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and 257 ؎ 84 M, respectively, suggesting that FEX could not be used as a specific inhibitor for OATP1B1 and OATP1B3, although FEX was preferentially accepted by OATP1B3. In conclusion, this is, to our knowledge, the first demonstration that OATP1B3 is thought to be a major transporter involved in hepatic uptake of FEX in humans.Several members of different uptake transporter families are thought to be involved in the hepatic uptake of substances in human liver. Since the uptake of substances from blood into hepatocytes is the first step in the hepatocellular elimination, it is increasingly recognized that uptake transporters in the basolateral membrane play an important role in substrate disposition. Organic anion-transporting polypeptides (OATPs) form a superfamily of the sodium-independent transport system that mediates the transmembrane transport of a wide range of amphiphilic organic compounds including bile salts, organic dyes, steroid conjugates, thyroid hormones, anionic oligopeptides, and many drugs, such as pravastatin (Hagenbuch and Meier, 2004).Fexofenadine hydrochloride (FEX) (Aventis Pharmaceuticals, Inc., Kansas City, MO), a selective histamine H 1 -receptor antagonist, is clinically effective in the treatment of seasonal allergic rhinitis and chronic idiopathic urticaria, for which it is considered as first-line therapy (Markham and Wagstaff, 1998;Simpson and Jarvis, 2000). FEX is the active metabolite of terfenadine (Seldane) and FEX showed no significant effect on the prolongation of the corrected QT interval (QT c ) in contrast to terfenadine (Pratt et al., 1999). After oral administration of [ 14 C]FEX (60 mg), most of the total dose was recovered in the urine (12%) and feces (80%), with the majority of the dose (Ͼ85%) recovered as the unchanged form (Lippert et al., 1996). This shows that metabolism is an insignificant elimination route and that FEX is poorly absorbed and/or is mainly eliminated from the liver into bile in its unchanged form.Hepatic metabolism is of minimal importance in the elimination of FEX. On the other hand, coadministration of erythromycin (500 mg three times a day) or ketoconazole (400 mg once daily) with FEX resulted in substantial increase in steady-state plasma concentration of FEX and its plasma AUC by 109 and 164%, respectively (product information, Aventis Pharmaceuticals, Inc.). However, a regional perfusion study showed that ketoconazole did not have a significant effect on the in vivo intestinal absorption of FEX when coadministered or given as a pretreatment (Tannergren et al., 2003).FEX was shown to be a substrate of P-glycoprotein and OATP1A2 (OATP-A), and its disposition was altered in mdr1a (Ϫ/Ϫ) mice (Cvetkovic et al., 1999). In addition, rifampin increased the oral clearance of FEX, suggesting an up-regulation of P-glycoprotein and, possibly, other transport processes (Hamman et al., 2001). Currently, several OATP family transporters such as OATP1B1 (OATP2/OATP-C), OATP1B3 (OATP8), and OATP2B1 (OATP-B) have been identified on the basal membrane of human...

The amino-terminal and carboxyl-terminal portions of the 1-34 fragment of parathyroid hormone (PTH) contain the major determinants of receptor activation and receptor binding, respectively. We investigated how the amino-terminal signaling portion of PTH interacts with the receptor by utilizing analogs of the weakly active fragment, rat (r) PTH(1-14)NH 2 , and cells transfected with the wild-type human PTH-1 receptor (hP1R-WT) or a truncated PTH-1 receptor which lacked most of the amino-terminal extracellular domain (hP1R-delNt). Of 132 mono-substituted PTH(1-14) analogs, most having substitutions in the (1-9) region were inactive in assays of cAMP formation in LLC-PK1 cells stably expressing hP1R-WT, whereas most having substitutions in the (10-14) region were active. Several substitutions (e.g. Ser 3 3 Ala, Asn 10 3 Ala or Gln, Leu 11 3 Arg, Gly 12 3 Ala, His 14 3 Trp) enhanced activity 2-10-fold. These effects were additive, as [Ala In mammals, parathyroid hormone (PTH) 1 plays a vital role in regulating blood calcium concentrations, and PTH-related peptide (PTHrP) plays a critical role in the development of the fetal skeleton (1). The biological actions of both of these peptides are mediated by the PTH/PTHrP receptor (or PTH-1 receptor) (2), a family B G protein-coupled receptor (3) that strongly activates the adenylyl cyclase/protein kinase A-signaling cascade (2), and more weakly the phospholipase C protein kinase C-signaling pathway (4). The mechanisms by which parathyroid hormone and PTHrP bind to the PTH-1 receptor and induce receptor activation are poorly understood but appear to involve multiple sites of intermolecular interaction. Early studies of PTH fragment analogs assigned the major determinants of receptor-binding affinity and cAMP-stimulating potency to the COOH-terminal and NH 2 -terminal portions of the fully active PTH(1-34) peptide, respectively (5, 6). PTH(1-34)-based analogs with NH 2 -terminal deletions, such as PTH(3-34) and PTH(7-34), bind efficiently to the receptor and are severely defective in stimulating a cAMP response; such peptides thus function as PTH-1 receptor antagonists (7-9). The dominant role that the NH 2 -terminal residues of PTH and PTHrP play in receptor activation is further reflected by their high level of evolutionary conservation.The anabolic effects of PTH on bone density (10, 11) have prompted considerable interest in the development of new PTH-1 receptor agonist analogs. Recently PTH(1-28) was shown to be an effective agonist for cAMP production in cellbased assays, although potency was ϳ10-fold reduced from that of PTH(1-34) (12, 13). Recently we found that in COS-7 or LLC-PK1 cells transfected with high levels of rat or human PTH-1 receptors, a fragment as short as PTH(1-14) elicited ϳ20-fold increases in cAMP formation levels (14). Although the potency of PTH(1-14) in these transfected cells was weak compared with PTH(1-34) (EC 50 ϭ 1 nM and 100 M, respectively), the response was sufficient for us to perform an initial structure-activity relationship analysis...