Virologic and enzymatic studies revealing the mechanism of K65R- and Q151M-associated HIV-1 drug resistance towards emtricitabine and lamivudine

Emtricitabine (FTC) and lamivudine (3TC) are deoxycytidine analogues with potent and selective inhibition of human immunodeficiency virus (HIV) and hepatitis B virus (HBV) replication. The K65R mutation in the HIV reverse transcriptase (RT) confers reduced susceptibility to 3TC, ddC, ddI, abacavir, and tenofovir in vitro. The Q151M mutation confers reduced susceptibility to many of the approved anti-HIV nucleoside analogues with the exception of 3TC and tenofovir. The double mutation K65R/Q151M has been shown to be more resistant to many NRTIs than either of the single mutations alone. In this study, we measured the antiviral activity of FTC and 3TC against HIV-1 containing K65R, Q151M, and K65R/Q151M mutations. We also studied the steady-state kinetic properties for the inhibition of dCTP incorporation by FTC 5'-triphosphate (TP) and 3TC-TP In addition, we measured the incorporation of dCTP, FTC-TP, and 3TC-TP into a random sequence DNA/DNA primer/template by the HIV-1 RTs using pre-steady-state kinetic analysis. Finally, we studied the incorporation of these deoxycytidine analogues into a HIV-1 genomic DNA/DNA primer/template by K65R HIV-1 RT to address certain concerns associated with DNA sequence specificity. Overall, this study demonstrated that K65R and K65R/Q151M related drug resistance to FTC and 3TC was mainly due to a significant decrease in the rate of incorporation. There was little to no effect on the binding affinities of the mutant HIV-1 RTs for the deoxycytidine analogues. The Q151M mutation remained sensitive to both FTC and 3TC in both cell culture and enzymatic assays. At a molecular level, FTC-TP was incorporated at least as efficiently as 3TC-TP for all of the HIV-1 RT and primer/templates tested.

Dioxolane guanosine, the active form of the prodrug diaminopurine dioxolane, is a potent inhibitor of drug-resistant HIV-1 isolates from patients for whom standard nucleoside therapy fails

Amdoxovir ([-]-beta-D-2,6-diaminopurine dioxolane [DAPD]) is a nucleoside reverse transcriptase inhibitor (NRTI) with activity against HIV-1. DAPD is deaminated in vivo by adenosine deaminase to (-)-beta-D-dioxolane guanosine (DXG), a highly active anti-HIV compound. The median 50% effective concentrations (EC 50 ) +/- SD (representing antiviral activity against a laboratory-derived HIV-1 isolate) for DAPD and DXG in peripheral blood mononuclear cells were 4.0 +/- 2.2 micromol/L and 0.25 +/- 0.17 micromol/L, respectively. The 50% cytotoxic dose (CC 50 ) of both DAPD and DXG was >500 micromol/L. Recombinant viruses and clinical isolates of HIV-1 from patients for whom NRTI therapy and/or nonnucleoside reverse transcriptase inhibitor (NNRTI) combination therapies failed remained susceptible to inhibition by DXG (less than fourfold change in EC 50). Similar analysis showed that recombinant viruses harboring mutations known to confer resistance to NRTIs (zidovudine, lamivudine, and abacavir) and NNRTIs (efavirenz and nevirapine) as well as the multidrug resistance-associated mutation Q151M and double codon insertions (SS and SG) were also susceptible to inhibition by DXG. Resistance to DXG was observed only in recombinant isolates containing the 65R and 151M double mutations. Phenotypic analysis of a site-directed mutant containing only the 151M mutation demonstrated moderate resistance to DXG (<10-fold change in EC 50). We also examined site-directed mutants containing only L74V or K65R, the characteristic resistance mutations for DXG. The L74V mutant remained susceptible to inhibition by DXG, and the K65R mutant demonstrated moderate resistance to DXG.