Effects of Grapefruit Juice and Orange Juice on the Intestinal Efflux of P-Glycoprotein Substrates

Purpose. The aim of this study is to investigate the effects of 50% ethyl acetate extracts of grapefruit juice (GFJ) and orange juice (OJ) on the transport activity of P-glycoprotein (P-gp) in the rat small intestine. Methods.The efflux of P-gp substrates from rat everted sac in the absence or presence of verapamil, GFJ, OJ or erythromycin was measured. Rhodamine123, fexofenadine and saquinavir were used as P-gp substrates. P-gp expression levels in the rat jejunum and ileum were determined by Western blot analysis. Results. The efflux of rhodamine123 from the everted sac increased from the apex of the jejunum to the low ileum and the expression of P-gp in the ileum was 2.31-fold higher than that in the jejunum. Verapamil and the 50% GFJ and OJ extracts inhibited the efflux from the intestine of all three drugs tested. Erythromycin decreased the efflux of rhodamine123 and fexofenadine, but did not affect the efflux of saquinavir in the intestine. Conclusions. GFJ and OJ extracts inhibited the efflux of P-gp substrates from the small intestine. Therefore, they may enhance the oral bioavailability of P-gp substrates by increasing absorption in the small intestine.     

The Absence of Stereoselective P-Glycoprotein-mediated Transport of R/S-Verapamil Across the Rat Jejunum

We have studied the potential stereoselective transport and metabolism of R/S-verapamil in rat jejunum, in-situ. A regional single-pass perfusion of the rat jejunum was performed on 24 rats in six separate groups. The effective permeability (P_eff) was assessed for three different concentrations of verapamil, 4, 40 and 400 mg L^?1. The P_eff of each enantiomer was also determined at 400 mg L^?1 when chlorpromazine (10 mM) was added to the perfusion solution. Two other groups of rats received R/S-verapamil as an intravenous infusion and the intestinal secretion and metabolism were studied by simultaneously perfusing the jejunum with a control or with chlorpromazine (10 mM) added. The concentrations in the outlet perfusate of each enantiomer of verapamil and norverapamil were assayed with HPLC. R/S-Verapamil is a high permeability drug in the proximal rat small intestine throughout the luminal concentration range studied and complete intestinal absorption was expected. There was an increase of P_eff from 0.42 times 10^?4 cm s^?1 to 0.80 times 10^?4 cm s^?1 (P < 0.05) at concentrations from 4 to 400 mg L^?1, respectively. The observed concentration-dependent jejunal P_eff and fraction absorbed (P < 0.05) of R/S-verapamil is consistent with the saturation of an efflux mechanism. When chlorpromazine (a P-glycoprotein inhibitor/substrate) was added the jejunal P_eff increased to 1.47 times 10^?4 cm s^?1. There was no difference between the P_eff of the two enantiomers in any of these experiments. The efflux of R/S-norverapamil into the rat jejunum was high after intravenous administration of R/S-verapamil, suggesting extensive metabolism in the enterocyte. In conclusion, both R/S-verapamil enantiomers are P-glycoprotein substrates, but there is no stereoselective transport of R/S-verapamil in the rat jejunum. The results also suggests that R/S-norverapamil is formed inside the enterocytes.     

Determination of the enantioselectivity of six chiral aryloxy aminopropanol drugs transport across Caco-2 cell monolayers

This study aimed to determine the transepithelial transport characteristics of chiral drug enantiomers across Caco-2 cell monolayers, a model of human intestinal epithelial membrane. Six chiral aryloxy enantiomers (atenolol, sotalol, celiprolol, carvedilol, metoprolol and propafenone) were tested in bi-directional transport studies. The separation and quantitation of these enantiomers were performed by reversed-phase high-performance liquid chromatography (RP-HPLC) using 2, 3, 4, 6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate (GITC) as a pre-column derivatizing agent. Bi-directional transport studies demonstrated that celiprolol and carvedilol exhibited significant enantioselectivity in polarized transport at the concentration range tested. The efflux ratio (ER) for (R)-(+)-celiprolol was 8.96, but it was much lower for (S)-(-)-celiprolol which is 3.42 at the concentration of 96.0 μM; carvedilol had the same transport behavior as celiprolol while the difference between the ER values of two enantiomers was not as significant as celiprolol at the concentration of 5.0 μM. They are 2.41 for (R)-(+)-carvedilol and 1.98 for (S)-(-)-carvedilol. But in the transport studies of racemic atenolol, sotalol, metoprolol and propafenone, no enantioselective transport were observed over the concentration range tested. Because P-glycoprotein (P-gp) is highly expressed in Caco-2 cells, we inferred that P-gp might participate in the transport processes of celiprolol and carvedilol in chirally discriminative ways.     

Stereoselective drug distribution and anticoagulant potency of the enantiomers of phenprocoumon in rats

The elimination, distribution and anticoagulant activity of S(—)-, R(+)-, and R,S(±)-phenprocoumon were determined in male Wistar-Lewis rats after intravenous injection of a single dose of 0·6 mg kg^?1. From the plasma concentrations which elicited the same anticoagulant effect, S(—)-phenprocoumon was 4 to 5 times more potent than R(+)-phenprocoumon. The potency of the racemate was between those of the enantiomers. The mean biologic half-life of the S(—)-enantiomer was shorter (12·5 h) than that of R(+)-phenprocoumon (17·8 h). No differences were observed in the apparent volume of distribution. However, the mean liver: plasma concentration ratio was higher for the S(—)-(6·9) than for the R(+)-enantiomer (5·2). At a total concentration of 16·8 μg ml^?1 the percentage of unbound drug in rat serum was significantly higher for the S(—)- (1·13%) than that for the R(+)-enantiomer (0·76%). Values obtained for the racemate were always between those of the enantiomers. It is concluded that stereoselective differences in the distribution between plasma and liver, and consequently in the rate of elimination are most likely due to stereoselective differences in serum protein binding. The greater anticoagulant potency of the S(—)- over the R(+)-enantiomer, cannot be explained primarily by the observed pharmacokinetic differences.     

Role of intestinal efflux transporters in the intestinal absorption of methotrexate in rats

The role of intestinal efflux transporters such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs) in intestinal absorption of methotrexate was examined in rats. In everted intestine, the mucosal efflux of methotrexate after application to serosal side was higher in jejunum than ileum, and the efflux in jejunum was suppressed by pantoprazole, a BCRP inhibitor, and probenecid, an MRP inhibitor, but not by verapamil, a P-gp inhibitor. The mucosal methotrexate efflux in ileum was suppressed by pantoprazole, but not by other inhibitors. On the other hand, the serosal efflux of methotrexate after application to mucosal side was greater in ileum than jejunum, and was suppressed by probenecid. In in-vivo rat studies, the intestinal absorption of methotrexate was significantly higher when methotrexate was administered to ileum than jejunum. Pantoprazole increased methotrexate absorption from jejunum and ileum. Probenecid increased the absorption of methotrexate from jejunum but decreased the absorption from ileum, as evaluated by peak plasma methotrexate levels. In conclusion, BCRP and MRPs are involved in the regional difference in absorption of methotrexate along the intestine, depending on their expression sites.     

外翻肠囊法研究奥硝唑对映体在大鼠不同肠段的吸收特性

目的考察奥硝唑在大鼠各肠段的吸收特性及奥硝唑两手性对映体在大鼠不同肠段吸收的差异性。方法采用大鼠外翻肠囊法,以HPLC手性色谱柱法测定奥硝唑在不同肠段的吸收量以及S-奥硝唑及R-奥硝唑的同一肠段肠吸收量,并分别计算吸收速率常数(Ka)和表观渗透系数(P_(app)),同法考察了P-蛋白抑制剂(维拉帕米和环孢素A)对奥硝唑肠吸收特性的影响。结果奥硝唑在不同肠段吸收均呈线性,符合零级药物吸收速率,吸收趋势为空肠>回肠>结肠。在空肠段两对映体以近于1∶1的比例同时吸收,吸收速率不存在显著差异,随着供试液中奥硝唑质量浓度的上升,Ka呈线性增加(R~2>0.99),Paap基本保持不变,P-蛋白抑制剂对奥硝唑的肠吸收没有显著性影响(P>0.05)。结论奥硝唑在不同肠段的吸收有差异,空肠部位是吸收的最佳部位(P<0.05),S-奥硝唑与R-奥硝唑在肠道中吸收速率不存在显著差异,吸收以被动扩散机制为主,奥硝唑不是P-糖蛋白的底物。     

延胡索乙素在大鼠体内的立体选择性药代动力学延胡索乙素在大鼠体内的立体选择性药代动力学

目的研究dl-延胡索乙素在大鼠体内的立体选择性动力学过程。方法血浆样品经液-液萃取后，用非手性-手性HPLC联用法测定血浆中延胡索乙素对映体的浓度，配对t-检验比较两对映体的血药浓度和药代动力学参数。结果大鼠血浆中l-延胡索乙素的浓度均明显高于d-延胡索乙素的浓度，两对映体的AUC，MRT和Cmax均有显著性差异。结论大鼠灌胃延胡索乙素消旋体后，两对映体在体内的药代动力学过程具有显著的立体选择性。     

大鼠肠道对左旋延胡索乙素及其消旋体的吸收差异研究

考察延胡索乙素(THP)的吸收机制，并研究其消旋体与左旋延胡索乙素(l-THP)在大鼠肠道的吸收差异。应用单向灌流模型，采用HPLC法测定THP及l-THP在灌流液中的浓度变化。灌流液中THP质量浓度为8，16，32 μg·mL^-1时，THP吸收速率常数和有效吸收系数均无统计差异(P>0.05)，各肠段的吸收速率常数和有效吸收系数也无统计差异(P>0.05)； l-THP和THP在大鼠肠道吸收存在显著性差异(P<0.05)； 在肠道灌流液中加入P-糖蛋白(P-gp)抑制剂维拉帕米后，THP吸收显著增加，而l-THP吸收几乎不变。THP在肠黏膜的转运为被动扩散过程，无特殊吸收窗口；THP消旋体与l-THP的吸收差异可能与P-gp与右旋THP的选择性结合有关。     

Differential tissue distribution of the enantiomers of racemic pindolol in the rat

Racemic pindolol, a β-adrenoceptor and a 5-HT_1A/1B receptor antagonist, has been reported to augment and accelerate the clinical efficacy of antidepressants. The (S)-enantiomer is more potent than the (R)-enantiomer both at the β-adrenergic and at the 5-HT_1A/1B receptors. A chiral HPLC column was used for determination of tissue concentrations of the enantiomers of pindolol at 90 min after 8 mg/kg s.c. of the racemate. The (S)/(R) ratio was found to vary between tissues from 1.74 in brain to 0.82 in plasma. The present findings may be important for understanding the pharmacodynamic actions of racemic pindolol.     

The action of flecainide acetate and its enantiomers on mammalian non-myelinated nerve fibres

The effects of flecainide acetate racemate and its two enantiomers on voltage-operated sodium and potassium channels and on the sodium pump activity of non-myelinated fibres of the guinea-pig vagus nerve were studied with the sucrose-gap method. The racemic mixture as well as theR enantiomer andS enantiomer in a concentration range of 3·10^?5?3·10^?4 M caused suppression of the compound action potential, a diminished propagation velocity and a reduction of the post-tetanic potential (PTH), which was also observed with lidocaine. There was no significant difference in the effect caused by the enantiomers seperately. TheR enantiomer tended to evoke a stronger effect compared with theS enantiomer. However, the magnitude of the action is concentration-dependent. At a concentration<10^?4 M the action of the racemate was stronger than an equimolar concentration of the enantiomers. The activity of the sodium pump, defined by the time constant of post-tetanic potential decay, was affected at a concentration of 10^?4 M of the racemate, in contrast to lidocaine. The racemate and both enantiomers of flecainide acetate possess a similar local anaesthetic action, as reflected by the inhibition of voltage-operated sodium channels.     

反式曲马朵在大鼠小肠中吸收的立体选择性

目的研究反式曲马朵(Trans T)在大鼠小肠中吸收的立体选择性。方法高效毛细管电泳法测定小肠分段灌流液中Trans T对映体的浓度。结果Trans T对映体在小肠不同部位的吸收分数基本一致;在低浓度时(+)-Trans T的吸收分数明显低于(-)-Trans T;在高浓度时Trans T对映体的吸收分数降低,(+)-Trans T与(-)-Trans T的吸收分数无明显差别。结论Trans T在大鼠小肠的不同部位均能被吸收,具立体选择性,以(-)-Trans T占优。     

Evidence of efflux-mediated and saturable absorption of rifampicin in rat intestine using the ligated loop and everted gut sac techniques

This study was carried out to explore whether efflux-mediated and saturable mechanisms play any role toward poor and variable intestinal absorption of rifampicin. In situ segmental permeability of rifampicin at various residence times was determined in rat gastrointestinal tract using the ligated loop technique. The involvement of efflux-mediated and saturable absorption of rifampicin was studied in rat intestine using the everted sac method. The samples were analyzed by a validated HPLC method. Rifampicin showed decreased permeability in jejunum and ileum with an increase in residence time. The permeation of rifampicin from the serosal to the mucosal side (secretion) was significantly higher than permeation from the mucosal to the serosal side (absorption) of jejunum and ileum. This indicated the involvement of efflux-mediated transport. Addition of verapamil, an inhibitor for P-glycoprotein (Pgp), multidrug resistance associated protein-2 (MRP-2), and other related transporters, increased absorption of rifampicin in jejunum and ileum by 2-3-fold and decreased secretion by almost 4-fold. The permeation rate (flux) of rifampicin through duodenum increased with concentration up to 300 microg/mL, becoming constant thereafter, indicating the existence of saturable absorption. There was no saturable permeation in jejunum and ileum. Thus the present study indicates the involvement of efflux-mediated and saturable absorption mechanisms of rifampicin in rat intestine, which act as barriers to the absorption of the drug. This explains the drug's poor absolute bioavailability. As Pgp varies from person to person to an extent of 2-8-fold, it can be one direct reason for the interindividual variable bioavailability shown by rifampicin.     

Regioselective O-methylation of tetrahydropapaveroline and tetrahydroxyberbine in vivo in rat brain

Enzymatic O-methylation is a primary pathway for the metabolism of catecholamines in mammals and of isoquinoline alkaloids in plants. This report describes the differential O-methylation patterns of the racemates and enantiomers of two catecholamine-derived alkaloids, tetrahydropapaveroline (THP) and 2,3,10,11-tetrahydroxyberbine (THB), in the brain of the rat. One hour after intracerebroventricular administration of a specific isomeric form of each alkaloid, the O-methylated metabolites were isolated from the rat brain and subsequently quantified using high performance liquid chromatography. The isomeric form of THP or THB which was administered markedly influenced the pattern of O-methylation. The racemate and R-(+)-enantiomer of THP were mono-O-methylated predominantly at the 7 and 3′ positions, while the S-(?)-enantiomer of THP was mono-O-methylated to an essentially equal degree at the 6, 7 and 3′ positions. Minimal mono-O-methylation at the 4′ position was detectable only with the racemate and (?)-enantiomer of THP. The racemate and enantiomers of THB were mono-O-methylated predominantly at the 2 and 11 positions and to a lesser extent at the 3 and 10 positions. Although minimal with the R-(+)-enantiomer, the 3 and the 10-O-methylation pathways were enhanced significantly with the S-(?)-enantiomer of THB. These results demonstrate that both enantiomers of THP and THB are O-methylated in vivo in rat brain and that the chiral centers of these alkaloids influence the position of O-methylation, thereby dictating the relative amounts of specific products formed.     

Possible Involvement of Multiple P-Glycoprotein-Mediated Efflux Systems in the Transport of Verapamil and Other Organic Cations Across Rat Intestine

Purpose. We investigated the intestinal transport of verapamil, chlorpromazine, and propantheline, particularly their P-glycoprotein-mediated secretion. Methods. Permeation of rat intestinal segments in vitro was determined using diffusion cells. Results. Verapamil permeation in the serosal-to-mucosal direction was much greater than in the mucosal-to-serosal direction using duodenal, jejunal, and colonic membranes. The concentration dependence of jejunal permeation in the absorptive and secretory directions was consistent with saturability of a secretory transport system. Using a monoclonal antibody to inhibit P-glycoprotein-mediated secretion caused a significant enhancement of verapamil absorption through the jejunum. In contrast, the rat ileum did not preferentially transport verapamil in the secretory direction, and the P-glycoprotein antibody had no effect on ileal absorption. Chlorpromazine and propantheline enhanced the mucosal-to-serosal permeation of verapamil through the jejunum, most likely due to competitive inhibition of the P-glycoprotein-mediated secretory process. Vinblastine, tetraethylammonium, and guanidine did not affect verapamil permeation. Propantheline was also a substrate for P-glycoprotein-mediated secretory transport, but in contrast to verapamil, propantheline secretory transport was expressed in rat ileum. Conclusions. These results suggest that these cationic compounds are transported by plural P-glycoprotein-mediated efflux systems with different substrate specificities depending on the intestinal site.     

Modulation of intestinal transport of 2,4-dinitrophenyl-S-glutathione, a multidrug resistance-associated protein 2 substrate, by bilirubin treatment in rats

The effect of bilirubin treatment on intestinal transport of 2,4-dinitrophenyl-S-glutathione (DNP-SG), a substrate of multidrug resistance-associated protein 2 (MRP2), after application of 1-chloro-2, 4-dinitrobenzene (CDNB), a precursor of DNP-SG, was examined in rat intestine by the in-vitro everted sac, in-situ re-circulating perfusion, and in-situ loop methods. CDNB was taken up rapidly by jejunum and ileum, and the consequent intestinal efflux of DNP-SG, a glutathione conjugated metabolite of CDNB, was significantly higher in jejunum than in ileum in the in-situ and in-vitro studies. Co-administration of bilirubin (100 microM), as well as probenecid (1 mM) or ciclosporin (100 microM), with CDNB decreased the DNP-SG efflux in jejunum significantly, but not in ileum. The suppression of DNP-SG efflux in jejunum was also observed after intravenous administration of bilirubin (85.5 micromol kg-1), in which plasma bilirubin glucuronide levels were approximately 100 microM. In the in-vitro metabolism study, bilirubin exerted no significant effect on CDNB metabolism in the intestinal S9 fraction (supernatant of 9000 g). These results suggested that the diseased states accompanied with hyperbilirubinaemia might have increased the intestinal absorption, or oral bioavailability, of MRP2 substrates by suppressing MRP2 function at the proximal intestinal region.     

Enantiomeric antidepressant drugs should be considered on individual merit

Many antidepressants have been introduced as racemic drugs, the enantiomers of which may differ in some of their pharmacodynamic and pharmacokinetic properties. This review argues that each enantiomer of a chiral antidepressant should be evaluated according to its individual characteristics rather than by extrapolation from the racemate, or by assumptions based on the stereoselective characteristics of other enantiomeric drugs. For example, in some cases the enantiomers' pharmacodynamic and therapeutic properties can be complementary, which suggests that the racemate should be used clinically. In other cases where enantiomers show qualitatively similar but quantitatively different properties to the racemate, using a single enantiomer might be more appropriate. In yet further cases, a distomer may induce the metabolism of the eutomer, enantiomers may be metabolised by different enzymes, there may be a different profile of drug-drug interactions, and therapeutic drug monitoring may be simpler. Therefore, this review exemplifies the principle that each enantiomer of a chiral antidepressant should be evaluated according to its individual pharmacological, pharmacokinetic and pharmacogenetic characteristics. These factors are discussed in relation to five chiral antidepressants: trimipramine, mianserin, mirtazapine, fluoxetine and citalopram. It is hoped that an appreciation of the stereoselective differences between enantiomers will facilitate improvements in the benefit:risk ratio of drugs used in the management of depression. Copyright 2001 John Wiley & Sons, Ltd.     

3种药物转运体抑制剂对沙奎那韦在大鼠肠道吸收影响的体外试验

目的:体外研究药物转运体P糖蛋白(P-gp)和多药耐药相关蛋白2(Mrp2)特异性抑制剂对沙奎那韦在大鼠肠道吸收的影响。方法:取大鼠用乌拉坦麻醉后,分别取十二指肠、空肠、回肠、结肠各8cm,制备离体肠外翻模型,检测不同肠段在P-gp抑制剂地高辛(10μmol.L-1)和维拉帕米(100μmol.L-1)、Mrp2抑制剂丙磺舒(600μmol.L-1)分别与沙奎那韦(12.5μg.mL-1)的混合Krebs-Ringer缓冲液(K-R液)中孵育5、10、20、30、45、60、90min后对沙奎那韦的累积吸收量;另设含沙奎那韦的K-R液为对照组。结果:沙奎那韦在大鼠十二指肠、空肠、回肠、结肠K-R液中的累积吸收量分别为(7.25±1.23)、(4.96±1.58)、(3.89±0.95)、(5.85±1.21)μg,吸收速率为十二指肠>结肠>空肠>回肠。维拉帕米((10.03±3.56)、(7.52±2.21)、(7.45±1.8)μg)和地高辛((8.76±2.25)、(5.98±1.89)、(6.04±1.92)μg)可显著提高沙奎那韦在结肠、空肠、回肠K-R液中的累积吸收量(P<0.05),对十二指肠无显著影响(P>0.05);丙磺舒对沙奎那韦在各肠段K-R液中的累积吸收量均无显著影响(P>0.05)。结论:P-gp可显著影响沙奎那韦的肠道吸收,Mrp2对沙奎那韦的肠道吸收无影响。     

Mechanisms Responsible for Poor Oral Bioavailability of Paeoniflorin: Role of Intestinal Disposition and Interactions with Sinomenine

Purpose To determine the intestinal disposition mechanisms of paeoniflorin, a bioactive glucoside, and to investigate the mechanisms by which sinomenine increases paeoniflorin bioavailability. Materials and Methods A single-pass “four-site” rat intestinal perfusion model and a cultured Caco-2 cell model were employed. Results In both model systems, paeoniflorin permeability was poor. In the perfusion model, maximal absorption and metabolism of paeoniflorin occurred in duodenum and jejunum, which were significantly decreased by a glucosidase inhibitor gluconolactone (20 mM). On the other hand, paeoniflorin absorption in terminal ileum increased significantly but its metabolism did not in the presence of sinomenine and cyclosporine A. In the Caco-2 cell model, paeoniflorin was transported 48-fold slower than its aglycone (paeoniflorigenin). Absorptive transport of paeoniflorin was significantly (p < 0.05) increased by sinomenine (38%), verapamil (27%), and cyclosporine A (41%), whereas its secretory transport was significantly (p < 0.01) decreased by sinomenine (50%), verapamil (35%) and cyclosporine A (37%). In contrast, MRP inhibitors MK-571 and leukotriene C4 did not affect transport of paeoniflorin. Lastly, sinomenine was also shown to significantly increase the absorptive transport of digoxin (a prototypical p-glycoprotein substrate) and to significantly decrease its secretory transport. Conclusions Poor permeation, p-gp-mediated efflux, and hydrolysis via a glucosidase contributed to the poor bioavailability of paeoniflorin. Sinomenine (an inhibitor of the p-gp-mediated digoxin efflux) increased paeoniflorin's bioavailability via the inhibition of p-gp-mediated paeoniflorin efflux in the intestine.     

Cerebral uptake of mefloquine enantiomers with and without the P-gp inhibitor elacridar (GF1210918) in mice

1. Mefloquine is a chiral neurotoxic antimalarial agent showing stereoselective brain uptake in humans and rats. It is a substrate and an inhibitor of the efflux protein P-glycoprotein. 2. We investigated the stereoselective uptake and efflux of mefloquine in mice, and the consequences of the combination with an efflux protein inhibitor, elacridar (GF120918) on its brain transport. 3. Racemic mefloquine (25 mg kg(-1)) was administered intraperitoneally with or without elacridar (10 mg kg(-1)). Six to seven mice were killed at each of 11 time-points between 30 min and 168 h after administration. Blood and brain concentrations of mefloquine enantiomers were determined using liquid chromatography. 4. A three-compartment model with zero-order absorption from the injection site was found to best represent the pharmacokinetics of both enantiomers in blood and brain. (-)Mefloquine had a lower blood and brain apparent volume of distribution and a lower efflux clearance from the brain, resulting in a larger brain/blood ratio compared to (+)mefloquine. Elacridar did not modify blood concentrations or the elimination rate from blood for either enantiomers. However, cerebral AUC(inf) of both enantiomers were increased, with a stronger effect on (+)mefloquine. The efflux clearance from the brain decreased for both enantiomers, with a larger decrease for (+)mefloquine. 5. After administration of racemic mefloquine in mice, blood and brain pharmacokinetics are stereoselective, (+)mefloquine being excreted from brain more rapidly than its antipode, showing that mefloquine is a substrate of efflux proteins and that mefloquine enantiomers undergo efflux in a stereoselective manner. Moreover, pretreatment with elacridar reduced the brain efflux clearances with a more pronounced effect on (+)mefloquine.     

An update on the clinical pharmacokinetics of fexofenadine enantiomers

Introduction: Fexofenadine is administered as a racemic mixture of (R)- and (S)-enantiomers. The plasma concentrations of (R)-fexofenadine in humans are about 1.5-fold higher than those of the (S)-enantiomer. Such differences in the pharmacokinetics between fexofenadine enantiomers are likely to be dependent on stereoselectivity for af?nity to drug-transporters.

Areas covered: This review focuses on elucidation of differences in clinical pharmacokinetics between fexofenadine enantiomers.

Expert opinion: Differences in pharmacokinetics between fexofenadine enantiomers were caused by organic anion transporting polypeptide (OATP) 2B1, with a minor contribution from P-glycoprotein (P-gp). In vitro studies using OATP2B1 cRNA showed that (R)-fexofenadine uptake into oocytes is greater than (S)-enantiomer uptake. P-gp inducers, carbamazepine, and inhibitors such as itraconazole and verapamil show greater effects on the pharmacokinetics of (S)-fexofenadine. Apple juice and grape fruit juice, OATP2B1 inhibitors, significantly decrease the exposure of both fexofenadine enantiomers, particularly the (S)-enantiomer, but do not change the t_1/2. Rifampicin significantly increases plasma concentrations of both enantiomers through inhibition of OATP1B3, whereas enantioselectivity of fexofenadine uptake by OATP1B3-expressing cells has not been observed. Combinations of multiple transporters such as OATP2B1 and P-gp facilitate enantioselective disposition of fexofenadine. Drug-transporters appear to be capable of chiral discrimination for transport of drugs with an asymmetric center.