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Cytochrome P450s play a central role in the metabolism and disposition of an extremely wide range of drugs and chemical carcinogens. Individual differences in the expression of these enzymes may be an important determinant in susceptibility to adverse drug reactions, chemical toxins and mutagens. In this paper, we have measured the relative levels of expression of cytochrome P450 isoenzymes from eight gene families or subfamilies in a panel of twelve human liver samples in order to determine the individuality in their expression and whether any forms are co-regulated. Isoenzymes were identified in most cases on Western blots based on the mobility of authentic recombinant human cytochrome P450 standards. The levels of the following P450 proteins correlated with each other: CYP2A6, CYP2B6 and a protein from the CYP2C gene subfamily, CYP2E1 and a member of the CYP2A gene subfamily, CYP2C8, CYP3A3/A4 and total cytochrome P450 content. Also, the levels of two proteins in the CYP4A gene subfamily were highly correlated. These correlations are consistent with the relative regulation of members of these gene families in rats or mice. In addition, the level of expression of specific isoenzymes has also been compared with the rate of metabolism of a panel of drugs, carcinogens and model P450 substrates. These latter studies demonstrate and confirm that the correlations obtained in this manner represent a powerful approach towards the assignment of the metabolism of substrates by specific human P450 isoenzymes.

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1 The cytotoxicity of metabolites generated from phenytoin, sorbinil and mianserin by human and mouse liver microsomes was assessed by co-incubation with human mononuclear leucocytes as target cells. Cytotoxicity was determined by trypan blue dye exclusion. 2 Phenytoin and sorbinil were metabolised by NADPH-dependent murine microsomal enzymes to cytotoxic metabolites. Cytotoxicity produced by both drugs was significantly enhanced by the epoxide hydrolase inhibitor trichloropropane oxide (TCPO). No significant cytotoxicity was observed in the presence of human liver microsomes. 3 Mianserin was metabolised by both human and mouse liver microsomes to a cytotoxin. Cytotoxicity was greater in the presence of human liver microsomes (13.7 ± 2.2%; mean + s.d. for four livers, compared with 6.0 ± 2.4%, mean ± s.d., n = 4, with mouse liver microsomes), and was unaffected by pretreatment with TCPO. 4 Stable metabolites were quantified by radiometric high performance liquid chromatography. Phenytoin and sorbinil were metabolised to 5-(p-hydroxyphenyl)-5-phenylhydantoin (0.3-0.5% of incubated radioactivity) and 2-hydroxysorbinil (0.4-2.7% of incubated radioactivity), respectively, by both human and mouse liver microsomes. 5 Mianserin was metabolised to 8-hydroxymianserin and desmethylmianserin by both human and mouse liver microsomes. Desmethylmianserin was the major product in incubations with human liver microsomes (32.3 ± 12%, mean ± s.d. for four livers), whereas 8-hydroxymianserin was the predominant metabolite generated by mouse liver microsomes (25.9 ± 1.5%, mean ± s.d., n = 4). 6 Generation of electrophilic metabolites was assessed by determination of the amount of radiolabelled material which became irreversibly bound to protein. Only mouse liver microsomes activated phenytoin to a chemically reactive metabolite, whereas both mouse and human liver microsomes generated reactive metabolites from sorbinil and mianserin. 7 These studies show that drug cytotoxicity can be mediated by low concentrations (circa F.M) of metabolites generated by NADPH-dependent hepatic microsomal enzymes; however demonstration of cytotoxicity in vitro has not been established as a means of predicting in vivo toxicity.

Amodiaquine is an antimalarial drug that has been associated with adverse reactions which may be immune mediated. Specific IgG anti-amodiaquine antibodies were detected after administration of the drug to rats (269 μmol/kg for 4 days), using an enzyme-linked immunosorbent assay employing amodiaquine conjugated to metallothionein as an antigen. A positive immune response was observed regardless of the route of administration, but the magnitude of the response in terms of antibody titre was in the order intraperitoneal administration > intramuscular administration > oral administration. Hapten inhibition experiments with structurally related drugs defined the specificity of the antibody which appears to recognize a conjugate of amodiaquine quinone imine and cysteine residues present in protein. Amodiaquine was converted to a protein-reactive species by activated human polymorphonuclear leucocytes in vitro, and this may provide a mechanism for immunogen formation in vivo. A humoral immune response was observed with doses of amodiaquine that did not produce either direct hepatotoxicity or leucopenia. Thus an animal model has been developed with which to investigate the toxicological consequences of amodiaquine immunogenicity.