PT - JOURNAL ARTICLE AU - Miki Nakajima AU - Sumika Nakamura AU - Shogo Tokudome AU - Noriaki Shimada AU - Hiroshi Yamazaki AU - Tsuyoshi Yokoi TI - Azelastine <em>N</em>-Demethylation by Cytochrome P-450 (CYP)3A4, CYP2D6, and CYP1A2 in Human Liver Microsomes: Evaluation of Approach to Predict the Contribution of Multiple CYPs DP - 1999 Dec 01 TA - Drug Metabolism and Disposition PG - 1381--1391 VI - 27 IP - 12 4099 - http://dmd.aspetjournals.org/content/27/12/1381.short 4100 - http://dmd.aspetjournals.org/content/27/12/1381.full SO - Drug Metab Dispos1999 Dec 01; 27 AB - Azelastine, an antiallergy and antiasthmatic drug, has been reported to be mainly N-demethylated to desmethylazelastine in humans. In the present study, Eadie-Hofstee plots of azelastine N-demethylation in human liver microsomes were biphasic. In microsomes from human B-lymphoblast cells, recombinant cytochrome P-450 (CYP)2D6 and CYP1A1 exhibited higher azelastine N-demethylase activity than did other CYP enzymes. On the other hand, recombinant CYP3A4 and CYP1A2 as well as CYP1A1 and CYP2D6 in microsomes from baculovirus-infected insect cells were active in azelastine N-demethylation. TheKM value of the recombinant CYP2D6 (2.1 μM) from baculovirus-infected insect cells was similar to theKM value of the high-affinity (2.4 ± 1.3 μM) component in human liver microsomes. On the other hand, theKM values of the recombinant CYP3A4 (51.1 μM) and CYP1A2 (125.4 μM) from baculovirus-infected insect cells were similar to the KM value of the low-affinity (79.7 ± 12.8 μM) component in human liver microsomes. Bufuralol inhibited the high-affinity component, making the Eadie-Hofstee plot in human liver microsomes monophasic. AzelastineN-demethylase activity in human liver microsomes (5 μM azelastine) was inhibited by ketoconazole, erythromycin, and fluvoxamine (IC50 = 0.08, 18.2, and 17.2 μM, respectively). Azelastine N-demethylase activity in microsomes from twelve human livers was significantly correlated with testosterone 6β-hydroxylase activity (r = 0.849,p &lt; .0005). The percent contributions of CYP1A2, CYP2D6, and CYP3A4 in human livers were predicted using several approaches based on the concept of correction with CYP contents or relative activity factors (RAFs). Our data suggested that the approach using RAFCL (RAF as the ratio of clearance) is most predictive of the N-demethylation clearance of azelastine because it best reflects the observedN-demethylation clearance in human liver microsomes. Summarizing the results, azelastine N-demethylation in humans liver microsomes is catalyzed mainly by CYP3A4 and CYP2D6, and CYP1A2 to a small extent (in average, 76.6, 21.8, and 3.9%, respectively), although the percent contribution of each isoform varied among individuals. The American Society for Pharmacology and Experimental Therapeutics