title: Effects of thioridazine, an inhibitor of CYP2D6, on the steady-state plasma concentrations of the enantiomers of mianserin and its active metabolite, desmethylmianserin, in depressed Japanese patients.
abstract: The antidepressant mianserin is administered as a racemate of the S(+)- and R(-)-enantiomers. Previous in-vitro studies have suggested that CYP2D6 is involved in the stereoselective metabolism of mianserin and its active metabolite, desmethylmianserin. To determine a role for CYP2D6 in vivo, the effects of thioridazine, an inhibitor of CYP2D6, on the steady-state plasma concentrations of the enantiomers of mianserin and desmethylmianserin were examined in 13 depressed Japanese patients. All patients were taking 30 mg of racemic mianserin at bedtime for 8-50 days. Thioridazine (40 mg/day) was coadministered for 1 week, and blood samplings were performed before and after thioridazine coadministration, 12 h after bedtime dosing. Plasma concentrations of the enantiomers of mianserin and desmethylmianserin were measured by HPLC, and the CYP2D6 genotype was determined by allele-specific PCR analysis. Thioridazine significantly increased plasma concentration of S(+)-mianserin (mean SD: 78.2 +/- 35.0 vs. 150.8 +/- 48.7 nM, P < 0.001), but not R(-)-mianserin (39.8 +/- 21.2 vs. 39.5 +/- 20.6 nM, NS). Thioridazine also significantly increased plasma concentrations of both S-desmethylmianserin (11.9 +/- 2.8 vs. 24.4 +/- 10.7 nM, P < 0.01) and R-desmethylmianserin (42.6 +/- 28.4 vs. 115.6 +/- 36.9 nM, P < 0.001). One patient homozygous for the defective allele CYP2D6*5 had the second highest and highest plasma concentrations of S(+)-mianserin and R-desmethylmianserin, respectively, before thioridazine coadministration, and exhibited little increase in plasma concentration of the drugs after thioridazine coadministration. These results suggest that thioridazine specifically inhibits the metabolism of S(+)-mianserin and R-desmethylmianserin, probably through inhibition of CYP2D6, but not R(-)-mianserin.
authors.affiliation: Department of Neuropsychiatry, Hirosaki University Hospital, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Yasui
authors.forename: G
authors.initials: G
authors.lastname: Tybring
authors.forename: K
authors.initials: K
authors.lastname: Otani
authors.forename: K
authors.initials: K
authors.lastname: Mihara
authors.forename: A
authors.initials: A
authors.lastname: Suzuki
authors.forename: J O
authors.initials: JO
authors.lastname: Svensson
authors.forename: S
authors.initials: S
authors.lastname: Kaneko
mesh_terms: D018687:Antidepressive Agents, Second-Generation / Q000097:blood* / Q000627:therapeutic use; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D003863:Depression / Q000097:blood* / Q000188:drug therapy; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006720:Homozygote; D006801:Humans; D007564:Japan; D008297:Male; D008803:Mianserin / Q000031:analogs & derivatives* / Q000097:blood* / Q000493:pharmacokinetics / Q000627:therapeutic use; D013237:Stereoisomerism; D013881:Thioridazine / Q000494:pharmacology*
chemical_list: D018687:Antidepressive Agents, Second-Generation; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D004791:Enzyme Inhibitors; D008803:Mianserin; D013881:Thioridazine; C056517:desmethylmianserin