The in vivo metabolism of methapyrilene, a hepatocarcinogen, in the rat

Methapyrilene hepatotoxicity is associated with increased hepatic glutathione, the formation of glucuronide conjugates, and enterohepatic recirculation

The mechanisms by which acute administration of methapyrilene, an H(1)-receptor antihistamine causes periportal necrosis to rats are unknown. This study investigated the role of the hepato-biliary system in methapyrilene hepatotoxicity following daily administration of 150 mg/kg per day over 3 consecutive days. Biliary metabolites of methapyrilene were tentatively identified. In male Han Wistar rats administration of methapyrilene significantly increased hepatic reduced glutathione (GSH) to 140% of control levels 24 h following the last dose. There were no significant changes in the activities of glutathione-related enzymes, glutathione peroxidase (GPx) and reductase (GSH), glutathione S-transferase (GST), and gamma-glutamyl cysteine synthetase (gamma-GCS) over 3 days of methapyrilene administration. Methapyrilene treatment resulted in no significant increase in excretion of biliary oxidized glutathione (GSSG), a sensitive marker of oxidative stress in vivo, following the third dose. [3H]Methapyrilene-derived radioactivity was detected in bile, to a greater extent than in feces, indicating that methapyrilene and/or metabolites underwent enterohepatic recirculation. Cannulation and exteriorization of the bile duct (to interrupt enterohepatic recirculation) afforded some protection against the hepatotoxicity, assessed by clinical chemistry and histopathology. Liquid chromatography-mass spectrometry (LC-MS) analysis of bile indicated the presence of unmetabolized methapyrilene, methapyrilene O-glucuronide and desmethyl methapyrilene O-glucuronide. These data demonstrate that acute methapyrilene hepatotoxicity in vivo is not a consequence of GSH depletion, or oxidative stress, but that enterohepatic recirculation of biliary metabolites may be important. Progressive exposure to non-oxidizing, reactive metabolic intermediates may be responsible for hepatotoxicity.

Methapyrilene hepatotoxicity is associated with oxidative stress, mitochondrial disfunction and is prevented by the Ca2+ channel blocker verapamil

Methapyrilene (MP) is an unusual hepatotoxin in that it causes periportal necrosis in rats. The mechanism of acute methapyrilene hepatotoxicity has, therefore, been investigated in cultured male rat hepatocytes. Addition of methapyrilene to rat hepatocytes resulted in a time- and dose-dependent loss in cell viability between 4 and 8 h of incubation as judged by cellular enzyme leakage. The cytochrome P450 (CYP) inhibitor metyrapone protected against methapyrilene-mediated toxicity suggesting that MP is metabolised by CYP for toxicity. The concentration-dependent protection from methapyrilene toxicity afforded by metyrapone correlated with an inhibition of microsomal CYP2C11-associated androstenedione 16alpha hydroxylase activity, and hepatocytes prepared from hypophysectomised rats (containing reduced levels of microsomal immunodetectable CYP2C11 and associated androstenedione 16alpha hydroxylase activity) showed resistance to the toxic effects of methapyrilene. These data suggest that the toxicity of methapyrilene is predominantly dependent on the CYP2C11 isoform. Treatment of hepatocytes with a toxic concentration of MP caused oxidative stress as indicated by increases in NADP+ levels within 2 h and cellular thiol oxidation as evidenced by a reduction--but not complete loss--in glutathione levels. Methapyrilene hepatotoxicity was associated with an early loss in mitochondrial function, as indicated by mitochondrial swelling and significant losses in cellular ATP within 2 h. Co-incubation of methapyrilene-treated hepatocytes with inhibitors of inner mitochondrial transition permeability pore opening--cyclosporin A or the thiol reductant dithiothreitol--abrogated cell death suggesting that pore opening and loss of mitochondrial Ca2+ homeostasis play a significant role in methapyrilene-mediated cell death. Co-incubation of methapyrilene-treated hepatocytes with the phenylalkylamine calcium channel blocker verapamil--but not by treating cells in a nominally calcium-free medium--also abrogated cell death, suggesting that if Ca2+ is involved in cell killing then it is dependent on an intracellular Ca2+ pool. Pre-treatment of hepatocytes for 1 h with verapamil--to inhibit intracellular Ca2+ pool filling--increased the potency of verapamil protection against methapyrilene toxicity by approximately 100-fold. Taken together, these data indicate that methapyrilene intoxication leads to mitochondrial disfunction and suggest a critical role for a loss of mitochondrial Ca2+ homeostasis in this model of hepatocyte death.

Identification of urinary metabolites of pyrilamine after oral administration to man

1. The metabolism of pyrilamine, 2-[4-methoxybenzyl-(2-dimethylaminoethyl) amino] pyridine, was studied in adult male volunteers after a single oral dose of 50 mg. 2. Solvent extracts of urine obtained with or without enzyme hydrolysis were analysed by gc/ms after derivatization with MSTFA/TMSCI (N-methyl-N-trimethylsilyl-trifluoroacetamide/trimethyl chlorosilane). The structure of metabolites were determined based on EI mass spectra and confirmed with those of authentic standards. 3. Conjugated metabolites identified in the urine were pyrilamine, O-desmethylpyrilamine, and ring hydroxylated derivatives of pyrilamine. O-desmethylpyrilamine was also detected in low abundance as a free form. 4. These metabolites observed in human urine were quite different from those previously reported in rat.

Metabolism of methapyrilene by Fischer-344 rat and B6C3F1 mouse hepatocytes

1. Suspension cultures of freshly isolated F344 rat and B6C3F1 mouse hepatocytes were compared for their ability to transform various concentrations of methapyrilene (MP). 2. MP metabolites were isolated and purified by h.p.l.c., and were identified by comparing their chromatographic and mass spectral properties with those of authentic standards. 3. Both rat and mouse hepatocytes transformed MP to tentatively identified 2-thiophenecarboxylic acid (I), and definitively identified mono-N-desmethyl methapyrilene glucuronide (II), methapyrilene glucuronide (III), methapyrilene N-oxide (V), and mono-N-desmethyl methapyrilene (VII).

Species differences in the metabolism and macromolecular binding of methapyrilene: a comparison of rat, mouse and hamster

1. The metabolism of methapyrilene (MPH) by rat, hamster and mouse liver microsomes in vitro was investigated together with the binding of 14C-MPH to calf thymus DNA after metabolic activation. 2. Both quantitative and qualitative differences in MPH metabolism were observed in these three species. Mouse liver microsomes catalyse the formation of two novel isomers of hydroxypyrdylmethapyrilene (hydroxypyridyl-MPH) as determined by mass spectral analysis. N,N'-Didesmethylmethapyrilene (didesmethyl-MPH) was formed in detectable quantities only when hamster liver microsomes were used. 3. Incubation of liver microsomes from all three species catalysed the binding of 14C-MPH to exogenous DNA, which was quantitatively similar for all three species. The effect of the cytochrome P-450 inhibitor, 2,4-dichloro-6-phenylphenoxyethylamine (DPEA), and methimazole, a flavin-dependent monooxygenase inhibitor, on binding differed significantly for the three species studied.