PMPA, an acyclic nucleoside phosphonate analog, is a potent inhibitor of HIV. In the cells, PMPA is efficiently phosphorylated by intracellular kinases to produce PMPApp, the pharmacologically active metabolite. Despite its demonstrated antiviral potency, PMPA has limited cell permeability presumably resulting from the presence of two negative charges on the phosphonyl group. To enhance intracellular concentrations of PMPA, we developed a prodrug, selectively metabolized inside cells. GS-7340 (9-[(R)-2-[[[[(S)-1-(isopropoxycarbonyl)ethyl] amino] phenoxy-phosphinyl]-methoxy] propyl] adenine) is a prodrug which is orally bioavailable in dogs as the intact prodrug and has demonstrated anti-HIV activity in cell culture of over 1000-fold greater than that of PMPA. The metabolism of PMPA in peripheral blood mononuclear cells (PBMC), red blood cells (RBC) and plasma was examined following exposure of whole blood to PMPA or GS-7340 at concentrations similar to ones observed systemically following oral administration in dogs. Following 1 hour incubation with whole blood, GS-7340 was stable in plasma, produced high levels of PMPA and its phosphorylated metabolites in PBMC but not in RBC. No intact prodrug was present in PBMC. The only other species present in PBMC was monoalaninyl PMPA. The levels of PMPA and the phosphorylated metabolites were over 20 times greater than those after incubation with PMPA. The dog and human blood data were similar. The intracellular levels of PMPA and PMPApp were roughly proportional to GS-7340 over a 10-fold concentration range indicating a lack of saturability of uptake and phosphorylation. Since PMPApp is the species responsible for antiviral activity of PMPA, the high intracellular levels of PMPApp should be an important indicator of greater clinical efficacy of GS-7340.