Experimental chemotherapy of neuroblastoma. II. Increased thymidylate synthetase activity in a 5 fluorodeoxyuridine resistant variant of mouse neuroblastoma

Mechanisms of resistance to chemotherapeutic agents were investigated using mouse neuroblastoma cells grown in tissue culture as an experimental model. The authors report the characterization of a variant which can grow in the presence of 2000 times more 5 fluorodeoxyuridine (FUdR) than the sensitive parent strain. This mutant displays an 8 fold elevation in thymidylate synthetase, but its drug retention and pyrimidine kinase, phosphorylase, and phosphoribosyltransferase activities are identical with those of sensitive cells. Drug retention is temperature dependent, inhibited by thymidine, and blocked by dinitrophenol but not ouabain, suggesting a need for cytoplasmic phosphorylation. Thymidylate synthetase isolated from either sensitive or resistant cells is strongly inhibited by 5 fluorodeoxyuridine 5 phosphate, and thymidine protects cells against the toxic effects of FUdR. Both the resistance of the variant to FUdR and its elevated thymidylate synthetase specific activity are unstable, largely disappearing after 9 wk (63 generations) without drug. Thymidine kinase is responsible for the phosphorylation of FUdR in this cell. Thus this FUdR resistant neuroblastoma line differs from all corresponding mutants in other tumor cell lines.