Apricitabine is a novel deoxycytidine analog reverse transcriptase inhibitor. In vitro apricitabine competes with other deoxycytidine analogues for intracellular phosphorylation mediated by deoxycytidine kinase. The topic of this study, the effect of concomitant administration of apricitabine and lamivudine on the plasma and intracellular pharmacokinetics of the two compounds, was investigated in healthy volunteers. Participants (n = 21; age, 18 to 30 years) received apricitabine at 600 mg twice daily, lamivudine at 300 mg once daily, and the two treatments in combination for 4 days each in random order. Plasma, urine, and intracellular pharmacokinetics were assessed on day 4 of each treatment period. Apricitabine was rapidly absorbed after oral administration, with peak concentrations being attained after a mean of 1.76 h. Coadministration with lamivudine had no significant effect on the plasma and urine pharmacokinetics of apricitabine. However, the formation of apricitabine triphosphate in peripheral blood mononuclear cells was markedly reduced after the coadministration of apricitabine and lamivudine than after the administration of apricitabine alone: the area under the concentration-time curve from 0 to 12 h for apricitabine triphosphate during combination treatment was ca. 15% of that seen after the administration of apricitabine alone. In contrast, apricitabine had no effect on the plasma pharmacokinetics of lamivudine or on the formation of lamivudine triphosphate in peripheral blood mononuclear cells. These results are consistent with in vitro findings that lamivudine inhibits the intracellular phosphorylation of apricitabine. In conjunction with similar in vitro observations for emtricitabine and apricitabine, these results suggest that apricitabine should not be coadministered with other deoxycytidine analogues for the treatment of human immunodeficiency virus infection.