Drug resistance and drug combination features of the human immunodeficiency virus inhibitor, BCH-10652 [(+/-)-2 '-deoxy-3 '-oxa-4 '-thiocytidine, dOTC]

The heterosubstituted nucleoside analogue dOTC [(+/-)-2'-deoxy-3'-oxa-4'-th iocytidine, BCH-10652] is a racemic compound structurally related to 3TC (l amivudine), but has the oxygen and sulphur in the furanosyl ring transposed . Both the enantiomers (-)dOTC (BCH-10618) and (+)dOTC (BCH-10619) had equi valent activity against wild-type strains of HIV-1 in C8166 T-cells (EC50 1 .0-10.0 mu M) and in PBMCs (EC50 0.1-3.0 mu M). Investigation of the activi ty of dOTC and its enantiomers against laboratory strains of HIV-1 with def ined resistance to 3TC, AZT (zidovudine), ddI (didanosine), PMEA (adefovir) , nevirapine and saquinavir indicated that sensitivity was maintained (<3-f old change in EC50) in all cases, with the exception of HIV-1(RF) 3TC-resis tant viruses. The degree of resistance recorded for dOTC (four- to sevenfol d), (-)dOTC (five- to eightfold) and (+)dOTC (five- to >18-fold) against th ese M1841 or M184V mutants, was significantly less than that recorded for 3 TC (>100-fold). In addition, the inhibitory effect of the compounds against clinical isolat es of HIV-1 recovered from patients with suspected resistance to 3TC and AZ T was investigated. Clinical isolates were genotyped using the Murex Line P robe Assay (LiPA) and subgrouped into wild-type, 3TC-resistant and dual 3TC /AZT-resistant, as well as undefined or mixed genotype populations. Compare d with the mean EC50 values obtained with genotypically and phenotypically wild-type clinical isolates, the mean EC50 values calculated for isolates p henotypically 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 i n vitro was investigated, virus resistant to (+)dOTC was readily selected f or (<10 passages), and a methionine (ATG) to isoleucine (ATA) amino acid ch ange 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 (CTC) amino acid change at position 184 identified in both cases. In addition, virus passaged 20 times in the presence of dOTC also had a parti al lysine (AAA) to arginine (AGA) exchange at position 65. These viruses sh owed 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 inhibit ors AZT, 3TC, d4T (stavudine) and ddI, the non-nucleoside RT inhibitor nevi rapine and the protease inhibitors saquinavir, ritonavir and indinavir was investigated. Two-way drug combination assays were carried out in periphera l 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.