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High frequency of cytidine to thymidine conversions was identified in the genome of several types of cancer cells. In breast cancer cells, these mutations are clustered in long DNA regions associated with single-strand DNA (ssDNA), double-strand DNA breaks (DSB), and genomic rearrangements. The observed mutational pattern resembles the deamination signature of cytidine to uridine carried out by members of the APOBEC3 family of cellular deaminases. Consistently, APOBEC3B (A3B) was recently identified as the mutational source in breast cancer cells. A3G is another member of the cytidine deaminases family predominantly expressed in lymphoma cells, where it is involved in mutational DSB repair following ionizing radiation treatments. This activity provides us with a new paradigm for cancer cell survival and tumor promotion and a mechanistic link between ssDNA, DSBs, and clustered mutations. Cancer Res; 73(12); 3494-8. ©2013 AACR., (©2013 AACR.)

We showed that the efficacy of the new 2'-deoxycytidine (2'-dCyd) analogue antimetabolite 2'-deoxy-2'-methylidenecytidine (DMDC) correlates well with tumor levels of cytidine (Cyd) deaminase in human cancer xenograft models. DMDC was highly effective in tumors with higher levels of Cyd deaminase, whereas lower levels yielded only slight activity. In contrast, gemcitabine (2',2'-difluorodeoxycytidine), which has action mechanisms similar to those of DMDC, is only slightly active in tumors with higher levels of the enzyme. In the present study, we investigated the roles of Cyd deaminase in the antitumor activity of the two 2'-dCyd antimetabolites in 13 human cancer cell lines. Tetrahydrouridine, an inhibitor of Cyd deaminase, reduced the antiproliferative activity of DMDC (P = 0.0015). Furthermore, tumor cells transfected with the gene of human Cyd deaminase become more susceptible to DMDC both in vitro and in vivo. These results indicate that Cyd deaminase is indeed essential for the activity of DMDC. In contrast, the antiproliferative activity of gemcitabine was increased to some extent by tetrahydrouridine (P = 0.0277), particularly in tumor cell lines with higher levels of Cyd deaminase. This suggests that higher levels of Cyd deaminase may inactivate gemcitabine. Among nucleosides and deoxynucleosides tested, only dCyd, a natural substrate of both Cyd deaminase and dCyd kinase, suppressed the antiproliferative activity of DMDC by up to 150-fold. Because the Vmax/Km of DMDC for dCyd kinase was 8-fold lower than that for dCyd, the activation of DMDC to DMDC monophosphate (DMDCMP) by dCyd kinase might be competitively inhibited by dCyd. In addition, the dCyd concentrations in human cancer xenografts were inversely correlated with levels of Cyd deaminase activity. It is therefore suggested that higher levels of Cyd deaminase reduce the intrinsic cellular concentrations of dCyd in tumors, resulting in efficient activation of DMDC to DMDCMP by dCyd kinase. These results indicate that the efficacy of DMDC may be predicted by measuring the activity of Cyd deaminase in tumor tissues before treatment starts and that DMDC may be exploited in a new treatment modality: tumor enzyme-driven cancer chemotherapy.

5-Trifluoromethyl-2'-deoxycytidine (F3methyl-dCyd), when coadministered with tetrahydrouridine (H4Urd), surpasses the efficacy of 5-trifluorothymidine and 5-trifluoromethyl-2'-deoxycytidine when administered alone as demonstrated with adenocarcinoma 755 and Lewis lung carcinoma as solid tumors implanted in C57BL X DBA/2 F1 mice. It appears that the reason for the heightened efficacy of F3methyl-dCyd, when coadministered with low concentrations of H4Urd, is decreased systemic deamination and subsequent systemic catabolism by pyrimidine nucleoside phosphorylases, which do not act on deoxycytidine and its analogues. Furthermore, the elevated levels of cytidine deaminase in these mouse tumors may result in selective conversion of F3methyl-dCyd to 5-trifluorothymidine at the tumor site. This suggests an approach to the treatment of human tumors possessing elevated levels of cytidine deaminase such as certain leukemias, bronchogenic carcinoma of the lung, adenocarcinomas of the colon and rectum, astrocytomas, and certain tumors which are refractory to chemotherapy with 1-beta-D-arabinofuranosylcytosine. In contrast to fluorinated pyrimidines in current use, F3methyl-dCyd + H4Urd potentially allows an exclusive DNA-, rather than both a DNA- and RNA-, directed approach. The major mechanism of the antitumor activity of F3methyl-dCyd appears to be via inhibition by 5-trifluorothymidine-5'-monophosphate of thymidylate synthetase, the target enzyme of fluoropyrimidine analogues in current use. However, the established and potential differences in the mode of action, anabolism, nature of incorporation into DNA, repair and cofactor requirements of F3methyl-dCyd and its anabolites, compared to that of the commonly utilized fluorinated pyrimidines, indicate that F3methyl-dCyd + H4Urd is a novel combination of agents. In comparative studies with Lewis lung carcinoma, F3methyl-dCyd (+ H4Urd) was shown to surpass the efficacies of 5-fluorouracil and 5-fluorodeoxyuridine and to be essentially equal in efficacy to 5-fluorodeoxycytidine (+ H4Urd). The optimum established protocol against Lewis lung carcinoma is F3methyl-dCyd, 175 mg/kg, + H4Urd, 25 mg/kg, once per day for 7 days. Studies utilizing high concentrations of H4Urd coadministered with F3methyl-dCyd indicate that the major pathway of tumor inhibition is via conversion of F3methyl-dCyd to 5-trifluorothymidine in view of the fact that tumor inhibition diminishes at doses of H4Urd which result in extensive (93%) inhibition of tumor cytidine deaminase.(ABSTRACT TRUNCATED AT 400 WORDS)