Background:  IDX899, a potent and selective NNRTI inhibitor of HIV-1 replication in cell culture is in early clinical development. Here we describe in vitro selection and phenotypic cross-resistance experiments that elucidate the resistance profile of IDX899 compared with efavirenz (EFV).
Methods:  Standard in vitro HIV-1 (subtype B, strain BH10) drug-resistance selection studies were performed in MT-2 cells under escalating concentrations of both drugs for >150 days; studies with other HIV-1 strains and subtypes have been initiated. Resistance mutations were characterized by direct population sequencing. Pools of selected viruses, normalized by p24 production, were tested against each drug to determine cross-resistance patterns. Contributions of individual, double, or triple mutations were further defined by phenotyping >25 HIV-1 (BH10) viruses containing site-directed mutations selected by the NNRTI class.
Results:  Different resistance profiles for the 2 drugs were revealed by 3 independent in vitro selection studies and the associated phenotypic testing. For EFV selection, pooled viruses containing a G190A change at passage 8 (P8) or a V179D mutation at P14 were enough to create virus with high EFV level resistance. EFV-resistant viruses selected through P28 and carrying as many as 4 resistance mutations typically remained sensitive to IDX899. For IDX899 selection, 26 to 30 passages and at least 3 mutations were typically required to generate virus with >100-fold resistance. Early emergent IDX899-resistant viruses remained sensitive to EFV until 3 or more mutations were selected. Virus containing V90I, S134I, and Y181C mutations at P26 showed 101-fold resistance and virus containing E138K, Y181I, and M230L mutations at P31 showed >1136-fold resistance to IDX899. These results have been confirmed using site-directed mutants which also suggested that IDX899 displayed only mild to moderate diminished susceptibility (0.5- to 15-fold) with all single and double mutants tested to date.
Conclusions:  Selection of HIV-1 isolates resistant to IDX899 took longer and required more mutations than EFV. IDX899 retained good activity against EFV-resistant isolates and the results obtained with site-directed mutants indicate differences in resistance profiles between the 2 drugs. IDX899 is a promising second-generation NNRTI which is entering clinical evaluation in HIV-positive individuals.