Transporters and Toxicity: Insights From the International Transporter Consortium Workshop 4.

Over the last decade, significant progress been made in elucidating the role of membrane transporters in altering drug disposition, with important toxicological consequences due to changes in localized concentrations of compounds. The topic of "Transporters and Toxicity" was recently highlighted as a scientific session at the International Transporter Consortium (ITC) Workshop 4 in 2021. The current white paper is not intended to be an extensive review on the topic of transporters and toxicity but an opportunity to highlight aspects of the role of transporters in various toxicities with clinically relevant implications as covered during the session. This includes a review of the role of solute carrier transporters in anticancer drug-induced organ injury, transporters as key players in organ barrier function, and the role of transporters in metal/metalloid toxicity.

Lipidomics profiles in hepatocytes from nonalcoholic steatohepatitis patients differ markedly from in vitro-induced steatotic hepatocytes.

Nonalcoholic steatohepatitis (NASH) is a severe form of liver injury that can be caused by a variety of stimuli and has a significant mortality rate. A common technique to induce in vitro steatosis involves culturing primary human hepatocytes (PHH) in fatty acid-enriched media. This study compared the lipidome of PHH cultured in fatty acid-enriched media to hepatocytes from patients with NASH and healthy controls. Hepatocytes from NASH patients displayed increased total cellular abundance of glycerolipids and phospholipids compared to healthy control hepatocytes. PHH cultured in fatty acid-enriched media demonstrated increased glycerolipids. However, these culture conditions did not induce elevated phospholipid levels. Thus, culturing PHH in fatty acid-enriched media has limited capacity to emulate the environment of hepatocytes in NASH patients.

Probe Cocktail to Assess Transporter Function in Sandwich-Cultured Human Hepatocytes.

PURPOSE: Probe substrates are used routinely to assess transporter function in vitro. Administration of multiple probe substrates together as a "cocktail" in sandwich-cultured human hepatocytes (SCHH) could increase the throughput of transporter function assessment in a physiologically-relevant in vitro system. This study was designed to compare transporter function between cocktail and single agent administration in SCHH. METHODS: Rosuvastatin, digoxin, and metformin were selected as probe substrates of hepatic transporters OATP1B1, OATP1B3, BCRP, P-gp, and OCT1. Total accumulation (Cells+Bile) and biliary excretion index (BEI) values derived from administration of the cocktail were compared to values obtained after administration of single agents in the absence and presence of a model inhibitor, erythromycin estolate. RESULTS: For rosuvastatin and metformin accumulation, the ratio of means [90% confidence interval (CI)] for cocktail to single agent administration was 100% [94%, 106%] and 90% [82%, 99%], respectively. Therefore, the cocktail and single-agent mode of administration were deemed equivalent per standard equivalence criterion of 80-120% for rosuvastatin and metformin accumulation, but not for digoxin accumulation (77% [62%, 92%]). The ratio of means [90% CI] for rosuvastatin BEI values between the two administration modes (105% [97%, 114%]) also was deemed equivalent. The ratio for digoxin BEI values between the two administration modes was 99% [78%, 120%]. In the presence of erythromycin estolate, the two administration modes were deemed equivalent for evaluation of rosuvastatin, digoxin, and metformin accumulation; the ratio of means [90% CI] was 104% [94%, 115%], 94% [82%, 105%], and 100% [88%, 111%], respectively. However, rosuvastatin and digoxin BEI values were low and quite variable in the presence of the inhibitor, so the BEI results were inconclusive. CONCLUSIONS: These data suggest that rosuvastatin and metformin can be administered as a cocktail to evaluate the function of OATP1B1, OATP1B3, BCRP, and OCT1 in SCHH, and that digoxin may not be an ideal component of such a cocktail.

Severe Hepatotoxicity of Mithramycin Therapy Caused by Altered Expression of Hepatocellular Bile Transporters.

Mithramycin demonstrates preclinical anticancer activity, but its therapeutic dose is limited by the development of hepatotoxicity that remains poorly characterized. A pharmacogenomics characterization of mithramycin-induced transaminitis revealed that hepatotoxicity is associated with germline variants in genes involved in bile disposition: ABCB4 (multidrug resistance 3) rs2302387 and ABCB11 [bile salt export pump (BSEP)] rs4668115 reduce transporter expression (P < 0.05) and were associated with ≥grade 3 transaminitis developing 24 hours after the third infusion of mithramycin (25 mcg/kg, 6 hours/infusion, every day ×7, every 28 days; P < 0.0040). A similar relationship was observed in a pediatric cohort. We therefore undertook to characterize the mechanism of mithramycin-induced acute transaminitis. As mithramycin affects cellular response to bile acid treatment by altering the expression of multiple bile transporters (e.g., ABCB4, ABCB11, sodium/taurocholate cotransporting polypeptide, organic solute transporter α/ß) in several cell lines [Huh7, HepaRG, HepaRG BSEP (-/-)] and primary human hepatocytes, we hypothesized that mithramycin inhibited bile-mediated activation of the farnesoid X receptor (FXR). FXR was downregulated in all hepatocyte cell lines and primary human hepatocytes (P < 0.0001), and mithramycin inhibited chenodeoxycholic acid- and GW4046-induced FXR-galactose-induced gene 4 luciferase reporter activity (P < 0.001). Mithramycin promoted glycochenodeoxycholic acid-induced cytotoxicity in ABCB11 (-/-) cells and increased the overall intracellular concentration of bile acids in primary human hepatocytes grown in sandwich culture (P < 0.01). Mithramycin is a FXR expression and FXR transactivation inhibitor that inhibits bile flow and potentiates bile-induced cellular toxicity, particularly in cells with low ABCB11 function. These results suggest that mithramycin causes hepatotoxicity through derangement of bile acid disposition; results also suggest that pharmacogenomic markers may be useful to identify patients who may tolerate higher mithramycin doses. SIGNIFICANCE STATEMENT: The present study characterizes a novel mechanism of drug-induced hepatotoxicity in which mithramycin not only alters farnesoid X receptor (FXR) and small heterodimer partner gene expression but also inhibits bile acid binding to FXR, resulting in deregulation of cellular bile homeostasis. Two novel single-nucleotide polymorphisms in bile flow transporters are associated with mithramycin-induced liver function test elevations, and the present results are the rationale for a genotype-directed clinical trial using mithramycin in patients with thoracic malignancies.

The C-DILI™ Assay: An Integrated In Vitro Approach to Predict Cholestatic Hepatotoxicity.

Drug-induced liver injury is an important reason for drug candidate failure. Alterations in the hepatobiliary disposition of bile acids are a proposed mechanism of cholestatic hepatotoxicity. Bile acids are synthesized in the hepatocyte, and excreted into the bile primarily by the bile salt export pump. Therefore, inhibition of the bile salt export pump by drugs has been postulated as a risk factor in the development of cholestatic hepatotoxicity. However, recent publications have shown a lack of correlation between bile salt export pump inhibition potency and drug-induced liver injury incidence. Following inhibition of the bile salt export pump mediated efflux of bile acids, the liver compensates through various mechanisms (adaptive response) including upregulation of basolateral bile acid efflux mediated by the farnesoid X receptor, the master regulator of bile acid homeostasis. The C-DILI™ assay integrates the effects of bile salt export pump inhibition, farnesoid X receptor antagonism, and basolateral efflux inhibition of bile acids to more accurately predict a drug's potential to cause cholestatic hepatotoxicity and drug-induced liver injury.

Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective.

Bile salt export pump (BSEP) inhibition has emerged as an important mechanism that may contribute to the initiation of human drug-induced liver injury (DILI). Proactive evaluation and understanding of BSEP inhibition is recommended in drug discovery and development to aid internal decision making on DILI risk. BSEP inhibition can be quantified using in vitro assays. When interpreting assay data, it is important to consider in vivo drug exposure. Currently, this can be undertaken most effectively by consideration of total plasma steady state drug concentrations (Css,plasma ). However, because total drug concentrations are not predictive of pharmacological effect, the relationship between total exposure and BSEP inhibition is not causal. Various follow-up studies can aid interpretation of in vitro BSEP inhibition data and may be undertaken on a case-by-case basis. BSEP inhibition is one of several mechanisms by which drugs may cause DILI, therefore, it should be considered alongside other mechanisms when evaluating possible DILI risk.

Farnesoid X Receptor Agonists Obeticholic Acid and Chenodeoxycholic Acid Increase Bile Acid Efflux in Sandwich-Cultured Human Hepatocytes: Functional Evidence and Mechanisms.

The farnesoid X receptor (FXR) is a nuclear receptor that regulates genes involved in bile acid homeostasis. FXR agonists, obeticholic acid (OCA) and chenodeoxycholic acid (CDCA), increase mRNA expression of efflux transporters in sandwich-cultured human hepatocytes (SCHH). This study evaluated the effects of OCA and CDCA treatment on the uptake, basolateral efflux, and biliary excretion of a model bile acid, taurocholate (TCA), in SCHH. In addition, changes in the protein expression of TCA uptake and efflux transporters were investigated. SCHH were treated with 1 µM OCA, 100 µM CDCA, or vehicle control for 72 hours followed by quantification of deuterated TCA uptake and efflux over time in Ca2+-containing and Ca2+-free conditions (n = 3 donors). A mechanistic pharmacokinetic model was fit to the TCA mass-time data to obtain estimates for total uptake clearance (CLUptake), total intrinsic basolateral efflux clearance (CLint,BL), and total intrinsic biliary clearance (CLint,Bile). Modeling results revealed that FXR agonists significantly increased CLint,BL by >6-fold and significantly increased CLint,Bile by 2-fold, with minimal effect on CLUptake Immunoblotting showed that protein levels of the basolateral transporter subunits organic solute transporter α and ß (OSTα and OSTß) in FXR agonist-treated SCHH were significantly induced by >2.5- and 10-fold, respectively. FXR agonist-mediated changes in the expression of other TCA transporters in SCHH were modest. In conclusion, this is the first report demonstrating that OCA and CDCA increased TCA efflux in SCHH, which contributed to reduced intracellular TCA concentrations. Increased basolateral efflux of TCA was consistent with increased OSTα/ß protein expression in OCA- and CDCA-treated SCHH.

Transient Changes in Hepatic Physiology That Alter Bilirubin and Bile Acid Transport May Explain Elevations in Liver Chemistries Observed in Clinical Trials of GGF2 (Cimaglermin Alfa).

GGF2 is a recombinant human neuregulin-1ß in development for chronic heart failure. Phase 1 clinical trials of GGF2 were put on hold when transient elevations in serum aminotransferases and total bilirubin were observed in 2 of 43 subjects who received single doses of GGF2 at 1.5 or 0.378 mg/kg. However, aminotransferase elevations were modest and not typical of liver injury sufficient to result in elevated serum bilirubin. Cynomolgus monkeys administered a single 15 mg/kg dose of GGF2 had similar transient elevations in serum aminotransferases and bilirubin as well as transient elevations in serum bile acids. However, no hepatocellular necrosis was observed in liver biopsies obtained during peak elevations. When sandwich-cultured human hepatocytes were treated with GGF2 for up to 72 h at concentrations approximately 0.8-fold average plasma Cmax for the 0.378 mg/kg dose, no cytotoxicity was observed. Gene expression profiling identified approximately 50% reductions in mRNAs coding for bilirubin transporters and bile acid conjugating enzymes, as well as changes in expression of additional genes mimicking the interleukin-6-mediated acute phase response. Similar gene expression changes were observed in GGF2-treated HepG2 cells and primary monkey hepatocytes. Additional studies conducted in sandwich-cultured human hepatocytes revealed a transient and GGF2 concentration-dependent decrease in hepatocyte bile acid content and biliary clearance of taurocholate without affecting biliary taurocholate efflux. Taken together, these data suggest that GGF2 does not cause significant hepatocellular death, but transiently modifies hepatic handling of bilirubin and bile acids, effects that may account for the elevations in serum bilirubin observed in the clinical trial subjects.

Comparative potency of obeticholic acid and natural bile acids on FXR in hepatic and intestinal in vitro cell models.

Obeticholic acid (OCA) is a semisynthetic farnesoid X receptor (FXR) agonist, an analogue of chenodeoxycholic acid (CDCA) which is indicated for the treatment of primary biliary cholangitis (PBC) in combination with ursodeoxycholic acid (UDCA). OCA efficiently inhibits bile acid synthesis and promotes bile acid efflux via activating FXR-mediated mechanisms in a physiologically relevant in vitro cell system, Sandwich-cultured Transporter Certified ™ human primary hepatocytes (SCHH). The study herein evaluated the effects of UDCA alone or in combination with OCA in SCHH. UDCA (≤100 µmol/L) alone did not inhibit CYP7A1 mRNA, and thus, no reduction in the endogenous bile acid pool observed. UDCA ≤100 µmol/L concomitantly administered with 0.1 µmol/L OCA had no effect on bile acid synthesis beyond what was observed with OCA alone. Furthermore, this study evaluated human Caco-2 cells (clone C2BBe1) as in vitro intestinal models. Glycine conjugate of OCA increased mRNA levels of FXR target genes in Caco-2 cells, FGF-19, SHP, OSTα/ß, and IBABP, but not ASBT, in a concentration-dependent manner, while glycine conjugate of UDCA had no effect on the expression of these genes. The results suggested that UDCA ≤100 µmol/L did not activate FXR in human primary hepatocytes or intestinal cell line Caco-2. Thus, co-administration of UDCA with OCA did not affect OCA-dependent pharmacological effects.

Obeticholic acid, a selective farnesoid X receptor agonist, regulates bile acid homeostasis in sandwich-cultured human hepatocytes.

Farnesoid X receptor (FXR) is a master regulator of bile acid homeostasis through transcriptional regulation of genes involved in bile acid synthesis and cellular membrane transport. Impairment of bile acid efflux due to cholangiopathies results in chronic cholestasis leading to abnormal elevation of intrahepatic and systemic bile acid levels. Obeticholic acid (OCA) is a potent and selective FXR agonist that is 100-fold more potent than the endogenous ligand chenodeoxycholic acid (CDCA). The effects of OCA on genes involved in bile acid homeostasis were investigated using sandwich-cultured human hepatocytes. Gene expression was determined by measuring mRNA levels. OCA dose-dependently increased fibroblast growth factor-19 (FGF-19) and small heterodimer partner (SHP) which, in turn, suppress mRNA levels of cholesterol 7-alpha-hydroxylase (CYP7A1), the rate-limiting enzyme for de novo synthesis of bile acids. Consistent with CYP7A1 suppression, total bile acid content was decreased by OCA (1 µmol/L) to 42.7 ± 20.5% relative to control. In addition to suppressing de novo bile acids synthesis, OCA significantly increased the mRNA levels of transporters involved in bile acid homeostasis. The bile salt excretory pump (BSEP), a canalicular efflux transporter, increased by 6.4 ± 0.8-fold, and the basolateral efflux heterodimer transporters, organic solute transporter α (OSTα ) and OSTß increased by 6.4 ± 0.2-fold and 42.9 ± 7.9-fold, respectively. The upregulation of BSEP and OSTα and OSTß, by OCA reduced the intracellular concentrations of d8 -TCA, a model bile acid, to 39.6 ± 8.9% relative to control. These data demonstrate that OCA does suppress bile acid synthesis and reduce hepatocellular bile acid levels, supporting the use of OCA to treat bile acid-induced toxicity observed in cholestatic diseases.

Prediction of Clinically Relevant Herb-Drug Clearance Interactions Using Sandwich-Cultured Human Hepatocytes: Schisandra spp. Case Study.

The Schisandraceae family is reported to have a range of pharmacological activities, including anti-inflammatory effects. As with all herbal preparations, extracts of Schisandra species are mixtures composed of >50 lignans, especially schizandrins, deoxyschizandrins, and gomisins. In China, Schisandra sphenanthera extract (SSE) is often coadministered with immunosuppressant treatment of transplant recipients. In cases of coadministration, the potential for herb-drug interactions (HDIs) increases. Clinical studies have been used to assess HDI potential of SSE. Results demonstrated that chronic SSE administration reduced midazolam (MDZ) clearance by 52% in healthy volunteers. Although clinical studies are definitive and considered the "gold standard," these studies are impractical for routine HDI assessments. Alternatively, in vitro strategies can be used to reduce the need for clinical studies. Transporter-certified sandwich-cultured human hepatocytes (SCHHs) provide a fully integrated hepatic cell system that maintains drug clearance pathways (metabolism and transport) and key regulatory pathways constitutive active/androstane receptor and pregnane X receptor (CAR/PXR) necessary for quantitative assessment of HDI potential. Mechanistic studies conducted in SCHHs demonstrated that SSE and the more commonly used dietary supplement Schisandra chinensis extract (SCE) inhibited CYP3A4/5-mediated metabolism and induced CYP3A4 mRNA in a dose-dependent manner. SSE and SCE reduced MDZ clearance to 0.577- and 0.599-fold of solvent control, respectively, in chronically exposed SCHHs. These in vitro results agreed with SSE clinical findings and predicted a similar in vivo HDI effect with SCE exposure. These findings support the use of an SCHH system that maintains transport, metabolic, and regulatory functionality for routine HDI assessments to predict clinically relevant clearance interactions.

Prediction of Hepatic Efflux Transporter-Mediated Drug Interactions: When Is it Optimal to Measure Intracellular Unbound Fraction of Inhibitors?

The intracellular unbound inhibitor concentration ([I]unbound,cell) is the most relevant concentration for predicting the inhibition of hepatic efflux transporters. However, the intracellular unbound fraction of inhibitor in hepatocytes (fu,cell,inhibitor) is not routinely determined. Studies are needed to evaluate the benefit of measuring fu,cell,inhibitor and using [I]unbound,cell versus intracellular total inhibitor concentration ([I]total,cell) when predicting inhibitory effects. This study examined the benefit of using [I]unbound,cell to predict hepatocellular bile acid disposition. Cellular total concentrations of taurocholate ([TCA]total,cell), a prototypical bile acid, were simulated using pharmacokinetic parameters estimated from sandwich-cultured human hepatocytes. The effect of various theoretical inhibitors was simulated by varying ([I]total,cell/ half maximal inhibitory concentration [IC50]) values. In addition, the fold change was calculated as the simulated [TCA]total,cell when fu,cell,inhibitor = 1 divided by the simulated [TCA]total,cell when fu,cell,inhibitor = 0.5-0.01. The lowest ([I]total,cell/IC50) value leading to a >2-fold change in [TCA]total,cell was chosen as a cutoff, and a framework was developed to categorize risk inhibitors for which the measurement of fu,cell,inhibitor is optimal. Fifteen compounds were categorized, 5 of which were compared with experimental observations. Future work is needed to evaluate this framework based on additional experimental data. In conclusion, the benefit of measuring fu,cell,inhibitor to predict hepatic efflux transporter-mediated drug-bile acid interactions can be determined a priori.

Prediction of Altered Bile Acid Disposition Due to Inhibition of Multiple Transporters: An Integrated Approach Using Sandwich-Cultured Hepatocytes, Mechanistic Modeling, and Simulation.

Transporter-mediated alterations in bile acid disposition may have significant toxicological implications. Current methods to predict interactions are limited by the interplay of multiple transporters, absence of protein in the experimental system, and inaccurate estimates of inhibitor concentrations. An integrated approach was developed to predict altered bile acid disposition due to inhibition of multiple transporters using the model bile acid taurocholate (TCA). TCA pharmacokinetic parameters were estimated by mechanistic modeling using sandwich-cultured human hepatocyte data with protein in the medium. Uptake, basolateral efflux, and biliary clearance estimates were 0.63, 0.034, and 0.074 mL/min/g liver, respectively. Cellular total TCA concentrations (Ct,Cells) were selected as the model output based on sensitivity analysis. Monte Carlo simulations of TCA Ct,Cells in the presence of model inhibitors (telmisartan and bosentan) were performed using inhibition constants for TCA transporters and inhibitor concentrations, including cellular total inhibitor concentrations ([I]t,cell) or unbound concentrations, and cytosolic total or unbound concentrations. For telmisartan, the model prediction was accurate with an average fold error (AFE) of 0.99-1.0 when unbound inhibitor concentration ([I]u) was used; accuracy dropped when total inhibitor concentration ([I]t) was used. For bosentan, AFE was 1.2-1.3 using either [I]u or [I]t This difference was evaluated by sensitivity analysis of the cellular unbound fraction of inhibitor (fu,cell,inhibitor), which revealed higher sensitivity of fu,cell,inhibitor for predicting TCA Ct,Cells when inhibitors exhibited larger ([I]t,cell/IC50) values. In conclusion, this study demonstrated the applicability of a framework to predict hepatocellular bile acid concentrations due to drug-mediated inhibition of transporters using mechanistic modeling and cytosolic or cellular unbound concentrations.

Hepatocellular Disposition and Transporter Interactions with Tolvaptan and Metabolites in Sandwich-Cultured Human Hepatocytes.

Tolvaptan is a selective V2-receptor antagonist primarily metabolized by CYP3A. The present study investigated the hepatocellular disposition of tolvaptan and the generated tolvaptan metabolites, DM-4103 and DM-4107, as well as the potential for drug-drug interaction (DDIs) with metabolic and transport proteins in sandwich-cultured human hepatocytes (SCHH). Tolvaptan was incubated with SCHH and quantified by LC-MS/MS. Pioglitazone, verapamil, MK-571 and elacridar were used as inhibitors to investigate mechanisms of transport and metabolism of tolvaptan and metabolites. Taurocholate (TCA), pravastatin, digoxin, and metformin were used as transporter probes to investigate which transport proteins were inhibited by tolvaptan and metabolites. Cellular accumulation of tolvaptan (0.15-50 µM), DM-4103 and DM-4107 in SCHH was concentration dependent. Tolvaptan accumulation (15 µM) in SCHH was not altered markedly by 50 µM pioglitazone, verapamil or MK-571, or 10 µM elacridar. Co-incubation of tolvaptan with pioglitazone, verapamil, MK-571 and elacridar reduced DM-4107 accumulation by 45.6, 79.8, 94.5 and 23.0%, respectively, relative to control. Co-incubation with increasing tolvaptan concentrations (0.15-50 µM) decreased TCA (2.5 µM) cell+bile accumulation and the TCA biliary excretion index (BEI; from 76% to 51%), consistent with inhibition of the bile salt export pump (BSEP). Tolvaptan (15 µM) had no effect on the cellular accumulation of 2.5 µM pravastatin or metformin. Digoxin cellular accumulation increased and the BEI of digoxin decreased from 23.9% to 8.1% in the presence of 15 µM tolvaptan, consistent with inhibition of P-glycoprotein (P-gp). In summary, SCHH studies revealed potential metabolic- and transporter-mediated DDIs involving tolvaptan and metabolites.

Assessing Natural Product-Drug Interactions: An End-to-End Safety Framework.

The use of natural products (NPs), including herbal medicines and other dietary supplements, by North Americans continues to increase across all age groups. This population has access to conventional medications, with significant polypharmacy observed in older adults. Thus, the safety of the interactions between multi-ingredient NPs and drugs is a topic of paramount importance. Considerations such as history of safe use, literature data from animal toxicity and human clinical studies, and NP constituent characterization would provide guidance on whether to assess NP-drug interactions experimentally. The literature is replete with reports of various NP extracts and constituents as potent inhibitors of drug metabolizing enzymes, and transporters. However, without standard methods for NP characterization or in vitro testing, extrapolating these reports to clinically-relevant NP-drug interactions is difficult. This lack of a clear definition of risk precludes clinicians and consumers from making informed decisions about the safety of taking NPs with conventional medications. A framework is needed that describes an integrated robust approach for assessing NP-drug interactions; and, translation of the data into formulation alterations, dose adjustment, labelling, and/or post-marketing surveillance strategies. A session was held at the 41st Annual Summer Meeting of the Toxicology Forum in Colorado Springs, CO, to highlight the challenges and critical components that should be included in a framework approach.

Inhibition of Human Hepatic Bile Acid Transporters by Tolvaptan and Metabolites: Contributing Factors to Drug-Induced Liver Injury?

Tolvaptan is a vasopressin V(2)-receptor antagonist that has shown promise in treating Autosomal Dominant Polycystic Kidney Disease (ADPKD). Tolvaptan was, however, associated with liver injury in some ADPKD patients. Inhibition of bile acid transporters may be contributing factors to drug-induced liver injury. In this study, the ability of tolvaptan and two metabolites, DM-4103 and DM-4107, to inhibit human hepatic transporters (NTCP, BSEP, MRP2, MRP3, and MRP4) and bile acid transport in sandwich-cultured human hepatocytes (SCHH) was explored. IC(50) values were determined for tolvaptan, DM-4103 and DM-4107 inhibition of NTCP (∼41.5, 16.3, and 95.6 µM, respectively), BSEP (31.6, 4.15, and 119 µM, respectively), MRP2 (>50, ∼51.0, and >200 µM, respectively), MRP3 (>50, ∼44.6, and 61.2 µM, respectively), and MRP4 (>50, 4.26, and 37.9 µM, respectively). At the therapeutic dose of tolvaptan (90 mg), DM-4103 exhibited a C(max)/IC(50) value >0.1 for NTCP, BSEP, MRP2, MRP3, and MRP4. Tolvaptan accumulation in SCHH was extensive and not sodium-dependent; intracellular concentrations were ∼500 µM after a 10-min incubation duration with tolvaptan (15 µM). The biliary clearance of taurocholic acid (TCA) decreased by 43% when SCHH were co-incubated with tolvaptan (15 µM) and TCA (2.5 µM). When tolvaptan (15 µM) was co-incubated with 2.5 µM of chenodeoxycholic acid, taurochenodeoxycholic acid, or glycochenodeoxycholic acid in separate studies, the cellular accumulation of these bile acids increased by 1.30-, 1.68-, and 2.16-fold, respectively. Based on these data, inhibition of hepatic bile acid transport may be one of the biological mechanisms underlying tolvaptan-associated liver injury in patients with ADPKD.

Evaluation of the endothelin receptor antagonists ambrisentan, bosentan, macitentan, and sitaxsentan as hepatobiliary transporter inhibitors and substrates in sandwich-cultured human hepatocytes.

BACKGROUND: Inhibition of the transporter-mediated hepatobiliary elimination of bile salts is a putative mechanism for liver toxicity observed with some endothelin receptor antagonists (ERAs). METHODS: Sandwich-cultured human hepatocytes were used to study the hepatobiliary distribution and accumulation of exogenous taurocholate, ERAs and endogenous bile acids. The molecular mechanisms for findings in hepatocytes or clinical observations were further explored using either vesicular assays (efflux transporters) or transfected cell-lines (uptake transporters). Inhibition constants (IC50) were measured for the human hepatobiliary transporters bile salt export pump (BSEP), sodium taurocholate cotransporting polypeptide (NTCP), multidrug resistance protein 2 (MRP2), P-glycoprotein (Pgp), breast cancer resistance protein (BCRP), organic anion-transporting polypeptide 1B1 (OATP1B1) and OATP1B3. RESULTS: The ERAs showed dose-dependent reductions in exogenous taurocholate cellular accumulation in human hepatocytes, with macitentan having the greatest effect. Consistent with their effects on bile acids, the ERAs inhibited bile transporters. IC50 values for OATP1B1 and OATP1B3 ranged from 2 µM for macitentan to 47 µM for ambrisentan. Macitentan and bosentan also inhibited NTCP with IC50 values of 10 and 36 µM, respectively. Similar to previously reported findings with sitaxsentan, BSEP inhibition was observed for bosentan and macitentan with IC50 values of 42 and 12 µM, respectively. In contrast, ambrisentan showed little or no inhibition of these transporters. Other transporters tested were weakly inhibited by the ERAs. Accumulation in hepatocytes was also a factor in the effects on bile transport. Macitentan demonstrated the greatest accumulation in human hepatocytes (∼100x) followed by sitaxsentan (∼40x), bosentan (∼20x) and ambrisentan (∼2x). CONCLUSIONS: Significant differences in the inhibition of hepatic transporters were observed between the evaluated ERAs in vitro. Macitentan had the highest level of cellular accumulation and caused the greatest effects on bile acid distribution in human hepatocytes followed by sitaxsentan and bosentan. Ambrisentan showed a low potential to affect bile acids.

Automated applications of sandwich-cultured hepatocytes in the evaluation of hepatic drug transport.

Predictions of the absorption, distribution, metabolism, excretion, and toxicity of compounds in pharmaceutical development are essential aspects of the drug discovery process. B-CLEAR is an in vitro system that uses sandwich-cultured hepatocytes to evaluate and predict in vivo hepatobiliary disposition (hepatic uptake, biliary excretion, and biliary clearance), transporter-based hepatic drug-drug interactions, and potential drug-induced hepatotoxicity. Automation of predictive technologies is an advantageous and preferred format in drug discovery. In this study, manual and automated studies are investigated and equivalence is demonstrated. In addition, automated applications using model probe substrates and inhibitors to assess the cholestatic potential of drugs and evaluate hepatic drug transport are examined. The successful automation of this technology provides a more reproducible and less labor-intensive approach, reducing potential operator error in complex studies and facilitating technology transfer.

An in vitro assay to assess transporter-based cholestatic hepatotoxicity using sandwich-cultured rat hepatocytes.

Drug-induced cholestasis can result from the inhibition of biliary efflux of bile acids in the liver. Drugs may inhibit the hepatic uptake and/or the biliary efflux of bile acids resulting in an increase in serum concentrations. However, it is the intracellular concentration of bile acids that results in hepatotoxicity, and thus serum concentrations may not necessarily be an appropriate indicator of hepatotoxicity. In this study, sandwich-cultured rat hepatocytes were used as an in vitro model to assess the cholestatic potential of drugs using deuterium-labeled sodium taurocholate (d(8)-TCA) as a probe for bile acid transport. Eight drugs were tested as putative inhibitors of d(8)-TCA uptake and efflux. The hepatobiliary disposition of d(8)-TCA in the absence and presence of drugs was measured by using liquid chromatography/tandem mass spectrometry, and the accumulation (hepatocytes and hepatocytes plus bile), biliary excretion index (BEI), and in vitro biliary clearance (Cl(biliary)) were reported. Compounds were classified based on inhibition of uptake, efflux, or a combination of both processes. Cyclosporine A and glyburide showed a decrease in total (hepatocytes plus bile) accumulation, an increase in intracellular (hepatocytes only) accumulation, and a decrease in BEI and Cl(biliary) of d(8)-TCA, suggesting that efflux was primarily affected. Erythromycin estolate, troglitazone, and bosentan resulted in a decrease in accumulation (total and intracellular), BEI, and Cl(biliary) of d(8)-TCA, suggesting that uptake was primarily affected. Determination of a compound's relative effect on bile acid uptake, efflux, and direct determination of alterations in intracellular amounts of bile acids may provide useful mechanistic information on compounds that cause increases in serum bile acids.

Characterization of sandwich-cultured hepatocytes as an in vitro model to assess the hepatobiliary disposition of copper.

Sandwich-cultured hepatocytes (SCH) from rats (SCRH), dogs (SCDH), and humans (SCHH) were used as an in vitro model to assess the hepatobiliary disposition of copper (Cu). The expression of Cu transporters, ceruloplasmin synthesis, Cu uptake, and biliary excretion and species differences in drug-induced alterations in Cu disposition were determined in SCH from all species. Western blot analysis verified basolateral Cu uptake transporter 1 (CTR1) and canalicular Cu efflux transporter (ATP7B) expression: enzyme-linked immunosorbent assay verified synthesis/secretion of ceruloplasmin (major Cu binding protein found in blood). Endogenous Cu in SCRH, SCDH, and SCHH were 17.2 +/- 7.00, 490 +/- 44.8, and 43.5 +/- 15.8 ng/well, respectively. The hepatobiliary disposition of Cu as measured by uptake (increase in intracellular Cu in comparison to endogenous levels) and biliary excretion (increase in Cu in wash solutions obtained from hepatocytes exposed to calcium-free versus standard buffer) was determined as a function of Cu concentration and incubation time. In general, an increase in Cu concentration or incubation time resulted in an increase in Cu uptake and/or biliary excretion; however, the extent to which they affected Cu disposition was species dependent. 5-(1,1-Dioxido-1,2-thiazinan-2-yl)-N-(4-fluorobenzyl)-8-hydroxy-1,6-naphthyridine-7-carboxamide (L-000870810) (an anti-HIV compound, the development of which was halted due to an observed Cu-specific toxicity in the liver and kidneys of dogs after long-term exposure) showed no effect on Cu disposition in SCRH; however, it increased the biliary excretion of Cu in SCDH and SCHH. This is the first report to demonstrate the utility of SCH as a model to assess hepatobiliary disposition of Cu in an in vitro system.