Synergistic activity of purine metabolism inhibitors in cultured human tumor cells.

Cytotoxic effects of 3-deazaguanosine (3-DGUO) result from the inhibition of DNA synthesis and incorporation of the drug into DNA. Synergistic antiproliferative effects of a combination of 3-deazaguanosine and 2-beta-D-ribofuranosylthiazole-4-carboxamide, a potent inhibitor of inosine monophosphate dehydrogenase, was observed in human tumor cells. Inosine reversed the antiproliferative effects of the 3-deazaguanosine but not 2-beta-D-ribofuranosylthiazole-4-carboxamide. 3-Deazaguanosine monophosphate was shown to inhibit the activity of the de novo purine synthesis enzyme, 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide transformylase. The data suggested a cytotoxic effect of 3-DGUO associated with the inhibition of de novo purine synthesis by drug nucleotides, an effect which may account for the synergistic action noted.