Description: Tetrahydrouridine, a synthetic pyrimidine nucleoside analog, is novel and potent inhibitor of cytidine deaminase (CDA) with biomodulating activity, it competitively blocks the enzyme's active site more effectively than intrinsic cytidine. Tetrahydrouridine increases the efficacy of the radiosensitizer cytochlor (5-chloro-2'-deoxycytidine) by inhibiting the enzyme deoxycytidine monophosphate (dCMP) deaminase and preventing the premature deamination of the cytochlor metabolite 5-chloro-2'-deoxycytidine monophosphate (CldCMP) to 5-chloro-2'-deoxyuridine monophosphate (CldUMP); in turn, this increases tumor concentrations of CldUMP which is then further anabolized and incorporated selectively into tumor DNA as CldU (5-chloro-2'-deoxyuridine).