Drug-drug interactions involving membrane transporters in the human kidney

Molecular basis for selective uptake and elimination of organic anions in the kidney by OAT1

In mammals, the kidney plays an essential role in maintaining blood homeostasis through the selective uptake, retention or elimination of toxins, drugs and metabolites. Organic anion transporters (OATs) are responsible for the recognition of metabolites and toxins in the nephron and their eventual urinary excretion. Inhibition of OATs is used therapeutically to improve drug efficacy and reduce nephrotoxicity. The founding member of the renal organic anion transporter family, OAT1 (also known as SLC22A6), uses the export of α-ketoglutarate (α-KG), a key intermediate in the Krebs cycle, to drive selective transport and is allosterically regulated by intracellular chloride. However, the mechanisms linking metabolite cycling, drug transport and intracellular chloride remain obscure. Here, we present cryogenic-electron microscopy structures of OAT1 bound to α-KG, the antiviral tenofovir and clinical inhibitor probenecid, used in the treatment of Gout. Complementary in vivo cellular assays explain the molecular basis for α-KG driven drug elimination and the allosteric regulation of organic anion transport in the kidney by chloride.

Kidney Drug Transporters in Pharmacotherapy

The kidney functions not only as a metabolite elimination organ but also plays an important role in pharmacotherapy. The kidney tubule epithelia cells express membrane carriers and transporters, which play an important role in drug elimination, and can determine drug nephrotoxicity and drug-drug interactions, as well as constituting direct drug targets. The above aspects of kidney transport proteins are discussed in the review.

Functional Expression of Multidrug-Resistance (MDR) Transporters in Developing Human Fetal Brain Endothelial Cells

There is little information about the functional expression of the multidrug resistance (MDR) transporters P-glycoprotein (P-gp, encoded by ABCB1) and breast cancer resistance protein (BCRP/ABCG2) in the developing blood−brain barrier (BBB). We isolated and cultured primary human fetal brain endothelial cells (hfBECs) from early and mid-gestation brains and assessed P-gp/ABCB1 and BCRP/ABCG2 expression and function, as well as tube formation capability. Immunolocalization of the von Willebrand factor (marker of endothelial cells), zonula occludens-1 and claudin-5 (tight junctions) was detected in early and mid-gestation-derived hfBECs, which also formed capillary-like tube structures, confirming their BEC phenotype. P-gp and BCRP immunostaining was detected in capillary-like tubes and in the cytoplasm and nucleus of hfBECs. P-gp protein levels in the plasma membrane and nuclear protein fractions, as well as P-gp protein/ABCB1 mRNA and BCRP protein levels decreased (p < 0.05) in hfBECs, from early to mid-gestation. No differences in P-gp or BCRP activity in hfBECs were observed between the two age groups. The hfBECs from early and mid-gestation express functionally competent P-gp and BCRP drug transporters and may thus contribute to the BBB protective phenotype in the conceptus from early stages of pregnancy.

Effects of Probenecid on Hepatic and Renal Disposition of Hexadecanedioate, an Endogenous Substrate of Organic Anion Transporting Polypeptide 1B in Rats

The aim of the present study was to investigate changes in plasma concentrations and tissue distribution of endogenous substrates of organic anion transporting polypeptide (OATP) 1B, hexadecanedioate (HDA), octadecanedioate (ODA), tetradecanedioate (TDA), and coproporphyrin-III, induced by its weak inhibitor, probenecid (PBD), in rats. PBD increased the plasma concentrations of these four compounds regardless of bile duct cannulation, whereas liver-to-plasma (K_p,liver) and kidney-to-plasma concentration ratios of HDA and TDA were reduced. Similar effects of PBD on plasma concentrations and K_p,liver of HDA, ODA, and TDA were observed in kidney-ligated rats, suggesting a minor contribution of renal disposition to the overall distribution of these three compounds. Tissue uptake clearance of deuterium-labeled HDA (d-HDA) in liver was 16-fold higher than that in kidney, and was reduced by 80% by PBD. This was compatible with inhibition by PBD of d-HDA uptake in isolated rat hepatocytes. Such inhibitory effects of PBD were also observed in the human OATP1B1-mediated uptake of d-HDA. Overall, the disposition of HDA is mainly determined by hepatic OATP-mediated uptake, which is inhibited by PBD. HDA might, thus, be a biomarker for OATPs minimally affected by urinary and biliary elimination in rats.

Pharmacokinetic drug–drug interactions: an insight into recent US FDA-approved drugs for prostate cancer

Pharmacokinetic drug–drug interaction is a significant safety and efficiency concern as it results in considerable concentration changes. Drug–drug interactions are a substantial concern in anticancer drugs that possess a narrow therapeutic index. These interactions remain as the principal regulatory obstacle that can lead to termination in the preclinical stage, restrictions in the prescription, dosage adjustments or withdrawal of the drugs from the market. Drug metabolizing enzymes or transporters mediate the majority of clinically relevant drug interactions. Cancer diagnosed aged patients use multiple medications and are more prone to significant drug–drug interactions. This review provides detailed information on clinically relevant drug–drug interactions resulting from drug metabolism by enzymes and transporters with a particular emphasis on recent FDA approved antiprostate cancer drugs.

Drug Metabolites Potently Inhibit Renal Organic Anion Transporters, OAT1 and OAT3

Human OAT1 and OAT3 play major roles in renal drug elimination and drug-drug interactions. However, there is little information on the interactions of drug metabolites with transporters. The goal of this study was to characterize the interactions of drug metabolites with OAT1 and OAT3 and compare their potencies of inhibition with those of their corresponding parent drugs. Using HEK293 cells stably transfected with OAT1 and OAT3, 25 drug metabolites and their corresponding parent drugs were screened for inhibitory effects on OAT1-and OAT3-mediated 6-carboxyfluorescein uptake at a screening concentration of 200 μM for all but 3 compounds. 20 and 24 drug metabolites were identified as inhibitors (inhibition > 50%) of OAT1 and OAT3, respectively. Seven drug metabolites were potent inhibitors of either or both OAT1 and OAT3 with K_i values less than 1 μM. 22 metabolites were more potent inhibitors of OAT3 than OAT1. Importantly, one drug and four metabolites were predicted to inhibit OAT3 at unbound plasma concentrations achieved clinically (C_max,u/K_i values ≥ 0.1). In conclusion, our study highlights the potential interactions of drug metabolites with OAT1 and OAT3 at clinically relevant concentrations, suggesting that drug metabolites may modulate therapeutic and adverse drug response by inhibiting renal drug transporters.

Physiologically based pharmacokinetic evaluation of cefuroxime in perioperative antibiotic prophylaxis

Aims

Adequate plasma concentrations of antibiotics during surgery are essential for the prevention of surgical site infections. We examined the pharmacokinetics of 1.5 g cefuroxime administered during induction of anaesthesia with follow‐up doses every 2.5 hours until the end of surgery. We built a physiologically based pharmacokinetic model with the aim to ensure adequate antibiotic plasma concentrations in a heterogeneous population.

Methods

A physiologically based pharmacokinetic model (PK‐Sim^®/MoBi^®) was developed to investigate unbound plasma concentrations of cefuroxime. Blood samples from 25 thoracic surgical patients were analysed with high‐performance liquid chromatography. To evaluate optimized dosing regimens, physiologically based pharmacokinetic model simulations were conducted.

Results

Dosing simulations revealed that a standard dosing regimen of 1.5 g every 2.5 hours reached the pharmacokinetic/pharmacodynamic target for Staphylococcus aureus. However, for Escherichia coli, >50% of the study participants did not reach predefined targets. Effectiveness of cefuroxime against E. coli can be improved by administering a 1.5 g bolus immediately followed by a continuous infusion of 3 g cefuroxime over 3 hours.

Conclusion

The use of cefuroxime for perioperative antibiotic prophylaxis to prevent staphylococcal surgical site infections appears to be effective with standard dosing of 1.5 g preoperatively and follow‐up doses every 2.5 hours. In contrast, if E. coli is relevant in surgeries, this dosing regimen appears insufficient. With our derived dose recommendations, we provide a solution for this issue.

Transporter-Mediated Drug-Drug Interactions and Their Significance

Drug transporters are considered to be determinants of drug disposition and effects/toxicities by affecting the absorption, distribution, and excretion of drugs. Drug transporters are generally divided into solute carrier (SLC) family and ATP binding cassette (ABC) family. Widely studied ABC family transporters include P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), and multidrug resistance proteins (MRPs). SLC family transporters related to drug transport mainly include organic anion-transporting polypeptides (OATPs), organic anion transporters (OATs), organic cation transporters (OCTs), organic cation/carnitine transporters (OCTNs), peptide transporters (PEPTs), and multidrug/toxin extrusions (MATEs). These transporters are often expressed in tissues related to drug disposition, such as the small intestine, liver, and kidney, implicating intestinal absorption of drugs, uptake of drugs into hepatocytes, and renal/bile excretion of drugs. Most of therapeutic drugs are their substrates or inhibitors. When they are comedicated, serious drug-drug interactions (DDIs) may occur due to alterations in intestinal absorption, hepatic uptake, or renal/bile secretion of drugs, leading to enhancement of their activities or toxicities or therapeutic failure. This chapter will illustrate transporter-mediated DDIs (including food drug interaction) in human and their clinical significances.

Genetic Heterogeneity of SLC22 Family of Transporters in Drug Disposition

An important aspect of modern medicine is its orientation to achieve more personalized pharmacological treatments. In this context, transporters involved in drug disposition have gained well-justified attention. Owing to its broad spectrum of substrate specificity, including endogenous compounds and xenobiotics, and its strategical expression in organs accounting for drug disposition, such as intestine, liver and kidney, the SLC22 family of transporters plays an important role in physiology, pharmacology and toxicology. Among these carriers are plasma membrane transporters for organic cations (OCTs) and anions (OATs) with a marked overlap in substrate specificity. These two major clades of SLC22 proteins share a similar membrane topology but differ in their degree of genetic variability. Members of the OCT subfamily are highly polymorphic, whereas OATs have a lower number of genetic variants. Regarding drug disposition, changes in the activity of these variants affect intestinal absorption and target tissue uptake, but more frequently they modify plasma levels due to enhanced or reduced clearance by the liver and secretion by the kidney. The consequences of these changes in transport-associated function markedly affect the effectiveness and toxicity of the treatment in patients carrying the mutation. In solid tumors, changes in the expression of these transporters and the existence of genetic variants substantially determine the response to anticancer drugs. Moreover, chemoresistance usually evolves in response to pharmacological and radiological treatment. Future personalized medicine will require monitoring these changes in a dynamic way to adapt the treatment to the weaknesses shown by each tumor at each stage in each patient.

Potent inhibition of human organic cation transporter 2 (hOCT2) by β-carboline alkaloids

1. Beta-carbolines are indole alkaloids with a wide range of pharmacological and toxicological activities. Beta-carbolines are structurally related to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), a known substrate of organic cation transporters (OCTs). The goal of this study is to determine the interaction of β-carbolines with human OCT1, 2, and 3 (SLC22A1-3). 2. Dose-dependent inhibition studies were performed for five commercially available β-carbolines using a fluorescent substrate assay in HEK293 cells stably expressing hOCT1-3. The substrate potential was evaluated by uptake assays and the impact of active transport on cellular toxicity examined. 3. All tested β-carbolines potently inhibited hOCT2 with IC50 values in the sub- or low micromolar range. Harmaline is the most potent hOCT2 inhibitor (IC50 = 0.50 ± 0.08 μM). hOCT1 and hOCT3 are less sensitive to β-carboline inhibition. Harmaline, norharmanium, and 2,9-dimethyl-4,9-dihydro-3H-β-carbolinium accumulated 2- to 7-fold higher in cells expressing hOCT1-3. HEK293 cells expressing hOCT1-3 were 6.5- to 13-fold more sensitive to harmane and norharmanium toxicity. 4. Our data support a significant role of hOCT1-3 in tissue uptake and disposition of β-carbolines. Importantly, the potent inhibition of hOCT2 by β-carbolines also raises the concern of potential drug interactions between naturally occurring bioactive alkaloids and drugs eliminated by hOCT2.

Evaluation and Quantitative Prediction of Renal Transporter-Mediated Drug-Drug Interactions

With numerous drugs cleared renally, inhibition of uptake transporters localized on the basolateral membrane of renal proximal tubule cells, eg, organic anion transporters (OATs) and organic cation transporters (OCTs), may lead to clinically meaningful drug-drug interactions (DDIs). Additionally, clinical evidence for the possible involvement of efflux transporters, such as P-glycoprotein (P-gp) and multidrug and toxin extrusion protein 1/2-K (MATE1/2-K), in the renal DDIs is emerging. Herein, we review recent progress regarding mechanistic understanding of transporter-mediated renal DDIs as well as the quantitative predictability of renal DDIs using static and physiologically based pharmacokinetic (PBPK) models. Generally, clinical DDI data suggest that the magnitude of plasma exposure changes attributable to renal DDIs is less than 2-fold, unlike the DDIs associated with inhibition of cytochrome P-450s and/or hepatic uptake transporters. It is concluded that although there is a need for risk assessment early in drug development, current available data imply that safety concerns related to the renal DDIs are generally low. Nevertheless, consideration must be given to the therapeutic index of the victim drug and potential risk in a specific patient population (eg, renal impairment). Finally, in vitro transporter data and clinical pharmacokinetic parameters obtained from the first-in-human studies have proven useful in support of quantitative prediction of DDIs associated with inhibition of renal secretory transporters, OATs or OCTs.

Molecular properties associated with transporter-mediated drug disposition

Membrane transporters play a key role in the absorption, distribution, clearance, elimination, and transport of drugs. Understanding the drug properties and structure activity relationships (SAR) for affinity to membrane transporters is critical to optimize clearance and pharmacokinetics during drug design. To facilitate the early identification of clearance mechanism, a framework named the extended clearance classification system (ECCS) was recently introduced. Using in vitro and physicochemical properties that are readily available in early drug discovery, ECCS has been successfully applied to identify major clearance mechanism and to implicate the role of membrane transporters in determining pharmacokinetics. While the crystal structures for most of the drug transporters are currently not available, ligand-based modeling approaches that use information obtained from the structure and molecular properties of the ligands have been applied to associate the drug-related properties and transporter-mediated disposition. The approach allows prospective prediction of transporter both substrate and/or inhibitor affinity and build quantitative structure-activity relationship (QSAR) to enable early optimization of pharmacokinetics, tissue distribution and drug-drug interaction risk. Drug design applications can be further improved through uncovering transporter protein crystal structure and generation of quality data to refine and develop viable predictive models.

Evaluation of Renal Transporter Inhibition Using Creatinine as a Substrate In Vitro to Assess the Clinical Risk of Elevated Serum Creatinine

Creatinine is a widely accepted biomarker for renal toxicity, but its renal clearance via transporter-mediated active secretion is significant. For a given new chemical entity, therefore, elevations in serum creatinine (SCr) can be caused by the inhibition of renal transporter(s) without renal toxicity. In the present study, an effort was made to assess the correlation between the inhibition of renal transporters in vitro and elevations in SCr. A total of 15 compounds were chosen based on their known effect on SCr and minimal impact on glomerular filtration rate. Their inhibition potencies against the major creatinine renal transporters, including organic cation transporter 2, organic anion transporter 2, and 2 forms of multidrug and toxin extrusion (MATE1 and MATE2K), were assessed in transporter-transfected cell lines using creatinine as a probe substrate. Collectively, the data suggest that the observed elevations in SCr can be attributed to the inhibition of renal transporter(s), but inhibition of renal transporters does not necessarily lead to elevated SCr. Thus, renal transporter inhibition data can be used to rationalize SCr changes. Additionally, differing renal transporter inhibition potencies using creatinine and metformin as probe substrates suggest that substrate-dependent inhibition exists for some compounds.

Impact of Substrate-Dependent Inhibition on Renal Organic Cation Transporters hOCT2 and hMATE1/2-K-Mediated Drug Transport and Intracellular Accumulation

Renal transporter-mediated drug-drug interactions (DDIs) are of significant clinical concern, as they can adversely impact drug disposition, efficacy, and toxicity. Emerging evidence suggests that human renal organic cation transporter 2 (hOCT2) and multidrug and toxin extrusion proteins 1 and 2-K (hMATE1/2-K) exhibit substrate-dependent inhibition, but their impact on renal drug secretion and intracellular accumulation is unknown. Using metformin and atenolol as the probe substrates, we found that the classic inhibitors (e.g., cimetidine) of renal organic cation secretion were approximately 10-fold more potent for hOCT2 when atenolol was used, suggesting that atenolol is a more sensitive in vitro substrate for hOCT2 than metformin. In contrast, inhibition of hMATE1/2-K was influenced much less by the choice of substrate. Cimetidine is a much more potent inhibitor for hMATE1/2-K when metformin is the substrate but acts as an equally potent inhibitor of hOCT2 and hMATE1/2-K when atenolol is the substrate. Using hOCT2/hMATE1 double-transfected Madin-Darby canine kidney cells, we evaluated the impact of substrate-dependent inhibition on hOCT2/hMATE1-mediated transepithelial flux and intracellular drug accumulation. At clinically relevant concentrations, cimetidine dose dependently inhibited basal-to-apical flux of atenolol and metformin but impacted their intracellular accumulation differently, indicating that substrate-dependent inhibition may shift the major substrate-inhibitor interaction site between apical and basolateral transporters. Cimetidine is effective only when applied to the basal compartment. Our findings revealed the complex and dynamic nature of substrate-dependent inhibition of renal organic cation drug transporters and highlighted the importance of considering substrate-dependent inhibition in predicting transporter-mediated renal drug interaction, accumulation, and toxicity.

Renal drug transporters and their significance in drug-drug interactions

The kidney is a vital organ for the elimination of therapeutic drugs and their metabolites. Renal drug transporters, which are primarily located in the renal proximal tubules, play an important role in tubular secretion and reabsorption of drug molecules in the kidney. Tubular secretion is characterized by high clearance capacities, broad substrate specificities, and distinct charge selectivity for organic cations and anions. In the past two decades, substantial progress has been made in understanding the roles of transporters in drug disposition, efficacy, toxicity and drug-drug interactions (DDIs). In the kidney, several transporters are involved in renal handling of organic cation (OC) and organic anion (OA) drugs. These transporters are increasingly recognized as the target for clinically significant DDIs. This review focuses on the functional characteristics of major human renal drug transporters and their involvement in clinically significant DDIs.

Effect of lapatinib on oral digoxin absorption in patients

The potential for an interaction between lapatinib and absorption of the P-glycoprotein (ABCB1) substrate digoxin at a therapeutic dose in breast cancer patients was characterized. Seventeen women with HER2-positive metastatic breast cancer received a single oral 0.5-mg dose of digoxin on days 1 and 9 and oral lapatinib 1500 mg once daily on days 2 through 9. Digoxin pharmacokinetic parameters were determined on day 1 (digoxin administration alone) and on day 9 (coadministration of lapatinib and digoxin), and parameters were compared to determine the effects of lapatinib on digoxin absorption. Concomitant medications that could affect ABCB1 were accounted for. Lapatinib 1500 mg/day increased digoxin absorption approximately 80%, implicating lapatinib inhibition of intestinal ABCB1-mediated efflux. In summary, coadministration of lapatinib with narrow therapeutic index drugs that are substrates of ABCB1 should be undertaken with caution and dose adjustment should be considered.

Edoxaban transport via P-glycoprotein is a key factor for the drug's disposition

Edoxaban (the free base of DU-176b), an oral direct factor Xa inhibitor, is mainly excreted unchanged into urine and feces. Because active membrane transport processes such as active renal secretion, biliary excretion, and/or intestinal secretion, and the incomplete absorption of edoxaban after oral administration have been observed, the involvement of drug transporters in the disposition of edoxaban was investigated. Using a bidirectional transport assay in human colon adenocarcinoma Caco-2 cell monolayers, we observed the vectorial transport of [(14)C]edoxaban, which was completely inhibited by verapamil, a strong P-glycoprotein (P-gp) inhibitor. In an in vivo study, an increased distribution of edoxaban to the brain was observed in Mdr1a/1b knockout mice when compared with wild-type mice, indicating that edoxaban is a substrate for P-gp. However, there have been no observations of significant transport of edoxaban by renal or hepatic uptake transporters, organic anion transporter (OAT)1, OAT3, organic cation transporter (OCT)2, or organic anion transporting polypeptide (OATP)1B1. Edoxaban exhibited no remarkable inhibition of OAT1, OAT3, OCT1, OCT2, OATP1B1, OATP1B3, or P-gp up to 30 μM; therefore, the risk of clinical drug-drug interactions due to any edoxaban-related transporter inhibition seems to be negligible. Our results demonstrate that edoxaban is a substrate of P-gp but not of other major uptake transporters tested. Because metabolism is a minor contributor to the total clearance of edoxaban and strong P-gp inhibitors clearly impact edoxaban transport, the P-gp transport system is a key factor for edoxaban's disposition.

In vitro-in vivo extrapolation method to predict human renal clearance of drugs

Renal clearance is a key determinant of the elimination of drugs. To date, only few in vitro-in vivo extrapolation (IVIVE) approaches have been described to predict the renal organ clearance as the net result of glomerular filtration, tubular secretion, and tubular reabsorption. In this study, we measured in LLC-PK1 cells the transport of 20 compounds that cover all four classes of the Biopharmaceutical Drug Disposition System. These data were incorporated into a novel kidney model to predict all renal clearance processes in human. We showed that filtration and secretion were main contributors to the renal organ clearance for all compounds, whereas reabsorption was predominant for compounds assigned to classes 1 and 2. Our results suggest that anionic drugs were not significantly secreted in LLC-PK1 cells, resulting in under-predicted clearances. When all study compounds were included a high overall correlation between the reported and predicted renal organ clearances was obtained (R² = 0.83). The prediction accuracy in terms of percentage within twofold and threefold error was 70% and 95%, respectively. In conclusion, our novel IVIVE method allowed to predict the human renal organ clearance and the contribution of each underlying process.

Prediction of clinical drug-drug interactions of veliparib (ABT-888) with human renal transporters (OAT1, OAT3, OCT2, MATE1, and MATE2K)

Veliparib (ABT-888) is largely eliminated as parent drug in human urine (70% of the dose). Renal unbound clearance exceeds glomerular filtration rate, suggesting the involvement of transporter-mediated active secretion. Clinically relevant pharmacokinetic interactions in the kidney have been associated with OAT1, OAT3, OCT2, MATE1, and MATE2K. In the present study, interactions of veliparib with these transporters were investigated. Veliparib inhibited OAT1, OAT3, OCT2, MATE1, and MATE2K with IC50 values of 1371, 505, 3913, 69.9, and 69.5 μM, respectively. The clinical unbound maximum plasma concentration of veliparib after single oral dose of 50 mg (0.45 μM) is manyfold lower than IC50 values for OAT1, OAT3, OCT2, MATE1, or MATE2K. These results indicate a low potential for drug-drug interaction (DDI) with OAT1/3, OCT2, or MATE1/2K. Additional studies demonstrated that veliparib is a substrate of OCT2. In Oct1/Oct2 double-knockout mice, the plasma exposure of veliparib was increased by 1.5-fold, and the renal clearance was decreased by 1.8-fold as compared with wild-type mice, demonstrating that organic cation transporters contribute to the renal elimination in vivo. In summary, the in vitro transporter data for veliparib predicts minimal potential for an OAT1/3-, OCT2-, and MATE1/2K-mediated DDI given the clinical exposure after single oral dose of 50 mg.

Drug-transporter interaction testing in drug discovery and development

The human body consists of several physiological barriers that express a number of membrane transporters. For an orally absorbed drug the intestinal, hepatic, renal and blood-brain barriers are of the greatest importance. The ATP-binding cassette (ABC) transporters that mediate cellular efflux and the solute carrier transporters that mostly mediate cellular uptake are the two superfamilies responsible for membrane transport of vast majority of drugs and drug metabolites. The total number of human transporters in the two superfamilies exceeds 400, and about 40-50 transporters have been characterized for drug transport. The latest Food and Drug Administration guidance focuses on P-glycoprotein, breast cancer resistance protein, organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, organic cation transporter 2 (OCT2), and organic anion transporters 1 (OAT1) and OAT3. The European Medicines Agency’s shortlist additionally contains the bile salt export pump, OCT1, and the multidrug and toxin extrusion transporters, multidrug and toxin extrusion protein 1 (MATE1) and MATE2/MATE2K. A variety of transporter assays are available to test drug-transporter interactions, transporter-mediated drug-drug interactions, and transporter-mediated toxicity. The drug binding site of ABC transporters is accessible from the cytoplasm or the inner leaflet of the plasma membrane. Therefore, vesicular transport assays utilizing inside-out vesicles are commonly used assays, where the directionality of transport results in drugs being transported into the vesicle. Monolayer assays utilizing polarized cells expressing efflux transporters are the test systems suggested by regulatory agencies. However, in some monolayers, uptake transporters must be coexpressed with efflux transporters to assure detectable transport of low passive permeability drugs. For uptake transporters mediating cellular drug uptake, utilization of stable transfectants have been suggested. In vivo animal models complete the testing battery. Some issues, such as in vivo relevance, gender difference, age and ontogeny issues can only be addressed using in vivo models. Transporter specificity is provided by using knock-out or mutant models. Alternatively, chemical knock-outs can be employed. Compensatory changes are less likely when using chemical knock-outs. On the other hand, specific inhibitors for some uptake transporters are not available, limiting the options to genetic knock-outs.

Why Do SGLT2 inhibitors inhibit only 30-50% of renal glucose reabsorption in humans?

Sodium glucose cotransporter 2 (SGLT2) inhibition is a novel and promising treatment for diabetes under late-stage clinical development. It generally is accepted that SGLT2 mediates 90% of renal glucose reabsorption. However, SGLT2 inhibitors in clinical development inhibit only 30-50% of the filtered glucose load. Why are they unable to inhibit 90% of glucose reabsorption in humans? We will try to provide an explanation to this puzzle in this perspective analysis of the unique pharmacokinetic and pharmacodynamic profiles of SGLT2 inhibitors in clinical trials and examine possible mechanisms and molecular properties that may be responsible.

Drug transport by Organic Anion Transporters (OATs)

Common to all so far functionally characterized Organic Anion Transporters (OATs) is their broad substrate specificity and their ability to exchange extracellular against intracellular organic anions. Many OATs occur in renal proximal tubules, the site of active drug secretion. Exceptions are murine Oat6 (nasal epithelium), human OAT7 (liver), and rat Oat8 (renal collecting ducts). In human kidneys, OAT1, OAT2, and OAT3 are localized in the basolateral membrane, and OAT4, OAT10, and URAT1 in the apical cell membrane of proximal tubule cells, respectively. In rats and mice, Oat1 and Oat3 are located basolaterally, and Oat2, Oat5, Oat9, Oat10, and Urat1 apically. Several classes of drugs interact with human OAT1-3, including ACE inhibitors, angiotensin II receptor antagonists, diuretics, HMG CoA reductase inhibitors, β-lactam antibiotics, antineoplastic and antiviral drugs, and uricosuric drugs. For most drugs, interaction was demonstrated in vitro by inhibition of OAT-mediated transport of model substrates; for some drugs, transport by OATs was directly proven. Based on IC₅₀ values reported in the literature, OAT1 and OAT3 show comparable affinities for diuretics, cephalosporins, and nonsteroidal anti-inflammatory drugs whereas OAT2 has a lower affinity to most of these compounds. Drug-drug interactions at OAT1 and OAT3 may retard renal drug secretion and cause untoward effects. OAT4, OAT10, and URAT1 in the apical membrane contribute to proximal tubular urate absorption, and OAT10 to nicotinate absorption. OAT4 is in addition able to release drugs, e.g. diuretics, into the tubule lumen.

Pharmaceutical drug transport: the issues and the implications that it is essentially carrier-mediated only

All cells necessarily contain tens, if not hundreds, of carriers for nutrients and intermediary metabolites, and the human genome codes for more than 1000 carriers of various kinds. Here, we illustrate using a typical literature example the widespread but erroneous nature of the assumption that the 'background' or 'passive' permeability to drugs occurs in the absence of carriers. Comparison of the rate of drug transport in natural versus artificial membranes shows discrepancies in absolute magnitudes of 100-fold or more, with the carrier-containing cells showing the greater permeability. Expression profiling data show exactly which carriers are expressed in which tissues. The recognition that drugs necessarily require carriers for uptake into cells provides many opportunities for improving the effectiveness of the drug discovery process.

In vitro and in vivo assessment of renal drug transporters in the disposition of mesna and dimesna

Mesna and its dimer, dimesna, are coadministered for mitigation of ifosfamide- and cisplatin-induced toxicities, respectively. Dimesna is selectively reduced to mesna in the kidney, producing its protective effects. In vitro screens of uptake and efflux transporters revealed saturable uptake by renal organic anion transporters OAT1, OAT3, and OAT4. Efflux transporters breast cancer resistance protein; multidrug and toxin extrusion 1 (MATE1); multidrug resistance proteins MRP1, MRP2, MRP4, and MRP5; and P-glycoprotein (Pgp) significantly reduced dimesna accumulation. Further investigation demonstrated that renal apical efflux transporters MATE1, MRP2, and Pgp were also capable of mesna efflux. Administration of OAT inhibitor probenecid to healthy subjects significantly increased combined mesna and dimesna plasma exposure (91% ± 34%) while decreasing the renal clearance due to net secretion (67.0% ± 12.7%) and steady-state volume of distribution (45.2% ± 13.4%). Thus, the kidney represents a significant sink of total mesna, whereas function of renal drug transporters facilitates clearance in excess of glomerular filtration rate and likely the presence of active mesna in the urine. Loss of renal transporter function due to genetic variability or drug-drug interactions may decrease the efficacy of chemoprotectants, increasing the risk of ifosfamide- and cisplatin-induced toxicities.

In vitro and in vivo evidence of the importance of organic anion transporters (OATs) in drug therapy

Organic anion transporters 1-10 (OAT1-10) and the urate transporter 1 (URAT1) belong to the SLC22A gene family and accept a huge variety of chemically unrelated endogenous and exogenous organic anions including many frequently described drugs. OAT1 and OAT3 are located in the basolateral membrane of renal proximal tubule cells and are responsible for drug uptake from the blood into the cells. OAT4 in the apical membrane of human proximal tubule cells is related to drug exit into the lumen and to uptake of estrone sulfate and urate from the lumen into the cell. URAT1 is the major urate-absorbing transporter in the apical membrane and is a target for uricosuric drugs. OAT10, also located in the luminal membrane, transports nicotinate with high affinity and interacts with drugs. Major extrarenal locations of OATs include the blood-brain barrier for OAT3, the placenta for OAT4, the nasal epithelium for OAT6, and the liver for OAT2 and OAT7. For all transporters we provide information on cloning, tissue distribution, factors influencing OAT abundance, interaction with endogenous compounds and different drug classes, drug/drug interactions and, if known, single nucleotide polymorphisms.

Membrane transporters in drug development

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labelling.

Organic cation transporters and their roles in antiretroviral drug disposition

BACKGROUND

Understanding metabolising processes and drug-transporter interactions is particularly crucial to the management of patients with HIV infection, given the several antiretroviral drugs that must be taken lifelong and the use of other medications for HIV-related and non-HIV-related conditions. Several interactions of antiretroviral drugs with metabolising enzymes, especially cytochrome P450 and ATP-dependent transporter P-glycoprotein, have been described but the role of the organic cation transporters (OCTs) is less clearly defined.

OBJECTIVE

To review the relevance of the OCTs for antiretroviral drug disposition.

METHODS

Interactions of OCTs with antiretroviral drugs and evidence for clinical relevance are discussed.

RESULTS/CONCLUSION

Several antiretroviral drugs show relevant interactions with the OCTs in cell-based experiments and the OCTs are highly upregulated in HIV-infected patients. For evaluating the clinical significance, interaction studies in HIV patients and reliable in vitro models for delineation of in vivo effects are needed.

Drug metabolism and pharmacokinetics

In this article, aspects of absorption, distribution, metabolism, and excretion have been described bearing in mind the pathogenesis of allergic diseases and their possible therapeutic opportunities. The importance of the routes of administration of the different therapeutic groups has been emphasized. The classical aspects of drug metabolism and disposition related to oral administration have been reviewed, but special emphasis has been given to intranasal, cutaneous, transdermal, and ocular administration as well as to the absorption and the subsequent bioavailability of drugs. Drug-metabolizing enzymes and transporters present in extrahepatic tissues, such as nasal mucosa and the respiratory tract, have been particularly discussed. As marketed antiallergic drugs include both racemates and enantiomers, aspects of stereoselective absorption, distribution, metabolism, and excretion have been discussed. Finally, a new and promising methodology, microdosing, has been presented, although it has not yet been applied to drugs used in the treatment of allergic diseases.

In vitro characterization and pharmacokinetics of dapagliflozin (BMS-512148), a potent sodium-glucose cotransporter type II inhibitor, in animals and humans

(2S,3R,4R,5S,6R)-2-(3-(4-Ethoxybenzyl)-4-chlorophenyl)-6-hydroxymethyl-tetrahydro-2H-pyran-3,4,5-triol (dapagliflozin; BMS-512148) is a potent sodium-glucose cotransporter type II inhibitor in animals and humans and is currently under development for the treatment of type 2 diabetes. The preclinical characterization of dapagliflozin, to allow compound selection and prediction of pharmacological and dispositional behavior in the clinic, involved Caco-2 cell permeability studies, cytochrome P450 (P450) inhibition and induction studies, P450 reaction phenotyping, metabolite identification in hepatocytes, and pharmacokinetics in rats, dogs, and monkeys. Dapagliflozin was found to have good permeability across Caco-2 cell membranes. It was found to be a substrate for P-glycoprotein (P-gp) but not a significant P-gp inhibitor. Dapagliflozin was not found to be an inhibitor or an inducer of human P450 enzymes. The in vitro metabolic profiles of dapagliflozin after incubation with hepatocytes from mice, rats, dogs, monkeys, and humans were qualitatively similar. Rat hepatocyte incubations showed the highest turnover, and dapagliflozin was most stable in human hepatocytes. Prominent in vitro metabolic pathways observed were glucuronidation, hydroxylation, and O-deethylation. Pharmacokinetic parameters for dapagliflozin in preclinical species revealed a compound with adequate oral exposure, clearance, and elimination half-life, consistent with the potential for single daily dosing in humans. The pharmacokinetics in humans after a single dose of 50 mg of [(14)C]dapagliflozin showed good exposure, low clearance, adequate half-life, and no metabolites with significant pharmacological activity or toxicological concern.

Podocyte-specific expression of organic cation transporter PMAT: implication in puromycin aminonucleoside nephrotoxicity

Plasma membrane monoamine transporter (PMAT) is a novel polyspecific organic cation transporter that transports organic cations and the purine nucleoside, adenosine. PMAT is expressed in the kidney, but the specific localization and function of this transporter in renal cells are unclear. In this study, we developed a polyclonal antibody toward a 14-amino acid sequence in the last intracellular loop of PMAT and determined the precise cellular localization of PMAT in human and rat kidneys. Surprisingly, we found that the PMAT protein was predominantly expressed in the glomerulus with minimal expression in tubular cells. Within the glomerulus, dual-color immunofluorescence labeling showed that the PMAT protein was specifically localized to the visceral glomerular epithelial cells, i.e., podocytes. There was no significant PMAT immunoreactivity in mesangial or glomerular endothelial cells. We further showed that puromycin aminonucleoside (PAN), a classic podocyte toxin that induces massive proteinuria and severe glomerulopathy, is transported by PMAT. Expression of PMAT in Madin-Darby canine kidney cells significantly increased cell sensitivity to PAN. Decynium 22, a potent PMAT inhibitor, abolished PAN toxicity in PMAT-expressing cells. Together, our data suggest that PMAT is specifically expressed in podocytes and may play an important role in PAN-induced kidney injury.

Involvement of human multidrug and toxin extrusion 1 in the drug interaction between cimetidine and metformin in renal epithelial cells

In human proximal tubules, organic cations are taken up from blood into cells by human organic cation transporter 2 [hOCT2/solute carrier (SLC) 22A2] and then eliminated into the lumen by apical H(+)/organic cation antiporters, human multidrug and toxin extrusion 1 (hMATE1/SLC47A1) and hMATE2-K (SLC47A2). To evaluate drug interactions of cationic drugs in the secretion process, epithelial cells engineered to express both hOCT2 and hMATE transporters are required to simultaneously evaluate drug interactions with renal basolateral and apical organic cation transporters. In the present study, therefore, we assessed the drug interaction between cimetidine and metformin with double-transfected Madin-Darby canine kidney cells stably expressing both hOCT2 and hMATE1 as an in vitro model of the proximal tubular epithelial cells. The basolateral-to-apical transport and intracellular accumulation of [(14)C]metformin by a double transfectant were markedly inhibited by 1 mM cimetidine at the basolateral side. On the other hand, 1 microM cimetidine at the basolateral side moderately decreased the basolateral-to-apical transport of [(14)C]metformin and significantly increased the intracellular accumulation of [(14)C]metformin from the basolateral side, suggesting that cimetidine at a low concentration inhibits apical hMATE1, rather than basolateral hOCT2. Actually, in concentration-dependent inhibition studies by a single transporter expression system, such as human embryonic kidney 293 stably expressing hMATE1, hMATE2-K, or hOCT2, cimetidine showed higher affinity for hMATEs than for hOCT2. These results suggest that apical hMATE1 is involved in drug interactions between cimetidine and cationic compounds in the proximal tubular epithelial cells.

Metabolism and renal elimination of gaboxadol in humans: role of UDP-glucuronosyltransferases and transporters

PURPOSE

Gaboxadol, a selective extrasynaptic agonist of the delta-containing gamma-aminobutyric acid type A (GABAA) receptor, is excreted in humans into the urine as parent drug and glucuronide conjugate. The goal of this study was to identify the UDP-Glucuronosyltransferase (UGT) enzymes and the transporters involved in the metabolism and active renal secretion of gaboxadol and its metabolite in humans.Methods. The structure of the glucuronide conjugate of gaboxadol in human urine was identified by LC/MS/MS. Human recombinant UGT isoforms were used to identify the enzymes responsible for the glucuronidation of gaboxadol. Transport of gaboxadol and its glucuronide was evaluated using cell lines and membrane vesicles expressing human organic anion transporters hOAT1 and hOAT3, organic cation transporter hOCT2, and the multidrug resistance proteins MRP2 and MRP4.Results. Our study indicated that the gaboxadol-O-glucuronide was the major metabolite excreted in human urine. UGT1A9, and to a lesser extent UGT1A6, UGT1A7 and UGT1A8, catalyzed the O-glucuronidation of gaboxadol in vitro. Gaboxadol was transported by hOAT1, but not by hOCT2, hOAT3, MRP2, and MRP4. Gaboxadol-O-glucuronide was transported by MRP4, but not MRP2.Conlusion. Gaboxadol could be taken up into the kidney by hOAT1 followed by glucuronidation and efflux of the conjugate into urine via MRP4.

Transport of dicationic drugs pentamidine and furamidine by human organic cation transporters

The antiparasitic activity of aromatic diamidine drugs, pentamidine and furamidine, depends on their entry into the pathogenic protozoa via membrane transporters. However, no such diamidine transporter has been identified in mammalian cells. The goal of this study is to investigate whether these dicationic drugs are substrates for human organic cation transporters (hOCTs, solute carrier family 22A1-3) and whether hOCTs play a role in their tissue distribution, elimination, and toxicity. Inhibitory and substrate activities of pentamidine and furamidine were studied in stably transfected Chinese hamster ovary (CHO) cells. The results of [(3)H]1-methyl-4-phenylpyridinium uptake study showed that pentamidine is a potent inhibitor for all three OCT isoforms (IC50 < 20 microM), whereas furamidine is a potent inhibitor for hOCT1 and hOCT3 (IC50 < 21 microM) but a less potent inhibitor for hOCT2 (IC50 = 189.2 microM). Both diamidines are good substrates for hOCT1 (Km = 36.4 and 6.1 microM, respectively), but neither is a substrate for hOCT2 or hOCT3. The cytotoxicity of pentamidine and furamidine was 4.4- and 9.3-fold greater, respectively, in CHO-hOCT1 cells compared with the mock cells. Ranitidine, an hOCT1 inhibitor, reversed this hOCT1-mediated potentiation of cytotoxicity. This is the first finding that dicationic drugs, such as pentamidine and furamidine, are substrates for hOCT1. In humans, aromatic diamidines are primarily eliminated in the bile but are distributed and cause toxicity in both liver and kidney. These transporters may play important roles in the disposition of aromatic diamidines in humans, as well as resultant drug-drug interactions and toxicity involving diamidine drugs.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Medications in the kidney

Patients with chronic kidney disease (CKD) constitute a population at high risk for adverse drug reactions and/or drug-drug interactions. Renal dysfunction-induced pathophysiological changes may alter both medication pharmacodynamics and handling. Most pharmacokinetic parameters are adversely affected by impaired kidney function, among which reduced glomerular filtration and altered tubular secretion and reabsorption lead to the most specific alterations. Dosages of drugs cleared by the kidney should usually be adjusted according to creatinine clearance. Recommended methods for maintenance dosing adjustments are dose reductions, lengthening the dosing interval, or both. Appropriate drug selection and dosing in patients with CKD is imperative to avoid drug adverse events and to ensure optimal patient outcomes.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

Species Difference in the Inhibitory Effect of Nonsteroidal Anti-Inflammatory Drugs on the Uptake of Methotrexate by Human Kidney Slices

Simultaneous use of nonsteroidal anti-inflammatory drugs (NSAIDs), probenecid, and other drugs has been reported to delay the plasma elimination of methotrexate in patients. Previously, we have reported that inhibition of the uptake process cannot explain such drug-drug interactions using rats. The present study quantitatively evaluated the possible role of the transporters in such drug-drug interactions using human kidney slices and membrane vesicles expressing human ATP-binding cassette (ABC) transporters. The uptake of methotrexate by human kidney slices was saturable with a K(m) of 45 to 49 microM. Saturable uptake of methotrexate by human kidney slices was markedly inhibited by p-aminohippurate and benzylpenicillin, but only weakly by 5-methyltetrahydrofolate. These transport characteristics are similar to those of a basolateral organic anion transporter (OAT) 3/SLC22A8. NSAIDs and probenecid inhibited the uptake of methotrexate by human kidney slices, and, in particular, salicylate, indomethacin, phenylbutazone, and probenecid were predicted to exhibit significant inhibition at clinically observed plasma concentrations. Among ABC transporters, such as BCRP/ABCG2, multidrug resistance-associated protein (MRP) 2/ABCC2, and MRP4/ABCC4, which are candidates for the luminal efflux of methotrexate, ATP-dependent uptake of methotrexate by MRP4-expressing membrane vesicles was most potently inhibited by NSAIDs. Salicylate and indomethacin were predicted to inhibit MRP4 at clinical plasma concentrations. Diclofenac-glucuronide significantly inhibited MRP2-mediated transport of methotrexate in a concentration-dependent manner, whereas naproxen-glucuronide had no effect. Inhibition of renal uptake (via OAT3) and efflux processes (via MRP2 and MRP4) explains the possible sites of drug-drug interaction for methotrexate with probenecid and some NSAIDs, including their glucuronides.