tetrahydrouridine and 5-benzylacyclouridine

Bolus doses of 5-chlorodeoxycytidine (CldC) administered with modulators of pyrimidine metabolism, followed by X-irradiation, resulted in a 2-fold dose increase effect against RIF-1 tumors in C3H mice. Pool size studies of the fate of [14C]-CldC in BDF1 mice bearing Sarcoma-180 tumors, which demonstrated the rapid formation of 5-chlorodeoxycytidylate (CldCMP), and incorporation of CldC as such in RIF-1 tumor DNA, indicate that CldC is a substrate for deoxycytidine kinase, as our past Km studies have shown. Our data indicate that 5-chlorodeoxyuridine triphosphate (CldUTP) accumulates from both the cytidine deaminase-thymidine kinase pathway, as well as from the deoxycytidine kinase-dCMP deaminase pathway, in tumor tissue. As shown in a previous study, tetrahydrouridine (H4U), a potent inhibitor of cytidine deaminase, can effectively inhibit the enzyme in the normal tissues of BDF1 mice. When H4U was administered with the modulators N-(phosphonacetyl)-L-aspartic acid (PALA) and 5-fluorodeoxycytidine (FdC), the levels of CldC-derived RNA and DNA directed metabolites increased in tumor and decreased in normal tissues compared to when CldC was administered alone. These modulators inhibit the de novo pathway of thymidine biosynthesis, lowering thymidine triphosphate (TTP) levels, which compete with CldUTP for incorporation into DNA. 5-Benzylacyclouridine (BAU), an inhibitor of uridine phosphorylase, was also utilized. DNA incorporation studies using C3H mice bearing RIF-1 tumors showed that the extent of incorporation of 5-chlorodeoxyuridine (CldU) into DNA correlates with the levels of cytidine and dCMP deaminases; this is encouraging in view of their high activity in many human malignancies and the low activities in normal tissues, including those undergoing active replication. Up to 3.9% replacement of thymidine by CldU took place in RIF-1 tumors, whereas incorporation into bone marrow was below our limit of detection. CldC did not result in photosensitization under conditions in cell culture in which radiosensitization to X rays was obtained. Thus, the combination of CldC with modulators of its metabolism has potential as a modality of selective radiosensitization for ultimate clinical use in a wider range of tumors than those of the brain.

Topics: Animals; Aspartic Acid; Combined Modality Therapy; Cytidine Deaminase; Deoxycytidine; DNA, Neoplasm; Drug Therapy, Combination; Mice; Mice, Inbred C3H; Phosphonoacetic Acid; Radiation-Sensitizing Agents; Sarcoma, Experimental; Tetrahydrouridine; Uracil; Uridine Phosphorylase

Administration of high-dose uridine or cytidine (3500 mg/kg) resulted in severe hypothermia of 6-10 degrees C in mice. This effect of uridine was observed in three different mouse strains, C57B1/6, Balb/c, and Swiss. A high-dose of uridine also caused hypothermia in Wistar rats. Co-infusion of uridine with benzylacyclouridine, an inhibitor of uridine phosphorylase, partially prevented uridine-mediated hypothermia in mice. A low dose of uridine (100 mg/kg) resulted in a slight increase in temperature. Plasma pharmacokinetics of uridine (at 3500 mg/kg) were studied in two mouse strains, C57B1/6 and Balb/c, and those of cytidine only in C57B1/6 mice. Peak plasma concentrations of uridine in both strains after uridine administration were about 20 mM (at 30-60 min). The peak plasma concentration of cytidine in C57B1/6 mice after cytidine administration was about 12 mM and that of uridine, 1.3 mM. The mean residence time for uridine was about 105 min. The area under the plasma concentration-time curve for uridine was about 50 mmol h/l, and that for cytidine, about 25 mmol h/l. In various tissues of C57B1/6 mice the levels of uridine, uracil and total uracil and cytosine nucleotide pools were determined before and 2 h after uridine administration. Uridine levels increased about 53-fold in liver, about 70-fold in a colon tumor, and only about 7-fold in brain, while the corresponding uracil levels increased about 9-fold, 4-fold and 11-fold, respectively. Total uracil nucleotide pools increased about 8-fold, 3.2-fold and 1.6-fold, respectively. Cytosine nucleotide pools did not increase in the brain. In conclusion, high-dose uridine administration caused severe hypothermia. Plasma levels of uridine and uracil were enhanced to a considerably higher extent than the levels in the tissues. The hypothermia might be related to breakdown products of uridine, since inhibition of uridine breakdown partially prevented hypothermia and since in brain uracil nucleotide levels were only slightly increased after uridine administration, while those of uracil were more markedly increased than in other tissues.

Topics: Animals; Body Temperature; Brain; Colonic Neoplasms; Cytidine; Cytidine Deaminase; Dose-Response Relationship, Drug; Female; Hypothermia; Liver; Male; Mice; Mice, Inbred BALB C; Mice, Inbred C57BL; Rats; Rats, Inbred Strains; Tetrahydrouridine; Thymidine; Uracil; Uridine; Uridine Phosphorylase