A synthetic pyrimidine nucleoside analogue with biomodulating activity. 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). Check foractive clinical trials using this agent. OR A substance being studied in the treatment of some types of cancer. It may help make cancer cells easier to kill with radiation therapy. Tetrahydrouridine is a type of radiosensitizing agent, a type of multidrug resistance modulator, and a type of cytidine deaminase inhibitor. Also called THU.