Abstract
5-Chlorodeoxycytidine, a halogenated pyrimidine nucleoside analog, was coadministered with several modulators of pyrimidine metabolism, in order to achieve tumor selective radiosensitization in murine tumor models. The modulators were tetrahy-drouridine (an inhibitor of cytidine deaminase), PALA (an inhibitor of aspartate transcarbamylase), 5-fluorodeoxycytidine (which acts to inhibit thymidylate synthetase, and also produces single and double strand breaks in DNA, through incorporation of FdUTP and dUTP into DNA, and subsequent repair) and 5-benzylacyclourindine (an inhibitor of uridine phosphorylase). Several experimental approaches were utilized to determine the extent of tumor radiosensitization and selectivity. Studies measuring the levels of the various metabolites of CldC one hour following CldC administration with or without the various modulators of metabolism showed that is is possible to selectively generate CldU and CldUMP in the tumor compared to normal tissues. DNA incorporation studies measuring the extent of substitution of CldU (derived from CldC) for thymidine also demonstrated the potential for tumor selectivity. Incorporation positively correlated with the levels of cytidine deaminase and dCMP deaminase (two enzymes involved in the metabolism of CldC) in a particular tissue. The levels of these enzymes have been reported to be elevated in many human malignancies. Two model systems were used to assay tumor radiosensitization. The optimum CldC treatment protocol (with tetrahydrouridine, PALA, and 5-fluorodeoxycytidine) yielded a two-fold dose enhancement ratio with the RIF-1 tumor model irraditated in vivo (cell survival was assayed via clonogenic assay). Utilizing the Lewis lung carcinoma model and measuring tumor growth over time, tumor growth was completely arrested for six days following irradiation, with the CldC protocol. When IdU was used at an equimolar dose greater toxicity was observed. These studies indicate that greater selectivity and increased tumor radiosensitization can be achieved with halogenated pyrimidines when modulators of their metabolism are coadministered.