Since nucleoside reverse transcriptase inhibitors (NRTIs) require intracellular phosphorylation to form their active triphosphates, it is preferable to combine NRTIs with different critical kinases. Thymidine kinase-1 is the critical enzyme for the phosphorylation of AZT to its monophosphates. The enzyme involved in the initial phosphorylation of the active metabolite of DAPD, DXG, is guanosine kinase. Nucleotide competition studies conducted using activated human peripheral blood mononuclear (PBM) cells performed with DAPD and AZT with concentrations between 0.1-10 µM demonstrated no competitive inhibition of DXG-triphosphate formation.