Background:  Many mutations selected by reverse transcriptase inhibitor (RTI) have been shown to correlate with decreased virologic response. Although the lamivudine (3TC)/emtricitabine (FTC)/abacavir (ABC) mutation M184V can diminish the effect of thymidine analog mutations (TAM—41L, 67N, 70R, 210W, 215Y, 219E/Q) on phenotypic sensitivity to ZDV, d4T, or TDF, other potential interactions between RTI mutations have not been rigorously explored.

Methods:  The RT gene (1.3 kb) was cloned from plasma HIV-1 RNA from treatment-experienced patients, and used to generate full-length proviral clones (pNL4-3 chimeras) for analysis of phenotypic drug resistance (Phenosense, ViroLogic). Specific modifications of key RT amino acids were performed by site-directed mutagenesis.

Results:  When 184V was introduced into HIV strains with multiple TAM combinations, there was an increase in IC₅₀ fold change for ABC, ddI, and 3TC, and a decrease for tenofovir (TDF), zidovudine (ZDV), and stavudine (d4T). The effect of introduction of 74V in HIV-1 strains with multiple TAM showed similar effects, but with a few differences.  Similar to 184V, introduction of 74V caused an increase in fold change to ABC, ddI, 3TC, and a reduction in fold change to ZDV and TDF, but in contrast to M184V, caused increases in fold change IC₅₀ to d4T. In addition, 74V tended to improve replication capacity, whereas replication capacity generally decreases with introduction of 184V. There also appears to be interactions between some mutations selected by nucleoside reverse transcriptase inhibitor (NRTI) and the non-NRTI (NNRTI) mutations mentioned above. Overall, when we restored the NNRTI mutations 98S, 101E, and 190S to wild type, the viral clones became more sensitive to NRTI, as well as to NNRTI. In a virus strain with the TAM combination of 41L, 210W, and 215Y, together with 74V, restoration of these NNRTI mutations to wild type showed a shift in TDF fold change from 1.24 to 0.61. Virus strains with other TAM combinations and 74V had a fold change < 0.4. The introduction of 103N into the background of multiple TAM ± 184V or 74V had no effect on NRTI sensitivity.

Conclusions:  Interactions of RTI mutations can affect the phenotypic sensitivity of RTI, both for 184V and 74V, as well as for certain combinations of NNRTI mutations. Demonstration of potential increases in phenotypic susceptibility to TDF and ZDV related to 74V in viruses having other drug resistance patterns, and exploration of the mechanism of mutational interactions that may affect drug sensitivity and clinical outcomes are warranted.

Keywords: Reverse Transcriptase Inhibitor (RTI); Drug Resistance; Phenotypic Sensitivity