PyrazofurinEnhancementof 5-AzacytidineAntitumorActivityin L5178Y and HumanLeukemiaCells1

Pyrazofurin(P9, which is an inhibitorof orotidybate decarboxylase (the conversion of orotidine 5'-monophos phate to uridine 5-monophosphate)results in marked reductions of intracellular levels of uridine triphosphate and cytidine5'-triphosphate. Thesetriphosphateribonu cleotides are known inhibitors of uridine-cytidine kinase. Exogenous uridineis rapidlyphosphorylated to uridine5'- monophosphate bythisenzymeinthe presenceof PFand therebycircumvents the lethaleffectof PF.Therefore,the effect of PF treatment on the cellular metabolism of 5- azacytidine (5-aza-C), a nucleoside drug analog of cyti dine, was studiedin the experimentaltumorcell L5178Y andhumanleukemiacells. PF, in concentrations (5 x 106 N)whichinhibitedoroti dybatedecarboxybase and produced80% reductionin uridinetriphosphateresultedin morerapidaccumulation of 5-aza-Cinto, and enhancedkillingof, rapidlydividing leukemiacells. Ribonucleotide analysisby high-pressure liquidchromatography demonstrated a 400%increaseof 5-aza-cytidine5'-triphosphatein PF-treatedcells. This triphosphorybated form of 5-aza-C was incorporated in greaterquantitiesintoRNAthat resultedin a 40%reduc tion of (3Hjleucineincorporation into protein,indicating thatthe synergisticlethaleffectsobservedwiththisdrug sequenceof PFâ€"+ 5-aza-Cwere by augmentedinhibition of proteinsynthesis,the previouslyproposedmajoranti tumormechanismof 5-aza-C.PF followedby 5-aza-Cfor thetreatmentof rapidlyproliferating humanleukemiamay be a useful sequentialdrug combinationwhen these patientshavefailedmorestandardformsof antileukemic therapy.