Carvedilol is a b-adrenoceptor antagonist, and has been clinically used to treat chronic heart failure as well as hypertension, angina pectoris, and cardiac arrhythmias. [1][2][3][4] Carvedilol is highly lipophilic and is absorbed rapidly from the gastrointestinal tract after oral administration. Orally administered carvedilol undergoes stereoselective first-pass metabolism, and the blood concentration of R-enantiomer with very low b-blocking activity is approximately 2-fold higher than that of S-enantiomer with high b-blocking activity. 5,6) Both enantiomers are eliminated predominantly by hepatic metabolism, with renal excretion accounting for only 0.3% of the administered dose. 7) Carvedilol is metabolized extensively via aliphatic side-chain oxidation, aromatic ring oxidation, and conjugation pathways.8) Oldham and Clarke reported that oxidative activity for carvedilol is observed in cytochrome P450 (CYP) 2D6, 2C9, 3A4, and 1A2. 9) In addition, Ohno et al. reported that UDP-glucuronosyltransferase (UGT) 2B7, 2B4, and 1A1 are capable of catalyzing the glucuronidation of carvedilol. 10) However, it is still unclear which enzyme is responsible for the stereoselective presystemic clearance of carvedilol. Furthermore, although CYP3A4 and several UGTs are expressed in intestinal epithelial cells, it is also unknown whether the intestine as well as the liver is involved in the first-pass metabolism of carvedilol.The approach for investigating intestinal drug absorption and metabolism is to use human intestinal cell lines, such as the frequently used line Caco-2. This line spontaneously differentiates, after which it shows enterocyte-like morphology and forms polarized monolayers via tight junctions. It also expresses various transporters and efflux pumps such as P-glycoprotein.11,12) Therefore, it has been utilized to examine the mechanism responsible for the intestinal absorption of various compounds. 12,13) In addition, several UGTs, such as UGT1A1, 1A6, and 2B7, are expressed in Caco-2 cells, and the expression of some UGTs is induced by aromatic hydrocarbon receptor (AhR) ligands, such as b-naphthoflavone (b-NF).14,15) Therefore, glucuronidation of drugs in the intestine has been studied using Caco-2 cells. [16][17][18] In contrast, the Caco-2 cell line had not been employed to examine the intestinal metabolism of CYP3A4 substrates due to its lack of CYP3A4 expression. Recently, however, it has been reported that CYP3A4 in Caco-2 cells is induced by 1a,25-dihydroxyvitamin D 3 (VD 3 ). 11,19) The finding suggests that this cell line may also be utilized to investigate intestinal drug metabolism by CYP3A4. In the present study, we investigated whether carvedilol is metabolized stereoselectively in Caco-2 cells, and also evaluated the changes in the metabolic rate of carvedilol in b-NF-and VD 3 -treated Caco-2 cells.
MATERIALS AND METHODS
MaterialsCarvedilol was kindly supplied by Daiichi Pharmaceutical (Tokyo, Japan). b-NF was obtained from Nacalai Tesque (Kyoto, Japan). VD 3 and 3Ј-azido-3Ј-deoxythimidine (AZT...