In Vitro and in Vivo Radiation Sensitization by the Halogenated Pyrimidine 5-Chloro-2′-deoxycytidine¹

5-Chloro-2′-deoxycytidine (Cld/Cyd) is hypothesized to have preferential incorporation into tumor DNA on the basis of elevated deoxycytidine-5′-phosphate deaminase and deoxycytidine kinase levels in tumors. Radiosensitization by Cld/Cyd was evaluated in exponentially growing Chinese hamster ovary cells by determining the ratio of radiation doses in control and treated cells to produce the same degree of cell killing (sensitizer enhancement ratio). Sensitizer enhancement ratios of 1.2–1.8 are seen at Cld/Cyd concentrations of 3–100 µm, 64 h incubation, and 200–600 cGy irradiation. Coincubation with tetrahydrouridine (H₄Urd), a proposed inhibitor of Cld/Cyd catabolism by plasma cytidine deaminase resulted in no enhanced drug or radiation cytotoxicity. C3H mice given implants of RIF-1 tumors received 72-h continuous i.p. infusions of Cld/Cyd with or without H₄Urd, or 5-bromo-2′-deoxyuridine (BrdUrd). Excised tumors were irradiated as single cell suspensions in vitro. Infusions of equimolar (0.4 mmol/kg/day) Cld/Cyd or BrdUrd resulted in greater radiosensitization by BrdUrd with no potentiation of Cld/Cyd by coinfusion with 0.8 mmol/kg/day H₄Urd. Infusions with equitoxic doses of Cld/Cyd (0.8 mmol/kg/day) or BrdUrd (0.4 mmol/kg/day) yielded equal BrdUrd and Cld/Cyd sensitizer enhancement ratios of 1.6, without H₄Urd potentiation of Cld/Cyd. Fluorescence-activated cell sorter analysis of tumor cell suspensions using a monoclonal antibody reactive with BrdUrd and Cld/Cyd disclosed a population of noncycling cells in tumors treated with Cld/Cyd/H₄Urd that is not seen in tumors exposed to either BrdUrd or Cld/Cyd alone.