Coordination of histamine H3 receptor antagonists with human adrenal cytochrome P450 enzymes

Optical difference spectroscopy was used to identify and quantify human adrenal microsomal and mitochondrial cytochrome P450 enzyme interactions with the histamine H(3) receptor antagonists thioperamide, clobenpropit and ciproxifan. Addition of these structurally diverse imidazole H(3) receptor antagonists to cytochrome-P450-containing human adrenal microsomal and mitochondrial preparations resulted in concentration-dependent type II optical difference spectra. Respective spectral dissociation constants (K(S)) for the drug interactions with human adrenal microsomal and mitochondrial cytochrome P450 were 1.5 and 1.6 micromol/l for thioperamide, 3.1 and 0.28 micromol/l for clobenpropit and 0.10 and 0.11 micromol/l for ciproxifan. The three compounds demonstrated a similar activity profile in cytochrome-P450-containing bovine adrenal microsomal and mitochondrial preparations. Findings indicate direct coordination of these imidazole-containing H(3) receptor antagonists with the heme moiety of human adrenal cytochrome P450 isozymes.