An investigation of the plasma and urinary metabolite profiles of the hepatotoxin methapyrilene in the male Wistar rat

The plasma and urinary metabolite profiles of the 2-thiophene-based H1-receptor antagonist methapyrilene (N,N-dimethyl-N'-pyridin-2-yl-N'-(thiophen-2-ylmethyl)ethane-1,2-diamine) were investigated in the rat following the oral administration of 0, 50 or 150 mg/kg/day of the drug for 5 days. The systemic exposure and metabolic fate of methapyrilene were investigated initially using a rapid “fit for purpose” reversed-phase (RP) UHPLC-MS/MS assay followed by a more in-depth characterization of its metabolites using RP-UHPLC coupled to a high resolution multi reflecting time of flight mass spectrometer. No detectable methapyrilene was present in plasma when measured on days 1, 3 and 5, at any dose, using samples obtained 24 h post administration. However, methapyrilene-derived metabolites were detected, in increasing amounts, in these plasma samples on days 3 and 5 of the study. Both methapyrilene and large numbers of metabolites were present in the urines collected 24 h after dosing on days 3 and 5. The profiles of these metabolites were both dose and time dependent. From the urine and plasma profiles obtained using both +ve and -ve ESI HRMS some 24 metabolites were characterized showing functionalization by aliphatic and aromatic hydroxylation, N-oxidation, N-demethylation and loss of the thiophene ring. Glucuronic acid, glycine and glutathione-derived conjugates were also detected.