Molecular Pharmacology, Vol 11, 105-117, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Experimental Chemotherapy of Neuroblastoma

II. Increased Thymidylate Synthetase Activity in a 5-Fluorodeoxyuridine-Resistant Variant of Mouse Neuroblastoma

¹ Departments of Neurosciences and Biology, The University of California at San Diego School of Medicine, La Jolla, California 92037, and Department of Neurology, University of Texas Health Science Center at Dallas, Southwestern Medical School, Dallas, Texas 75235

Mechanisms of resistance to chemotherapeutic agents have been investigated using mouse neuroblastoma cells grown in tissue culture as an experimental model. We 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 weeks (63 generations) without drug. Thymidine kinase is responsible for the phosphorylation of FUdR in this cell. Thus our FUdR-resistant neuroblastoma line differs from all corresponding mutants in other tumor cell lines.

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ACKNOWLEDGMENT We should like to acknowledge the assistance of Mrs. Peggy Cooney in the preparation of the manuscript.