellcome), nevirapine (BI-RG-587; Boehringer Ingelheim), and 0-TIBO (Janssen) were also synthesized by the chemical department of Boehringer Mannheim GmbH. The inhibitors were dissolved in dimethyl sulfoxide, and the mixture was added to the RT assay mixture (the dimethyl sulfoxide concentration was less than 5%). For cell culture assays, stock solutions were prepared by dissolving the inhibitors in 50% dimethyl sulfoxide-50% culture medium. In the cell culture supernatant, the dimethyl sulfoxide concentration was less than 1%.Nonradioactive Bl-1 RT assay. Selective inhibitors of HIV-1 RT were identified by using a previously published high-efficiency screening system (4). The assay contains purified HIV-1 RT (11) expressed in Escherichia coli, an in vitro transcript of the HIV-1 long terminal repeat (21), and the primer-binding site as the template and as well as an 18-mer oligonucleotide as the primer. Inhibition of RT was determined by a nonradioactive RT assay (ELISA) with biotin-and digoxigenin-labeled nucleoside triphosphates (4). In brief, 1 ,ug