POL MUTATIONS CONFERRING ZIDOVUDINE AND DIDANOSINE RESISTANCE WITH DIFFERENT EFFECTS IN-VITRO YIELD MULTIPLY RESISTANT HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ISOLATES IN-VIVO

Specific mutations in the human immunodeficiency virus type 1 (HIV-1) pol gene that cause zidovudine (3'-azido-2',3'-dideoxythymidine; AZT) and didanosine (2',3'-dideoxyinosine; ddI) resistance were studied. Th e 50% inhibitory concentrations (IC50s) of nucleosides for cloned viru ses containing these mutations were compared with the IC50s of the cor responding triphosphate analogs for mutant recombinant-expressed rever se transcriptases (RTs). Changes in ddATP inhibition of RNA-dependent DNA polymerase activity fully accounted for the ddl resistance of the virus caused by a Leu-74 --> Val substitution in RT, including an augm entation by the AZT-selected substitutions Thr-215 --> Tyr and Lys-219 --> Gln in RT. In contrast, the AZT-selected substitutions studied di d not cause as great a change in the IC50 of AZT-triphosphate (AZT-TP) for polymerase as they did in the IC50 of AZT for mutant virus. In ad dition, the mutation at codon 74 suppressed AZT resistance in the viru s caused by the mutations at codons 215 and 219 but did not suppress t he AZT-TP resistance of enzyme containing these same mutations in RT. The mutation at codon 74 was found in clinical isolates whether or not the patient had received AZT prior to starting ddl therapy. AZT resis tance coexisted with ddl resistance following acquisition of Leu-74 - Val in three clinical isolates, indicating that the suppressive effect of Val-74 on the AZT resistance of the virus does not occur in all ge netic contexts. When this suppression of AZT resistance was seen in th e virus, Val-74 did not appear to cause mutually exclusive changes in AZT-TP and ddATP binding to RT in vitro. The results of the in vitro e xperiments and characterization of clinical isolates suggest that ther e are differences in the functional effects of these AZT and ddl resis tance mutations.