Relationship between drug/metabolite exposure and impairment of excretory transport function

Novel inhibitors of breast cancer resistance protein (BCRP, ABCG2) among marketed drugs

Drug-drug interactions (DDIs) are a major concern for the safe use of medications. Breast cancer resistance protein (BCRP) is a clinically relevant ATP-binding cassette (ABC) transporter for drug disposition. Inhibition of BCRP increases the plasma concentrations of BCRP substrate drugs, which potentially could lead to adverse drug reactions. The aim of the present study was to identify BCRP inhibitors amongst a library of 232 commonly used drugs and anticancer drugs approved by the United States Food and Drug Administration (FDA). BCRP inhibition studies were carried out using the vesicular transport assay. We found 75 drugs that reduced the relative transport activity of BCRP to less than 25% of the vehicle control and were categorized as strong inhibitors. The concentration required for 50% inhibition (IC₅₀) was determined for 13 strong inhibitors that were previously poorly characterized for BCRP inhibition. The IC₅₀ ranged from 1.1 to 11 µM, with vemurafenib, dabigatran etexilate and everolimus being the strongest inhibitors. According to the drug interaction guidance documents from the FDA and the European Medicines Agency (EMA), in vivo DDI studies are warranted if the theoretical intestinal luminal concentration of a drug exceeds its IC₅₀ by tenfold. Here, the IC₅₀ values for eight of the drugs were 100-fold lower than their theoretical intestinal luminal concentration. Moreover, a mechanistic static model suggested that vemurafenib, bexarotene, dabigatran etexilate, rifapentine, aprepitant, and ivacaftor could almost fully inhibit intestinal BCRP, increasing the exposure of concomitantly administered rosuvastatin over 90%. Therefore, clinical studies are warranted to investigate whether these drugs cause BCRP-mediated DDIs in humans.

Studying the right transporter at the right time: an in vitro strategy for assessing drug-drug interaction risk during drug discovery and development

INTRODUCTION

Transporters are significant in dictating drug pharmacokinetics, thus inhibition of transporter function can alter drug concentrations resulting in drug-drug interactions (DDIs). Because they can impact drug toxicity, transporter DDIs are a regulatory concern for which prediction of clinical effect from in vitro data is critical to understanding risk.

AREA COVERED

The authors propose in vitro strategies to assist mitigating/removing transporter DDI risk during development by frontloading specific studies, or managing patient risk in the clinic. An overview of clinically relevant drug transporters and observed DDIs are provided, alongside presentation of key considerations/recommendations for in vitro study design evaluating drugs as inhibitors or substrates. Guidance on identifying critical co-medications, clinically relevant disposition pathways and using mechanistic static equations for quantitative prediction of DDI is compiled.

EXPERT OPINION

The strategies provided will facilitate project teams to study the right transporter at the right time to minimise development risks associated with DDIs. To truly alleviate or manage clinical risk, the industry will benefit from moving away from current qualitative basic static equation approaches to transporter DDI hazard assessment towards adopting the use of mechanistic models to enable quantitative DDI prediction, thereby contextualising risk to ascertain whether a transporter DDI is simply pharmacokinetic or clinically significant requiring intervention.

Assessing the Functional Redundancy between P-gp and BCRP in Controlling the Brain Distribution and Biliary Excretion of Dual Substrates with PET Imaging in Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are co-localized at the blood–brain barrier, where they display functional redundancy to restrict the brain distribution of dual P-gp/BCRP substrate drugs. We used positron emission tomography (PET) with the metabolically stable P-gp/BCRP substrates [¹¹C]tariquidar, [¹¹C]erlotinib, and [¹¹C]elacridar to assess whether a similar functional redundancy as at the BBB exists in the liver, where both transporters mediate the biliary excretion of drugs. Wild-type, Abcb1a/b^(−/−), Abcg2^(−/−), and Abcb1a/b^(−/−)Abcg2^(−/−) mice underwent dynamic whole-body PET scans after i.v. injection of either [¹¹C]tariquidar, [¹¹C]erlotinib, or [¹¹C]elacridar. Brain uptake of all three radiotracers was markedly higher in Abcb1a/b^(−/−)Abcg2^(−/−) mice than in wild-type mice, while only moderately changed in Abcb1a/b^(−/−) and Abcg2^(−/−) mice. The transfer of radioactivity from liver to excreted bile was significantly lower in Abcb1a/b^(−/−)Abcg2^(−/−) mice and almost unchanged in Abcb1a/b^(−/−) and Abcg2^(−/−) mice (with the exception of [¹¹C]erlotinib, for which biliary excretion was also significantly reduced in Abcg2^(−/−) mice). Our data provide evidence for redundancy between P-gp and BCRP in controlling both the brain distribution and biliary excretion of dual P-gp/BCRP substrates and highlight the utility of PET as an upcoming tool to assess the effect of transporters on drug disposition at a whole-body level.

Drug delivery and targeting to brain tumors: considerations for crossing the blood-brain barrier

Introduction: The blood-brain barrier (BBB) selectively impedes the transportation of drug molecules into the brain, which makes the drug delivery and targeting of brain tumors very challenging.Areas covered: Having surveyed the recent literature, comprehensive insights are given into the impacts of the BBB on the advanced drug delivery and targeting modalities for brain tumors.Expert opinion: Brain capillary endothelial cells form the BBB in association with astrocytes, pericytes, neurons, and extracellular matrix. Coop of these forms the complex setting of neurovascular unite. The BBB maintains the brain homeostasis by restrictive controlling of the blood circulating nutrients/substances trafficking. Despite substantial progress on therapy of brain tumors, there is no impeccable strategy to safely deliver chemotherapeutics into the brain. Various strategies have been applied to deliver chemotherapeutics into the brain (e.g. BBB opening, direct delivery by infusion, injection, microdialysis, and implants, and smart nanosystems), which hold different pros and cons. Of note, smart nanoscale multifunctional nanomedicines can serve as targeting, imaging, and treatment modality for brain tumors. Given that aggressive brain tumors (e.g. gliomas) are often unresponsive to any treatments, an in-depth understanding of the molecular/cellular complexity of brain tumors might help the development of smart and effective treatment modalities.

Hepatic transporter-mediated pharmacokinetic drug-drug interactions: Recent studies and regulatory recommendations

Transporter-mediated drug-drug interactions are one of the major mechanisms in pharmacokinetic-based drug interactions and correspondingly affecting drugs' safety and efficacy. Regulatory bodies underlined the importance of the evaluation of transporter-mediated interactions as a part of the drug development process. The liver is responsible for the elimination of a wide range of endogenous and exogenous compounds via metabolism and biliary excretion. Therefore, hepatic uptake transporters, expressed on the sinusoidal membranes of hepatocytes, and efflux transporters mediating the transport from hepatocytes to the bile are determinant factors for pharmacokinetics of drugs, and hence, drug-drug interactions. In parallel with the growing research interest in this area, regulatory guidances have been updated with detailed assay models and criteria. According to well-established preclinical results, observed or expected hepatic transporter-mediated drug-drug interactions can be taken into account for clinical studies. In this paper, various methods including in vitro, in situ, in vivo, in silico approaches, and combinational concepts and several clinical studies on the assessment of transporter-mediated drug-drug interactions were reviewed. Informative and effective evaluation by preclinical tools together with the integration of pharmacokinetic modeling and simulation can reduce unexpected clinical outcomes and enhance the success rate in drug development.

Physiologically Based Pharmacokinetic Modeling of Central Nervous System Pharmacokinetics of CDK4/6 Inhibitors to Guide Selection of Drug and Dosing Regimen for Brain Cancer Treatment

A better understanding of the human central nervous system (CNS) pharmacokinetics is critical to the selection of the right drug and refinement of dosing regimen for more effective treatment of primary and metastatic brain cancer. Using the physiologically‐based pharmacokinetic (PBPK) modeling approach, we systematically compared the CNS pharmacokinetics of three cyclin D‐cyclin dependent kinase 4 and 6 (CDK4/6) inhibitors (ribociclib, palbociclib, and abemaciclib) in patients with cancer. A PBPK model platform was developed and verified for predicting plasma and CNS pharmacokinetics. Target engagement ratio (TER), defined as the ratio of the average steady‐state unbound drug brain concentration to the in vitro half‐maximal inhibitory concentration (IC₅₀) for CDK4/6 inhibition, was used as a crude predictor of efficacy. As compared with ribociclib and palbociclib, abemaciclib penetrated into the human brain to a larger extent, but at a slower rate, and was retained in the brain longer. Following the standard dosing regimens, the predicted CDK4/6 TERs were 26/5.2 for abemaciclib, 2.4/0.62 for ribociclib, and 0.36/0.27 for palbociclib. Simulations suggested that abemaciclib achieved comparable TERs following twice daily or daily dosing; ribociclib may sufficiently inhibit both CDK4 and CDK6 at the maximum tolerated dose; whereas, palbociclib achieved TERs < 0.5 even at a dose 50% higher than the standard dose. In conclusion, the PBPK modeling, supported by available preclinical and clinical evidence, suggests that abemaciclib is the best CDK4/6 inhibitor for brain cancer treatment, whereas palbociclib is not recommended. The model refined dosing regimen is 300 mg daily on a 4‐weeks‐on schedule for abemaciclib, and 900 mg daily on a 3‐weeks‐on/1‐week‐off schedule for ribociclib.

Measurement of Hepatic ABCB1 and ABCG2 Transport Activity with [11C]Tariquidar and PET in Humans and Mice

P-Glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) in the canalicular membrane of hepatocytes mediate the biliary excretion of drugs and drug metabolites. To measure hepatic ABCB1 and ABCG2 activity, we performed positron emission tomography (PET) scans with the ABCB1/ABCG2 substrate [¹¹C]tariquidar in healthy volunteers and wild-type, Abcb1a/b^(-/-), Abcg2^(-/-), and Abcb1a/b^(-/-)Abcg2^(-/-) mice without and with coadministration of unlabeled tariquidar. PET data were analyzed with a three-compartment pharmacokinetic model. [¹¹C]Tariquidar underwent hepatobiliary excretion in both humans and mice, and tariquidar coadministration caused a significant reduction in the rate constant for the transfer of radioactivity from the liver into bile (by -74% in humans and by -62% in wild-type mice), suggesting inhibition of canalicular efflux transporter activity. Radio-thin-layer chromatography analysis revealed that the majority of radioactivity (>87%) in the mouse liver and bile was composed of unmetabolized [¹¹C]tariquidar. PET data in transporter knockout mice revealed that both ABCB1 and ABCG2 mediated biliary excretion of [¹¹C]tariquidar. In vitro experiments indicated that tariquidar is not a substrate of major hepatic basolateral uptake transporters (SLCO1B1, SLCO1B3, SLCO2B1, SLC22A1, and SLC22A3). Our data suggest that [¹¹C]tariquidar can be used to measure hepatic canalicular ABCB1/ABCG2 transport activity without a confounding effect of uptake transporters.

Protein Expression and Functional Relevance of Efflux and Uptake Drug Transporters at the Blood-Brain Barrier of Human Brain and Glioblastoma

The knowledge of transporter protein expression and function at the human blood-brain barrier (BBB) is critical to prediction of drug BBB penetration and design of strategies for improving drug delivery to the brain or brain tumor. This study determined absolute transporter protein abundances in isolated microvessels of human normal brain (N = 30), glioblastoma (N = 47), rat (N = 10) and mouse brain (N = 10), and cell membranes of MDCKII cell lines, using targeted proteomics. In glioblastoma microvessels, efflux transporters (ABCB1 and ABCG2), monocarboxylate transporter 1 (MCT1), glucose transporter 1 (GLUT1), sodium-potassium pump (Na/K ATPase), and Claudin-5 protein levels were significantly reduced, while large neutral amino acid transporter 1 (LAT1) was increased and GLU3 remained the same, as compared with human normal brain microvessels. ABCC4, OATP1A2, OATP2B1, and OAT3 were undetectable in microvessels of both human brain and glioblastoma. Species difference in BBB transporter abundances was noted. Cellular permeability experiments and modeling simulations suggested that not a single apical uptake transporter but a vectorial transport system consisting of an apical uptake transporter and basolateral efflux mechanism was required for efficient delivery of poor transmembrane permeability drugs from the blood to brain.

Mechanistic Modeling of the Hepatic Disposition of Estradiol-17β-Glucuronide in Sandwich-Cultured Human Hepatocytes

Estradiol-17β-glucuronide (E₂17G) is an estrogen metabolite that has cholestatic properties. In humans, circulating E₂17G is transported into hepatocytes by organic anion transporting polypeptides (OATPs) and is excreted into bile by multidrug-resistance associated protein 2 (MRP2). E₂17G is also a substrate of the basolateral efflux transporters MRP3 and MRP4, which translocate E₂17G from hepatocytes to blood. However, the contribution of basolateral efflux to hepatocyte disposition of E₂17G has not been evaluated previously. To address this question, E₂17G disposition was studied in sandwich-cultured human hepatocytes and mechanistic modeling was applied to calculate clearance values (mean ± S.D.) for uptake, intrinsic biliary excretion (CL_int,bile) and intrinsic basolateral efflux (CL_int,BL). The biliary excretion index of E₂17G was 45% ± 6%. The CL_int,BL of E₂17G [0.18 ± 0.03 (ml/min)/g liver) was 1.6-fold higher than CL_int,bile [0.11 ± 0.06 (ml/min)/g liver]. Simulations were performed to study the effects of increased CL_int,BL and a concomitant decrease in CL_int,bile on hepatic E₂17G exposure. Results demonstrated that increased CL_int,BL can effectively reduce hepatocellular and biliary exposure to this potent cholestatic agent. Simulations also revealed that basolateral efflux can compensate for impaired biliary excretion and, vice versa, to avoid accumulation of E₂17G in hepatocytes. However, when both clearance processes are impaired by 90%, hepatocyte E₂17G exposure increases up to 10-fold. These data highlight the contribution of basolateral efflux transport, in addition to MRP2-mediated biliary excretion, to E₂17G disposition in human hepatocytes. This elimination route could be important, especially in cases where basolateral efflux is induced, such as cholestasis. SIGNIFICANCE STATEMENT: The disposition of the cholestatic estrogen metabolite estradiol-17β-glucuronide (E₂17G) was characterized in sandwich-cultured human hepatocytes. The intrinsic basolateral efflux clearance was estimated to be 1.6-fold higher than the intrinsic biliary excretion clearance, emphasizing the contribution of basolateral elimination in addition to biliary excretion. Simulations highlight how hepatocytes can effectively cope with increased E₂17G through the regulation of both basolateral and biliary transporters.

Clinical Extrapolation of the Effects of Dolutegravir and Other HIV Integrase Inhibitors on Folate Transport Pathways

Preliminary analysis of ongoing birth surveillance study identified evidence of potential increased risk for neural tube defects (NTDs) in newborns associated with exposure to dolutegravir at the time of conception. Folate deficiency is a common cause of NTDs. Dolutegravir and other HIV integrase inhibitor drugs were evaluated in vitro for inhibition of folate transport pathways: proton-coupled folate transporter (PCFT), reduced folate carrier (RFC), and folate receptor α (FRα)-mediated endocytosis. Inhibition of folate transport was extrapolated to the clinic by using established approaches for transporters in intestine, distribution tissues, and basolateral and apical membranes of renal proximal tubules (2017 FDA Guidance). The positive controls, methotrexate and pemetrexed, demonstrated clinically relevant inhibition of PCFT, RFC, and FRα in folate absorption, distribution, and renal sparing. Valproic acid was used as a negative control that elicits folate-independent NTDs; valproic acid did not inhibit PCFT, RFC, or FRα At clinical doses and exposures, the observed in vitro inhibition of FRα by dolutegravir and cabotegravir was not flagged as clinically relevant; PCFT and RFC inhibition was not observed in vitro. Bictegravir inhibited both PCFT and FRα, but the observed inhibition did not reach the criteria for clinical relevance. Elvitegravir and raltegravir inhibited PCFT, but only raltegravir inhibition of intestinal PCFT was flagged as potentially clinically relevant at the highest 1.2-g dose (not the 400-mg dose). These studies showed that dolutegravir is not a clinical inhibitor of folate transport pathways, and it is not predicted to elicit clinical decreases in maternal and fetal folate levels. Clinically relevant HIV integrase inhibitor drug class effect on folate transport pathways was not observed. SIGNIFICANCE STATEMENT: Preliminary analysis of ongoing birth surveillance study identified evidence of potential increased risk for neural tube defects (NTDs) in newborns associated with exposure to the HIV integrase inhibitor dolutegravir at the time of conception; folate deficiency is a common cause of NTDs. Dolutegravir and other HIV integrase inhibitor drugs were evaluated in vitro for inhibition of the major folate transport pathways: proton-coupled folate transporter, reduced folate carrier, and folate receptor α-mediated endocytosis. The present studies showed that dolutegravir is not a clinical inhibitor of folate transport pathways, and it is not predicted to elicit clinical decreases in maternal and fetal folate levels. Furthermore, clinically relevant HIV integrase inhibitor drug class effect on folate transport pathways was not observed.

Overcoming the Blood–Brain Barrier. Challenges and Tricks for CNS Drug Delivery

Treatment of certain central nervous system disorders, including different types of cerebral malignancies, is limited by traditional oral or systemic administrations of therapeutic drugs due to possible serious side effects and/or lack of the brain penetration and, therefore, the efficacy of the drugs is diminished. During the last decade, several new technologies were developed to overcome barrier properties of cerebral capillaries. This review gives a short overview of the structural elements and anatomical features of the blood–brain barrier. The various in vitro (static and dynamic), in vivo (microdialysis), and in situ (brain perfusion) blood–brain barrier models are also presented. The drug formulations and administration options to deliver molecules effectively to the central nervous system (CNS) are presented. Nanocarriers, nanoparticles (lipid, polymeric, magnetic, gold, and carbon based nanoparticles, dendrimers, etc.), viral and peptid vectors and shuttles, sonoporation and microbubbles are briefly shown. The modulation of receptors and efflux transporters in the cell membrane can also be an effective approach to enhance brain exposure to therapeutic compounds. Intranasal administration is a noninvasive delivery route to bypass the blood–brain barrier, while direct brain administration is an invasive mode to target the brain region with therapeutic drug concentrations locally. Nowadays, both technological and mechanistic tools are available to assist in overcoming the blood–brain barrier. With these techniques more effective and even safer drugs can be developed for the treatment of devastating brain disorders.

Cellular Pharmacokinetic Model-Based Analysis of Genistein, Glyceollin, and MK-571 Effects on 5 (and 6)-Carboxy-2',7'-Dichloroflourescein Disposition in Caco-2 Cells

Pharmacokinetic modeling was used to describe 5 (and 6)-carboxy-2',7'-dichloroflourescein (CDF) disposition in Caco-2 cells following CDF or CDFDA (CDF diacetate) dosing. CDF transcellular flux was modeled by simple passive diffusion. CDFDA dosing models were based on simultaneous fitting of CDF levels in apical, basolateral, and intracellular compartments. Predicted CDF efflux was 50% higher across the apical versus the basolateral membrane. This difference was similar following apical and basolateral CDFDA dosing, despite intracellular levels being 3-fold higher following basolateral dosing, thus supporting nonsaturable CDF efflux kinetics. A 3-compartment catenary model with intracellular CDFDA hydrolysis described CDF disposition. This model predicted that apical CDF efflux was not altered in the presence of MK-571, and that basolateral membrane clearance was enhanced to account for reduced intracellular CDF in the presence of this multidrug resistance-associated protein (MRP) inhibitor. Similar effects were predicted for glyceollin, while genistein exposure had no predicted effects on CDF efflux. These modulator effects are discussed in the context of model predicted intracellular CDF concentrations relative to reports of CDF affinity (measured by Km) for MRP2 and MRP3. This model-based analysis confirms the complexity of efflux kinetics and suggests that other transporters may have contributed to CDF efflux.

Pharmacokinetic Interaction between Faldaprevir and Cyclosporine or Tacrolimus in Healthy Volunteers: A Prospective, Open-Label, Fixed-Sequence, Crossover Study

Faldaprevir (FDV) is a potent, orally administered inhibitor of hepatitis C virus. In this single-centre, open-label, fixed-sequence, crossover study of 32 healthy adult male and female volunteers, subjects received either a single dose of cyclosporine (CsA) 50 mg (N = 16) or tacrolimus (TAC) 0.5 mg (N = 16), followed by a washout of at least 14 days. Each subject then received a loading dose of FDV 240 mg followed by 120 mg FDV once daily for 6 days. FDV 120 mg was then co-administered with an additional single dose of CsA (50 mg) or TAC (0.5 mg), followed by an additional 6 days of FDV 120 mg once daily. Intensive blood sampling was performed to assess the PK interaction potential. Assessment of relative BA indicated that exposure to CsA co-administered with FDV was similar to CsA alone. However, the AUC_τ,ss and C_max,ss of FDV were increased by 23% and 41%, respectively, when FDV was co-administered with CsA. Exposure to TAC was slightly increased (AUC_0-∞ increased by 27%, no change in C_max ) when TAC was co-administered with FDV. In contrast, exposure to FDV co-administered with TAC was similar to FDV alone. No unexpected safety findings arose from the trial. The limitations of the study (use of single, low dose of TAC and CsA, and only healthy volunteers in the trial) are discussed.

Physiologically Based Pharmacokinetic Modeling of Transporter-Mediated Hepatic Clearance and Liver Partitioning of OATP and OCT Substrates in Cynomolgus Monkeys

In the present investigations, we evaluate in vitro hepatocyte uptake and partitioning for the prediction of in vivo clearance and liver partitioning. Monkeys were intravenously co-dosed with rosuvastatin and bosentan, substrates of the organic anion transporting polypeptides (OATPs), and metformin, a substrate of organic cation transporter 1 (OCT1). Serial plasma and liver samples were collected over time. Liver and plasma unbound fraction was determined using equilibrium dialysis. In vivo unbound partitioning (Kp_u,u) for rosuvastatin, bosentan, and metformin, calculated from total concentrations in the liver and plasma, were 243, 553, and 15, respectively. A physiologically based pharmacokinetic monkey model that incorporates active and passive hepatic uptake was developed to fit plasma and liver concentrations. In addition, a two-compartment model was used to fit in vitro hepatic uptake curves in suspended monkey hepatocyte to determine active uptake, passive diffusion, and intracellular unbound fraction parameters. At steady-state in the model, in vitro Kp_u,u was determined. The results demonstrated that in vitro values under-predicted in vivo active uptake for rosuvastatin, bosentan, and metformin by 6.7-, 28-, and 1.5-fold, respectively, while passive diffusion was over-predicted. In vivo Kp_u,u values were under-predicted from in vitro data by 30-, 79-, and 3-fold. In conclusion, active uptake and liver partitioning in monkeys for OATP substrates were greatly under-predicted from in vitro hepatocyte uptake, while OCT-mediated uptake and partitioning scaled reasonably well from in vitro, demonstrating substrate- and transporter-dependent scaling factors. The combination of in vitro experimental and modeling approaches proved useful for assessing prediction of in vivo intracellular partitioning.

Prediction of the Transporter-Mediated Drug-Drug Interaction Potential of Dabrafenib and Its Major Circulating Metabolites

The BRAF inhibitor dabrafenib was recently approved for the treatment of certain BRAF V600 mutation-positive tumors, either alone or in combination therapy with the mitogen-activated extracellular signal regulated kinase 1 (MEK1) and MEK2 inhibitor, trametinib. This article presents the dabrafenib transporter-mediated drug-drug interaction (DDI) risk assessment, which is currently an important part of drug development, regulatory submission, and drug registration. Dabrafenib and its major circulating metabolites (hydroxy-, carboxy-, and desmethyl-dabrafenib) were investigated as inhibitors of the clinically relevant transporters P-gp, BCRP, OATP1B1, OATP1B3, OCT2, OAT1, and OAT3. The DDI Guidance risk assessment decision criteria for inhibition of BCRP, OATP1B1 and OAT3 were slightly exceeded and therefore a minor DDI effect resulting from inhibition of these transporters remained possible. Biliary secretion is the major excretion pathway of dabrafenib-related material (71.1% of orally administered radiolabeled dose recovered in feces), whereas urinary excretion was observed as well (22.7% of the dose). In vitro uptake into human hepatocytes of the dabrafenib metabolites, but not of dabrafenib parent compound, was mediated, at least in part, by hepatic uptake transporters. The transporters responsible for uptake of the pharmacologically active hydroxy- and desmethyl dabrafenib could not be identified, whereas carboxy-dabrafenib was a substrate of several OATPs. Dabrafenib, hydroxy-, and desmethyl-dabrafenib were substrates of P-gp and BCRP, whereas carboxy-dabrafenib was not. Although a small increase in exposure to carboxy-dabrafenib upon inhibition of OATPs and an increase in exposure to desmethyl-dabrafenib upon inhibition of P-gp or BCRP cannot be excluded, the clinical significance of such increases is likely to be low.

A Comprehensive Review of Drug-Drug Interactions with Metformin

Metformin is the world's most commonly used oral glucose-lowering drug for type 2 diabetes, and this is mainly because it protects against diabetes-related mortality and all-cause mortality. Although it is an old drug, its mechanism of action has not yet been clarified and its pharmacokinetic pathway is still not fully understood. There is considerable inter-individual variability in the response to metformin, and this has led to many drug-drug interaction (DDI) studies of metformin. In this review, we describe both in vitro and human interaction studies of metformin both as a victim and as a perpetrator. We also clarify the importance of including pharmacodynamic end points in DDI studies of metformin and taking pharmacogenetic variation into account when performing these studies to avoid hidden pitfalls in the interpretation of DDIs with metformin. This evaluation of the literature has revealed holes in our knowledge and given clues as to where future DDI studies should be focused and performed.

Pitavastatin is a more sensitive and selective organic anion-transporting polypeptide 1B clinical probe than rosuvastatin

AIMS

Rosuvastatin and pitavastatin have been proposed as probe substrates for the organic anion-transporting polypeptide (OATP) 1B, but clinical data on their relative sensitivity and selectivity to OATP1B inhibitors are lacking. A clinical study was therefore conducted to determine their relative suitability as OATP1B probes using single oral (PO) and intravenous (IV) doses of the OATP1B inhibitor rifampicin, accompanied by a comprehensive in vitro assessment of rifampicin inhibitory potential on statin transporters.

METHODS

The clinical study comprised of two separate panels of eight healthy subjects. In each panel, subjects were randomized to receive a single oral dose of rosuvastatin (5 mg) or pitavastatin (1 mg) administered alone, concomitantly with rifampicin (600 mg) PO or IV. The in vitro transporter studies were performed using hepatocytes and recombinant expression systems.

RESULTS

Rifampicin markedly increased exposures of both statins, with greater differential increases after PO vs. IV rifampicin only for rosuvastatin. The magnitudes of the increases in area under the plasma concentration-time curve were 5.7- and 7.6-fold for pitavastatin and 4.4- and 3.3-fold for rosuvastatin, after PO and IV rifampicin, respectively. In vitro studies showed that rifampicin was an inhibitor of OATP1B1 and OATP1B3, breast cancer resistance protein and multidrug resistance protein 2, but not of organic anion transporter 3.

CONCLUSIONS

The results indicate that pitavastatin is a more sensitive and selective and thus preferred clinical OATP1B probe substrate than rosuvastatin, and that a single IV dose of rifampicin is a more selective OATP1B inhibitor than a PO dose.

Species differences in hepatobiliary disposition of taurocholic acid in human and rat sandwich-cultured hepatocytes: implications for drug-induced liver injury

The bile salt export pump (BSEP) plays an important role in bile acid excretion. Impaired BSEP function may result in liver injury. Bile acids also undergo basolateral efflux, but the relative contributions of biliary (CLBile) versus basolateral efflux (CLBL) clearance to hepatocellular bile acid excretion have not been determined. In the present study, taurocholic acid (TCA; a model bile acid) disposition was characterized in human and rat sandwich-cultured hepatocytes (SCH) combined with pharmacokinetic modeling. In human SCH, biliary excretion of TCA predominated (CLBile = 0.14 ± 0.04 ml/min per g liver; CLBL = 0.042 ± 0.019 ml/min per g liver), whereas CLBile and CLBL contributed approximately equally to TCA hepatocellular excretion in rat SCH (CLBile = 0.34 ± 0.07 ml/min per g liver; CLBL = 0.26 ± 0.07 ml/min per g liver). Troglitazone decreased TCA uptake, CLBile, and CLBL; membrane vesicle assays revealed for the first time that the major metabolite, troglitazone sulfate, was a noncompetitive inhibitor of multidrug resistance-associated protein 4, a basolateral bile acid efflux transporter. Simulations revealed that decreased CLBile led to a greater increase in hepatic TCA exposure in human than in rat SCH. A decrease in both excretory pathways (CLBile and CLBL) exponentially increased hepatic TCA in both species, suggesting that 1) drugs that inhibit both pathways may have a greater risk for hepatotoxicity, and 2) impaired function of an alternate excretory pathway may predispose patients to hepatotoxicity when drugs that inhibit one pathway are administered. Simulations confirmed the protective role of uptake inhibition, suggesting that a drug's inhibitory effects on bile acid uptake also should be considered when evaluating hepatotoxic potential. Overall, the current study precisely characterized basolateral efflux of TCA, revealed species differences in hepatocellular TCA efflux pathways, and provided insights about altered hepatic bile acid exposure when multiple transport pathways are impaired.

Species differences in hepatobiliary disposition of taurocholic acid in human and rat sandwich-cultured hepatocytes: implications for drug-induced liver injury

The bile salt export pump (BSEP) plays an important role in bile acid excretion. Impaired BSEP function may result in liver injury. Bile acids also undergo basolateral efflux, but the relative contributions of biliary (CLBile) versus basolateral efflux (CLBL) clearance to hepatocellular bile acid excretion have not been determined. In the present study, taurocholic acid (TCA; a model bile acid) disposition was characterized in human and rat sandwich-cultured hepatocytes (SCH) combined with pharmacokinetic modeling. In human SCH, biliary excretion of TCA predominated (CLBile = 0.14 ± 0.04 ml/min per g liver; CLBL = 0.042 ± 0.019 ml/min per g liver), whereas CLBile and CLBL contributed approximately equally to TCA hepatocellular excretion in rat SCH (CLBile = 0.34 ± 0.07 ml/min per g liver; CLBL = 0.26 ± 0.07 ml/min per g liver). Troglitazone decreased TCA uptake, CLBile, and CLBL; membrane vesicle assays revealed for the first time that the major metabolite, troglitazone sulfate, was a noncompetitive inhibitor of multidrug resistance-associated protein 4, a basolateral bile acid efflux transporter. Simulations revealed that decreased CLBile led to a greater increase in hepatic TCA exposure in human than in rat SCH. A decrease in both excretory pathways (CLBile and CLBL) exponentially increased hepatic TCA in both species, suggesting that 1) drugs that inhibit both pathways may have a greater risk for hepatotoxicity, and 2) impaired function of an alternate excretory pathway may predispose patients to hepatotoxicity when drugs that inhibit one pathway are administered. Simulations confirmed the protective role of uptake inhibition, suggesting that a drug's inhibitory effects on bile acid uptake also should be considered when evaluating hepatotoxic potential. Overall, the current study precisely characterized basolateral efflux of TCA, revealed species differences in hepatocellular TCA efflux pathways, and provided insights about altered hepatic bile acid exposure when multiple transport pathways are impaired.

Apical ABC transporters and cancer chemotherapeutic drug disposition

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters that mediate cellular extrusion of a broad range of substrates ranging from amino acids, lipids, and ions to xenobiotics including many anticancer drugs. ABCB1 (P-GP) and ABCG2 (BCRP) are the most extensively studied apical ABC drug efflux transporters. They are highly expressed in apical membranes of many pharmacokinetically relevant tissues such as epithelial cells of the small intestine and endothelial cells of the blood capillaries in brain and testis, and in the placental maternal-fetal barrier. In these tissues, they have a protective function as they efflux their substrates back to the intestinal lumen or blood and thus restrict the intestinal uptake and tissue disposition of many compounds. This presents a major challenge for the use of many (anticancer) drugs, as most currently used anticancer drugs are substrates of these transporters. Herein, we review the latest findings on the role of apical ABC transporters in the disposition of anticancer drugs. We discuss that many new, rationally designed anticancer drugs are substrates of these transporters and that their oral availability and/or brain disposition are affected by this interaction. We also summarize studies that investigate the improvement of oral availability and brain disposition of many cytotoxic (e.g., taxanes) and rationally designed (e.g., tyrosine kinase inhibitor) anticancer drugs, using chemical inhibitors of these transporters. These findings provide a better understanding of the importance of apical ABC transporters in chemotherapy and may therefore advance translation of promising preclinical insights and approaches to clinical studies.

Pharmacometrics in pregnancy: An unmet need

Pregnant women and their fetuses are orphan populations with respect to the safety and efficacy of drugs. Physiological and absorption, distribution, metabolism, and excretion (ADME) changes during pregnancy can significantly affect drug pharmacokinetics (PK) and may necessitate dose adjustment. Here, the specific aspects related to the design, execution, and analysis of clinical studies in pregnant women are discussed, underlining the unmet need for top-down pharmacometrics analyses and bottom-up modeling approaches. The modeling tools that support data analysis for the pregnancy population are reviewed, with a focus on physiologically based pharmacokinetics (PBPK) and population pharmacokinetics (POP-PK). By integrating physiological data, preclinical data, and clinical data (e.g., via POP-PK) to quantify anticipated changes in the PK of drugs during pregnancy, the PBPK approach allows extrapolation beyond the previously studied model drugs to other drugs with well-characterized ADME characteristics. Such a systems pharmacology approach can identify drugs whose PK may be altered during pregnancy, guide rational PK study design, and support dose adjustment for pregnant women.

Trimethoprim-metformin interaction and its genetic modulation by OCT2 and MATE1 transporters

AIMS

Metformin pharmacokinetics depends on the presence and activity of membrane-bound drug transporters and may be affected by transport inhibitors. The aim of this study was to investigate the effects of trimethoprim on metformin pharmacokinetics and genetic modulation by organic cation transporter 2 (OCT2) and multidrug and toxin extrusion 1 (MATE1) polymorphisms.

METHODS

Twenty-four healthy volunteers received metformin 500 mg three times daily for 10 days and trimethoprim 200 mg twice daily from day 5 to 10. Effects of trimethoprim on steady-state metformin pharmacokinetics were analysed.

RESULTS

In the population as a whole, trimethoprim significantly reduced the apparent systemic metformin clearance (CL/F) from 74 to 54 l h(-1) and renal metformin clearance from 31 to 21 l h(-1) , and prolonged half-life from 2.7 to 3.6 h (all P < 0.01). This resulted in an increase in the maximal plasma concentration by 38% and in the area under the plasma concentration-time curve by 37%. In volunteers polymorphic for both OCT2 and MATE1, trimethoprim had no relevant inhibitory effects on metformin kinetics. Trimethoprim was associated with a decrease in creatinine clearance from 133 to 106 ml min(-1) (P < 0.01) and an increase in plasma lactate from 0.94 to 1.2 mmol l(-1) (P = 0.016).

CONCLUSIONS

The extent of inhibition by trimethoprim was moderate, but might be clinically relevant in patients with borderline renal function or high-dose metformin.

OATP and MRP2-mediated hepatic uptake and biliary excretion of eprosartan in rat and human

BACKGROUND

Eprosartan is an angiotensin II receptor antagonist, used in the treatment of hypertension and heart failure in clinical patients. The objective of this study was to clarify the mechanism underlying hepatic uptake and biliary excretion of eprosartan in rats and humans.

METHODS

Perfused rat liver in situ, rat liver slices, isolated rat hepatocytes and human organic anion-transporting polypeptide (OATP)-transfected cells in vitro were used in this study.

RESULTS

Extraction ratio of eprosartan was decreased by rifampicin in perfused rat livers. Uptake of eprosartan in rat liver slices and isolated rat hepatocytes was significantly inhibited by Oatp modulators such as ibuprofen, digoxin, rifampicin and cyclosporine A, but not by tetraethyl ammonium or p-aminohippurate. Uptake of eprosartan in rat hepatocytes indicated a saturable process. Although uptake of eprosartan in OATP1B3-human embryonic kidney cells (HEK) 293 cells was not observed, significant differences in cellular accumulations of eprosartan between vector- and OATP1B1-Madin-Darby canine kidney strain (MDCK) II cells were found in transcellular transport study. Moreover, cumulative biliary excretion rate of eprosartan in the presence of probenecid (Multidrug resistance-associated protein 2 (Mrp2) inhibitor) was significantly decreased in perfused rat livers. Vectorial basal-to-apical transport of eprosartan was also observed in OATP1B1/MRP2 double transfectants.

CONCLUSIONS

Eprosartan was transported by multiple Oatps (at least Oatp1a1 and Oatp1a4)/Mrp2 in rat and OATP1B1/MRP2, at least, in human.

An experimental approach to evaluate the impact of impaired transport function on hepatobiliary drug disposition using Mrp2-deficient TR- rat sandwich-cultured hepatocytes in combination with Bcrp knockdown

Breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 2 (MRP2) are members of the ATP binding cassette (ABC) transporter family located in the canalicular membrane of hepatocytes that mediate biliary excretion of many drugs and endogenous compounds. BCRP and MRP2 have overlapping substrate profiles. Predicting drug disposition in the setting of altered transport function has important clinical significance. This investigation was designed to establish an in vitro model system to evaluate the impact of impaired Mrp2 and Bcrp function on hepatobiliary drug disposition. To achieve Bcrp knockdown by RNA interference (RNAi), sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) and wild-type (WT) rats were infected with adenoviral vectors to express shRNA targeting Bcrp (Ad-siBcrp) at multiplicity of infection (MOI) of 1-10. MOI of 5 was identified as optimal. At MOI of 5, viral infection as well as WT or TR(-) status was statistically significant predictors of the rosuvastatin (RSV) biliary excretion index (BEI), consistent with the known role of Bcrp and Mrp2 in the biliary excretion of RSV in vivo in rats. Relative to WT rat SCH, marginal mean BEI (%) of RSV in TR(-) rat SCH decreased by 28.6 (95% CI: 5.8-51.3). Ad-siBcrp decreased marginal mean BEI (%) of RSV by 13.3 (7.5-9.1) relative to SCH infected with adenoviral vectors expressing a nontargeting shRNA (Ad-siNT). The BEI of RSV was almost ablated in TR(-) rat SCH with Bcrp knockdown (5.9 ± 3.0%) compared to Ad-siNT-infected WT rat SCH (45.4 ± 6.6%). These results demonstrated the feasibility of Bcrp knockdown in TR(-) rat SCH as an in vitro system to assess the impact of impaired Bcrp and Mrp2 function. At MOI of 5, viral infection had minimal effects on RSV total accumulation, but significantly decreased marginal mean taurocholate total accumulation (pmol/mg of protein) and BEI (%) by 9.9 (7.0-12.8) and 7.5 (3.7-11.3), respectively, relative to noninfected SCH. These findings may be due to off-target effects on hepatic bile acid transporters, even though no changes in protein expression levels of the hepatic bile acid transporters were observed. This study established a strategy for optimization of the knockdown system, and demonstrated the potential use of RNAi in SCH as an in vitro tool to predict altered hepatobiliary drug disposition when canalicular transporters are impaired.

Predicting the outer boundaries of P-glycoprotein (P-gp)-based drug interactions at the human blood-brain barrier based on rat studies

Using positron emission tomography (PET), (11)C-verapamil as the P-gp substrate, and cyclosporine A (CsA) as the P-gp inhibitor, we showed that the magnitude of P-gp-based drug interactions at the human blood-brain barrier (BBB) is modest. However, such interactions at clinically relevant CsA blood concentrations may be greater for substrates where P-gp plays an even larger role (fractional contribution of P-gp, ft > 0.97) in preventing the CNS entry of the drug (e.g., nelfinavir). Since we have shown that the rat is an excellent predictor of the verapamil-CsA interaction at the human BBB, we determined the magnitude of drug interaction at the rat BBB between nelfinavir and CsA. Under isoflurane anesthesia, male Sprague-Dawley rats were coadministered IV infusions of nelfinavir and escalating doses of CsA to achieve pseudo steady-state plasma/blood and brain concentrations of both drugs (blood CsA ranged 0-264.9 μM, n = 3-6/group). The percent increase in the brain:blood nelfinavir concentration ratio (determined by LC/MS) was described by the Hill equation with Emax = 6481%, EC50 = 12.3 μM, and γ = 1.6. Then, using these data, as well as in vitro data in LLCPK1 cells expressing the human P-gp, we predicted that CsA (at clinically relevant blood concentration of 1.5 μM) will increase the distribution of nelfinavir into the human brain by 236%. Collectively, our data suggest that clinically significant P-gp based drug interactions at the human BBB are possible for P-gp substrates highly excluded from the brain (ft > 0.97) and should be investigated using noninvasive approaches (e.g., PET).

Functional ATP-binding cassette drug efflux transporters in isolated human and rat hepatocytes significantly affect assessment of drug disposition

Freshly isolated hepatocytes are considered the gold standard for in vitro studies of hepatic drug disposition. To ensure a reliable supply of cells, cryopreserved human hepatocytes are often used. ABC-superfamily drug efflux transporters are key elements in hepatic drug disposition. These transporters are often considered lost after isolation of hepatocytes. In the present study, the expression and activity of ABC transporters BCRP, BSEP, P-gp, MRP2, MRP3, and MRP4 in human and rat cryopreserved hepatocytes were investigated. In commercially available human cryopreserved hepatocytes, all drug efflux transporters except human BCRP (hBCRP) exhibited similar expression levels as in fresh liver biopsies. Expression levels of hBCRP were 60% lower in cryopreserved human hepatocytes than in liver tissue, which could lead to, at most, a 2.5-fold reduction in hBCRP-mediated efflux. Fresh rat hepatocytes showed significantly lower levels of rat BCRP compared with liver expression levels; expression levels of other ABC transporters were unchanged. ABC transporters in human cryopreserved cells were localized to the plasma membrane. Functional studies could demonstrate P-gp and BCRP activity in both human cryopreserved and fresh rat hepatocytes. Inhibiting P-gp-mediated efflux by elacridar in in vitro experiments significantly decreased fexofenadine efflux from hepatocytes, resulting in an increase in apparent fexofenadine uptake. The results from the present study clearly indicate that ABC transporter-mediated efflux in freshly isolated as well as cryopreserved rat and human hepatocytes should be taken into account in in vitro experiments used for modeling of drug metabolism and disposition.

Utility of Oatp1a/1b-knockout and OATP1B1/3-humanized mice in the study of OATP-mediated pharmacokinetics and tissue distribution: case studies with pravastatin, atorvastatin, simvastatin, and carboxydichlorofluorescein

Although organic anion transporting polypeptide (OATP)-mediated hepatic uptake is generally conserved between rodents and humans at a gross pharmacokinetic level, the presence of three major hepatic OATPs with broad overlap in substrate and inhibitor affinity, and absence of rodent-human orthologs preclude clinical translation of single-gene knockout/knockin findings. At present, changes in pharmacokinetics and tissue distribution of pravastatin, atorvastatin, simvastatin, and carboxydichlorofluorescein were studied in oatp1a/1b-knockout mice lacking the three major hepatic oatp isoforms, and in knockout mice with liver-specific knockin of human OATP1B1 or OATP1B3. Relative to wild-type controls, oatp1a/1b-knockout mice exhibited 1.6- to 19-fold increased intravenous and 2.1- to 115-fold increased oral drug exposure, due to 33%-75% decreased clearance, 14%-60% decreased volume of distribution, and ≤74-fold increased oral bioavailability, with the magnitude of change depending on the contribution of oatp1a/1b to pharmacokinetics. Hepatic drug distribution was 4.2- to 196-fold lower in oatp1a/1b-knockout mice; distributional attenuation was less notable in kidney, brain, cardiac, and skeletal muscle. Knockin of OATP1B1 or OATP1B3 partially restored control clearance, volume, and bioavailability values (24%-142% increase, ≤47% increase, and ≤77% decrease vs. knockout, respectively), such that knockin pharmacokinetic profiles were positioned between knockout and wild-type mice. Consistent with liver-specific humanization, only hepatic drug distribution was partially restored (1.3- to 6.5-fold increase vs. knockout). Exposure and liver distribution changes in OATP1B1-humanized versus knockout mice predicted the clinical impact of OATP1B1 on oral exposure and contribution to human hepatic uptake of statins within 1.7-fold, but only after correcting for human/humanized mouse liver relative protein expression factor (OATP1B1 = 2.2, OATP1B3 = 0.30).

Role of hepatic efflux transporters in regulating systemic and hepatocyte exposure to xenobiotics

Hepatic efflux transporters include numerous well-known and emerging proteins localized to the canalicular or basolateral membrane of the hepatocyte that are responsible for the excretion of drugs into the bile or blood, respectively. Altered function of hepatic efflux transporters due to drug-drug interactions, genetic variation, and/or disease states may lead to changes in xenobiotic exposure in the hepatocyte and/or systemic circulation. This review focuses on transport proteins involved in the hepatocellular efflux of drugs and metabolites, discusses mechanisms of altered transporter function as well as the interplay between multiple transport pathways, and highlights the importance of considering intracellular unbound concentrations of transporter substrates and/or inhibitors. Methods to evaluate hepatic efflux transport and predict the effects of impaired transporter function on systemic and hepatocyte exposure are discussed, and the sandwich-cultured hepatocyte model to evaluate comprehensively the role of hepatic efflux in the hepatobiliary disposition of xenobiotics is characterized.

Hepatic basolateral efflux contributes significantly to rosuvastatin disposition II: characterization of hepatic elimination by basolateral, biliary, and metabolic clearance pathways in rat isolated perfused liver

Basolateral efflux clearance (CLBL) contributes significantly to rosuvastatin (RSV) elimination in sandwich-cultured hepatocytes (SCH). The contribution of CLBL to RSV hepatic elimination was determined in single-pass isolated perfused livers (IPLs) from wild-type (WT) and multidrug resistance-associated protein 2 (Mrp2)-deficient (TR(-)) rats in the absence and presence of the P-glycoprotein and breast cancer resistance protein (Bcrp) inhibitor, elacridar (GF120918); clearance values were compared with SCH. RSV biliary clearance (CLBile) was ablated almost completely by GF120918 in TR(-) IPLs, confirming that Mrp2 and Bcrp primarily are responsible for RSV CLBile. RSV appearance in outflow perfusate was attributed primarily to CLBL, which was impaired in TR(-) IPLs. CLBL was ≈ 6-fold greater than CLBile in the linear range in WT IPLs in the absence of GF120918. Recovery of unchanged RSV in liver tissue increased in TR(-) compared with WT (≈ 25 versus 6% of the administered dose) due to impaired CLBL and CLBile. RSV pentanoic acid, identified by high-resolution liquid chromatography-tandem mass spectroscopy, comprised ≈ 40% of total liver content and ≈ 16% of the administered dose in TR(-) livers at the end of perfusion, compared with ≈ 30 and 3% in WT livers, consistent with impaired RSV excretion and "shunting" to the metabolic pathway. In vitro-ex vivo extrapolation between WT SCH and IPLs (without GF120918) revealed that uptake clearance and CLBL were 4.2- and 6.4-fold lower, respectively, in rat SCH compared with IPLs; CLBile translated almost directly (1.1-fold). The present IPL data confirmed the significant role of CLBL in RSV hepatic elimination, and demonstrated that both CLBL and CLBile influence RSV hepatic and systemic exposure.

Impact of P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) gene dosage on plasma pharmacokinetics and brain accumulation of dasatinib, sorafenib, and sunitinib

Low brain accumulation of anticancer drugs due to efflux transporters may limit chemotherapeutic efficacy, necessitating a better understanding of the underlying mechanisms. P-glycoprotein (Abcb1a/1b) and breast cancer resistance protein (Abcg2) combination knockout mice often display disproportionately increased brain accumulation of shared drug substrates compared with single transporter knockout mice. Recently developed pharmacokinetic models could explain this phenomenon. To experimentally test these models and their wider relevance for tyrosine kinase inhibitors and other drugs, we selected dasatinib, sorafenib, and sunitinib because of their divergent oral availability and brain accumulation profiles: the brain accumulation of dasatinib is mainly restricted by Abcb1, that of sorafenib mainly by Abcg2, and that of sunitinib equally by Abcb1 and Abcg2. We analyzed the effect of halving the efflux activity of these transporters at the blood-brain barrier by generating heterozygous Abcb1a/1b;Abcg2 knockout mice and testing the plasma and brain levels of the drugs after oral administration at 10 mg/kg. Real-time reverse transcription-polymerase chain reaction analysis confirmed the ∼2-fold decreased expression of both transporters in brain. Interestingly, whereas complete knockout of the transporters caused 24- to 36-fold increases in brain accumulation of the drugs, the heterozygous mice only displayed 1.6- to 1.9-fold increases of brain accumulation relative to wild-type mice. These results are well in line with the predictions of the pharmacokinetic models and provide strong support for their validity for a wider range of drugs. Moreover, retrospective analysis of fetal accumulation of drugs across the placenta in Abcb1a/1b heterozygous knockout pups suggests that these models equally apply to the maternal-fetal barrier.

Intracellular drug concentrations and transporters: measurement, modeling, and implications for the liver

Intracellular concentrations of drugs and metabolites are often important determinants of efficacy, toxicity, and drug interactions. Hepatic drug distribution can be affected by many factors, including physicochemical properties, uptake/efflux transporters, protein binding, organelle sequestration, and metabolism. This white paper highlights determinants of hepatocyte drug/metabolite concentrations and provides an update on model systems, methods, and modeling/simulation approaches used to quantitatively assess hepatocellular concentrations of molecules. The critical scientific gaps and future research directions in this field are discussed.

Minor compensatory changes in SAGE Mdr1a (P-gp), Bcrp, and Mrp2 knockout rats do not detract from their utility in the study of transporter-mediated pharmacokinetics

Mdr1a-, Bcrp-, and Mrp2-knockout rats are a more practical species for absorption, distribution, metabolism, and excretion (ADME) studies than murine models and previously demonstrated expected alterations in the pharmacokinetics of various probe substrates. At present, gene expression and pathology changes were systematically studied in the small intestine, liver, kidney, and brain tissue from male SAGE Mdr1a, Bcrp, and Mrp2 knockout rats versus wild-type Sprague-Dawley controls. Gene expression data supported the relevant knockout genotype. As expected, Mrp2 knockout rats were hyperbilirubinemic and exhibited upregulation of hepatic Mrp3. Overall, few alterations were observed within 112 ADME-relevant genes. The two potentially most consequential changes were upregulation of intestinal carboxylesterase in Mdr1a knockouts and catechol-O-methyltransferase in all tissues of Bcrp knockout rats. Previously reported upregulation of hepatic Mdr1b P-glycoprotein in proprietary Wistar Mdr1a knockout rats was not observed in the SAGE counterpart investigated herein. Relative liver and kidney weights were 22-53% higher in all three knockouts, with microscopic increases in hepatocyte size in Mdr1a and Mrp2 knockout rats and glomerular size in Bcrp and Mrp2 knockouts. Increased relative weight of clearing organs is quantitatively consistent with reported increases in the clearance of drugs that are not substrates of the knocked-out transporter. Overall, SAGE knockout rats demonstrated modest compensatory changes, which do not preclude their general application to study transporter-mediated pharmacokinetics. However, until future studies elucidate the magnitude of functional change, caution is warranted in rare instances of extensive metabolism by catechol-O-methyltransferase in Bcrp knockouts and intestinal carboxylesterase in Mdr1a knockout rats, specifically for molecules with free catechol groups and esters subject to gut-wall hydrolysis.

The elimination of MTC-220, a novel anti-tumor agent of conjugate of paclitaxel and muramyl dipeptide analogue, in rats

PURPOSE

MTC-220, a conjugate of paclitaxel and muramyl dipeptide analogue, was reported to exhibit anti-tumor ability and anti-metastatic effect. The aim of present study was to investigate the elimination of MTC-220 and the related mechanisms in rats.

METHODS

The excretion of MTC-220 and its metabolites in bile and urine were determined in rats after intravenous administration at 4 mg/kg. Caco-2 cell monolayer, in situ liver perfusion model and in vivo pharmacokinetics with selected inhibitors in rats were used to confirm the involvement of hepatic transporters in the elimination of MTC-220. The metabolic stability of MTC-220 was assessed by the incubation with rat liver microsomes and plasma.

RESULTS

Approximately 72 % of MTC-220 was excreted into bile and less than 0.02 % into urine after administration in rats. The Caco-2 cell monolayer was impermeable to MTC-220. In in situ liver perfusion model, the hepatic extraction ratio of MTC-220 was reduced to 40 % of control in the presence of rifampicin, an Oatps inhibitor, and the cumulative biliary excretion rates of MTC-220 were reduced to 52.9, 71.5 and 62.9 % of control when concomitant perfusion with probenecid, novobiocin and verapamil, the inhibitors of Mrp2, Bcrp and P-gp, respectively. Co-administration of rifampicin, probenecid, novobiocin and verapamil with MTC-220 increased the AUC0-t and decreased the CL of MTC-220 in certain extents in rats. MTC-220 remained metabolically intact in rat liver microsomes, but less stable in plasma incubation.

CONCLUSIONS

In summary, the elimination of MTC-220 was mainly through the biliary excretion in unchanged form in rats. Liver transporters including Oatps, Mrp2, Bcrp and P-gp might be all involved in the hepatic elimination of MTC-220. MTC-220 exhibited the high metabolic stability in liver microsomes, but less stable in plasma. The esterases might involve in the metabolism of MTC-220 in plasma.

ITC recommendations for transporter kinetic parameter estimation and translational modeling of transport-mediated PK and DDIs in humans

This white paper provides a critical analysis of methods for estimating transporter kinetics and recommendations on proper parameter calculation in various experimental systems. Rational interpretation of transporter-knockout animal findings and application of static and dynamic physiologically based modeling approaches for prediction of human transporter-mediated pharmacokinetics and drug-drug interactions (DDIs) are presented. The objective is to provide appropriate guidance for the use of in vitro, in vivo, and modeling tools in translational transporter science.

Understanding the critical disposition pathways of statins to assess drug-drug interaction risk during drug development: it's not just about OATP1B1

The use of statins is widespread across disease areas because many patients have comorbidities. Given that these drugs have become common as comedications, it is essential to have an understanding of the potential risks of drug-drug interactions (DDIs) between statins and candidate drugs in development. Although the hepatic uptake transporter organic anion-transporting polypeptide 1B1 (OATP1B1) is known to play a substantial role in statin-related DDI risk, other transporters and metabolizing enzymes can also be involved. Consequently, a holistic approach to risk assessment is required, tailored to each statin. Using evidence from pharmacogenetics, DDIs, and literature on absorption, distribution, metabolism, and elimination (ADME) in humans, this review identifies pathways that contribute the most to, and are therefore the most critical to, the disposition of each statin. It also provides an understanding of the expected theoretical maximum increase in systemic exposure if the disposition of a statin is inhibited. Finally, on a statin-by-statin basis, we propose in vitro inhibition studies that should be routinely conducted during drug development so as to better assess DDI risk.