The heterosubstituted nucleoside analogue dOTC [( )-2'-deoxy-3'-oxa-4'-thiocytidine, BCH-10652] is a racemic compound structurally related to 3TC (lamivudine), but has the oxygen and sulphur in the furanosyl ring transposed. Both the enantiomers (-)dOTC (BCH-10618) and (+)dOTC (BCH-10619) had equivalent activity against wild-type strains of HIV-1 in C8166 T-cells (EC50 1.0-10.0 microM) and in PBMCs (EC50 0.1-3.0 microM). Investigation of the activity of dOTC and its enantiomers against laboratory strains of HIV-1 with defined resistance to 3TC, AZT (zidovudine), ddl (didanosine), PMEA (adefovir), nevirapine and saquinavir indicated that sensitivity was maintained (<3-fold change in EC50) in all cases, with the exception of HIV-1RF 3TC-resistant viruses. The degree of resistance recorded for dOTC (four- to sevenfold), (-)dOTC (five- to eightfold) and (+)dOTC (five- to >18-fold) against these M1841 or M184V mutants, was significantly less than that recorded for 3TC (>100-fold). In addition, the inhibitory effect of the compounds against clinical isolates of HIV-1 recovered from patients with suspected resistance to 3TC and AZT was investigated. Clinical isolates were genotyped using the Murex Line Probe Assay (LiPA) and subgrouped into wild-type, 3TC-resistant and dual 3TC/AZT-resistant, as well as undefined or mixed genotype populations. Compared with the mean EC50 values obtained with genotypically and phenotypically wild-type clinical isolates, the mean EC50 values calculated for isolates phenotypically resistant to 3TC or 3TC and AZT were only 2.6-, 1.6- and 8.2-fold higher for dOTC, (-)dOTC and (+)dOTC, respectively. When the rate of emergence of virus resistant to dOTC and its enantiomers in vitro was investigated, virus resistant to (+)dOTC was readily selected for (<10 passages), and a methionine (ATG) to isoleucine (ATA) amino acid change at codon 184 was identified. In contrast, virus resistant to dOTC and (-)dOTC took longer to appear (15-20 passages), with a methionine (ATG) to valine (GTG) amino acid change at position 184 identified in both cases. In addition, virus passaged 20 times in the presence of dOTC also had a partial lysine (AAA) to arginine (AGA) exchange at position 65. These viruses showed only low-level resistance to dOTC and its enantiomers, but were highly resistant to 3TC. The antiviral effects of dOTC in combination with the nucleoside RT inhibitors AZT, 3TC, d4T (stavudine) and ddl, the non-nucleoside RT inhibitor nevirapine and the protease inhibitors saquinavir, ritonavir and indinavir was investigated. Two-way drug combination assays were carried out in peripheral blood mononuclear cell (PBMC) cultures by measuring the reduction in p24 viral antigen levels, and data was analysed using the MacSynergy II program. dOTC in combination with 3TC or d4T showed a moderate synergistic effect while all other combinations had an additive interaction.