Effects of pH and temperature on the stability and decomposition of N,N'N"-triethylenethiophosphoramide in urine and buffer.

N,N',N"-Triethylenethiophosphoramide (thiotepa) was dissolved at 100 micrograms/ml in urine or in 0.1 M sodium acetate buffer and incubated at 37 degrees or 22 degrees. After 0, 15, 30, 60, 90, and 120 min of incubation, 0.1-ml samples were extracted into ethyl acetate and analyzed by gas-liquid chromatography (1.8-m X 2-mm column packed with 3% OV225 on 100/120 Supelcoport; oven at 180 degrees; injection port and nitrogen-phosphorus detector at 230 degrees). Thiotepa was more stable at 22 degrees than at 37 degrees and at pH 6 to 7 than at pH 4 to 5.5. After 2 hr of incubation at 37 degrees, thiotepa concentrations decreased by 40% at pH 5.0 but only 10% at pH 6 or 7. Although thiotepa concentrations declined as described above, alkylating activity, as assessed by p-nitrobenzyl pyridine reactivity, was stable at all temperatures and pHs tested. Partition coefficients of thiotepa degradation products into toluene, ethyl acetate, diethyl ether, and hexane were determined after 0 and 120 min of incubation in urine at pH 4.0. The extractability of alkylating activity into these organic solvents decreased dramatically after 120 min. Thiotepa degradation products were extracted from urine at pH 4.0 after 0, 30, 60, and 120 min incubation at 37 degrees and were separated by thin-layer chromatography. In addition to thiotepa (Rf 0.15), 3 degradation products possessing p-nitrobenzyl pyridine alkylating activity (Rf 0.35, 0.52, and 0.60) were observed during the course of incubation. The structures of the materials with Rf 0.35 and 0.52 were identified by mass spectrometry and indicated that thiotepa degradation occurs by successive addition of HCl molecules with opening of the aziridine rings and conversion to 2-chloroethyl moieties.