Pyrazofurin (PF), which is an inhibitor of orotidylate decarboxylase (the conversion of orotidine 5′-monophosphate to uridine 5′-monophosphate) results in marked reductions of intracellular levels of uridine triphosphate and cytidine 5′-triphosphate. These triphosphate ribonucleotides are known inhibitors of uridine-cytidine kinase. Exogenous uridine is rapidly phosphorylated to uridine 5′-monophosphate by this enzyme in the presence of PF and thereby circumvents the lethal effect of PF. Therefore, the effect of PF treatment on the cellular metabolism of 5-azacytidine (5-aza-C), a nucleoside drug analog of cytidine, was studied in the experimental tumor cell L5178Y and human leukemia cells.
PF, in concentrations (5 × 10−6 m) which inhibited orotidylate decarboxylase and produced 80% reduction in uridine triphosphate resulted in more rapid accumulation of 5-aza-C into, and enhanced killing of, rapidly dividing leukemia cells. Ribonucleotide analysis by high-pressure liquid chromatography demonstrated a 400% increase of 5-aza-cytidine 5′-triphosphate in PF-treated cells. This triphosphorylated form of 5-aza-C was incorporated in greater quantities into RNA that resulted in a 40% reduction of [3H]leucine incorporation into protein, indicating that the synergistic lethal effects observed with this drug sequence of PF → 5-aza-C were by augmented inhibition of protein synthesis, the previously proposed major antitumor mechanism of 5-aza-C. PF followed by 5-aza-C for the treatment of rapidly proliferating human leukemia may be a useful sequential drug combination when these patients have failed more standard forms of antileukemic therapy.
This work was supported by Grant CA08341-12 from the National Cancer Institute and by Grant ACS-IN-31-P-07 from the American Cancer Society.