In vitro identification of the human cytochrome p450 enzymes involved in the oxidative metabolism of loxapine.

In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) enzymes responsible for the oxidative metabolism of loxapine to 8-hydroxyloxapine, 7-hydroxyloxapine, N-desmethylloxapine (amoxapine) and loxapine N-oxide. These studies included use of cDNA-expressed enzymes, correlation analysis with 12 phenotyped human liver microsomal samples, and use of selective inhibitors of cytochrome P450s. The resultant data indicated that loxapine was mainly metabolized by human liver microsomes to (i) 8-hydroxyloxapine by CYP1A2, (ii) 7-hydroxyloxapine by CYP2D6, (iii) N-desmethyloxapine by CYP3A4 and (iv) loxapine N-oxide by CYP3A4.

Microsomal N-glucuronidation of nicotine and cotinine: human hepatic interindividual, human intertissue, and interspecies hepatic variation.

Two of the abundant conjugates of human nicotine metabolism result from the N-glucuronidation of S-(-)-nicotine and S-(-)-cotinine, transformations we recently demonstrated in liver microsomes. We further studied these microsomal N-glucuronidation reactions with respect to human hepatic interindividual, human intertissue, and interspecies hepatic variation. The reactivities of microsomes from human liver (n = 12), various human tissues, and liver from eight species toward the N-glucuronidation of S-(-)-nicotine and S-(-)-cotinine, and also R-(+)-nicotine in human liver were examined.

Quaternary ammonium-linked glucuronidation of 1-substituted imidazoles by liver microsomes: interspecies differences and structure-metabolism relationships.

N-Glucuronidation at an aromatic tertiary amine of 5-membered polyaza ring systems was investigated for a model series of eight 1-substituted imidazoles in liver microsomes from five species. The major objectives were to investigate substrate specificities of the series in human microsomes and interspecies variation for the prototype molecule, 1-phenylimidazole. The formed quaternary ammonium-linked metabolites were characterized by positive ion electrospray mass spectrometry.

N-glucuronidation of nicotine and cotinine in human: formation of cotinine glucuronide in liver microsomes and lack of catalysis by 10 examined UDP-glucuronosyltransferases.

Two predominant human glucuronide metabolites of nicotine result from pyridine nitrogen atom conjugation. The present objectives included determination of the kinetics of formation of S(-)-cotinine N1-glucuronide in pooled human liver microsomes and investigation of the UDP-glucuronosyltransferases (UGTs) involved in N-glucuronidation of nicotine isomers and S(-)-cotinine by use of recombinant enzymes (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B15). Quantification was by radiochemical high-performance liquid chromatography with use of radiolabeled substrates.