MECHANISMS OF SENSITIVITY AND RESISTANCE OF MURINE TUMORS TO 5-FLUOROURACIL

Abstract

The biochemical basis for the resistance of murine leukemia p-388 cells to 5-fluorouracil (FUra) was systematically investigated by examining the transport and metabolism of FUra, or its anabolites, and the inhibition of enzymes and processes known to be affected by the drug. Of these parameters, only the following 3 were altered significantly in the resistant line: the enzyme required for the phosphorylation of uridine 5''-monophosphate to uridine 5''-diphosphate was present at a significantly lower specific activity in the resistant line than in its sensitive counterpart; the rates of generation and persistance of 5-fluoro-2''-deoxyuridine 5''-monophosphate were significantly lower and shorter in the variant; there was a 1.6- and 3-fold decrease in the incorporation of FUra into polyadenylic acid-containing RNA and polyadenylic acid-lacking RNA, respectively, in resistant vs. sensitive cells. These findings suggest a dual mechanism for resistance to FUra in these leukemic cells, namely, a depressed capacity to generate di- and triphosphates of the riboside and deoxyriboside of the drug leading to lower pools of the proximate antimetabolite, fluorouridine 5''-triphosphate, and accelerated excretion of 5-fluoro-2''-deoxyuridine 5''-monophosphate, so that thymidylate synthetase is perturbed in a less than lethal way.