Increased drug susceptibility of HIV-1 reverse transcriptase mutants containing M184V and zidovudine-associated mutations: analysis of enzyme processivity, chain-terminator removal and viral replication

The presence of the HIV reverse transcriptase (RT) resistance mutation, M18 4V, induced by lamivudine and abacavir treatment results in increased tenof ovir, adefovir and zidovudine susceptibility for HIV-1 with zidovudine-asso ciated RT mutations in vitro, Treatment with oral prodrugs of tenofovir and adefovir has resulted in substantial HIV-1 RNA reductions in antiretrovira l-experienced patient populations who have lamivudine- and zidovudine-resis tant HIV-1, An enzymatic analysis was undertaken to elucidate the mechanism s of altered drug susceptibilities of HIV-1 containing zidovudine-associate d mutations in the presence or absence of M184V, The inhibition constants ( K-i) for the active metabolites of tenofovir, adefovir and zidovudine did n ot vary significantly between recombinant mutant and wild-type RT enzymes. Although increased removal of chain-terminating inhibitors by pyrophosphoro lysis and ATP-dependent unblocking correlated with reduced susceptibility o f viruses with zidovudine-associated mutations, a reduction in the removal of chain-terminators was not observed, which would explain the increased dr ug susceptibility of mutants containing M184V plus zidovudine-associated mu tations. However, analyses of single-cycle processivity of the mutant RT en zymes on heteropolymeric RNA templates showed that all M184V-containing mut ant RT enzymes were less processive than wild-type RT, most notably for mut ants expressing both zidovudine-associated mutations and M184V, Similarly, the in vitro replication capacity of a mutant virus expressing a zidovudine -associated mutation and M184V was significantly reduced compared with wild -type virus. The observed decrease in enzymatic processivity of the M184V-e xpressing RT enzymes might result in decreased viral replication, which the n might contribute to the increased drug susceptibility of HIV-1 expressing these RT mutations.