Interactions of dihydralazine with cytochromes P4501A: a possible explanation for the appearance of anti-cytochrome P4501A2 autoantibodies

The antihypertensive drug dihydralazine may, on rare occasions, cause immunoallergic hepatitis characterized by anti-cytochrome P450 (P450)1A2 autoantibodies. To understand the first steps leading to this immune reaction, we studied the covalent binding fo dihydralazine metabolites to microsomes from rat and human livers. Upon incubation with NADPH and microsomes, dihydralazine formed metabolites that reacted with heme (as evidenced by destruction of heme, formation of 445-nm light-absorbing complexes, and covalent binding of heme to P450 apoprotein) and covalently bound to microsomal proteins. Formation of these metabolites was shown (by NADPH dependence, induction by beta-naphthoflavone, and immunoinhibition by anti-P4501A antibodies) to be mediated by P4501A. Finally, these metabolites appeared to bind to P4501A2, which produced them. These results support the following scheme for the first steps of this autoimmune reaction: P4501A2 metabolizes dihydralazine into reactive metabolites that then bind to it, forming a neoantigen that triggers an immune response characterized by autoantibodies against P4501A2.