Differential impact of thymidine analogue mutations on emtricitabine and lamivudine susceptibility.

The impact of drug resistance-associated mutations on subsequent antiretroviral therapy is an important consideration in managing treatment-experienced, HIV-1-infected patients. Lamivudine (3TC) and emtricitabine (FTC) are structurally related nucleoside reverse transcriptase inhibitors (NRTIs) approved for use in HIV-1-infected individuals. To evaluate whether susceptibility differences exist between lamivudine and emtricitabine, the phenotypic impact of common NRTI resistance-associated mutations was compared in HIV-1 from patient samples with paired FTC and 3TC susceptibility results. FTC phenotypic susceptibility was more greatly impacted than 3TC susceptibility in the presence of thymidine analogue mutations (TAMs), as the mean fold-change values were higher for FTC than for 3TC in groups of samples containing TAMs (P < 0.001 for 6 of 7 groups). For samples with K65R, L74I/V, or Q151M mutations, the phenotypic impact was similar, as the mean fold-change was not significantly different between drugs. Although the long-term clinical significance of these differences is unclear, they may suggest differential efficacy in some patients with prior NRTI experience, especially those with HIV harboring TAMs.