Influence of Quinidine, Cimetidine, and Ketoconazole on the Enantioselective Pharmacokinetics and Metabolism of Metoprolol in Rats VANESSA BERGAMIN BORALLI, 1 EDUARDO BARBOSA COELHO, 2 AND VERA LUCIA LANCHOTE 1* 1 Department of Clinical, Toxicologic, and Bromatologic Analysis, Faculty of Pharmaceutical Sciences of Ribeira ˜o Preto, University of Sa ˜o Paulo, Ribeira ˜o Preto, Sa ˜o Paulo, Brazil 2 Department of Internal Medicine, Discipline of Nephrology, Faculty of Medicine of Ribeira ˜o Preto, University of Sa ˜o Paulo, Ribeira ˜o Preto, Sa ˜o Paulo, Brazil ABSTRACT Metoprolol is a b-blocker and its racemic mixture is used for the treat- ment of hypertension. In the present study we investigated the influence of CYP2D and CYP3A on the stereoselective metabolism of metoprolol in rats. Male Wistar rats (n 5 6 per group) received racemic metoprolol (15 mg/kg) orally, with or without pretreatment with the CYP inhibitor ketoconazole (50 mg/kg), cimetidine (150 mg/kg), or quinidine (80 mg/kg). Blood samples were collected up to 48 h after metoprolol administration. The plasma concentrations of the stereoisomers of metoprolol, O-demethylmetoprolol (ODM), a-hydroxymetoprolol (OHM) (Chiralpak 1 AD column), and metoprolol acidic metabolite (AODM) (Chiralcel 1 OD-R column) were determined by HPLC using fluo- rescence detection (k exc 5 229 nm; k em 5 298 nm). CYP3A inhibition by ketoconazole reduced the plasma concentrations of ODM and AODM and favored the formation of OHM. CYP2D and CYP3A inhibition by cimetidine reduced the plasma concentrations of OHM and AODM and favored the formation of ODM. The inhibition of CYP2D by quinidine reduced the plasma concentrations of OHM and favored the formation of ODM. In conclusion, the results suggest that CYP3A is involved in the formation of ODM and CYP2D is involved in the formation of AODM. Chirality 21:886–893, 2009. V V C 2009 Wiley-Liss, Inc. KEY WORDS: metoprolol; cytochrome P450 inhibitors; metabolites; pharmacokinetics; rats; enantiomers INTRODUCTION Metoprolol, a selective ß 1 -adrenergic receptor antago- nist, is used in clinical practice in the racemic form for the treatment of myocardial infarction, heart failure, and arte- rial hypertension. The (S)-metoprolol enantiomer has about 500-fold more affinity for the ß 1 -adrenergic receptor than its (R)-antipode. 1,2 In rats, the kinetic disposition of metoprolol is enantio- selective, although inverse and less marked when com- pared to humans. Oral clearance of the (2)-(S) enantiomer is higher than that of the (1)-(R) enantiomer in rats treated with racemic metoprolol at a dose of 20 mg/kg (2.26 versus 1.99 ml/min/kg). The elimination half-life is similar (35 min) for the (1)-(R)- and (2)-(S)-metoprolol enantiomers, indicating that rats are an animal model with a behavior similar to that of Caucasian extensive metabo- lizers of metoprolol. 3 Metabolism is the main process responsible for meto- prolol elimination. 4 The metabolites of metoprolol (see Fig. 1) are generated by aliphatic hydroxylation (a-hydrox- ymetoprolol, OHM), deamination (H104/83), and O-deal- kylation (O-demethylmetoprolol, ODM) followed by oxidation (acid metabolite, AODM). 5 The rate of a-hydrox- ylation exceeds O-demethylation by about 30% in rat liver microsomal preparations. 6 In these oxidations, CYP2D is associated with a-hydroxylation and an additional CYP enzyme is responsible for O-demethylation in human liver microsomes. 6,7 The a-hydroxylation pathway shows a high degree of product stereoselectivity in rat liver microsomes, mainly yielding the 1 0 R-hydroxy product (1 0 R/1 0 S > 12). 6 Boralli et al. 8 observed that the kinetic disposition of unchanged metoprolol and the formation of ODM are not enantiose- lective in rats but the metabolism of a-OHM mainly yields the 1 0 R product. As CYP2D is only partly responsible for O-demethylation of metoprolol in rats, 6 the involvement of another CYP isoform in the metabolic pathway might be assessed by chemical inhibitor studies. 9 Contract grant sponsors: Fundac ¸a ˜o de Amparo a ` Pesquisa do Estado de Sa ˜o Paulo (FAPESP), Conselho Nacional de Desenvolvimento Cientı ´fico and Tecnolo ´ gico (CNPq) *Correspondence to: Vera Lucia Lanchote, Faculdade de Cie ˆncias Farma- ce ˆuticas de Ribeira ˜o Preto-USP, Departamento de Ana ´lises Clı ´nicas, Toxi- colo ´ gicas and Bromatolo ´ gicas, Avenida do Cafe ´ s.n. Campus da USP, 14040-903, Ribeira ˜o Preto, SP, Brazil. E-mail: lanchote@fcfrp.usp.br Received for publication 30 April 2008; Accepted 14 October 2008 DOI: 10.1002/chir.20682 Published online 22 January 2009 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 21:886–893 (2009) V V C 2009 Wiley-Liss, Inc.