Biochemical aspects of chemotherapy of mouse colon carcinoma: fluoropyrimidines and pyrazofurin.

Abstract

Fluorouracil was metabolized to nucleotides and incorporated into RNA of mouse colon carcinomas and normal tissues. No significant difference was observed in three mouse colon tumors, but the extent of incorporation of the analog into RNA of normal colon tissue was lower than that in colon tumors. Fluorouracil phosphoribosyltransferase activity, although low, was observed to be about five times higher in mouse colon carcinomas than it was in normal colon tissue. Fluorouracil and 5-fluoro-2′-deoxyuridine, as anticipated, inhibited the incorporation of [6-3H]-2′-deoxyuridine into DNA of colon carcinomas and normal tissues. Colon and spleen tissue made a more rapid recovery of capacity for DNA synthesis than did colon tumors. In normal tissues examined the recovery from inhibition of DNA synthesis by fluorodeoxyuridine appeared to be more rapid than was recovery from fluorouracil inhibition. Effects of pyrazofurin, an inhibitor of orotidylic acid decarboxylase, on pyrimidine synthesis in mouse colon carcinomas and in normal tissues was analyzed by means of high-pressure liquid chromatography. Consequences of pyrazofurin-induced inhibition of pyrimidine biosynthesis in mouse colon carcinomas were evident in decreased pools of pyrimidine ribonucleotides. Orotidylic acid did not accumulate behind this block but elevated levels of orotic acid and orotidine were observed in acid-soluble extracts. The maximum reduction in uracil ribonucleotide pools was observed 24 hours after pyrazofurin treatment. Recovery of uracil ribonucleotide pools was evident within 48 hours and was complete 72 hours after treatment. The maximum levels of orotic acid and orotidine in colon carcinomas were attained 24 hours after treatment; these levels remained elevated above control levels for 72 hours after pyrazofurin treatment. The pool of uracil ribonucleotides was not depressed in colon and spleen tissue from pyrazofurin-treated animals; nevertheless, pronounced elevation of orotic acid and orotidine levels in these normal tissues was observed. These results reveal differences in effects of pyrazofurin on pyrimidine ribonucleotide pools in mouse colon carcinoma and in colon tissue. These differences may be due in part to availability or extent of utilization of exogenous pyrimidines or precursors of pyrimidines. Such differences may be exploited by means of scheduled combination chemotherapy with inhibitors of pyrimidine synthesis and pyrimidine analogs.