Studies on intestinal absorption of sulpiride (2): transepithelial transport of sulpiride across the human intestinal cell line Caco-2

To determine the transport mechanism of sulpiride in an in vitro model of the human intestine, we investigated the transepithelial transport of this agent in Caco-2 cells. The transepithelial transport and intracellular accumulation of sulpiride were measured using Caco-2 cell monolayers cultured on a permeable membrane. The transepithelial transport of sulpiride in Caco-2 cells showed temperature dependence, and the transport was enhanced at weakly acidic pH on the apical side. These results demonstrate that the transepithelial transport of sulpiride is carrier mediated. To identify the drug transporter species that take part in the transepithelial transport of sulpiride, we examined the effects with the addition and preloading with specific substrates and inhibitors of various drug transporters. The results obtained from these examinations indicated that the apical-to-basolateral transport of sulpiride is mediated by the peptide transporter PEPT1, organic cation transporters OCTN1 and OCTN2 on the apical membrane, and the basolateral peptide transporter on the basolateral membrane. The basolateral-to-apical transport is mediated by the basolateral peptide transporter and organic cation transporter OCT1 on the basolateral membrane and by P-glycoprotein on the apical membrane. A decrease in the absorption of sulpiride may occur in coadministration protocols involving PEPT1-, OCTN1-, and OCTN2-transported drugs. Coadministration using the P-glycoprotein-transported drugs, in contrast, may enhance the absorption of sulpiride.     

Molecular and physiological evidence for multifunctionality of carnitine/organic cation transporter OCTN2

OCTN2 is an Na(+)-dependent transporter for carnitine, which is essential for fatty acid metabolism, and its functional defect leads to fatal systemic carnitine deficiency (SCD). It also transports the organic cation tetraethylammonium (TEA) in an Na(+)-independent manner. Here, we studied the multifunctionality of OCTN2, by examining the transport characteristics in cells transfected with mouse OCTN2 and in juvenile visceral steatosis (jvs) mice that exhibit a SCD phenotype owing to mutation of the OCTN2 gene. The physiological significance of OCTN2 as an organic cation transporter was confirmed by using jvs mice. The embryonic fibroblasts from jvs mice exhibited significantly decreased transport of [(14)C]TEA. Pharmacokinetic analysis of [(14)C]TEA disposition demonstrated that jvs mice showed decreased tissue distribution and renal secretory clearance. In transport experiments using OCTN2-expressing cells, TEA and carnitine showed mutual trans-stimulation effects in their transport, implying a carnitine/TEA exchange mechanism. In addition, Na(+) affected the affinity of carnitine for OCTN2, whereas Na(+) is unlikely to be involved in TEA transport. This is the first molecular and physiological demonstration of the operation of an organic cation transporter in renal apical membrane. The results are consistent with the physiological coupling of carnitine reabsorption with the secretion of organic cations.     

PDZK1 directly regulates the function of organic cation/carnitine transporter OCTN2

Urinary excretion of cationic xenobiotics is believed to be mediated by organic cation transporter (OCT and OCTN) families expressed on both basolateral and brush-border membranes of renal tubules, although the molecular mechanisms for targeting of these transporters to each membrane are poorly understood. Here, to examine the regulatory mechanisms for cell-surface expression and function of these transporters, we evaluated the interaction of these transporters with several PDZ proteins. A pull-down study using recombinant C-terminal proteins of OCTs and OCTNs identified a specific interaction of apical transporters OCTN1 and OCTN2, but not basolateral transporters OCT1 and OCT2, with PDZK1, intestinal and kidney-enriched PDZ protein, and Na+/H+ exchanger regulatory factor 2 (also called E3KARP, SIP-1, or TKA-1). Both yeast two-hybrid and pull-down studies suggested a requirement of the last four amino acids in OCTN1 and OCTN2 for the interaction. The interaction of PDZK1 with the C terminus of OCTN2 was also confirmed in a pull-down study using kidney brush-border membrane vesicles. Immunohistochemical analysis revealed that both PDZK1 and OCTN2 are colocalized in brush-border membranes of the kidney. Finally, double transfection of OCTN2 with PDZK1 stimulated the uptake by OCTN2 of its endogenous substrate carnitine, and this increase could be accounted for by the 6-fold increase in transport capacity. Such an increase was not observed for OCTN2 with deletion of the last four amino acids. Biotinylation study of surface proteins revealed minimal effect of PDZK1 on cell-surface expression of OCTN2. The present findings are the first to identify PDZK1 as a functional regulator of OCTN2 through direct interaction with the C terminus.     

Evaluation of air-interfaced Calu-3 cell layers for investigation of inhaled drug interactions with organic cation transporters in vitro

A physiologically pertinent in vitro model is urgently needed for probing interactions between inhaled drugs and the organic cation transporters (OCT) in the bronchial epithelium. This study evaluated OCT expression, functionality, inhibition by common inhaled drugs and impact on formoterol transepithelial transport in layers of human bronchial epithelial Calu-3 cells grown at an air-liquid interface. 21 day old Calu-3 layers expressed OCT1, OCT3, OCTN1 and OCTN2 whereas OCT2 could not be detected. Quantification of the cellular uptake of the OCT substrate ASP(+) in presence of inhibitors suggested several OCT were functional at the apical side of the cell layers. ASP(+) uptake was reduced by the bronchodilators formoterol, salbutamol (albuterol), ipratropium and the glucocorticoid budesonide. However, the OCT inhibitory properties of the two β(2)-mimetics were suppressed at therapeutically relevant concentrations. The absorptive permeability of formoterol across the cell layers was enhanced at a high drug concentration shown to decrease ASP(+) uptake by ～50% as well as in presence of the OCT inhibitor tetraethylammonium (TEA). Secretory transport was unaffected by the drug concentration but was reduced by TEA. Our data indicate air-interfaced Calu-3 layers offer a low-cost in vitro model suitable for assessing inhaled drug-OCT interactions in the bronchial epithelium.     

The blood-brain barrier choline transporter

Drug delivery to the brain is made difficult by the blood-brain barrier (BBB) which is selectively permeable to organic drug compounds. Several membrane solute and nutrient transporters are expressed in the BBB vasculature, which may be utilized as mechanism of delivery of drugs to the brain. Of interest to us, are the organic cation transporters which could be used to transport cationic compounds into the CNS. In this mini-review, we will review the current understanding of the structural requirements for designing compounds which could effectively use organic cation transporters (OCT). For the first time, structural requirements for both OCT1 and OCT2 versus the BBB choline transporter (BBBCHT) are discussed and compared. The information gained here could increase the success rate in successful CNS drug delivery and therapeutics.     

Agmatine and putrescine uptake in the human glioma cell line SK-MG-1

The pharmacological properties of a specific agmatine uptake mechanism were investigated in the human glioma cell line SK-MG-1 and compared with those of the putrescine transporter expressed by the same cells and with those of several other organic cation transport systems or ion channels reported in the literature. The specific accumulation of [14C]agmatine at 37 degrees C above nonspecific accumulation at 4 degrees C was energy-dependent and saturable with a Vmax of 64.3+/-3.5 nmol/min per mg protein and a Km of 8.6+/-1.4 microM. Specific accumulation was attenuated by replacement of extracellular Na+ by choline by 65%, not affected by lithium and enhanced by replacement by sucrose. Phentolamine, clonidine, 1,3-di(2-tolyl)guanidine, histamine, putrescine, spermine and spermidine were inhibitors of specific [14C]agmatine accumulation. In contrast, corticosterone, desipramine, O-methylisoprenaline, cirazoline, moxonidine, L-arginine, L-lysine, verapamil, nifedipine and CdCl2 at concentrations up to 10 mM failed to inhibit specific [14C]agmatine accumulation, thus excluding that the latter is mediated by amino acid or monoamine carriers, by Ca2+ channels or by the organic cation transporters OCT1, OCT2, OCT3, OCTN1 or OCTN2. The pattern of activity of inhibitory compounds was also different from that determined for specific putrescine accumulation found in the same cells (Km 1.3+/-0.1 microM, Vmax 26.1+/-0.4 nmol/min per mg protein) ruling out an identity of the specific [14C]agmatine and [14C]putrescine accumulation mechanisms. It is concluded that specific accumulation of agmatine in human glioma cells is mediated by a specific transporter whose pharmacological properties are not identical to those of the agmatine transporter previously identified in rat brain synaptosomes and to other so far known carrier mechanisms for organic cations and ion channels. The agmatine uptake system may be important for the regulation of the extracellular concentration of agmatine in man.     

Membrane transport function in primary cultures of human proximal tubular cells

To further develop primary cultures of human proximal tubular (hPT) cells for study of drug disposition, we determined kinetics and protein expression of several key transporters for organic anions and cations, peptides, and neutral amino acids. p-Aminohippurate uptake exhibited similar kinetics as published values, was inhibited by cephaloridine, cimetidine, methotrexate, and urate, consistent with function of both organic anion transporter 1 (OAT1) and OAT3. Transport rates by organic cation transporters (OCTs) were up to three-fold higher than those of OATs. Of the OCT substrates tested, triethanolamine exhibited the highest transport rates across the basolateral membrane (BLM). OCTN1 exhibited high-affinity, low-capacity BLM transport of l-carnitine. Glycylsarcosine transport by PepT2 was rapid and comparable to that of OCTs. Amino acid System L on the BLM exhibited comparable kinetic parameters for transport of l-leucine as the OATs. Efflux of verapamil across the brush-border membrane by P-glycoprotein was very rapid. Expression of carriers was generally maintained throughout 5 days of culture. Of the four OAT proteins studied (OAT1-4), expression of OAT1 and OAT3 was the most readily detected and exhibited interindividual variation. OCTN2 was the major OCT in hPT cells. Expression was also quantified for multidrug resistance-associated proteins 2 and 5 and P-glycoprotein. These results show that primary cultures of hPT cells express a diverse array of transporters for major classes of important drugs and are suitable for study of drug transport and disposition and assessment of potential drug-drug interactions in human kidney.     

PDZ adaptor protein PDZK2 stimulates transport activity of organic cation/carnitine transporter OCTN2 by modulating cell surface expression.

A part of the organic cation transporter families (OCT3, OCTN1, and OCTN2) has recently been identified to physically interact with PDZ (PSD95, Dlg, and ZO1) domain-containing proteins, although the physiological relevance of such interaction has not yet been fully examined. Here we have examined the stimulatory effect of PDZK2 [also named NaPi-Cap2 and intestinal and kidney-enriched PDZ protein (IKEPP)] on those cation transporters. In HEK293 cells, coexpression with PDZK2 increased the uptake of carnitine by OCTN2 with minimal effect on its substrate recognition specificity, but not for transport activity of OCT3 or OCTN1. The stimulatory effect of PDZK2 on OCTN2 was compatible with an approximately 2 times increase in transport capacity and can be accounted for by the increase in cell surface expression of OCTN2. Coexpression of PDZK2 did not affect carnitine transport activity of OCTN2 with deletion of the last four amino acids, which were found to be important for the interaction, suggesting involvement of physical interaction of the two proteins in the increase of cell surface expression of OCTN2. In mouse kidney, colocalization of PDZK2 and OCTN2 occurred predominantly in the region that was close to, but not the same as, the surface of apical membranes where OCTN2 alone was observed, suggesting the existence of OCTN2 in the subapical compartment that interacts with PDZK2. The present data have thus proposed an "intracellular pool" for OCTN2 that may be relevant to the stabilization of cell surface expression of OCTN2, thereby increasing transport activity for carnitine.     

Characterization of Calu-3 cell monolayers as a model of bronchial epithelial transport: organic cation interaction studies

Background: To fully exploit organic cation transporters for targeted drug delivery in the lung, the use of a readily available and well-characterized tissue culture model and cheap easily detectable substrates is indispensable.

Objectives: To investigate the suitability of Calu-3 as tissue model for characterizing organic cation permeation across the bronchial cells using a fluorescent dye, 4-(4-(Dimethylamino)styryl)-N-methylpyridinium iodide (4-DI-1-ASP).

Methods: Substrate uptake, inhibition, and transport were performed to establish active transport mechanism. Organic cation transporter expression was determined with quantitative polymerase chain reaction (qPCR), immune-histochemistry, and fluorescent microscopy.

Results: 4-Di-1-ASP uptake in Calu-3 cells was concentration (K_m = 2.7?±?0.3?mM, V_max = 4.6?±?2.6 nmol/µg protein/30?min), temperature (uptake at 37°C>>4°C), and pH dependent (higher uptake at pH ≥ 7). L-carnitine, verapamil, and corticosterone significantly inhibited its uptake with IC₅₀ of 28.2, 0.81, and 0.12?mM, respectively. Transport of the dye across the cells was polarized (AP→BL transport was 2.5-fold > BL→AP), saturable (Km = 43.9?±?3.2) (µM; Vmax =0.0228± nmol/cm²/sec) and reduced 3-fold by metabolic inhibition. The expression pattern of the organic cation transporters (OCT) and carnitine/organic cation transporter (OCTN) isoforms was: OCT1<<OCT3 <OCTN1<OCTN2; OCT2 was not detected.

Conclusions: Based on qPCR, immunohistochemistry, uptake and transport data, the Calu-3 cells can be used as a model for not only studying strategies for optimizing the effect of inhaled organic cations, but also for cross-validating newly-developed respiratory cell lines.     

Functional characterization of human organic cation transporter OCTN1 single nucleotide polymorphisms in the Japanese population

The organic cation transporter OCTN1 (SLC22A4) is expressed ubiquitously, with strong expression in kidney, trachea, bone marrow, and fetal liver, and it mediates transport of organic cations in a pH-dependent manner. Recent studies have identified single nucleotide polymorphisms (SNPs) of OCTN1 in the Japanese population. Two SNPs present in the exon regions, c1063t and g1531a, cause amino acid mutation, Thr306Ile (T306I) and Gly462Glu (G462E), respectively. We examined the influence of these SNPs on the intracellular localization, protein expression, and transport activity of OCTN1. Immunocytochemical analysis showed similar localizations of OCTN1 in cellular membranes of HEK293 cells transiently transfected with an expression plasmid DNA for OCTN1 or its SNP allelic variants. The Km and Vmax values for tetraethylammonium (TEA) uptake by T306I were similar to those of the wild-type even when the Vmax value was normalized for the expression level of OCTN1 protein. In contrast, G462E had almost negligible transport activity, although the protein expression level of G462E was equivalent to that of the wild-type. We conclude that the SNP that causes the single amino acid mutation T306I does not affect TEA transport activity, whereas the mutation G462E abrogates the TEA transport activity, presumably affecting the physiological function of OCTN1 and/or the pharmacological characteristics of its substrates.     

基于药物转运体的元胡止痛方药效成分延胡索乙素、巴马汀、原阿片碱、欧前胡素和异欧前胡素跨血脑屏障机制研究

目的 基于药物转运体研究元胡止痛方药效成分延胡索乙素、巴马汀、原阿片碱、欧前胡素和异欧前胡素跨血脑屏障转运机制。方法 建立hCMEC/D3-U87双层细胞模型研究延胡索乙素、巴马汀、原阿片碱、欧前胡素和异欧前胡素的跨血脑屏障吸收情况;通过实时荧光定量PCR （qRT-PCR）法测定hCMEC/D3细胞中相关转运体蛋白多药耐药蛋白1（MDR1）、乳腺癌耐药蛋白（BCRP）、有机阳离子转运体1（OCT1）、OCT2、OCT3、有机阴离子转运体1（OCTN1）、OCTN2、OATP1A2、OATP2B1的mRNA表达水平;使用6株过表达人转运体的细胞株（S2-OCT1、S2-OCT3、S2-OCTN1、S2-OCTN2、HEK293-OATP1A2、HEK293-OATP2B1）进行摄取实验,判断各药物成分是否为转运体蛋白的底物。结果 各成分均可以一定程度地跨越血脑屏障,延胡索乙素、欧前胡素、异欧前胡素具有更好的跨血脑屏障特性,巴马汀和原阿片碱的跨血脑屏障能力较弱。外排型转运蛋白MDR1、BCRP和摄入型转运蛋白OCTN1、OATP2B1、OATP1A2在hCMECD3细胞中有相对高的mRNA表达水平。延胡索乙素是OCT1、OCTN1和OATP1A2的底物;巴马汀是OCT1、OCT3、OCTN1、OCTN2和OATP2B1的底物;原阿片碱是OCT1、OCTN1和OCTN2的底物;欧前胡素、异欧前胡素是OCT3的底物。结论 延胡索乙素可能通过转运体OATP1A2、OCT1、OCTN1跨越血脑屏障进入脑组织细胞;巴马汀可能通过OATP2B1、OCT1、OCT3、OCTN1、OCTN2传递进入脑组织细胞;原阿片碱可能通过OCT1、OCTN1和OCTN2传递进入脑组织细胞;欧前胡素、异欧前胡素可能通过OCT3传递进入脑组织细胞。     

Wudang cherry ameliorates urate underexcretion and renal dysfunction in hyperuricemic mice

OBJECTIVE To investigate effects of Wudang cherry on urate excretion and renal function and examined whether renal organic ion transporters were involved in potassium oxonateinduced hyperuricemic mice.METHODS The model of hyperuricemic mice was induced by intraperitoneal injection of potassium oxonate(250 mg·kg~(-1))for 7 d.Water extracts of Wudang cherry at 500 mg·kg~(-1)were orally administered to hyperuricemic mice for 7 d,benzbromarone(20 mg·kg~(-1))and allopurinol(20 mg·kg~(-1))were given as positive controls,vehicle control group was given equal normal saline.Serum and urine levels of uric acid were measured in hyperuricemic and normal mice.Simultaneously,the m RNA and protein levels of mouse urate transporter 1(m URAT1),glucose transporter 9(mGLUT9),organic anion transporters(mOAT1 and mOAT3),ATP-binding cassette,subfamily G,membrane 2(mABCG2)and organic cation/carnitine transporters(m OCT1,m OCT2,m OCTN1 and m OCTN2)in the kidney were analyzed by Western blot,RT-PCR,immunohistochemical and immunofluorescent assay,respectively.RESULTS Wudang cherry significantly reduced serum uric acid levels and increased urine uric acid levels in hyperuricemic mice.And it effectively reversed potassium oxonate-induced alterations in renal m URAT1,mGLUT9,mOAT1,mOAT3 and mABCG2 m RNA and protein levels,resulting in the enhancement of renal urate excretion in mice.Moreover,Wudang Cherry increased renal m OCT1,m OCT2,m OCTN1 and m OCTN2 m RNA and protein levels,and improved renal impairment in this model.CONCLUSION Wudang cherry processes uricosuric and nephroprotective actions by regulating renal organic ion transporters in hyperuricemic mice.     

Activation of ATP-sensitive K+ channels by ADP and K+ channel openers: homology model of sulfonylurea receptor carboxyl-termini

The multispecific organic anion transporters have been indicated to be involved in the transmembrane transport of various anionic substances. The kidney and liver possess the distinct organic anion transport pathways for the elimination of potentially toxic anionic drugs and metabolites. In the kidney, proximal tubular cells actively excrete organic anions of both endogenous and exogenous origin. We have isolated the renal multispecific organic anion transporter, OAT1 (organic anion transporter 1), from the rat kidney. OAT1 is a 551-amino acid residue protein with 12 putative membrane spanning domains. OAT1 mediates sodium-independent, anion exchange for a variety of organic anions including p-aminohippurate, cyclic nucleotides, prostanoides, dicarboxylates, and anionic drugs including beta-lactams, non-steroidal antiinflammatory drugs, diuretics and antiviral drugs. So far, three other isoforms have been identified. OATs comprise a new family of multispecific organic anion transporter, i.e., the OAT family. OATs show weak structural similarity to organic cation transporters (OCTs) and OCTN/carnitine transporters. All of the members of the OAT family are commonly expressed in the kidney, suggesting its significance in the renal organic anion excretion. In addition, OAT members appear to be responsible for the distribution/elimination of water soluble anionic drugs into/from the liver, brain and fetus.     

Donepezil, tacrine and α-phenyl-n-tert-butyl nitrone (PBN) inhibit choline transport by conditionally immortalized rat brain capillary endothelial cell lines (TR-BBB)

In the present study, we have characterized the choline transport system and examined the influence of various amine drugs on the choline transporter using a conditionally immortalized rat brain capillary endothelial cell line (TR-BBB) in vitro. The cell-to-medium (C/M) ratio of [3H]choline in TR-BBB cells increased time-dependently. The initial uptake rate of [3H]choline was concentration-dependent with a Michaelis-Menten value, Km, of 26.2 +/- 2.7 microM. The [3H]choline uptake into TR-BBB was Na+-independent, but was membrane potential-dependent. The [3H]choline uptake was susceptible to inhibition by hemicholinium-3, and tetraethylammonium (TEA), which are organic cation transporter substrates. Also, the uptake of [3H]choline was competitively inhibited with Ki values of 274 microM, 251 microM and 180 microM in the presence of donepezil hydrochloride, tacrine and alpha-phenyl-n-tert-butyl nitrone (PBN), respectively. These characteristics of choline transport are consistent with those of the organic cation transporter (OCT). OCT2 mRNA was expressed in TR-BBB cells, while the expression of OCT3 or choline transporter (CHT) was not detected. Accordingly, these results suggest that OCT2 is a candidate for choline transport at the BBB and may influence the BBB permeability of amine drugs.     

Interaction of organic cations with a newly identified plasma membrane monoamine transporter

Many endogenous compounds and xenobiotics are organic cations that rely on polyspecific organic cation transporters (OCTs) to traverse cell membranes. We recently cloned a novel human plasma membrane monoamine transporter (PMAT) that belongs to the equillibrative nucleoside transporter (ENT) family. We have reported previously that, unlike other ENTs, PMAT (also known as ENT4) is a Na+-independent and membrane potential-sensitive transporter that transports monoamine neurotransmitters and the neurotoxin 1-methyl-4-phenylpyridinium (MPP+). These compounds are the known substrates for OCTs, which raises the possibility that PMAT functions as a polyspecific transporter like the OCTs. In the present study, we analyzed the interaction of PMAT with a series of structurally diverse organic cations using MDCK cells stably expressing human PMAT. Our study showed that PMAT interacts with many organic cations that have heterogeneous chemical structures. PMAT transports classic OCT substrates, such as tetraethylammonium, guanidine, and histamine. Prototype OCT inhibitors, including cimetidine, and type II cations (e.g., quinidine, quinine, verapamil, and rhodamine123) are also PMAT inhibitors. An analysis of molecular structures and apparent binding affinities revealed that charge and hydrophobicity are the principal determinants for transporter-substrate/inhibitor interaction. A planar aromatic mass seems to be important for high affinity interaction. trans-Stimulation and efflux studies demonstrate that PMAT is able to mediate bidirectional transport. These functional properties of PMAT are strikingly similar to those of the OCTs. We therefore conclude that PMAT can function as a polyspecific organic cation transporter, which may play a role in organic cation transport in vivo.     

The Poorly Membrane Permeable Antipsychotic Drugs Amisulpride and Sulpiride Are Substrates of the Organic Cation Transporters from the SLC22 Family

Variations in influx transport at the blood-brain barrier might affect the concentration of psychotropic drugs at their site of action and as a consequence might alter therapy response. Furthermore, influx transporters in organs such as the gut, liver and kidney may influence absorption, distribution, and elimination. Here, we analyzed 30 commonly used psychotropic drugs using a parallel artificial membrane permeability assay. Amisulpride and sulpiride showed the lowest membrane permeability (P_e < 1.5 × 10⁻⁶ cm/s) and will require influx transport to penetrate the blood-brain barrier and other physiological barriers. We then studied the uptake of amisulpride and sulpiride by the organic cation transporters of the SLC22 family OCT1, OCT2, OCT3, OCTN1, and OCTN2 Amisulpride was found to be transported by all five transporters studied. In contrast, sulpiride was only transported by OCT1 and OCT2. OCT1 showed the highest transport ability both for amisulpride (CL_int = 1.9 ml/min/mg protein) and sulpiride (CL_int = 4.2 ml/min/mg protein) and polymorphisms in OCT1 significantly reduced the uptake of both drugs. Furthermore, we observed carrier-mediated uptake that was inhibitable by known OCT inhibitors in the immortalized human brain microvascular endothelial cell line hCMEC/D3. In conclusion, this study demonstrates that amisulpride and sulpiride are substrates of organic cation transporters of the SLC22 family. SLC22 transporters may play an important role in the distribution of amisulpride and sulpiride, including their ability to penetrate the blood-brain barrier.

Electronic supplementary material

The online version of this article (doi:10.1208/s12248-014-9649-9) contains supplementary material, which is available to authorized users.KEY WORDS: amisulpride, blood-brain barrier, membrane permeability, organic cation transporters, sulpiride     

Biopharmaceutical studies on molecular mechanisms of membrane transport

By incorporating the transporter-mediated or receptor-mediated transport process in physiologically based pharmacokinetic models, we succeeded in the quantitative prediction of plasma and tissue concentrations of beta-lactam antibiotics, insulin, pentazocine, quinolone antibacterial agents, and inaperizone and digoxin. The author's research on transporter-mediated pharmacokinetics focuses on the molecular and functional characteristics of drug transporters such as oligopeptide transporter, monocarboxylic acid transporter, anion antiporter, organic anion transporters, organic cation/carnitine transporters (OCTNs), and the ATP-binding cassette transporters P-glycoprotein and MRP2. We have successfully demonstrated that these transporters play important roles in the influxes and/or effluxes of drugs in intestinal and renal epithelial cells, hepatocytes, and brain capillary endothelial cells that form the blood-brain barrier. In the systemic carnitine deficiency (SCD) phenotype mouse model, juvenile visceral steatosis (jvs) mouse, a mutation in the OCTN2 gene was found. Furthermore, several types of mutation in human SCD patients were found, demonstrating that OCTN2 is a physiologically important carnitine transporter. Interestingly, OCTNs transport carnitine in a sodium-dependent manner and various cationic drugs transport it in a sodium-independent manner. OCTNs are thought to be multifunctional transporters for the uptake of carnitine into tissue cells and for the elimination of intracellular organic cationic drugs.     

Polyspecific Organic Cation Transporters: Structure,Function, Physiological Roles,and Biopharmaceutical Implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.