Identification of Appropriate Endogenous Biomarker for Risk Assessment of Multidrug and Toxin Extrusion Protein-Mediated Drug-Drug Interactions in Healthy Volunteers

New Biomarkers for Renal Transporter-Mediated Drug-Drug Interactions: Metabolomic Effects of Cimetidine, Probenecid, Verapamil, and Rifampin in Humans

The inhibition of renal transport proteins organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATE1, MATE2-K), and organic anion transporters (OAT1, OAT3) causes clinically relevant drug-drug interactions (DDI). Endogenous biomarkers could be used to improve risk prediction of such renal DDIs. While a number of biomarkers for renal DDIs have been described so far, multiple criteria for valid biomarkers have frequently not been investigated, for example, specificity, metabolism, or food effects. Therefore, there is a need for novel biomarkers of renal DDIs. Here, we investigated the global metabolomic effects following the administration of two classical inhibitors of renal transport proteins [cimetidine (OCT2/MATEs), probenecid (OATs)] in human plasma and urine of healthy volunteers. Additionally, we investigated metabolomic effects of two inhibitors of other transporters [verapamil (P-glycoprotein), rifampin (organic anion transporting polypeptides)] as controls. This analysis shows that both cimetidine and probenecid affect compounds involved in caffeine metabolism, carnitines, and sulfates. Hierarchical cluster analysis of the effects of all four inhibitors on endogenous compounds identified multiple promising new sensitive and specific biomarker candidates for OCT2/MATE- or OAT-mediated DDIs. For OCT2/MATEs, 5-amino valeric acid betaine (median log2-fold change of estimated renal elimination: -3.62) presented itself as a promising candidate. For OATs, estimated renal elimination of 7-methyluric acid and cinnamoylglycine (median log2-fold changes -3.10 and -1.92, respectively) was both sensitive and specific. This study provides comprehensive information on metabolomic effects of transport protein inhibition in humans and identifies putative new sensitive and specific biomarkers for renal transporter-mediated DDIs.

Quantitative contributions of hepatic and renal organic cation transporters to the clinical pharmacokinetic cimetidine-metformin interaction

The widely prescribed oral anti-diabetic drug metformin is eliminated unchanged in the urine primarily through active tubular secretion. This process is mediated by organic cation transporter 2 (OCT2), an uptake transporter expressed on the basolateral membrane of renal proximal tubule cells. Metformin uptake into the liver, the site of action, is mediated by OCT1, which is expressed on the sinusoidal membrane of hepatocytes. Sixteen healthy adults participated in a clinical pharmacokinetic drug-drug interaction study in which they were orally administered metformin (50 mg) as a dual OCT1/2 substrate alone (baseline) and with cimetidine (400 mg) as an OCT inhibitor. Relative to baseline, metformin systemic plasma exposure increased by 24% ( p <0.05) in the presence of cimetidine, which was accompanied by a disproportional decrease (8%) in metformin renal clearance ( p =0.005). Genetic variants of OCT1 and OCT2 moderately impacted the significance and magnitude of the interaction. Collectively, we hypothesized that the cimetidine-metformin interaction involves inhibition of hepatic OCT1 as well as renal OCT2. We tested this hypothesis by developing a physiologically based pharmacokinetic (PBPK) model and assessing potential OCT biomarkers in plasma and urine to gain mechanistic insight into the transporters involved in this interaction. The PBPK model predicted that cimetidine primarily inhibits hepatic OCT1 and, to a lesser extent, renal OCT2. The unchanged renal clearance of potential OCT2 biomarkers following cimetidine exposure supports a minimal role for renal OCT2 in this interaction.

Prediction of Inhibitory Activity against the MATE1 Transporter via Combined Fingerprint- and Physics-Based Machine Learning Models

Renal secretion plays an important role in excretion of drug from the kidney. Two major transporters known to be highly involved in renal secretion are MATE1/2 K and OCT2, the former of which is highly related to drug-drug interactions. Among published in silico models for MATE inhibitors, a previous model obtained a ROC-AUC value of 0.78 using high throughput percentage inhibition data [J. Med. Chem. 2013, 56(3), 781-795] which we aimed to improve upon here using a combined fingerprint and physics-based approach. To this end, we collected 225 publicly available compounds with pIC50 values against MATE1. Subsequently, on the one hand, we performed a physics-based approach using an Alpha-Fold protein structure, from which we obtained MM-GB/SA scores for those compounds. On the other hand, we built Random Forest (RF) and message passing neural network models using extended-connectivity fingerprints with radius 4 (ECFP4) and chemical structures as graphs, respectively, which also included MM-GB/SA scores as input variables. In a five-fold cross-validation with a separate test set, we found that the best predictivity for the hold-out test was observed in the RF model (including ECFP4 and MM-GB/SA data) with an ROC-AUC of 0.833 ± 0.036; while that of the MM-GB/SA regression model was 0.742. However, the MM-GB/SA model did not show a dependency of the performance on the particular chemical space being predicted. Additionally, via structural interaction fingerprint analysis, we identified interacting residues with inhibitor as identical for those with noninhibitors, including substrates, such as Gln49, Trp274, Tyr277, Tyr299, Ile303, and Tyr306. The similar binding modes are consistent with the observed similar IC50 value inhibitor when using different substrates experimentally, and practically, this can release the experimental scientists from bothering of selecting substrates for MATE1. Hence, we were able to build highly predictive classification models for MATE1 inhibitory activity with both ECFP4 and MM-GB/SA score as input features, which is fit-for-purpose for use in the drug discovery process.

Innovative Approaches to Optimize Clinical Transporter Drug-Drug Interaction Studies

Of the 450 cell membrane transporters responsible for shuttling substrates, nutrients, hormones, neurotransmitters, antioxidants, and signaling molecules, approximately nine are associated with clinically relevant drug-drug interactions (DDIs) due to their role in drug and metabolite transport. Therefore, a clinical study evaluating potential transporter DDIs is recommended if an investigational product is intestinally absorbed, undergoes renal or hepatic elimination, or is suspected to either be a transporter substrate or perpetrator. However, many of the transporter substrates and inhibitors administered during a DDI study also affect cytochrome P450 (CYP) activity, which can complicate data interpretation. To overcome these challenges, the assessment of endogenous biomarkers can help elucidate the mechanism of complex DDIs when multiple transporters or CYPs may be involved. This perspective article will highlight how creative study designs are currently being utilized to address complex transporter DDIs and the role of physiology-based -pharmacokinetic (PBPK) models can play.

Expanding Role of Endogenous Biomarkers for Assessment of Transporter Activity in Drug Development: Current Applications and Future Horizon

The evaluation of transporter-mediated drug-drug interactions (DDIs) during drug development and post-approval contributes to benefit-risk assessment and helps formulate clinical management strategies. The use of endogenous biomarkers, which are substrates of clinically relevant uptake and efflux transporters, to assess the transporter inhibitory potential of a drug has received widespread attention. Endogenous biomarkers, such as coproporphyrin (CP) I and III, have increased mechanistic understanding of complex DDIs. Other endogenous biomarkers are under evaluation, including, but not limited to, sulfated bile acids and 4-pyridoxic acid (PDA). The role of endogenous biomarkers has expanded beyond facilitating assessment of transporter-mediated DDIs and they have also been used to understand alterations in transporter activity in the setting of organ dysfunction and various disease states. We envision that endogenous biomarker-informed approaches will not only help to formulate a prudent and informed DDI assessment strategy but also facilitate quantitative predictions of changes in drug exposures in specific populations.

Utilising Endogenous Biomarkers in Drug Development to Streamline the Assessment of Drug-Drug Interactions Mediated by Renal Transporters: A Pharmaceutical Industry Perspective

The renal secretion of many drugs is facilitated by membrane transporters, including organic cation transporter 2, multidrug and toxin extrusion protein 1/2-K and organic anion transporters 1 and 3. Inhibition of these transporters can reduce renal excretion of drugs and thereby pose a safety risk. Assessing the risk of inhibition of these membrane transporters by investigational drugs remains a key focus in the evaluation of drug-drug interactions (DDIs). Current methods to predict DDI risk are based on generating in vitro data followed by a clinical assessment using a recommended exogenous probe substrate for the individual drug transporter. More recently, monitoring plasma-based and urine-based endogenous biomarkers to predict transporter-mediated DDIs in early phase I studies represents a promising approach to facilitate, improve and potentially avoid conventional clinical DDI studies. This perspective reviews the evidence for use of these endogenous biomarkers in the assessment of renal transporter-mediated DDI, evaluates how endogenous biomarkers may help to expand the DDI assessment toolkit and offers some potential knowledge gaps. A conceptual framework for assessment that may complement the current paradigm of predicting the potential for renal transporter-mediated DDIs is outlined.

Inception and development of a LC-MS/MS assay for the multiplexed quantitation of nine human drug transporter biomarkers

Background: It has become common practice to assess solute carrier transporter (SLC)-mediated drug-drug interactions (DDIs) by quantitating various individual endogenous compounds as biomarkers in human plasma and urine. The goal of this work was to develop biomarker multiplex assays that could be utilized during first in human studies to support the simultaneous assessment of clinical DDI risk across various SLCs. Methodology: Hydrophilic interaction chromatography-MS/MS methods were developed, and validations were performed. Results: The multiplex assays were applied to a first in human study. Placebo/reference subject biomarker data were consistent with single assay in-house and published data. Conclusion: This work demonstrates the utility of these multiplex methods to support the concurrent evaluation of clinical DDI risk across various SLCs.

Quantitative prediction of transporter-mediated drug-drug interactions using the mechanistic static pharmacokinetic (MSPK) model

Guidance/guidelines on drug-drug interactions (DDIs) have been issued in Japan, the United States, and Europe. These guidance/guidelines provide decision trees for conducting metabolizing enzyme-mediated clinical DDI studies; however, the decision trees for transporter-mediated DDIs lack quantitative prediction methods. In this study, the accuracy of a net-effect mechanistic static pharmacokinetics (MSPK) model containing the fraction transported (ft) of transporters was examined to predict transporter-mediated DDIs. This study collected information on 25 oral drugs with new active reagents that were used in clinical DDI studies as perpetrators (42 cases) from drugs approved in Japan between April 2016 and June 2020. The AUCRs (AUC ratios with and without perpetrators) of victim drugs were predicted using the net-effect MSPK model. As a result, 83 and 95% of the predicted AUCRs were within 1.5- and 2-fold error in the observed AUCRs, respectively. In cases where the victims were statins in which pharmacokinetics several transporters are involved, 70 and 91% of the predicted AUCRs were within 1.5- and 2-fold errors, respectively. Therefore, the net-effect MSPK model was applicable for predicting the AUCRs of victims, which are substrates for multiple transporters.

Identification and characterization of an endogenous biomarker of the renal vectorial transport (OCT2-MATE1)

The renal tubular organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1) mediate the vectorial elimination of many drugs and toxins from the kidney, and endogenous biomarkers for vectorial transport (OCT2-MATE1) would allow more accurate drug dosing and help to characterize drug-drug interactions and toxicity. Human serum uptake in OCT2-overexpressing cells and metabolomics analysis were carried out. Potential biomarkers were verified in vitro and in vivo. The specificity of biomarkers was validated in renal transporter overexpressing cells and the sensitivity was investigated by Km . The results showed that the uptake of thiamine, histamine, and 5-hydroxytryptamine was significantly increased in OCT2-overexpressing cells. In vitro assays confirmed that thiamine, histamine, and 5-hydroxytryptamine were substrates of both OCT2 and MATE1. In vivo measurements indicated that the serum thiamine level was increased significantly in the presence of the rOCT2 inhibitor cimetidine, and the level in renal tissue was increased significantly by the rMATE1 inhibitor pyrimethamine. There were no significant changes in the uptake or efflux of thiamine in cell lines overexpressed OAT1, OAT2, OAT3, MRP4, organic anion transporting polypeptide 4C1, P-gp, peptide transporter 2, urate transporter 1, and OAT4. The Km for thiamine with OCT2 and MATE1 were 71.2 and 10.8 μM, respectively. In addition, the cumulative excretion of thiamine at 2 and 4 h was strongly correlated with metformin excretion (R2  > 0.6). Thus, thiamine is preferentially secreted by the OCT2 and MATE1 in renal tubules and can provide a reference value for evaluating the function of the renal tubular OCT2-MATE1.

The Effect of BI 730357 (Retinoic Acid-Related Orphan Receptor Gamma t Antagonist, Bevurogant) on the Pharmacokinetics of a Transporter Probe Cocktail, Including Digoxin, Furosemide, Metformin, and Rosuvastatin: An Open-Label, Non-randomized, 2-Period Fixed-Sequence Trial in Healthy Subjects

Evaluating Drug-Drug Interactions (DDIs) for new investigational compounds requires several trials evaluating different drugs with different transporter specificities. By using a cocktail of drugs with different transporter specificities, a single trial could evaluate the pharmacokinetics (PKs) of each cocktail drug simultaneously, reducing the number of clinical DDI trials required for clinical development. We aimed to investigate the effect of steady-state Boehringer Ingelheim (BI) 730357 (bevurogant) on the PKs of a validated and optimized 4-component transporter cocktail. This open-label, non-randomized, 2-period fixed-sequence phase I trial compared transporter cocktail (0.25 mg digoxin/1 mg furosemide/10 mg metformin hydrochloride/10 mg rosuvastatin) with and without BI 730357 in healthy subjects aged 18-55 years with body mass index 18.5-29.9 kg/m2 . During reference treatment/period 1, transporter cocktail was administered 90 minutes after breakfast. After a washout period, during test treatment/period 2, BI 730357 was dosed twice daily for 13 days, with transporter cocktail administered on day 1. The primary endpoints were the area under the concentration-time curve of the analyte in plasma over the time interval from 0 extrapolated to infinity (AUC0-∞ ) and the maximum measured concentration of the analyte in plasma (Cmax ), and the secondary endpoint was the area under the concentration-time curve of the analyte in plasma over the time interval from 0 to the last quantifiable data point (AUC0-tz ). Steady-state BI 730357 increased digoxin (+48% to +94%), minimally affected metformin (-2% to -9%), furosemide (+12% to +18%), and rosuvastatin (+19% to +39%) exposure. Therefore, no clinically relevant inhibition of transporters OCT2/MATE-1/MATE-2K, OAT1/OAT3, OATP1B1/OATP1B3 was observed. Potential inhibition of breast cancer resistance protein noted as PK parameters of coproporphyrin I/III (OATP1B1/OATP1B3 biomarkers) remained within bioequivalence boundaries while rosuvastatin PK parameters (AUC0-∞ /Cmax /AUC0-tz ) exceeded the bioequivalence boundary. BI 730357 was safe and well tolerated. This trial confirms the usefulness and tolerability of the transporter cocktail consisting of digoxin, furosemide, metformin, and rosuvastatin in assessing drug-transporter interactions in vivo.

A Metabolomic Analysis of Sensitivity and Specificity of 23 Previously Proposed Biomarkers for Renal Transporter-Mediated Drug-Drug Interactions

Endogenous biomarkers are discussed as tools for detection of drug-drug interactions (DDIs) mediated by renal transport proteins, such as organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATE1 and MATE2-K) and organic anion transporters (OAT1 and OAT3). Whereas sensitivity of some endogenous biomarkers against at least one clinical transporter inhibitor has frequently been shown, intra-study comparisons of the extent of effects of inhibitors on different biomarkers are frequently lacking. Moreover, in vivo specificity of such discussed biomarkers has frequently not been studied. We therefore investigated changes of 10 previously described putative biomarkers for inhibition of OCT2/MATEs, as well as 15 previously described putative biomarkers for OATs in human plasma and urine samples of healthy volunteers in response to treatment with 4 inhibitors of transport proteins [verapamil (P-glycoprotein), rifampin (organic anion transporting polypeptides), cimetidine (OCT2/MATEs), and probenecid (OATs)]. Two of the putative biomarkers had been suggested for both OCT2/MATEs and OATs. All substances were unequivocally identified in an untargeted metabolomics assay. The OCT2/MATE biomarkers choline and trimethylamine N-oxide were both sensitive and specific (median log2-fold changes -1.18 in estimated renal elimination and -0.85 in urinary excretion, respectively). For renal OATs, indoleacetyl glutamine and indoleacetic acid (median log2-fold changes -3.77 and -2.85 in estimated renal elimination, respectively) were the candidates for sensitive and specific biomarkers with the most extensive change, followed by taurine, indolelactic acid, and hypoxanthine. This comprehensive study adds further knowledge on sensitivity and specificity of 23 previously described biomarkers of renal OCT2/MATE- and OAT-mediated DDIs.

Quantitative Consideration of Clinical Increases in Serum Creatinine Caused by Renal Transporter Inhibition

Creatinine is a common biomarker of renal function and is secreted in the renal tubular cells via drug transporters, such as organic cation transporter 2 and multidrug and toxin extrusion (MATE) 1/2-K. To differentiate between drug-induced acute kidney injury (AKI) and drug interactions through the renal transporter, it has been examined whether these transporter inhibitions quantitatively explained increases in serum creatinine (SCr) at their clinically relevant concentrations using drugs without any changes in renal function. For such renal transporter inhibitors and recently approved tyrosine kinase inhibitors (TKIs), this mini-review describes clinical increases in SCr and inhibitory potentials against the renal transporters. Most cases of SCr elevations can be explained by considering the renal transporter inhibitions based on unbound maximum plasma concentrations, except for drugs associated with obvious changes in renal function. SCr increases for cobicistat, dolutegravir, and dronedarone, and some TKIs were significantly underestimated, and these underestimations were suggested to be associated with low plasma unbound fractions. Sensitivity analysis of SCr elevations regarding inhibitory potentials of MATE1/2-K demonstrated that typical inhibitors such as cimetidine, DX-619, pyrimethamine, and trimethoprim could give false interpretations of AKI according to the criteria based on relative or absolute levels of SCr elevations. Recent progress and current challenges of physiologically-based pharmacokinetics modeling for creatinine disposition were also summarized. Although it should be noted for the potential impact of in vitro assay designs on clinical translatability of transporter inhibitions data, mechanistic approaches could support decision-making in clinical development to differentiate between AKI and creatinine-drug interactions. SIGNIFICANCE STATEMENT: Serum creatinine (SCr) is widely used as an indicator of kidney function, but it increases due to inhibitions of renal transporters, such as multidrug and toxin extrusion protein 1/2-K despite no functional changes in the kidney. Such SCr elevations were quantitatively explained by renal transporter inhibitions except for some drugs with high protein binding. The present analysis demonstrated that clinically relevant inhibitors of the renal transporters could cause SCr elevations above levels corresponding to acute kidney injury criteria.

Progress in the Quantitative Assessment of Transporter-Mediated Drug-Drug Interactions Using Endogenous Substrates in Clinical Studies

Variations in drug transporter activities, caused by genetic polymorphism and drug-drug interactions (DDIs), alter the systemic exposure of substrate drugs, leading to differences in drug responses. Recently, some endogenous substrates of drug transporters, particularly the solute carrier family transporters such as OATP1B1, OATP1B3, OAT1, OAT3, OCT1, OCT2, MATE1, and MATE2-K, have been identified to investigate variations in drug transporters in humans. Clinical data obtained support their performance as surrogate probes in terms of specificity and reproducibility. Pharmacokinetic parameters of the endogenous biomarkers depend on the genotypes of drug transporters and the systemic exposure to perpetrator drugs. Furthermore, the development of physiologically based pharmacokinetic models for the endogenous biomarkers has enabled a top-down approach to obtain insights into the effect of perpetrators on drug transporters and to more precisely simulate the DDI with victim drugs, including probe drugs. The endogenous biomarkers can address the uncertainty in the DDI prediction in the preclinical and early phases of clinical development and have the potential to fulfill regulatory requirements. Therefore, the endogenous biomarkers should be able to predict disease effects on the variations in drug transporter activities observed in patients. This mini-review focuses on recent progress in the identification and use of the endogenous drug transporter substrate biomarkers and their application in drug development. SIGNIFICANCE STATEMENT: Advances in analytical methods have enabled the identification of endogenous substrates of drug transporters. Changes in the pharmacokinetic parameters (Cmax, AUC, or CLR) of these endogenous biomarkers relative to baseline values can serve as a quantitative index to assess variations in drug transporter activities during clinical studies and thereby provide more precise DDI predictions.

Research Methods and New Advances in Drug-Drug Interactions Mediated by Renal Transporters

The kidney is critical in the human body's excretion of drugs and their metabolites. Renal transporters participate in actively secreting substances from the proximal tubular cells and reabsorbing them in the distal renal tubules. They can affect the clearance rates (CLr) of drugs and their metabolites, eventually influence the clinical efficiency and side effects of drugs, and may produce drug-drug interactions (DDIs) of clinical significance. Renal transporters and renal transporter-mediated DDIs have also been studied by many researchers. In this article, the main types of in vitro research models used for the study of renal transporter-mediated DDIs are membrane-based assays, cell-based assays, and the renal slice uptake model. In vivo research models include animal experiments, gene knockout animal models, positron emission tomography (PET) technology, and studies on human beings. In addition, in vitro-in vivo extrapolation (IVIVE), ex vivo kidney perfusion (EVKP) models, and, more recently, biomarker methods and in silico models are included. This article reviews the traditional research methods of renal transporter-mediated DDIs, updates the recent progress in the development of the methods, and then classifies and summarizes the advantages and disadvantages of each method. Through the sorting work conducted in this paper, it will be convenient for researchers at different learning stages to choose the best method for their own research based on their own subject's situation when they are going to study DDIs mediated by renal transporters.

Development and validation of an LC-MS/MS method to quantify kynurenic acid in human plasma

Background: Monitoring levels of endogenous biomarkers has become an alternative approach to assess transporter-mediated drug-drug interactions in clinical trials. Among the biomarkers of interest, kynurenic acid is effective for the human organic anion transporters OAT1 and OAT3. Here, a simple and robust bioanalytical method was developed using LC-MS/MS to quantify kynurenic acid in human plasma. Results: This method achieved a LLOQ of 10 nm with acceptable signal-to-noise ratio (S/N >5). In addition, an interfering agent, tryptophan, was identified and separated chromatographically. A full method validation was performed in the spirit of GLP. Conclusion: This method can serve as a tool readily available to assess potential drug-drug interactions mediated by inhibition of OAT1 and OAT3 activities.

Development and implementation of urinary transporter biomarkers to facilitate assessment of drug-drug interaction

Aim: Novel urinary biomarker evaluation approaches to support inhibition assessment for renal transporters (e.g., OCT2, multidrug and toxin extrusion proteins [MATEs]). Methods: Highly sensitive and robust hydrophilic interaction chromatography-MS/high-resolution MS assays, for urine and plasma, were developed and characterized to evaluate transporter biomarkers including N¹-methyladenosine and N¹-methylnicotinamide. Results: The assays were simple and reliable with good selectivity and sensitivity, and successfully supported a clinical drug-drug interaction study with a drug candidate that presented in vitro inhibition of OCT2 and MATEs. Conclusion: The multiplexed assays enable a performance comparison, including biomarker specificity and sensitivity, that should increase the confidence in early clinical OCT2/MATEs drug-drug interaction risk assessment.

Endogenous markers of kidney function and renal drug clearance processes of filtration, secretion, and reabsorption

The kidneys are responsible for maintaining physiologic homeostasis. The kidneys clear a variety of drugs and other substances through passive (filtration) and active processes that utilize transport proteins. Renal clearance is comprised of the processes of glomerular filtration, tubular secretion, and tubular reabsorption. Endogenous biomarkers, such as creatinine and cystatin C, are routinely used to estimate renal clearance. Understanding the contributing components of renal function and clearance, through the use of biomarkers, is necessary in elucidating the renal pharmacology of drugs and other substances. While exogenous markers of kidney function have been known for decades, several complexities have limited their usage. Several endogenous markers are being evaluated and hold promise to elucidate the individual components of kidney function that represent filtration, secretion, and reabsorption.

New and Emerging Research on Solute Carrier and ATP Binding Cassette Transporters in Drug Discovery and Development: Outlook From the International Transporter Consortium

Enabled by a plethora of new technologies, research in membrane transporters has exploded in the past decade. The goal of this state-of-the-art article is to describe recent advances in research on membrane transporters that are particularly relevant to drug discovery and development. This review covers advances in basic, translational, and clinical research that has led to an increased understanding of membrane transporters at all levels. At the basic level, we describe the available crystal structures of membrane transporters in both the solute carrier (SLC) and ATP binding cassette superfamilies, which has been enabled by the development of cryogenic electron microscopy methods. Next, we describe new research on lysosomal and mitochondrial transporters as well as recently deorphaned transporters in the SLC superfamily. The translational section includes a summary of proteomic research, which has led to a quantitative understanding of transporter levels in various cell types and tissues and new methods to modulate transporter function, such as allosteric modulators and targeted protein degraders of transporters. The section ends with a review of the effect of the gut microbiome on modulation of transporter function followed by a presentation of 3D cell cultures, which may enable in vivo predictions of transporter function. In the clinical section, we describe new genomic and pharmacogenomic research, highlighting important polymorphisms in transporters that are clinically relevant to many drugs. Finally, we describe new clinical tools, which are becoming increasingly available to enable precision medicine, with the application of tissue-derived small extracellular vesicles and real-world biomarkers.

Reimagining the Framework Supporting the Static Analysis of Transporter Drug Interaction Risk; Integrated Use of Biomarkers to Generate Pan‐Transporter Inhibition Signatures

Solute carrier (SLC) transporters present as the loci of important drug-drug interactions (DDIs). Therefore, sponsors generate in vitro half-maximal inhibitory concentration (IC₅₀ ) data and apply regulatory agency-guided "static" methods to assess DDI risk and the need for a formal clinical DDI study. Because such methods are conservative and high false-positive rates are likely (e.g., DDI study triggered when liver SLC R value ≥ 1.04 and renal SLC maximal unbound plasma (C_max,u )/IC₅₀ ratio ≥ 0.02), investigators have attempted to deploy plasma- and urine-based SLC biomarkers in phase I studies to de-risk DDI and obviate the need for drug probe-based studies. In this regard, it was possible to generate in-house in vitro SLC IC₅₀ data for various clinically (biomarker)-qualified perpetrator drugs, under standard assay conditions, and then estimate "% inhibition" for each SLC and relate it empirically to published clinical biomarker data (area under the plasma concentration vs. time curve (AUC) ratio (AUCR, AUC_inhibitor /AUC_reference ) and % decrease in renal clearance (ΔCL_renal )). After such a "calibration" exercise, it was determined that only compounds with high R values (> 1.5) and C_max,u /IC₅₀ ratios (> 0.5) are likely to significantly modulate liver (AUCR > 1.25) and renal (ΔCL_renal  > 25%) biomarkers and evoke DDI risk. The % inhibition approach supports integration of liver and renal SLC data and allows one to generate pan-SLC inhibition signatures for different test perpetrators (e.g., SLC % inhibition ranking). In turn, such signatures can guide the selection of the most appropriate individual (or combinations of) biomarkers for testing in phase I studies.

Transporters in Regulatory Science: Notable Contributions from Dr. Giacomini in the Past Two Decades

Transporters govern the access of molecules to cells or their exit from cells, thereby controlling the overall distribution of drugs to their intracellular site of action. Clinically relevant drug-drug interactions mediated by transporters are of increasing interest in drug development. Drug transporters, acting alone or in concert with drug metabolizing enzymes, can play an important role in modulating drug absorption, distribution, metabolism, and excretion, thus affecting the pharmacokinetics and/or pharmacodynamics of a drug. Dr. Kathy Giacomini from the University of California, San Francisco is one of the world leaders in transporters and pharmacogenetics with key contributions to transporter science. Her contributions to transporter science are noteworthy. This review paper will summarize Dr. Giacomini's key contributions and influence on transporters in regulatory science in the past two decades. Regulatory science research highlighted in this review covers various aspects of transporter science including understanding the effect of renal impairment on transporters, transporter ontogeny, biomarkers for transporters, and interactions of excipients with transporters affecting drug absorption. Significance Statement This review paper highlights Dr. Giacomini's key contributions and influence on transporters in regulatory science in the past two decades. She has been at the cutting edge of science pertaining to drug transport, drug disposition, and regulatory science, leading to new era of translational sciences pertaining to drug disposition and transporter biology. Her research has and will continue to bring enormous impact on gaining new knowledge in guiding drug development and inspire scientists from all sectors in the field.

Renal Transporter-Mediated Drug-Biomarker Interactions of the Endogenous Substrates Creatinine and N1 -Methylnicotinamide: A PBPK Modeling Approach

Endogenous biomarkers for transporter-mediated drug-drug interaction (DDI) predictions represent a promising approach to facilitate and improve conventional DDI investigations in clinical studies. This approach requires high sensitivity and specificity of biomarkers for the targets of interest (e.g., transport proteins), as well as rigorous characterization of their kinetics, which can be accomplished utilizing physiologically-based pharmacokinetic (PBPK) modeling. Therefore, the objective of this study was to develop PBPK models of the endogenous organic cation transporter (OCT)2 and multidrug and toxin extrusion protein (MATE)1 substrates creatinine and N¹ -methylnicotinamide (NMN). Additionally, this study aimed to predict kinetic changes of the biomarkers during administration of the OCT2 and MATE1 perpetrator drugs trimethoprim, pyrimethamine, and cimetidine. Whole-body PBPK models of creatinine and NMN were developed utilizing studies investigating creatinine or NMN exogenous administration and endogenous synthesis. The newly developed models accurately describe and predict observed plasma concentration-time profiles and urinary excretion of both biomarkers. Subsequently, models were coupled to the previously built and evaluated perpetrator models of trimethoprim, pyrimethamine, and cimetidine for interaction predictions. Increased creatinine plasma concentrations and decreased urinary excretion during the drug-biomarker interactions with trimethoprim, pyrimethamine, and cimetidine were well-described. An additional inhibition of NMN synthesis by trimethoprim and pyrimethamine was hypothesized, improving NMN plasma and urine interaction predictions. To summarize, whole-body PBPK models of creatinine and NMN were built and evaluated to better assess creatinine and NMN kinetics while uncovering knowledge gaps for future research. The models can support investigations of renal transporter-mediated DDIs during drug development.

Evaluation of the Effect of Abrocitinib on Drug Transporters by Integrated Use of Probe Drugs and Endogenous Biomarkers

Abrocitinib is an oral Janus kinase 1 (JAK1) inhibitor currently approved in the United Kingdom for the treatment of moderate‐to‐severe atopic dermatitis (AD). As patients with AD may use medications to manage comorbidities, abrocitinib could be used concomitantly with hepatic and/or renal transporter substrates. Therefore, we assessed the potential effect of abrocitinib on probe drugs and endogenous biomarker substrates for the drug transporters of interest. In vitro studies indicated that, among the transporters tested, abrocitinib has the potential to inhibit the activities of P‐glycoprotein (P‐gp), breast cancer resistance protein (BCRP), organic anion transporter 3 (OAT3), organic cation transporter 1 (OCT1), and multidrug and toxin extrusion protein 1 and 2K (MATE1/2K). Therefore, subsequent phase I, two‐way crossover, open‐label studies in healthy participants were performed to assess the impact of abrocitinib on the pharmacokinetics of the transporter probe substrates dabigatran etexilate (P‐gp), rosuvastatin (BCRP and OAT3), and metformin (OCT2 and MATE1/2K), as well as endogenous biomarkers for MATE1/2K (N¹‐methylnicotinamide (NMN)) and OCT1 (isobutyryl‐_L‐carnitine (IBC)). Co‐administration with abrocitinib was shown to increase the plasma exposure of dabigatran by ~ 50%. In comparison, the plasma exposure and renal clearance of rosuvastatin and metformin were not altered with abrocitinib co‐administration. Similarly, abrocitinib did not affect the exposure of NMN or IBC. An increase in dabigatran exposure suggests that abrocitinib inhibits P‐gp activity. By contrast, a lack of impact on plasma exposure and/or renal clearance of rosuvastatin, metformin, NMN, or IBC suggests that BCRP, OAT3, OCT1, and MATE1/2K activity are unaffected by abrocitinib.

In Vitro and In Vivo Inhibition of MATE1 by Tyrosine Kinase Inhibitors

The membrane transport of many cationic prescription drugs depends on facilitated transport by organic cation transporters of which several members, including OCT2 (SLC22A2), are sensitive to inhibition by select tyrosine kinase inhibitors (TKIs). We hypothesized that TKIs may differentially interact with the renal transporter MATE1 (SLC47A1) and influence the elimination and toxicity of the MATE1 substrate oxaliplatin. Interactions with FDA-approved TKIs were evaluated in transfected HEK293 cells, and in vivo pharmacokinetic studies were performed in wild-type, MATE1-deficient, and OCT2/MATE1-deficient mice. Of 57 TKIs evaluated, 37 potently inhibited MATE1 function by >80% through a non-competitive, reversible, substrate-independent mechanism. The urinary excretion of oxaliplatin was reduced by about 2-fold in mice with a deficiency of MATE1 or both OCT2 and MATE1 (p < 0.05), without impacting markers of acute renal injury. In addition, genetic or pharmacological inhibition of MATE1 did not significantly alter plasma levels of oxaliplatin, suggesting that MATE1 inhibitors are unlikely to influence the safety or drug-drug interaction liability of oxaliplatin-based chemotherapy.

Simultaneous determination of nicotinamide and N1 -methylnicotinamide in human serum by LC-MS/MS to associate their serum concentrations with obesity

A rapid and sensitive LC-MS/MS method was developed and validated for the simultaneous determination of nicotinamide and its metabolite N¹ -methylnicotinamide in human serum. Serum samples were prepared by protein precipitation with acetonitrile. The chromatographic separation was performed on a Waters Spherisorb S5 CN microbore column (2.0 × 100 mm, 5 μm) with gradient elution within 7 min. Acetonitrile and 5 mm ammonium formate aqueous solution (containing 0.1% formic acid) were used as mobile phases. Nicotinamide, N¹ -methylnicotinamide and N'-methylnicotinamide (internal standard) were detected with a triple-quadrupole tandem mass spectrometer in the positive ion mode. Multiple reaction monitoring was used to monitor precursor to product ion transitions of m/z 123.1 → 80.1 for nicotinamide, m/z 137.1 → 94.1 for N¹ -methylnicotinamide and m/z 137.1 → 80.1 for the internal standard. The linear ranges of nicotinamide and N¹ -methylnicotinamide were 5.000-160.0 and 2.500-80.00 ng/ml, respectively. The intra- and inter-day precisions (RSD) of both analytes were within 6.90%. The recoveries were >88%. The analytes were proven to be stable during all sample storage, preparation and analytic procedures. The method was successfully applied to determine the concentrations of nicotinamide and N¹ -methylnicotinamide in human serum to investigate the association between their concentrations and obesity in 1160 Chinese subjects.

Recent Advances in Drug Transporter Sciences: Highlights From the Year 2020

Drug Metabolism Reviews has an impressive track record of providing scientific reviews in the area of xenobiotic biotransformation over 47 years. It has consistently proved to be resourceful to many scientists from pharmaceutical industry, academia, regulatory agencies working in diverse areas including enzymology, pharmacology, pharmacokinetics and toxicology. Over the last 5 years Drug metabolism Reviews has annually published an industry commentary aimed to highlight novel insights and approaches that have made significant impacts on the field of biotransformation (led by Cyrus Khojasteh). We hope to continue this tradition by providing an overview of advances made in the field of drug transporters during 2020. The field of drug transporters is rapidly evolving as they play an essential role in drug absorption, distribution, clearance and elimination. In this review we have selected outstanding drug transporter articles that have significantly contributed to moving forward the field of transporter science with respect to translation and improved understanding of diverse aspects including uptake clearance, clinical biomarkers, induction, proteomics, emerging transporters and tissue targeting.The theme of this review consists of synopsis that summarizes each article followed by our commentary. The objective of this work is not to provide a comprehensive review but rather exemplify novel insights and state-of-the-art highlights of recent research that have advanced our understanding of drug transporters in drug disposition. We are hopeful that this effort will prove useful to the scientific community and as such request feedback, and further extend an invitation to anyone interested in contributing to future reviews.

Impact of Direction of Transport on the Evaluation of Inhibition Potencies of Multidrug and Toxin Extrusion Protein 1 Inhibitors

Multidrug and toxin extrusion (MATE) transporters are expressed on the luminal membrane of renal proximal tubule cells and extrude their substrates into the luminal side of the tubules. Inhibition of MATE1 can reduce renal secretory clearance of its substrate drugs and lead to drug-drug interactions (DDIs). To address whether IC₅₀ values of MATE1 inhibitors with regard to their extracellular concentrations are affected by the direction of MATE1-mediated transport, we established an efflux assay of 1-methyl-4-phenylpyridinium (MPP⁺) and metformin using the human embryonic kidney 293 model transiently expressing human MATE1. The efflux rate was defined by reduction of the cellular amount of MPP⁺ and metformin for 0.25 minutes shortly after the removal of extracellular MPP⁺ and metformin. Inhibition potencies of 12 inhibitors toward MATE1-mediated transport were determined in both uptake and efflux assays. When MPP⁺ was used as a substrate, 8 out of 12 inhibitors showed comparable IC₅₀ values between assays (<4-fold). IC₅₀ values from the efflux assays were higher for cimetidine (9.9-fold), trimethoprim (10-fold), famotidine (6.4-fold), and cephalexin (>3.8-fold). When metformin was used as a substrate, IC₅₀ values of the tested inhibitors when evaluated using uptake and efflux assays were within 4-fold of each other, with the exception of cephalexin (>4.7-fold). IC₅₀ values obtained from the uptake assay using metformin showed smaller IC₅₀ values than those from the efflux assay. Therefore, the uptake assay is recommended to determine IC₅₀ values for the DDI predictions. SIGNIFICANCE STATEMENT: In this study, a new method to evaluate IC₅₀ values of extracellular added inhibitors utilizing an efflux assay was established. IC₅₀ values were not largely different between uptake and efflux directions but were smaller for uptake. This study supports the rationale for a commonly accepted uptake assay with metformin as an in vitro probe substrate for multidrug and toxin extrusion 1-mediated drug-drug interaction risk assessment in drug development.