Metabolism and alkylating activity of thio-TEPA in rat liver slice incubation

Precision-cut rat-liver slices were used to study the metabolism of the alkylating agent N,N',N''-triethylenethiophosphoramide (thio-TEPA). Exposure to high concentrations (1-10 mM) of thio-TEPA for 6 h did not prove to be toxic to the liver slices as indicated by insignificant leakage of potassium from the cells. The time course of the disappearance of thio-TEPA (initial concentration, 5.2 microM) from the buffer during incubation followed first-order kinetics. Formation of N,N'N''-triethylenephosphoramide (TEPA) apparently accounted for the elimination of thio-TEPA. Pretreatment of the rats with phenobarbital significantly increased the reaction rate. Conversely, pretreatment with the cytochrome P-450 inhibitor allylisopropylacetamide significantly reduced the metabolic rate. The elimination of thio-TEPA and formation of TEPA occurred independently of thio-TEPA concentration, which ranged from 5.2 to 104 microM. Thio-TEPA's oxo-analogue TEPA, which was not further metabolized, was the only metabolite identified. However, a significantly time-related increase in 4-(nitrobenzyl)-pyridine (NBP) alkylating activity was observed following incubation of liver slices with thio-TEPA but not after their incubation with TEPA. This may possibly indicate the formation of unknown active metabolites.