PyrazofurinEnhancement of 5-Azacytidine AntitumorActivity in L5178Y and Human Leukemia Cells1

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.These triphosphateribonu cleotides are known inhibitors of uridine-cytidine kinase. Exogenousuridineis rapidlyphosphorylatedto uridine5'monophosphatebythisenzymeinthe presenceof PFand therebycircumventsthe lethaleffectof PF.Therefore,the effect of PF treatment on the cellular metabolism of 5azacytidine (5-aza-C), a nucleoside drug analog of cyti dine, was studied in the experimentaltumorcell L5178Y and humanleukemiacells. PF, in concentrations(5 x 106 N)whichinhibitedoroti dybatedecarboxybaseand produced 80% reduction in uridinetriphosphateresultedin more rapidaccumulation of 5-aza-C into, and enhanced killingof, rapidlydividing leukemiacells. Ribonucleotideanalysisby high-pressure liquidchromatographydemonstrateda 400%increaseof 5-aza-cytidine 5'-triphosphate in PF-treated cells. This triphosphorybated form of 5-aza-C was incorporated in greater quantitiesinto RNAthat resultedin a 40% reduc tion of [3Hjleucineincorporationinto protein, indicating that the synergisticlethal effectsobservedwiththis drug sequenceof PF —+ 5-aza-Cwere by augmentedinhibition of proteinsynthesis,the previouslyproposedmajoranti tumormechanismof 5-aza-C. PF followedby 5-aza-Cfor thetreatmentof rapidlyproliferatinghumanleukemiamay be a useful sequential drug combinationwhen these patientshave failed morestandardformsof antileukemic therapy.