Intracellular Metabolism of 3′-azido-3′-deoxythymidine and 2′,3′-dideoxyinosine in Combination in the Absence and Presence of Ribavirin

We examined the intracellular phosphorylation of 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxyinosine (ddl) and the effects on rNTP and dNTP pools when AZT and ddl were incubated separately and in combination in lymphocytes. We also compared the effect of adding ribavirin (RBV) to the two-drug combination of AZT + ddl. AZT and ddl, used singly or in combination, had no effect on the dNTP pools of CEM lymphoblastoid cells. Neither did the combination of AZT + ddl have any effect on the rNTP pools. RBV, a known inhibitor of IMP dehydrogenase, caused a decrease in GTP and an increase in dTTP whether incubated alone or with the drug combination of AZT + ddl. The addition of AZT + ddl therefore did not alter the effects of RBV upon cellular nucleotide pools. AZT was phosphorylated to a much greater extent than ddl. The activation of ddl to ddA-TP was increased 2-fold in the presence of AZT, whereas AZT phosphorylation was unchanged when combined with ddl. This increase in ddl activation may explain in part the synergistic antiviral activity of the combination of AZT + ddl. The increased activation was not due to increased phosphorylation of ddl resulting from IMP dehydrogenase inhibition. The addition of 10 μm RBV to the two-drug combination of AZT + ddl did not change the intracellular phosphorylation of AZT or ddl. The activation of ddl to ddA-TP, when combined with AZT, appeared to be maximal and could not be further increased by addition of RBV to this combination.

Selective protection of toxicity of 2',3'-dideoxypyrimidine nucleoside analogs by beta-D-uridine in human granulocyte-macrophage progenitor cells

beta-D-Uridine protected human granulocyte-macrophage lineage cells in both semi-solid (granulocyte-macrophage colony-forming units, CFU-GM) and liquid cultures against the toxic effects of 3'-azido-3'-deoxythymidine (AZT), 3'-fluoro-3'-deoxythymidine (FLT) and a combination of AZT and FLT, without impairment of the activities of these respective drugs against human immunodeficiency virus (HIV) replication. In addition, beta-D-uridine also protected human CFU-GM against toxicity of the in vivo AZT metabolite, 3'-amino-3'-deoxythymidine (AMT). Beta-L-uridine and alpha-D-uridine, two stereoisomers of the natural form, and the base uracil, were unable to protect cells against either AZT or FLT toxicity, whereas beta-D-uridine-5'-bis(SATE)phosphotriester, a prodrug of beta-D-uridine-5'-monophosphate, successfully protected cells against AZT toxic effects, suggesting that beta-D-uridine needs to be metabolized to its nucleotides to exert a pharmacological effect. These data suggest in addition that AZT, FLT and AMT share a common target site(s) of toxicity involved in myelosuppression.