HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 VIRAL BACKGROUND PLAYS A MAJOR ROLE IN DEVELOPMENT OF RESISTANCE TO PROTEASE INHIBITORS

The observed in vitro and in vivo benefit of combination treatment wit h anti-human immunodeficiency virus (HIV) agents prompted us to examin e the potential of resistance development when two protease inhibitors are used concurrently, Recombinant HIV-1 (NL4-3) proteases containing combined resistance mutations associated with BMS-186318 and A-77003 (or saquinavir) were either inactive or had impaired enzyme activity, Subsequent construction of HIV-1 (NL4-3) proviral clones containing th e same mutations yielded viruses that were severely impaired in growth or nonviable, confirming that combination therapy may be advantageous . However, passage of BMS-186318-resistant HIV-1 (RF) in the presence of either saquinavir or SC52151, which represented sequential drug tre atment, produced viable viruses resistant to both BMS-186318 and the s econd compound. The predominant breakthrough virus contained the G48V/ A71T/V82A protease mutations. The clone-purified RF (G48V/A71T/V82A) v irus, unlike the corresponding defective NL4-3 triple mutant, grew wel l and displayed cross-resistance to four distinct protease inhibitors. Chimeric virus and in vitro mutagenesis studies indicated that the RF -specific protease sequence, specifically the lie at residue 10, enabl ed the NL4-3 strain with the triple mutant to grow, Our results clearl y indicate that viral genetic background will play a key role in deter mining whether cross-resistance variants will arise.