Abstract
The coadministration of tetrahydrouridine (H(,4)Urd) diverted the metabolism of ('3)H-5-fluoro-2'-deoxycytidine (FdCyd) in human epidermoid laryngeal carcinoma (HEp-2) cells through the deoxycytidine kinase-deoxycytidylate deaminase (dCK-dCMPD) pathway to the formation of 5-fluorodeoxyuridylate (FdUMP) without the incorporation of 5-fluorouridylate (FUMP) into RNA or the formation of RNA-level antimetabolite pools. Antimetabolite pool sizes, as assayed by HPLC, following treatment of BD2F(,1) mice bearing ascitic mammary adenocarcinoma-755 (ADC-755) or Lewis lung carcinoma (LLC) with ('3)H-FdCyd + H(,4)Urd resulted in pool sizes indicative of a tumor-selective, dual pathway metabolism of FdCyd via both the cytidine deaminase-deoxythymidine kinase (CD-dTK) and dCK-dCMPD pathways. In contrast to the high levels of all RNA- and DNA-level antimetabolites derived from FdCyd found in tumor tissue, in normal tissues (bone marrow, intestine, liver and spleen) and in serum, FdCyd was metabolized to only a small extent, all antimetabolite pools were markedly lower. RNA-level antimetabolite pools and FdUrd were generally 100-fold lower in normal than in tumor tissue, and FdUMP was 10- to 25-fold lower in normal, than tumor tissues. In contrast, the use of tritiated 5-fluoro-2'-deoxyuridine (('3)H-FdUrd) or tritiated 5-fluorouracil (('3)H-FUra) resulted in the formation of higher levels (10- to 40-fold) of DNA- and RNA-level antimetabolites in normal tissue and lower levels (1/8) of FdUMP in tumor tissue than when FdCyd + H(,4)Urd was utilized. ('3)H-FdCyd + H(,4)Urd exposure resulted in selective incorporation of antimetabolites into tumor RNA and DNA; a small extent of incorporation of antimetabolites into normal tissue RNA and DNA was observed. FdCyd incorporated 3- to 10-fold greater into tumor than intestine, liver and spleen, and incorporated 190-fold greater in tumor than in bone marrow. Treatment of mice bearing ADC-755 or LLC with ('3)H-FdUrd or ('3)H-FUra resulted in only marginal selectivity in terms of tumor-selective antimetabolite incorporation into nucleic acid. dCMPD and CD enzyme assays have confirmed that H(,4)Urd administration effectively inhibited the low CD activity in normal, but not the elevated levels found in tumor tissue. Thus, FdCyd + H(,4)Urd treatment in vivo results in a tumor-selective, dual pathway (i.e., both dCK-dCMPD and CD-dTK) activation of FdCyd to the formation of RNA- and DNA-level antimetabolites via selective inhibition of normal tissue CD levels by H(,4)Urd. On the other hand, 5-trifluoromethyl-2'-deoxycytidine (F(,3)methyldCyd), when coadministered with H(,4)Urd, has been shown through tumor and enzyme inhibition assays to be activated solely by the CD-dTK pathway. The drug combinations, FdCyd (or F(,3)methyldCyd) + H(,4)Urd were more efficacious than FdUrd, F(,3)dTyd, or FUra used at their optimal and LD(,10) drug doses against ADC-755 and LLC.