Evolution of AZT resistance in HIV-1: The 41-70 intermediate that is not observed in vivo has a replication defect

The human immunodeficiency virus type 1 (HIV-I) is notorious for its abilit y to evolve drug-resistance in patients treated with potent antivirals. Res istance to inhibitors of the viral reverse transcriptase (RT) enzyme is fre quently mediated by a single amino acid substitution within RT. Resistance against the nucleoside analogue AZT is remarkable in that multiple amino ac id changes accumulate over time to yield virus variants with high-level dru g resistance. We now report that in addition to drug-resistance properties, the relative replication capacity of the virus variants affects the evolut ion of AZT resistance. Some of the typical AZT-resistance mutations have a negative impact on virus replication, and the 41-70 double mutant was found to represent a particularly poor virus. Furthermore, introduction of addit ional AZT-resistance mutations (41-70-215) leads to nearly complete restora tion of virus replication. These results may explain the absence of the 41- 70 double mutant in clinical samples and indicate that the evolution of AZT resistance is also influenced by virus replication parameters. Prolonged p assage of the replication-impaired 41-70 virus in the absence of AZT yielde d several fast-replicating variants. These revertants have compensatory cha nges in the RT polymerase, some of which have been observed previously in A ZT-treated patients. Because we could select for these changes without drug pressure, these changes are likely to Improve the RT enzyme function and t he HIV-I replication capacity, (C) 2001 Academic Press.