Effects of Induction and Inhibition of Cytochromes P450 on the Hepatotoxicity of Methapyrilene

Abstract

The mechanisms by which the antihistamine drug methapyrilene causes acute periportal hepatotoxicity in rats are not yet elucidated. This study investigated the effects of modulators of cytochrome P450 (CYP) activity on the hepatotoxicity of methapyrilene and also the effect of methapyrilene on hepatic CYP. Pretreatment of male Han Wistar rats with p-naphthoflavone, phenobarbitone, butylated hydroxytoluene, piperonyl butoxide, Aroclor 1254, or cobalt protoporphyrin IX, agents known to modify hepatic CYP, all afforded some degree of protection against a hepatotoxic dose of methapyrilene (150 mg/kg x 3 days p.o.), as assessed by clinical chemistry and histology. Total hepatic CYP depletion by cobalt protoporphyrin IX treatment indicated CYPmediated bioactivation was a prerequisite for methapyrilene-induced hepatotoxicity. Protection against hepatic damage was strongly associated with /3-naphthoflavone induction of CYP1A and phenobarbitone-associated CYP2B induction. However, the role of CYP3A, which is constitutively expressed in the liver and induced by piperonyl butoxide, butylated hydroxytoluene, or Aroclor 1254, was unclear. Modulation of FAD monooxgenase activity by methimazole pretreatment was not associated with increased methapyrilene-induced hepatotoxicity. Methapyrilene treatment alone specifically decreased microsomal enzyme activity markers for CYP2C11, CYP3A, and CYP2A and pretreatment with all the hepatic enzyme-inducing agents specifically prevented the loss of CYP2C11. Together this suggested that CYP2C11 was responsible for the suicide substrate bioactivation of methapyrilene and the toxicologic outcome largely relied upon an abundance of detoxifying enzymes present in the liver, o Methapyrilene is an ethylenediamine H, histamine receptor antagonist which has been used for over 20 years, principally as a sleep aid and in cold and allergy formulations. Subsequently, methapyrilene was withdrawn from clinical use when it was found to be hepatocarcinogenic, following chronic administration in rats (Lijinsky et ai, 1980), although not in 'To whom correspondence should be addressed. Fax: 0171 982 6135. E-mail: c.j.powell@mds.qmw.ac.uk. mice, guinea pigs, or hamsters (Brennan and Creasia, 1982; Lijinsky et ai, 1983). In the rat, acute administration of methapyrilene has been found to damage the periportal region of the liver (Graichen et ai, 1985); however, the underlying biochemical mechanisms responsible are poorly understood.Methapyrilene undergoes extensive metabolism in vivo, in cultured hepatocytes and in microsomes Schmitz, 1986, 1987; Singer et ai, 1987;Kelly et al., 1992). While many metabolites of methapyrilene have been identified, the metabolite(s) responsible for hepatic damage in rats remains elusive. Species differences in the metabolism of methapyrilene seem largely quantitative rather than qualitative (Kammerer and Schmitz, 1987;Lampe and Kammerer, 1990). It is plausible, therefore, that the species differences in methapyrilene-induced hepatotoxicity may be explained by differences in ...

Identification of the Thiophene Ring of Methapyrilene as a Novel Bioactivation-Dependent Hepatic Toxicophore

“…The representative positions of GSH substitution on the thiophene rings of I and III are those reported for similar metabolites of 2-phenylthiophene . MP produces an acute hepatotoxicity in male rats at relatively low doses (Graichen et al, 1985;Ratra et al, 1998aRatra et al, , 2000Huang et al, 2004;Craig et al, 2006). The comparative mildness of this injury more typically resembles druginduced clinical hepatotoxicities than the severe damage produced in experimental animals by many other reference hepatotoxicants.…”

“…HLM from two female and two male donors aged 10 to 41 years were pooled. P450 content was determined by reduced-carbon monoxide difference spectroscopy as described previously (Ratra et al, 1998a), and protein content was measured by the Bradford assay (Bradford, 1976). Incubations were carried out in a final volume of 1 ml of Tris-HCl buffer (0.2 M), pH 7.4, containing 1 mg/ml microsomal protein, 20 M [ 3 H]MP (0.25 Ci), and a NADPH-regenerating system (0.4 mM NADP, 7.5 mM glucose 6-phosphate, 1 U/ml glucose-6-phosphate dehydrogenase, and 5.0 mM MgCl 2 ).…”

“…Fig. 1], a 2-thiophene H 1 -receptor antagonist, was not associated with hepatotoxicity in humans, but it causes unusual dose-dependent periportal damage in adult male rats (Graichen et al, 1985;Ratra et al, 1998a;Ratra et al, 2000). Typical hepatotoxic regimens (150 -300 mg/kg/day ϫ 3) produce a mild to moderate injury characterized by hepatocellular necrosis, bile duct proliferation, and inflammatory cell infiltration.…”

“…However, although the cytotoxicity has been associated with metabolic activation, oxidative stress, mitochondrial damage (Ratra et al, 1998a;Ratra et al, 1998b), protein modification, and altered protein expression (Craig et al, 2006), its mechanism is still obscure and the toxicophore is unidentified.…”

“…The drug-induced hepatotoxicity is considered to be metabolism-dependent because it is diminished by agents that inhibit cytochrome P450 (P450): cobalt protoporphyrin in vivo (Ratra et al, 1998a) and metyrapone in rat hepatocytes (Ratra et al, 1998b). The major metabolites ( Fig.…”