2',3'-DIDEHYDRO-3'-DEOXYTHYMIDINE - REGULATION OF ITS METABOLIC-ACTIVATION BY MODULATORS OF THYMIDINE-5'-TRIPHOSPHATE BIOSYNTHESIS

The anti-human immunodeficiency virus (anti-HN) agent 2',3'-didehydro3 '-deoxythymidine (D4T), like other 2',3'-dideoxynucleosides, requires conversion to its 5'-triphosphate to exert its pharmacological effect. Although D4T-triphosphate is unusually potent as an inhibitor of HIV- 1 reverse transcriptase, the phosphorylation of the drug at low dose l evels is inefficient because of its low affinity as an alternate subst rate for the initial phosphorylation enzyme thymidine kinase. Because thymidine kinase is under feedback regulatory control by the physiolog ical deoxynucleoside-5'-triphosphate dTTP, we examined the effect on D 4T phosphorylation and thus, potentially, on its antiviral activity, o f a variety of agents that lower intracellular dTTP pools. We found th at agents that inhibit the de novo pyrimidine biosynthetic pathway hav e the ability to increase D4T phosphorylation, the most effective bein g two inhibitors of thymidylate formation, methotrexate and 5-fluoro-2 '-deoxyuridine, compounds that block the enzymes dihydrofolate reducta se and thymidylate synthetase, respectively. Because HIV itself tacks the capacity to synthesize dTTP acid the other deoxynucleoside triphos phates essential for viral replication, combinations of D4T with modul atory agents that deplete host-cell dTTP, unlike conventional anti-HIV drug monotherapy directed solely at viral enzymes, have the ability t o inhibit replication of mutant HIV strains as well as of wild-type vi rus.