Mechanism of an unusual, but clinically significant, digoxin-bupropion drug interaction

Combinations of common SNPs of the transporter gene ABCB1 influence apparent bioavailability, but not renal elimination of oral digoxin

Effects of different genotypes on the pharmacokinetics of probe substrates may support their use as phenotyping agents for the activity of the respective enzyme or transporter. Digoxin is recommended as a probe substrate to assess the activity of the transporter P-glycoprotein (P-gp) in humans. Current studies on the individual effects of three commonly investigated single nucleotide polymorphisms (SNPs) of the ABCB1 gene encoding P-gp (C1236T, G2677T/A, and C3435T) on digoxin pharmacokinetics are inconclusive. Since SNPs are in incomplete linkage disequilibrium, considering combinations of these SNPs might be necessary to assess the role of polymorphisms in digoxin pharmacokinetics accurately. In this study, the relationship between SNP combinations and digoxin pharmacokinetics was explored via a population pharmacokinetic approach in 40 volunteers who received oral doses of 0.5 mg digoxin. Concerning the SNPs 1236/2677/3435, the following combinations were evaluated: CGC, CGT, and TTT. Carriers of CGC/CGT and TTT/TTT had 35% higher apparent bioavailability compared to the reference group CGC/CGC, while no difference was seen in CGC/TTT carriers. No significant effect on renal clearance was observed. The population pharmacokinetic model supports the use of oral digoxin as a phenotyping substrate of intestinal P-gp, but not to assess renal P-gp activity.

Transport of Bupropion and its Metabolites by the Model CHO and HEK293 Cell Lines

BACKGROUND

Bupropion (BUP) is widely used as an antidepressant and smoking cessation aid. There are three major pharmacologically active metabolites of BUP, Erythrohydrobupropion (EB), Hydroxybupropion (OHB) and Threohydrobupropion (TB). At present, the mechanisms underlying the overall disposition and systemic clearance of BUP and its metabolites have not been well understood, and the role of transporters has not been studied.

OBJECTIVE

The goal of this study was to investigate whether BUP and its active metabolites are substrates of the major hepatic uptake and efflux transporters.

METHOD

CHO or HEK293 cell lines or plasma membrane vesicles that overexpress OATP1B1, OATP1B3, OATP2B1, OATP4A1, OCT1, BCRP, MRP2 or P-gp were used in cellular or vesicle uptake and inhibition assays. Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was used to quantify transport activity.

RESULTS

BUP and its major active metabolites were actively transported into the CHO or HEK293 cells overexpressing OATP1B1, OATP1B3 or OATP2B1; however, such cellular active uptake could not be inhibited at all by prototypical inhibitors of any of the OATP transporters. These compounds were not transported by OCT1, BCRP, MRP2 or P-gp either. These results suggest that the major known hepatic transporters likely play a minor role in the overall disposition and systemic clearance of BUP and its active metabolites in humans. We also demonstrated that BUP and its metabolites were not transported by OATP4A1, an uptake transporter on the apical membrane of placental syncytiotrophoblasts, suggesting that OATP4A1 is not responsible for the transfer of BUP and its metabolites from the maternal blood to the fetal compartment across the placental barrier in pregnant women.

CONCLUSION

BUP and metabolites are not substrates of the major hepatic transporters tested and thus these hepatic transporters likely do not play a role in the overall disposition of the drug. Our results also suggest that caution should be taken when using the model CHO and HEK293 cell lines to evaluate potential roles of transporters in drug disposition.

Interaction between Fexofenadine and CYP Phenotyping Probe Drugs in Geneva Cocktail

Drug metabolic enzymes and transporters are responsible for an important variability in drug disposition. The cocktail approach is a sound strategy for the simultaneous evaluation of several enzyme and transporter activities for a personalized dosage of medications. Recently, we have demonstrated the reliability of the Geneva cocktail, combining the use of dried blood spots (DBS) and reduced dose of phenotyping drugs for the evaluation of the activity of six cytochromes and P-glycoprotein (P-gp). As part of a study evaluating potential drug–drug interactions between probe drugs of the Geneva cocktail, the present paper focuses on the impact of cytochromes (CYP) probe drugs on the disposition of fexofenadine, a P-gp test drug. In a randomized four-way Latin-square crossover study, 30 healthy volunteers (15 men and 15 women) received caffeine 50 mg, bupropion 20 mg, flurbiprofen 10 mg, omeprazole 10 mg, dextromethorphan 10 mg, midazolam 1 mg, and fexofenadine 25 mg alone (or as part of a previously validated combination) and all together (Geneva cocktail). The determination of drug concentrations was performed in DBS samples and pharmacokinetic parameters were calculated. Fexofenadine AUC_{0–8 h} and C_max decreased by 43% (geometric mean ratio: 0.57; CI 90: 0.50–0.65; p < 0.001) and 49% (geometric mean ratio: 0.51; CI 90: 0.44–0.59; p < 0.001), respectively, when fexofenadine was administered as part of the Geneva cocktail in comparison to fexofenadine alone. Consequently, the apparent oral clearance (Cl/F) increased 1.7-fold (CI 90: 1.49–1.93; p < 0.001). There was no interaction between the remaining probes. In conclusion, an unexpected interaction occurred between fexofenadine and one or several of the following substances: caffeine, bupropion, flurbiprofen, omeprazole, dextromethorphan, and midazolam. Further studies are necessary to elucidate the mechanism of this interaction.

A Clinical Drug-Drug Interaction Study Assessing a Novel Drug Transporter Phenotyping Cocktail With Adefovir, Sitagliptin, Metformin, Pitavastatin, and Digoxin

A new probe drug cocktail containing substrates of important drug transporters was tested for mutual interactions in a clinical trial. The cocktail consisted of (predominant transporter; primary phenotyping metric): 10 mg adefovir-dipivoxil (OAT1; renal clearance (CL_R )), 100 mg sitagliptin (OAT3; CL_R ), 500 mg metformin (several renal transporters; CL_R ), 2 mg pitavastatin (OATP1B1; clearance/F), and 0.5 mg digoxin (intestinal P-gp, renal P-gp, and OATP4C1; peak plasma concentration (C_max ) and CL_R ). Using a randomized six-period, open change-over design, single oral doses were administrated either concomitantly or separately to 24 healthy male and female volunteers. Phenotyping metrics were evaluated by noncompartmental analysis and compared between periods by the standard average bioequivalence approach (boundaries for ratios 0.80-1.25). Primary metrics supported the absence of relevant interactions, whereas secondary metrics suggested that mainly adefovir was a victim of minor drug-drug interactions (DDIs). All drugs were well tolerated. This cocktail may be another useful tool to assess transporter-based DDIs in vivo.

The renal transport protein OATP4C1 mediates uptake of the uremic toxin asymmetric dimethylarginine (ADMA) and efflux of cardioprotective L-homoarginine

Elevated plasma concentrations of the uremic toxin asymmetrical dimethylarginine (ADMA) and low plasma concentrations of L-homoarginine are independently associated with cardiovascular events and total mortality. Enzymes degrading ADMA [dimethylaminohydrolase 1 (DDAH1)] and synthesizing L-homoarginine [L-arginine:glycine amidinotransferase (AGAT)] are expressed in human proximal tubule cells. So far, it is not known which transport protein in the basolateral membrane of proximal tubule cells is mediating the uptake of ADMA into the cells for subsequent degradation or the export of intracellularly synthesized L-homoarginine. One study suggested that the uptake transporter OATP4C1 (gene symbol SLCO4C1) may be involved in the transport of ADMA and other uremic toxins. OATP4C1 is a member of the SLCO/SLC21 family of solute carriers, localized in the basolateral membrane of human proximal tubule cells. By using stably-transfected HEK cells overexpressing human OATP4C1, we demonstrate that ADMA and L-homoarginine are substrates of OATP4C1 with K_m values of 232.1 μM and 49.9 μM, respectively. ADMA and the structurally related uremic toxin SDMA (100 μM) inhibited OATP4C1-mediated L-homoarginine uptake (P < 0.01), whereas other tested uremic toxins such as urea and p-cresyl sulfate have no effect on OATP4C1-mediated transport. Preloading experiments (300 μM for 60 min) with subsequent efflux studies revealed that OATP4C1 also facilitates efflux e.g. of L-homoarginine. Both ADMA and L-homoarginine are substrates of human OATP4C1. Because proximal tubule cells are one site of ADMA metabolism and L-homoarginine synthesis, we postulate a protective role of OATP4C1 by mediating uptake of ADMA from and export of L-homoarginine into the systemic circulation.

Evaluation of a Potential Clinical Significant Drug-Drug Interaction between Digoxin and Bupropion in Cynomolgus Monkeys

PURPOSE

A three-period digoxin-bupropion drug-drug interaction study was performed in cynomolgus monkeys to assess the effect of bupropion and its metabolites on digoxin disposition.

METHODS

Monkeys were administered either an i.v. infusion (0.1 mg/kg) or an oral dose of digoxin (0.2 mg/kg) as control. In single-dosing period, monkeys received an i.v. infusion of bupropion at 1.5 mg/kg together with an infusion or oral dosing of digoxin, respectively. During multiple-dosing period, bupropion was orally administered q.d. at 7.72 mg/kg for 12-day. Then it was co-administered with an i.v. infusion or oral dosing of digoxin, respectively. Renal expression of OATP4C1 and P-gp was examined.

RESULTS

Bupropion significantly increased i.v. digoxin CL_renal0-48h by 1 fold in single-dosing period. But it had no effect on the systemic disposition of digoxin. In multiple-dosing period, bupropion significantly increased oral digoxin CL_renal0-48h, CL_total0-48h, CL_{non-renal0-48h} and decreased its plasma exposure. Bupropion and its metabolites did not alter creatinine clearance. OATP4C1 was located at the basolateral membrane of proximal tubule cells, while P-gp was on the apical membrane.

CONCLUSIONS

The effect of multiple dosing with bupropion on the pharmacokinetics of digoxin is more pronounced. The magnitude of increase in digoxin CL_renal0-48h contributed to the decrease in AUC of digoxin in some extent, but certainly is not the major driving force. The lack of systemic exposure after a single dose but a significant decrease in exposure mediated by an increase in the digoxin CL_{non-renal0-48h} with repeated dosing is likely to be the more clinically relevant.

Co-administration of nuciferine reduces the concentration of metformin in liver via differential inhibition of hepatic drug transporter OCT1 and MATE1

Nuciferine (NF), one of the main and effective components in Nelumbo nucifera Gaertn. leaf extracts, is a promising drug candidate for the treatment of obesity-related diseases, while metformin is a first line therapeutic drug for type 2 diabetes mellitus. Since nuciferine and metformin are likely to be co-administered, the aim of the present study was to evaluate whether co-administration of nuciferine would influence the liver (target tissue) distribution and the anti-diabetic effect of metformin by inhibiting hepatic organic cation transporter 1 (OCT1) and multidrug and toxin extrusion 1 (MATE1). The data demonstrated that nuciferine significantly reduced metformin accumulation in MDCK cells stably expressing human OCT1 (MDCK-hOCT1) or hMATE1 (MDCK-hMATE1), and primary cultured mouse hepatocytes. Furthermore, the presence of nuciferine in the basal compartment caused a concentration-dependent reduction of intracellular metformin accumulation in MDCK-hOCT1/hMATE1 cell monolayers. Compared with the metformin treatment-alone group, co-administration of nuciferine (40 mg/kg) markedly reduced the metformin concentration in mouse livers at 30 and 60 min after a single oral dose of metformin (200 mg/kg), and subsequently impaired the glucose-lowering effect of metformin (200 mg/kg), but the glucose-lowering effect became no different at 90 and 120 min. Therefore, nuciferine influenced the liver concentration and glucose-lowering effect of metformin only for a period of time after dose, administration of nuciferine and metformin with an interval might prevent the drug-drug interaction mediated by OCT1 and MATE1.

New Frontiers: Approaches to Understand the Mechanistic Basis of Renal Toxicity

A scientific session entitled "New Frontiers: Approaches to Understand the Mechanistic Basis of Renal Toxicity" focused on novel biomarkers to monitor kidney injury both preclinically and clinically, as well as providing mechanistic insight of the induced injury. Further, the role and impact of kidney membrane transporters in drug-induced kidney toxicity provided additional considerations when understanding kidney injury and the complex role of drug transporters in either sensitivity or resistance to drug-induced injury. The onset of nephropathy in diabetic patients was also presented, focusing on the quest to discover novel biomarkers that would differentiate diabetic populations more susceptible to nephropathy and renal failure. The session highlighted exciting new research areas and novel biomarkers that will enhance our understanding of kidney injury and provide tools for ensuring patient safety clinically.

Assessment of Pharmacokinetic Drug-Drug Interactions in Humans: In Vivo Probe Substrates for Drug Metabolism and Drug Transport Revisited

Pharmacokinetic parameters of selective probe substrates are used to quantify the activity of an individual pharmacokinetic process (PKP) and the effect of perpetrator drugs thereon in clinical drug-drug interaction (DDI) studies. For instance, oral caffeine is used to quantify hepatic CYP1A2 activity, and oral dagibatran etexilate for intestinal P-glycoprotein (P-gp) activity. However, no probe substrate depends exclusively on the PKP it is meant to quantify. Lack of selectivity for a given enzyme/transporter and expression of the respective enzyme/transporter at several sites in the human body are the main challenges. Thus, a detailed understanding of the role of individual PKPs for the pharmacokinetics of any probe substrate is essential to allocate the effect of a perpetrator drug to a specific PKP; this is a prerequisite for reliably informed pharmacokinetic models that will allow for the quantitative prediction of perpetrator effects on therapeutic drugs, also in respective patient populations not included in DDI studies.

Transporters in Drug Development: 2018 ITC Recommendations for Transporters of Emerging Clinical Importance

This white paper provides updated International Transporter Consortium (ITC) recommendations on transporters that are important in drug development following the 3^rd ITC workshop. New additions include prospective evaluation of organic cation transporter 1 (OCT1) and retrospective evaluation of organic anion transporting polypeptide (OATP)2B1 because of their important roles in drug absorption, disposition, and effects. For the first time, the ITC underscores the importance of transporters involved in drug-induced vitamin deficiency (THTR2) and those involved in the disposition of biomarkers of organ function (OAT2 and bile acid transporters).

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Delineating the Role of Various Factors in Renal Disposition of Digoxin through Application of Physiologically Based Kidney Model to Renal Impairment Populations

Development of submodels of organs within physiologically-based pharmacokinetic (PBPK) principles and beyond simple perfusion limitations may be challenging because of underdeveloped in vitro-in vivo extrapolation approaches or lack of suitable clinical data for model refinement. However, advantage of such models in predicting clinical observations in divergent patient groups is now commonly acknowledged. Mechanistic understanding of altered renal secretion in renal impairment is one area that may benefit from such models, despite knowledge gaps in renal pathophysiology. In the current study, a PBPK kidney model was developed for digoxin, accounting for the roles of organic anion transporting peptide 4C1 (OATP4C1) and P-glycoprotein (P-gp) in its tubular secretion, with the aim to investigate the impact of age and renal impairment (moderate to severe) on renal drug disposition. Initial PBPK simulations based on changes in glomerular filtration rate (GFR) underestimated the observed reduction in digoxin renal excretion clearance (CL_R) in subjects with moderately impaired renal function relative to healthy. Reduction in either proximal tubule cell number or the OATP4C1 abundance in the mechanistic kidney model successfully predicted 59% decrease in digoxin CL_R, in particular when these changes were proportional to reduction in GFR. In contrast, predicted proximal tubule concentration of digoxin was only sensitive to changes in the transporter expression/ million proximal tubule cells. Based on the mechanistic modeling, reduced proximal tubule cellularity and OATP4C1 abundance, and inhibition of OATP4C1-mediated transport, are proposed as possible causes of reduced digoxin renal secretion in renally impaired patients.

The role of organic anion transporting polypeptides in drug absorption, distribution, excretion and drug-drug interactions

INTRODUCTION

The in vivo fate and effectiveness of a drug depends highly on its absorption, distribution, metabolism, excretion and toxicity (ADME-Tox). Organic anion transporting polypeptides (OATPs) are membrane proteins involved in the cellular uptake of various organic compounds, including clinically used drugs. Since OATPs are significant players in drug absorption and distribution, modulation of OATP function via pharmacotherapy with OATP substrates/inhibitors, or modulation of their expression, affects drug pharmacokinetics. Given their cancer-specific expression, OATPs may also be considered anticancer drug targets. Areas covered: We describe the human OATP family, discussing clinically relevant consequences of altered OATP function. We offer a critical analysis of published data on the role of OATPs in ADME and in drug-drug interactions, especially focusing on OATP1A2, 1B1, 1B3 and 2B1. Expert opinion: Four members of the OATP family, 1A2, 1B1, 1B3 and 2B1, have been characterized in detail. As biochemical and pharmacological knowledge on the other OATPs is lacking, it seems timely to direct research efforts towards developing the experimental framework needed to investigate the transport mechanism and substrate specificity of the poorly described OATPs. In addition, elucidating the role of OATPs in tumor development and therapy response are critical avenues for further research.