Use of 5-Fluorodeoxycytidine and Tetrahydrouridine to Exploit High Levels of Deoxycytidylate Deaminase in Tumors to Achieve DNA-and Target-Directed Therapies

In view of the 20- to 80-fold elevation of deoxycytidine-5'-phosphate (dCMP) deaminase in many human malignant tumors, we have utilized 5-fluorodeoxycytidine (FdCyd) coadministered with tetrahydrouridine (H4Urd) as a combination of antitumor agents against two murine solid tumors which possess high levels of dCMP deaminase. This approach is based on our past studies in which we demonstrated that FdCyd is an excellent substrate for mammalian 2'-deoxycytidine kinase, and that H4Urd increases the toxicity of FdCyd in the mouse. Cell culture studies utilizing 2'-deoxytetrahydrouridine which inhibits cytidine deaminase and as 2'-deoxytetrahydrouridine-5'-monophos-phate inhibits dCMP deaminase, provide indirect evidence for the pathway that we had proposed in the past, 2'-Deoxytetrahydrouridine antagonized the toxicity of FdCyd to a greater extent than did H4Urd and showed marked antagonism in cytidine deaminase-deficient cells. Cell lines lacking both cytidine and 2'-deoxycytidine-5'-monophosphate deaminase were markedly resistant to FdCyd. Thymidine and deoxyuridine antagonized toxicity in a manner consistent with the proposed pathway of anabolism of FdCyd and consistent with its resulting in the inhibition of thymidylate synthetase. We have established the efficacy of FdCyd + H4Urd chemotherapy utilizing adenocarcinoma 755 and Lewis lung carcinoma in C57BL x DBA/2 Fi mice. An example of an optimum schedule versus Lewis lung carcinoma is FdCyd, 10 to 12 mg/kg, plus H4Urd, 25 mg/kg, coadministered simultaneously, once per day on Days 1 io 7 after tumor implantation. Tumor inhibitions on Days 12, 14, and 16 were 95, 90, and 80%, respectively, with 8% maximum weight loss. Comparative studies were undertaken only with Lewis lung carcinoma and it was established that FdCyd + H4Urd surpasses the efficacies of 5-fluorouracil and 5-fluorodeoxyuridine as well as FdCyd when administered without H4Urd. We propose that the administration of FdCyd with H4Urd can result in preferential, tumor-directed conversion of a nontoxic nucleoside analogue to a toxic antimetabolite by an enzyme that is markedly elevated in human tumor tissue. The analogues of deoxycytidine are resistant to catabolism and are anabolized by a different subset of enzymes than are 5-fluorouracil or 5-fluorodeoxyuridine; therefore, it is a novel approach. Not only are there intrinsic selectivity, metabolic stability, and the advantages that accrue from prodrug therapy in this strategy, but in addition, the potential for an exclusively DNA-directed effect exists. This is in contrast to approaches with 5-fluorouradl and 5-fluorodeoxyuridine, in which, in addition to DNA effects, parallel effects on RNA metabolism and processing occur.