Identification and Quantitative Assessment of Uremic Solutes as Inhibitors of Renal Organic Anion Transporters, OAT1 and OAT3

Evaluation of 14 PFAS for permeability and organic anion transporter interactions: Implications for renal clearance in humans

Per- and polyfluoroalkyl substances (PFAS) encompass a diverse group of synthetic fluorinated chemicals known to elicit adverse health effects in animals and humans. However, only a few studies investigated the mechanisms underlying clearance of PFAS. Herein, the relevance of human renal transporters and permeability to clearance and bioaccumulation for 14 PFAS containing three to eleven perfluorinated carbon atoms (ηpfc = 3-11) and several functional head-groups was investigated. Apparent permeabilities and interactions with human transporters were measured using in vitro cell-based assays, including the MDCK-LE cell line, and HEK293 stable transfected cell lines expressing organic anion transporter (OAT) 1-4 and organic cation transporter (OCT) 2. The results generated align with the Extended Clearance Classification System (ECCS), affirming that permeability, molecular weight, and ionization serve as robust predictors of clearance and renal transporter engagement. Notably, PFAS with low permeability (ECCS 3A and 3B) exhibited substantial substrate activity for OAT1 and OAT3, indicative of active renal secretion. Furthermore, we highlight the potential contribution of OAT4-mediated reabsorption to the renal clearance of PFAS with short ηpfc, such as perfluorohexane sulfonate (PFHxS). Our data advance our mechanistic understanding of renal clearance of PFAS in humans, provide useful input parameters for toxicokinetic models, and have broad implications for toxicological evaluation and regulatory considerations.

Residual Kidney Function in Hemodialysis: Its Importance and Contribution to Improved Patient Outcomes

Individuals afflicted with advanced kidney dysfunction who require dialysis for medical management exhibit different degrees of native kidney function, called residual kidney function (RKF), ranging from nil to appreciable levels. The primary focus of this manuscript is to delve into the concept of RKF, a pivotal yet under-represented topic in nephrology. To begin, we unpack the definition and intrinsic nature of RKF. We then juxtapose the efficiency of RKF against that of hemodialysis in preserving homeostatic equilibrium and facilitating physiological functions. Given the complex interplay of RKF and overall patient health, we shed light on the extent of its influence on patient outcomes, particularly in those living with advanced kidney dysfunction and on dialysis. This manuscript subsequently presents methodologies and measures to assess RKF, concluding with the potential benefits of targeted interventions aimed at preserving RKF.

Membrane transporters in drug development and as determinants of precision medicine

The effect of membrane transporters on drug disposition, efficacy and safety is now well recognized. Since the initial publication from the International Transporter Consortium, significant progress has been made in understanding the roles and functions of transporters, as well as in the development of tools and models to assess and predict transporter-mediated activity, toxicity and drug-drug interactions (DDIs). Notable advances include an increased understanding of the effects of intrinsic and extrinsic factors on transporter activity, the application of physiologically based pharmacokinetic modelling in predicting transporter-mediated drug disposition, the identification of endogenous biomarkers to assess transporter-mediated DDIs and the determination of the cryogenic electron microscopy structures of SLC and ABC transporters. This article provides an overview of these key developments, highlighting unanswered questions, regulatory considerations and future directions.

A Possible Therapeutic Application of the Selective Inhibitor of Urate Transporter 1, Dotinurad, for Metabolic Syndrome, Chronic Kidney Disease, and Cardiovascular Disease

The reabsorption of uric acid (UA) is mainly mediated by urate transporter 1 (URAT1) and glucose transporter 9 (GLUT9) in the kidneys. Dotinurad inhibits URAT1 but does not inhibit other UA transporters, such as GLUT9, ATP-binding cassette transporter G2 (ABCG2), and organic anion transporter 1/3 (OAT1/3). We found that dotinurad ameliorated the metabolic parameters and renal function in hyperuricemic patients. We consider the significance of the highly selective inhibition of URAT1 by dotinurad for metabolic syndrome, chronic kidney disease (CKD), and cardiovascular disease (CVD). The selective inhibition of URAT1 by dotinurad increases urinary UA in the proximal tubules, and this un-reabsorbed UA may compete with urinary glucose for GLUT9, reducing glucose reabsorption. The inhibition by dotinurad of UA entry via URAT1 into the liver and adipose tissues increased energy expenditure and decreased lipid synthesis and inflammation in rats. Such effects may improve metabolic parameters. CKD patients accumulate uremic toxins, including indoxyl sulfate (IS), in the body. ABCG2 regulates the renal and intestinal excretion of IS, which strongly affects CKD. OAT1/3 inhibitors suppress IS uptake into the kidneys, thereby increasing plasma IS, which produces oxidative stress and induces vascular endothelial dysfunction in CKD patients. The highly selective inhibition of URAT1 by dotinurad may be beneficial for metabolic syndrome, CKD, and CVD.

Development of 4-Pyridoxic Acid PBPK Model to Support Biomarker-Informed Evaluation of OAT1/3 Inhibition and Effect of Chronic Kidney Disease

Monitoring endogenous biomarkers is increasingly used to evaluate transporter-mediated drug-drug interactions (DDIs) in early drug development and may be applied to elucidate changes in transporter activity in disease. 4-pyridoxic acid (PDA) has been identified as the most sensitive plasma endogenous biomarker of renal organic anion transporters (OAT1/3). Increase in PDA baseline concentrations was observed after administration of probenecid, a strong clinical inhibitor of OAT1/3 and also in patients with chronic kidney disease (CKD). The aim of this study was to develop and verify a physiologically-based pharmacokinetic (PBPK) model of PDA, to predict the magnitude of probenecid DDI and predict the CKD-related changes in PDA baseline. The PBPK model for PDA was first developed in healthy population, building on from previous population pharmacokinetic modeling, and incorporating a mechanistic kidney model to consider OAT1/3-mediated renal secretion. Probenecid PBPK model was adapted from the Simcyp database and re-verified to capture its dose-dependent pharmacokinetics (n = 9 studies). The PBPK model successfully predicted the PDA plasma concentrations, area under the curve, and renal clearance in healthy subjects at baseline and after single/multiple probenecid doses. Prospective simulations in severe CKD predicted successfully the increase in PDA plasma concentration relative to healthy (within 2-fold of observed data) after accounting for 60% increase in fraction unbound in plasma and additional 50% decline in OAT1/3 activity beyond the decrease in glomerular filtration rate. The verified PDA PBPK model supports future robust evaluation of OAT1/3 DDI in drug development and increases our confidence in predicting exposure and renal secretion in patients with CKD.

Incorporating Uremic Solute-mediated Inhibition of OAT1/3 Improves PBPK Prediction of Tenofovir Renal and Systemic Disposition in Patients with Severe Kidney Disease

BACKGROUND

Dose modification of renally secreted drugs in patients with chronic kidney disease (CKD) has relied on serum creatinine concentration as a biomarker to estimate glomerular filtration (GFR) under the assumption that filtration and secretion decline in parallel. A discrepancy between actual renal clearance and predicted renal clearance based on GFR alone is observed in severe CKD patients with tenofovir, a compound secreted by renal OAT1/3. Uremic solutes that inhibit OAT1/3 may play a role in this divergence.

METHODS

To examine the impact of transporter inhibition by uremic solutes on tenofovir renal clearance, we determined the inhibitory potential of uremic solutes hippuric acid, indoxyl sulfate, and p-cresol sulfate. The inhibition parameters (IC50) were incorporated into a previously validated mechanistic kidney model; simulated renal clearance and plasma PK profile were compared to data from clinical studies.

RESULTS

Without the incorporation of uremic solute inhibition, the PBPK model failed to capture the observed data with an absolute average fold error (AAFE) > 2. However, when the inhibition of renal uptake transporters and uptake transporters in the slow distribution tissues were included, the AAFE value was within the pre-defined twofold model acceptance criterion, demonstrating successful model extrapolation to CKD patients.

CONCLUSION

A PBPK model that incorporates inhibition by uremic solutes has potential to better predict renal clearance and systemic disposition of secreted drugs in patients with CKD. Ongoing research is warranted to determine if the model can be expanded to include other OAT1/3 substrate drugs and to evaluate how these findings can be translated to clinical guidance for drug selection and dose optimization in patients with CKD.

Emerging Roles of Uremic Toxins and Inflammatory Cytokines in the Alteration of Hepatic Drug Disposition in Patients with Kidney Dysfunction

Patients with kidney dysfunction exhibit distinct pharmacokinetic profiles compared to those with normal kidney function. Hence, it is desirable to monitor the drug efficacy and toxicity caused by fluctuations in plasma drug concentrations associated with kidney dysfunction. Recently, pharmacokinetic information of drugs excreted mainly through the urine of patients with kidney dysfunction has been reported via drug-labeling information. Pharmacokinetic changes in drugs mainly eliminated by the liver cannot be overlooked as drug metabolism and/or transport activity in the liver may also be altered in patients with kidney dysfunction; however, the underlying mechanisms remain unclear. To plan an appropriate dosage regimen, it is necessary to clarify the underlying processes of functional changes in pharmacokinetic proteins. In recent years, uremic toxins have been shown to reduce the activity and/or expression of renal and hepatic transporters. This inhibitory effect has been reported to be time-dependent. In addition, inflammatory cytokines, such as interleukin-6, released from immune cells activated by uremic toxins and/or kidney injury can reduce the expression levels of drug-metabolizing enzymes and transporters in human hepatocytes. In this mini-review, we have summarized the renal and hepatic pharmacokinetic changes as well as the potential underlying mechanisms in kidney dysfunction, such as the chronic kidney disease and acute kidney injury. SIGNIFICANCE STATEMENT: Patients with kidney dysfunction exhibit distinct pharmacokinetic profiles compared to those with normal kidney function. Increased plasma concentrations of uremic toxins and inflammatory cytokines during kidney disease may potentially affect the activities and/or expression levels of drug-metabolizing enzymes and transporters in the liver and kidneys.

Assessment of the impact of glyphosate and 2,4-D herbicides on the kidney injury and transcriptome changes in obese mice fed a Western diet

The development of chronic kidney disease has been associated with comorbidities resulting from the consumption of Westernized dietary (WD) patterns, including obesity and other metabolic dysfunctions. Kidneys also have a crucial role in the metabolism and excretion of xenobiotics, including herbicides. There is limited knowledge regarding the simultaneous exposure to WD and glyphosate (glypho) and 2,4-D, the most used herbicides globally. Thus, this study examined whether exposure to glypho and/or 2,4-D, either individually or in mixed, could impact the early effects of WD intake on kidney histology and gene expression in a rodent model. Male C57BL6J mice were fed a WD containing 20% lard, 0.2% cholesterol, 20% sucrose, and high sugar solution with 23.1 and 18.9 g/L of D-fructose and D-glucose for six months. During this period, the mice also received glypho (0.05 or 5 mg/kg/day), 2,4-D (0.02 or 2 mg/kg/day), or a mixture of both (0.05 +0.02, 5 +2 mg/kg/day) via intragastric administration five times per week. The doses were within or below the established regulatory limits. While single or mixed exposures did not alter WD-induced obesity, tubular lipid vacuolation, or increased serum creatinine levels; the exposure to higher doses of the mixture (5 +2) reduced the mesangial matrix area and tubular cell proliferation, while increasing the density of F4/80 macrophages in the renal interstitium. In terms of transcriptomic analysis, the herbicide mixture altered the expression of 415 genes in the kidney, which were found to be associated with immune response processes, particularly those related to phagocyte activity. While discrete, findings indicate that herbicide mixtures, rather than single exposures, might induce minor deleterious effects on the kidneys of obese mice under WD intake.

Chloroquine and Hydroxychloroquine, as Proteasome Inhibitors, Upregulate the Expression and Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3), at the basolateral membrane of kidney proximal tubule cells, facilitates the elimination of numerous widely used drugs. Earlier investigation from our laboratory revealed that ubiquitin conjugation to OAT3 leads to OAT3 internalization from the cell surface, followed by degradation in the proteasome. In the current study, we examined the roles of chloroquine (CQ) and hydroxychloroquine (HCQ), two well-known anti-malarial drugs, in their action as proteasome inhibitors and their effects on OAT3 ubiquitination, expression, and function. We showed that in cells treated with CQ and HCQ, the ubiquitinated OAT3 was considerably enhanced, which correlated well with a decrease in 20S proteasome activity. Furthermore, in CQ- and HCQ-treated cells, OAT3 expression and OAT3-mediated transport of estrone sulfate, a prototypical substrate, were significantly increased. Such increases in OAT3 expression and transport activity were accompanied by an increase in the maximum transport velocity and a decrease in the degradation rate of the transporter. In conclusion, this study unveiled a novel role of CQ and HCQ in enhancing OAT3 expression and transport activity by preventing the degradation of ubiquitinated OAT3 in proteasomes.

The kidney drug transporter OAT1 regulates gut microbiome-dependent host metabolism

Organic anion transporter 1 (OAT1/SLC22A6, NKT) is a multispecific drug transporter in the kidney with numerous substrates, including pharmaceuticals, endogenous metabolites, natural products, and uremic toxins. Here, we show that OAT1 regulates levels of gut microbiome-derived metabolites. We depleted the gut microbiome of Oat1-KO and WT mice and performed metabolomics to analyze the effects of genotype (KO versus WT) and microbiome depletion. OAT1 is an in vivo intermediary between the host and the microbes, with 40 of the 162 metabolites dependent on the gut microbiome also impacted by loss of Oat1. Chemoinformatic analysis revealed that the altered metabolites (e.g., indoxyl sulfate, p-cresol sulfate, deoxycholate) had more ring structures and sulfate groups. This indicates a pathway from gut microbes to liver phase II metabolism, to renal OAT1-mediated transport. The idea that multiple gut-derived metabolites directly interact with OAT1 was confirmed by in vitro transport and magnetic bead binding assays. We show that gut microbiome-derived metabolites dependent on OAT1 are impacted in a chronic kidney disease (CKD) model and human drug-metabolite interactions. Consistent with the Remote Sensing and Signaling Theory, our results support the view that drug transporters (e.g., OAT1, OAT3, OATP1B1, OATP1B3, MRP2, MRP4, ABCG2) play a central role in regulating gut microbe-dependent metabolism, as well as interorganismal communication between the host and microbiome.

Mechanistic Study on the Effect of Renal Impairment on the Pharmacokinetics of Vildagliptin and its Carboxylic Acid Metabolite

PURPOSE

To clarify the mechanism of renal impairment leading to different degrees of increased plasma exposure to dipeptidyl peptidase 4 inhibitor vildagliptin and its major metabolite, M20.7.

METHODS

The 5/6 nephrectomized (5/6 Nx) rat model, to simulate chronic renal failure (CRF) patients, combined with kidney slices and transporter studies in vitro were used to assess this pharmacokinetic differences.

RESULTS

After intragastric administration to 5/6 Nx rats, vildagliptin showed increased plasma levels by 45.8%, and M20.7 by 7.51 times, which was similar to patients with severe renal impairment. The recovery rate of M20.7 in urine and feces increased by less than 20%, showing limited effect of renal impairment on vildagliptin metabolism. In vitro studies found M20.7 to be the substrate for organic anion transporter 3 (OAT3). However, the active uptake of M20.7 in renal slices showed no difference between the 5/6 Nx and normal rats. In OAT3 overexpressed cells, the protein-bound uremic toxins, 3-carboxy-4-methyl-5propyl-2-furanpropionate (CMPF), hippuric acid (HA) and indoxyl sulfate (IS), which accumulate in CRF patients, inhibited M20.7 uptake with IC₅₀ values of 5.75, 29.0 and 69.5 μM respectively, far lower than plasma concentrations in CRF patients, and showed a mixed inhibition type.

CONCLUSIONS

The large increase in plasma exposure of M20.7 could be attributed to the accumulation of uremic toxins in CRF patients, which inhibited OAT3 activity and blocked renal excretion of M20.7, while vildagliptin, with high permeability, showed a slight increase in plasma exposure due to reduced glomerular filtration.

Determining the Effects of Chronic Kidney Disease on Organic Anion Transporter1/3 Activity Through Physiologically Based Pharmacokinetic Modeling

BACKGROUND AND OBJECTIVE

The renal excretion of drugs via organic anion transporters 1 and 3 (OAT1/3) is significantly decreased in patients with renal impairment. This study uses physiologically based pharmacokinetic models to quantify the reduction in OAT1/3-mediated secretion of drugs throughout varying stages of chronic kidney disease.

METHODS

Physiologically based pharmacokinetic models were constructed for four OAT1/3 substrates in healthy individuals: acyclovir, meropenem, furosemide, and ciprofloxacin. Observed data from drug-drug interaction studies with probenecid, a potent OAT1/3 inhibitor, were used to parameterize the contribution of OAT1/3 to the renal elimination of each drug. The models were then translated to patients with chronic kidney disease by accounting for changes in glomerular filtration rate, kidney volume, renal blood flow, plasma protein binding, and hematocrit. Additionally, a relationship was derived between the estimated glomerular filtration rate and the reduction in OAT1/3-mediated secretion of drugs based on the renal extraction ratios of ƿ-aminohippuric acid in patients with varying degrees of renal impairment. The relationship was evaluated in silico by evaluating the predictive performance of each final model in describing the pharmacokinetics (PK) of drugs across stages of chronic kidney disease.

RESULTS

OAT1/3-mediated renal excretion of drugs was found to be decreased by 27-49%, 50-68%, and 70-96% in stage 3, stage 4, and stage 5 of chronic kidney disease, respectively. In support of the parameterization, physiologically based pharmacokinetic models of four OAT1/3 substrates were able to adequately characterize the PK in patients with different degrees of renal impairment. Total exposure after intravenous administration was predicted within a 1.5-fold error and 85% of the observed data points fell within a 1.5-fold prediction error. The models modestly under-predicted plasma concentrations in patients with end-stage renal disease undergoing intermittent hemodialysis. However, results should be interpreted with caution because of the limited number of molecules analyzed and the sparse sampling in observed chronic kidney disease pharmacokinetic studies.

CONCLUSIONS

A quantitative understanding of the reduction in OAT1/3-mediated excretion of drugs in differing stages of renal impairment will contribute to better predictive accuracy for physiologically based pharmacokinetic models in drug development, assisting with clinical trial planning and potentially sparing this population from unnecessary toxic exposures.

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.

Effect of Chronic Kidney Disease on the Renal Secretion via Organic Anion Transporters 1/3: Implications for Physiologically-Based Pharmacokinetic Modeling and Dose Adjustment

There is growing evidence that active tubular secretory clearance (CL_s) may not decline proportionally with the glomerular filtration rate (GFR) in chronic kidney disease (CKD), leading to the overestimation of renal clearance (CL_r) when using solely GFR to approximate disease effect on renal elimination. The clinical pharmacokinetic data of 33 renally secreted OAT1/3 substrates were collated to investigate the impact of mild, moderate, and severe CKD on CL_r, tubular secretion and protein binding (f_u,p). The f_u,p of the collated substrates ranged from 0.0026 to 1.0 in healthy populations; observed CKD‐related increase in the f_u,p (up to 2.7‐fold) of 8 highly bound substrates (f_u,p ≤ 0.2) was accounted for in the analysis. Use of prediction equation based on disease‐related changes in albumin resulted in underprediction of the CKD‐related increase in f_u,p of highly bound substrates, highlighting the necessity to measure protein binding in severe CKD. The critical analysis of clinical data for 33 OAT1/3 probes established that decrease in OAT1/3 activity proportional to the changes in GFR was insufficient to recapitulate effects of severe CKD on unbound tubular secretion clearance. OAT1/3‐mediated CL_s was estimated to decline by an additional 50% relative to the GFR decline in severe CKD, whereas change in active secretion in mild and moderate CKD was proportional to GFR. Consideration of this additional 50% decline in OAT1/3‐mediated CL_s is recommended for physiologically‐based pharmacokinetic models and dose adjustment of OAT1/3 substrates in severe CKD, especially for substrates with high contribution of the active secretion to CL_r.

Clinical Implications of Altered Drug Transporter Abundance/Function and PBPK Modeling in Specific Populations: An ITC Perspective

The role of membrane transporters on pharmacokinetics (PKs), drug-drug interactions (DDIs), pharmacodynamics (PDs), and toxicity of drugs has been broadly recognized. However, our knowledge of modulation of transporter expression and/or function in the diseased patient population or specific populations, such as pediatrics or pregnancy, is still emerging. This white paper highlights recent advances in studying the changes in transporter expression and activity in various diseases (i.e., renal and hepatic impairment and cancer) and some specific populations (i.e., pediatrics and pregnancy) with the focus on clinical implications. Proposed alterations in transporter abundance and/or activity in diseased and specific populations are based on (i) quantitative transporter proteomic data and relative abundance in specific populations vs. healthy adults, (ii) clinical PKs, and emerging transporter biomarker and/or pharmacogenomic data, and (iii) physiologically-based pharmacokinetic modeling and simulation. The potential for altered PK, PD, and toxicity in these populations needs to be considered for drugs and their active metabolites in which transporter-mediated uptake/efflux is a major contributor to their absorption, distribution, and elimination pathways and/or associated DDI risk. In addition to best practices, this white paper discusses current challenges and knowledge gaps to study and quantitatively predict the effects of modulation in transporter activity in these populations, together with the perspectives from the International Transporter Consortium (ITC) on future directions.

Insights into the structure and function of the human organic anion transporter 1 in lipid bilayer membranes

The human SLC22A6/OAT1 plays an important role in the elimination of a broad range of endogenous substances and xenobiotics thus attracting attention from the pharmacological community. Furthermore, OAT1 is also involved in key physiological events such as the remote inter-organ communication. Despite its significance, the knowledge about hOAT1 structure and the transport mechanism at the atomic level remains fragmented owing to the lack of resolved structures. By means of protein-threading modeling refined by μs-scaled Molecular Dynamics simulations, the present study provides the first robust model of hOAT1 in outward-facing conformation. Taking advantage of the AlphaFold 2 predicted structure of hOAT1 in inward-facing conformation, we here provide the essential structural and functional features comparing both states. The intracellular motifs conserved among Major Facilitator Superfamily members create a so-called “charge-relay system” that works as molecular switches modulating the conformation. The principal element of the event points at interactions of charged residues that appear crucial for the transporter dynamics and function. Moreover, hOAT1 model was embedded in different lipid bilayer membranes highlighting the crucial structural dependence on lipid-protein interactions. MD simulations supported the pivotal role of phosphatidylethanolamine components to the protein conformation stability. The present model is made available to decipher the impact of any observed polymorphism and mutation on drug transport as well as to understand substrate binding modes.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Recent lessons learned from population pharmacokinetic studies of mycophenolic acid: physiological, genomic, and drug interactions leading to the prediction of drug effects

INTRODUCTION

Mycophenolic acid (MPA) is a widely used immunosuppressant in transplantation and autoimmune disease. Highly variable pharmacokinetics have been observed with MPA, but the exact mechanisms remain largely unknown.

AREAS COVERED

The current review provided a critical, comprehensive update of recently published population pharmacokinetic/dynamic models of MPA (n=16 papers identified from PubMed and Embase, inclusive from January 2017 to August 2021), with specific emphases on the intrinsic and extrinsic factors influencing the pharmacology of MPA. The significance of the identified covariates, potential mechanisms, and comparisons to historical literature have been provided.

EXPERT OPINION

While select covariates affecting the population pharmacokinetics of MPA are consistently observed and mechanistically supported, some variables have not been regularly reported and/or lacked mechanistic explanation. Very few pharmacodynamic models were available, pointing to the need to extrapolate pharmacokinetic findings. Ideal models of MPA should consist of: i) utilizing optimal sampling points to allow the characterizations of absorption, re-absorption, and elimination phases; ii) characterizing unbound/total MPA, MPA metabolites, plasma/urinary concentrations, and genetic polymorphisms to facilitate mechanistic interpretations; and iii) incorporating actual outcomes and pharmacodynamic data to establish clinical relevance. We anticipate the field will continue to expand in the next 5 to 10 years.

PBPK Simulation-Based Evaluation of Ganciclovir Crystalluria Risk Factors: Effect of Renal Impairment, Old Age, and Low Fluid Intake

Dosing guidance is often lacking for chronic kidney disease (CKD) due to exclusion of such patients from pivotal clinical trials. Physiologically based pharmacokinetic (PBPK) modelling supports model-informed dosing when clinical data are lacking, but application of these approaches to patients with impaired renal function is not yet at full maturity. In the current study, a ganciclovir PBPK model was developed for patients with normal renal function and extended to CKD population. CKD-related changes in tubular secretion were explored in the mechanistic kidney model and implemented either as proportional or non-proportional decline relative to GFR. Crystalluria risk was evaluated in different clinical settings (old age, severe CKD and low fluid intake) by simulating ganciclovir medullary collecting duct (MCD) concentrations. The ganciclovir PBPK model captured observed changes in systemic pharmacokinetic endpoints in mild-to-severe CKD; these trends were evident irrespective of assumed pathophysiological mechanism of altered active tubular secretion in the model. Minimal difference in simulated ganciclovir MCD concentrations was noted between young adult and geriatric populations with normal renal function and urine flow (1 mL/min), with lower concentrations predicted for severe CKD patients. High crystalluria risk was identified at reduced urine flow (0.1 mL/min) as simulated ganciclovir MCD concentrations exceeded its solubility (2.6–6 mg/mL), irrespective of underlying renal function. The analysis highlighted the importance of appropriate distribution of virtual subjects’ systems data in CKD populations. The ganciclovir PBPK model illustrates the ability of this translational tool to explore individual and combined effects of age, urine flow, and renal impairment on local drug renal exposure.

Application of a PBPK Model of Rivaroxaban to Prospective Simulations of Drug-Drug-Disease Interactions with Protein Kinase Inhibitors in CA-VTE

BACKGROUND AND PURPOSE

Rivaroxaban is a viable anticoagulant for the management of cancer associated venous thromboembolism (CA-VTE). A previously verified physiologically-based pharmacokinetic (PBPK) model of rivaroxaban established how its multiple pathways of elimination via both CYP3A4/2J2-mediated hepatic metabolism and organic anion transporter 3 (OAT3)/P-glycoprotein-mediated renal secretion predisposes rivaroxaban to drug-drug-disease interactions (DDDIs) with clinically relevant protein kinase inhibitors (PKIs). We proposed the application of PBPK modelling to prospectively interrogate clinically significant DDIs between rivaroxaban and PKIs (erlotinib and nilotinib) for dose adjustments in CA-VTE.

EXPERIMENTAL APPROACH

The inhibitory potencies of the PKIs on CYP3A4/2J2-mediated metabolism of rivaroxaban were characterized. Using prototypical OAT3 inhibitor ketoconazole, in vitro OAT3 inhibition assays were optimized to ascertain the in vivo relevance of derived transport inhibitory constants (K_i ). Untested DDDIs between rivaroxaban and erlotinib or nilotinib were simulated.

KEY RESULTS

Mechanism-based inactivation (MBI) of CYP3A4-mediated rivaroxaban metabolism by both PKIs and MBI of CYP2J2 by erlotinib were established. The importance of substrate specificity and nonspecific binding to derive OAT3-inhibitory K_i values of ketoconazole and nilotinib for the accurate prediction of interactions was illustrated. When simulated rivaroxaban exposure variations with concomitant erlotinib and nilotinib therapy were evaluated using published dose-exposure equivalence metrics and bleeding risk analyses, dose reductions from 20 mg to 15 mg and 10 mg in normal and mild renal dysfunction, respectively, were warranted.

CONCLUSION AND IMPLICATIONS

We established a PBPK-DDDI model to prospectively evaluate clinically relevant interactions between rivaroxaban and PKIs for the safe and efficacious management of CA-VTE.

Bridging the gap between in silico and in vivo by modeling opioid disposition in a kidney proximal tubule microphysiological system

Opioid overdose, dependence, and addiction are a major public health crisis. Patients with chronic kidney disease (CKD) are at high risk of opioid overdose, therefore novel methods that provide accurate prediction of renal clearance (CL_r) and systemic disposition of opioids in CKD patients can facilitate the optimization of therapeutic regimens. The present study aimed to predict renal clearance and systemic disposition of morphine and its active metabolite morphine-6-glucuronide (M6G) in CKD patients using a vascularized human proximal tubule microphysiological system (VPT-MPS) coupled with a parent-metabolite full body physiologically-based pharmacokinetic (PBPK) model. The VPT-MPS, populated with a human umbilical vein endothelial cell (HUVEC) channel and an adjacent human primary proximal tubular epithelial cells (PTEC) channel, successfully demonstrated secretory transport of morphine and M6G from the HUVEC channel into the PTEC channel. The in vitro data generated by VPT-MPS were incorporated into a mechanistic kidney model and parent-metabolite full body PBPK model to predict CL_r and systemic disposition of morphine and M6G, resulting in successful prediction of CL_r and the plasma concentration–time profiles in both healthy subjects and CKD patients. A microphysiological system together with mathematical modeling successfully predicted renal clearance and systemic disposition of opioids in CKD patients and healthy subjects.

Impaired Tubular Secretion of Organic Solutes in Advanced Chronic Kidney Disease

BACKGROUND

The clearance of solutes removed by tubular secretion may be altered out of proportion to the GFR in CKD. Recent studies have described considerable variability in the secretory clearance of waste solutes relative to the GFR in patients with CKD.

METHODS

To test the hypothesis that secretory clearance relative to GFR is reduced in patients approaching dialysis, we used metabolomic analysis to identify solutes in simultaneous urine and plasma samples from 16 patients with CKD and an eGFR of 7±2 ml/min per 1.73 m2 and 16 control participants. Fractional clearances were calculated as the ratios of urine to plasma levels of each solute relative to those of creatinine and urea in patients with CKD and to those of creatinine in controls.

RESULTS

Metabolomic analysis identified 39 secreted solutes with fractional clearance >3.0 in control participants. Fractional clearance values in patients with CKD were reduced on average to 65%±27% of those in controls. These values were significantly lower for 18 of 39 individual solutes and significantly higher for only one. Assays of the secreted anions phenylacetyl glutamine, p-cresol sulfate, indoxyl sulfate, and hippurate confirmed variable impairment of secretory clearances in advanced CKD. Fractional clearances were markedly reduced for phenylacetylglutamine (4.2±0.6 for controls versus 2.3±0.6 for patients with CKD; P<0.001), p-cresol sulfate (8.6±2.6 for controls versus 4.1±1.5 for patients with CKD; P<0.001), and indoxyl sulfate (23.0±7.3 versus 7.5±2.8; P<0.001) but not for hippurate (10.2±3.8 versus 8.4±2.6; P=0.13).

CONCLUSIONS

Secretory clearances for many solutes are reduced more than the GFR in advanced CKD. Impaired secretion of these solutes might contribute to uremic symptoms as patients approach dialysis.

Association of Plasma Uremic Solute Levels with Residual Kidney Function in Children on Peritoneal Dialysis

BACKGROUND AND OBJECTIVES

Residual native kidney function confers health benefits in patients on dialysis. It can facilitate control of extracellular volume and inorganic ion concentrations. Residual kidney function can also limit the accumulation of uremic solutes. This study assessed whether lower plasma concentrations of uremic solutes were associated with residual kidney function in pediatric patients on peritoneal dialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Samples were analyzed from 29 pediatric patients on peritoneal dialysis, including 13 without residual kidney function and ten with residual kidney function. Metabolomic analysis by untargeted mass spectrometry compared plasma solute levels in patients with and without residual kidney function. Dialytic and residual clearances of selected solutes were also measured by assays using chemical standards.

RESULTS

Metabolomic analysis showed that plasma levels of 256 uremic solutes in patients with residual kidney function averaged 64% (interquartile range, 51%-81%) of the values in patients without residual kidney function who had similar total Kt/V_urea. The plasma levels were significantly lower for 59 of the 256 solutes in the patients with residual kidney function and significantly higher for none. Assays using chemical standards showed that residual kidney function provides a higher portion of the total clearance for nonurea solutes than it does for urea.

CONCLUSIONS

Concentrations of many uremic solutes are lower in patients on peritoneal dialysis with residual kidney function than in those without residual kidney function receiving similar treatment as assessed by Kt/V_urea.

Potent Inhibitors of Organic Anion Transporters 1 and 3 From Natural Compounds and Their Protective Effect on Aristolochic Acid Nephropathy

Organic anion transporters 1 and 3 (OAT1 and OAT3) play a critical role in renal drug-drug interactions and are involved in the nephrotoxicity of many anionic xenobiotics. To date, relatively little is known about the interaction of natural compounds with OAT1 and OAT3. Of the 270 natural compounds screened in the present study, 21 compounds inhibited OAT1 and 45 compounds inhibited OAT3. Further concentration-dependent studies identified 7 OAT1 inhibitors and 10 OAT3 inhibitors with IC50 values of <10 μM, and most of them were flavonoids, the most commonly ingested polyphenolic compounds in the diet and herbal products. Computational modeling of OAT1 and OAT3 revealed the important residues for the recognition of inhibitors. The two strong OAT inhibitors, namely wedelolactone and wogonin, were evaluated for their in vivo interactions with the OAT substrate aristolochic acid I (AAI), a natural compound causing aristolochic acid-induced nephropathy (AAN) in many species. The cytotoxicity of AAI increased in two OAT-overexpressing cell lines, with more cytotoxicity in OAT1-overexpressing cells, suggesting a more important role of OAT1 than OAT3 in AAN. Both wedelolactone and wogonin markedly increased serum AAI concentrations in AAI-treated rats and ameliorated kidney injuries in AAI-treated mice. To conclude, the present findings are of significant value in understanding natural compound-drug interactions and provide a natural source for developing treatments for AAN.

Population pharmacokinetic modeling and simulation to support qualification of pyridoxic acid as endogenous biomarker of OAT1/3 renal transporters

Renal clearance of many drugs is mediated by renal organic anion transporters OAT1/3 and inhibition of these transporters may lead to drug‐drug interactions (DDIs). Pyridoxic acid (PDA) and homovanillic acid (HVA) were indicated as potential biomarkers of OAT1/3. The objective of this study was to develop a population pharmacokinetic model for PDA and HVA to support biomarker qualification. Simultaneous fitting of biomarker plasma and urine data in the presence and absence of potent OAT1/3 inhibitor (probenecid, 500 mg every 6 h) was performed. The impact of study design (multiple vs. single dose of OAT1/3 inhibitor) and ability to detect interactions in the presence of weak/moderate OAT1/3 inhibitors was investigated, together with corresponding power calculations. The population models developed successfully described biomarker baseline and PDA/HVA OAT1/3‐mediated interaction data. No prominent effect of circadian rhythm on PDA and HVA individual baseline levels was evident. Renal elimination contributed greater than 80% to total clearance of both endogenous biomarkers investigated. Estimated probenecid unbound in vivo OAT inhibitory constant was up to 6.4‐fold lower than in vitro values obtained with PDA as a probe. The PDA model was successfully verified against independent literature reported datasets. No significant difference in power of DDI detection was found between multiple and single dose study design when using the same total daily dose of 2000 mg probenecid. Model‐based simulations and power calculations confirmed sensitivity and robustness of plasma PDA data to identify weak, moderate, and strong OAT1/3 inhibitors in an adequately powered clinical study to support optimal design of prospective clinical OAT1/3 interaction studies.

Renal and non-renal response of ABC and SLC transporters in chronic kidney disease

INTRODUCTION

The solute carrier (SLC) and the ATP-binding cassette (ABC) transporter superfamilies play essential roles in the disposition of small molecules (endogenous metabolites, uremic toxins, drugs) in the blood, kidney, liver, intestine, and other organs. In chronic kidney disease (CKD), the loss of renal function is associated with altered function of remote organs. As renal function declines, many molecules accumulate in the plasma. Many studies now support the view that ABC and SLC transporters as well as drug metabolizing enzymes (DMEs) in renal and non-renal tissues are directly or indirectly affected by the presence of various types of uremic toxins, including those derived from the gut microbiome; this can lead to aberrant inter-organ communication.

AREAS COVERED

Here, the expression, localization and/or function of various SLC and ABC transporters as well as DMEs in the kidney and other organs are discussed in the context of CKD and systemic pathophysiology.

EXPERT OPINION

According to the Remote Sensing and Signaling Theory (RSST), a transporter and DME-centric network that optimizes local and systemic metabolism maintains homeostasis in the steady state and resets homeostasis following perturbations due to renal dysfunction. The implications of this view for pharmacotherapy of CKD are also discussed.

Membrane Carriers and Transporters in Kidney Physiology and Disease

The growing information suggests that chronic kidney disease may affect expression and function of membrane carriers and transporters in the kidney. The dysfunction of carriers and transporters entails deficient elimination of uremic solutes as well as xenobiotics (drugs and toxins) with subsequent clinical consequences. The renal carriers and transporters are also targets of drugs used in clinical practice, and intentional drug-drug interactions in the kidney are produced to increase therapeutic efficacy. The understanding of membrane carriers and transporters function in chronic kidney disease is important not only to better characterize drug pharmacokinetics, drug actions in the kidney, or drug-drug interactions but also to define the organ pathophysiology.

(-)-Epigallocatechin-3-gallate Inhibits Human and Rat Renal Organic Anion Transporters

Organic anion transporter 1 (OAT1, SLC22A6) and 3 (OAT3, SLC22A8) are multispecific drug transporters highly expressed on the basolateral membranes of the renal proximal tubules. OAT1 and OAT3 mediate the tubular secretion of clinically significant drugs; thus, they influence the pharmacokinetics of drugs and further determine their efficacy and toxicity. OAT1 and OAT3 are also the target of drug-drug interactions. In this study, we examined the effects of the tea catechin (-)-epigallocatechin-3-gallate (EGCG) on human (h) and rat (r) OAT1 and OAT3 using the fluorescent organic anion 6-carboxyfluorescein (6-CF) and hOAT1-, hOAT3-, rOat1-, or rOat3-expressing HEK293 cells and on renal elimination of 6-CF in rats. 6-CF is transported by hOAT1, hOAT3, rOat1, and rOat3. 6-CF is urinary excreted by Oats in rats. EGCG, a dominant catechin in green tea leaf, inhibits human and rat OAT1 and OAT3 and reduces the renal elimination of 6-CF in rats. Our findings are useful for the assessment of food-drug interactions mediated by renal OATs.

Loop Diuretics Inhibit Renal Excretion of Trimethylamine N-Oxide

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Uremic retention solutes predominantly eliminate through the kidneys largely via specific efflux channels in the proximal renal tubules.

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For the first time, we demonstrated in vivo that renal tubular excretion of TMAO can be inhibited by concomitant loop diuretic administration via competition at the level of renal transporters.

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We further observed accumulation of TMAO in the renal parenchyma, which implied differential distributions of TMAO across various tissues and/or systems as a consequence of efflux channel control.

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Poorer outcomes in patients who receive long-term loop diuretic agents may therefore be associated with metabolic perturbations, such as retention of metabolites like TMAO, beyond impaired glomerular filtration.

This study demonstrates, for the first time, that renal tubular excretion of trimethylamine N-oxide (TMAO) is inhibited by concomitant loop diuretic administration. The observed marked accumulation in the renal parenchyma, and to lesser extent, plasma, implies differential distributions of TMAO across various tissues and/or systems as a consequence of efflux channel control. A better understanding of TMAO renal clearance and its potential interactions with current and future therapies in patients with heart failure are warranted.

Changes in the Serum Metabolome of Patients Treated With Broad-Spectrum Antibiotics

Background:

The gut microbiome (GMB) generates numerous small chemicals that can be absorbed by the host and variously biotransformed, incorporated, or excreted. The resulting metabolome can provide information about the state of the GMB, of the host, and of their relationship. Exploiting this information in the service of biomarker development is contingent on knowing the GMB-sensitivity of the individual chemicals comprising the metabolome. In this regard, human studies have lagged far behind animal studies. Accordingly, we tested the hypothesis that serum levels of chemicals unequivocally demonstrated to be GMB-sensitive in rodent models would also be affected in a clinical patient sample treated with broad spectrum antibiotics.

Methods:

We collected serum samples from 20 hospitalized patients before, during, and after treatment with broad-spectrum antibiotics. We also collected samples from 5 control patients admitted to the hospital but not prescribed antibiotics. We submitted the samples for a non-targeted metabolomic analysis and then focused on chemicals known to be affected both by germ-free status and by antibiotic treatment in the mouse and/or rat.

Results:

Putative identification was obtained for 499 chemicals in human serum. An aggregate analysis did not show any time x treatment interactions. However, our literature search identified 10 serum chemicals affected both by germ-free status and antibiotic treatment in the mouse or rat. Six of those chemicals were measured in our patient samples and additionally met criteria for inclusion in a focused analysis. Serum levels of 5 chemicals (p-cresol sulfate, phenol sulfate, hippurate, indole propionate, and indoxyl sulfate) declined significantly in our group of antibiotic-treated patients but did not change in our patient control group.

Conclusions:

Broad-spectrum antibiotic treatment in patients lowered serum levels of selected chemicals previously demonstrated to be GMB-sensitive in rodent models. Interestingly, all those chemicals are known to be uremic solutes that can be derived from aromatic amino acids (L-phenylalanine, L-tyrosine, or L-tryptophan) by anaerobic bacteria, particularly Clostridial species. We conclude that judiciously selected serum chemicals can reliably detect antibiotic-induced suppression of the GMB in man and thus facilitate further metabolome-based biomarker development.

Characterization of the cellular transport mechanisms for the anti-cachexia candidate compound TCMCB07

BACKGROUND

Cachexia is a debilitating, life-threatening condition whose pathology includes reduced food intake accompanied by hypermetabolism, leading to a catabolic state. The hypothalamic melanocortin system is a critical regulator of metabolic rate with effects being mediated through the melanocortin-4 receptor (MC4R). MC4R activation is also critical to the initiation and maintenance of cachexia. A major problem in the design of anti-cachexia drugs has been the need to cross the blood-brain barrier to access the metabolic rate-controlling centres in the hypothalamus. The overwhelming majority of anti-cachexia drugs are only effective when administered intracerebroventricularly. TCMCB07 is a cyclic nonapeptide peptide MC4R antagonist with parenteral anti-cachexia activity in both small and large animal models. This suggests it can cross the blood-brain barrier. The aim of this study was to examine potential transport mechanisms of TCMCB07 furthering its preclinical development for subsequent studies in humans.

METHODS

In vitro studies were performed in transporter-transfected cells to study whether or not TCMCB07 was an inhibitor as well as substrate for OATP1A2, OATP1B1, OATP1B3, OATP2B1, OCT2, OAT1, OAT3, MATE1, MATE2-K, P-gp (MDR1), and BCRP. In vivo mass balance studies were also performed in mice to evaluate the absorption and disposition of TCMCB07 after oral and intravenous bolus administrations.

RESULTS

TCMCB07 inhibited the uptake of prototypical substrates in cells transfected with OATP1A2 (IC50 24.0 μM), OATP1B1 (IC50 6.8 μM), OATP1B3 (IC50 307 μM), OATP2B1 (IC50 524 μM), OCT2 (IC50 1,169 μM), MATE1 (IC50 8.7 μM), and MATE2-K (IC50 20.7 μM) but not in cells transfected with OAT1 and OAT3. TCMCB07 did not affect the P-gp (MDR1)-mediated and BCRP-mediated permeability of prototypical substrates in transfected cells. Importantly, direct evidence was shown for the uptake of TCMCB07 in OATP1A2-transfected cells (i.e. Vmax 236 pmol/mg, Km 58.4 μM, and Kd 0.39 μL/mg), demonstrating that the nonapeptide was a substrate for this transporter. Mass balance studies demonstrated that 24.2% of TCMCB07 was absorbed orally in vivo (P = 0.0033) and excreted primarily in the bile after both oral and intravenous administrations.

CONCLUSIONS

OATP1A2 is the transporter responsible for the oral absorption of TCMCB07 in the intestine and for its pharmacologic response in the brain.

Physiologically-Based Pharmacokinetic Modelling of Creatinine-Drug Interactions in the Chronic Kidney Disease Population

Elevated serum creatinine (S_Cr ) caused by the inhibition of renal transporter(s) may be misinterpreted as kidney injury. The interpretation is more complicated in patients with chronic kidney disease (CKD) due to altered disposition of creatinine and renal transporter inhibitors. A clinical study was conducted in 17 patients with CKD (estimated glomerular filtration rate 15-59 mL/min/1.73 m² ); changes in S_Cr were monitored during trimethoprim treatment (100-200 mg/day), administered to prevent recurrent urinary infection, relative to the baseline level. Additional S_Cr -interaction data with trimethoprim, cimetidine, and famotidine in patients with CKD were collated from the literature. Our published physiologically-based creatinine model was extended to predict the effect of the CKD on S_Cr and creatinine-drug interaction. The creatinine-CKD model incorporated age/sex-related differences in creatinine synthesis, CKD-related glomerular filtration deterioration; change in transporter activity either proportional or disproportional to glomerular filtration rate (GFR) decline were explored. Optimized models successfully recovered baseline S_Cr from 64 patients with CKD (geometric mean fold-error of 1.1). Combined with pharmacokinetic models of inhibitors, the creatinine model was used to simulate transporter-mediated creatinine-drug interactions. Use of inhibitor unbound plasma concentrations resulted in 66% of simulated S_Cr interaction data within the prediction limits, with cimetidine interaction significantly underestimated. Assuming that transporter activity deteriorates disproportional to GFR decline resulted in higher predicted sensitivity to transporter inhibition in patients with CKD relative to healthy patients, consistent with sparse clinical data. For the first time, this novel modelling approach enables quantitative prediction of S_Cr in CKD and delineation of the effect of disease and renal transporter inhibition in this patient population.

Dose Optimization in Kidney Disease: Opportunities for PBPK Modeling and Simulation

Kidney disease affects pharmacokinetic (PK) profiles of not only renally cleared drugs but also nonrenally cleared drugs. The impact of kidney disease on drug disposition has not been fully elucidated, but describing the extent of such impact is essential for conducting dose optimization in kidney disease. Accurate evaluation of kidney function has been a clinical interest for dose optimization, and more scientists pay attention and conduct research for clarifying the role of drug transporters, metabolic enzymes, and their interplay in drug disposition as kidney disease progresses. Physiologically based pharmacokinetic (PBPK) modeling and simulation can provide valuable insights for dose optimization in kidney disease. It is a powerful tool to integrate discrete knowledge from preclinical and clinical research and mechanistically investigate system- and drug-dependent factors that may contribute to the changes in PK profiles. PBPK-based prediction of drug exposures may be used a priori to adjust dosing regimens and thereby minimize the likelihood of drug-related toxicity. With real-time clinical studies, parameter estimation may be performed with PBPK approaches that can facilitate identification of sources of interindividual variability. PBPK modeling may also facilitate biomarker research that aids dose optimization in kidney disease. U.S. Food and Drug Administration guidances related to conduction of PK studies in kidney impairment and PBPK documentation provide the foundation for facilitating model-based dose-finding research in kidney disease.

Novel Mechanistic PBPK Model to Predict Renal Clearance in Varying Stages of CKD by Incorporating Tubular Adaptation and Dynamic Passive Reabsorption

Chronic kidney disease (CKD) has significant effects on renal clearance (CLr ) of drugs. Physiologically-based pharmacokinetic (PBPK) models have been used to predict CKD effects on transporter-mediated renal active secretion and CLr for hydrophilic nonpermeable compounds. However, no studies have shown systematic PBPK modeling of renal passive reabsorption or CLr for hydrophobic permeable drugs in CKD. The goal of this study was to expand our previously developed and verified mechanistic kidney model to develop a universal model to predict changes in CLr in CKD for permeable and nonpermeable drugs that accounts for the dramatic nonlinear effect of CKD on renal passive reabsorption of permeable drugs. The developed model incorporates physiologically-based tubular changes of reduced water reabsorption/increased tubular flow rate per remaining functional nephron in CKD. The final adaptive kidney model successfully (absolute fold error (AFE) all < 2) predicted renal passive reabsorption and CLr for 20 permeable and nonpermeable test compounds across the stages of CKD. In contrast, use of proportional glomerular filtration rate reduction approach without addressing tubular adaptation processes in CKD to predict CLr generated unacceptable CLr predictions (AFE = 2.61-7.35) for permeable compounds in severe CKD. Finally, the adaptive kidney model accurately predicted CLr of para-amino-hippuric acid and memantine, two secreted compounds, in CKD, suggesting successful integration of active secretion into the model, along with passive reabsorption. In conclusion, the developed adaptive kidney model enables mechanistic predictions of in vivo CLr through CKD progression without any empirical scaling factors and can be used for CLr predictions prior to assessment of drug disposition in renal impairment.

Investigation of the Effects and Mechanisms of Dendrobium loddigesii Rolfe Extract on the Treatment of Gout

Objective

Gout is a chronic disease that causes inflammatory arthritis, which is closely related to urate accumulation induced by a disorder of uric acid metabolism and the consequent deposition of monosodium urate crystals. Dendrobium loddigesii Rolfe is an herbal medicine that has been used in some traditional Chinese medicine formulae in the treatment of gout. This study aimed to explore and verify the antigout activity of Dendrobium loddigesii extract (DLE) on alleviating the hyperuricaemia of mice and the acute gouty arthritis of rats.

Methods

An animal model of hyperuricaemia was established using potassium oxonate (PO). We analysed the expression of uric acid transporter mRNA in the kidney in the hyperuricaemic mice after treatment with DLE. Simultaneously, a monosodium urate crystal-induced acute gouty arthritis rat model was used to evaluate the effects of DLE, according to the level of ankle swelling, as well as the protein levels of inflammatory receptors and cytokines, as assayed by WB and ELISA.

Results

DLE alleviated hyperuricaemia in mice and inhibited acute gouty arthritis in rats (P < 0.05). Meanwhile, DLE regulated the levels of uric acid transporters mRNA transcripts, including mouse organic anion transporter 1 (mOAT1), organic anion transporter 3 (mOAT3), urate transporter 1 (mURAT1), and glucose transporter 9 (mGLUT9) in the kidney (P < 0.05), suggesting that DLE promoted uric acid metabolism. Furthermore, DLE significantly suppressed the protein levels of TLRs, MyD88, and NF-κB in the ankle joint's synovium (P < 0.05), and the serum levels of IL-1β, IL-6, and TNF-α were also reduced, which demonstrated the anti-inflammatory effects of DLE.

Conclusion

DLE alleviates hyperuricaemia by regulating the transcription level of uric acid transporters in the kidney. It also inhibits acute gouty arthritis by inhibiting the pathway of TLRs/MyD88/NF-κB in the ankle joint's synovium. The findings of the present study imply that DLE alleviates gout by promoting uric acid metabolism and inhibiting inflammation related to the TLRs/MyD88/NF-κB pathway.

Transporters (OATs and OATPs) contribute to illustrate the mechanism of medicinal compatibility of ingredients with different properties in yuanhuzhitong prescription

Various medicinal ingredients with different tastes are combined according to the theory of compatibility in Chinese materia medica to achieve a better efficacy, while the mechanism was not very clear. Here, the authors studied the interaction between ingredients and human transporters such as the kidney transporters OAT1 and OAT3, the liver transporters OATP1B1 and OATP1B3, and the intestine transporter OATP2B1 to discern the compatibility mechanism of ingredients with different tastes in the Yuanhuzhitong preparation (YHP) comprising Corydalis yanhusuo (CYH) and Angelica dahurica (AD), which could relieve pain by restraining the central system. The results show that tetrahydropalmatine (TDE), the major component of CYH, could be transported by OAT3 into kidney, OATP1B1 and OATP1B3 into liver, while imperatorin (IPT) and isoimperatorin (ISP), the two key components of AD, and AD extract showed strong inhibition to OAT1 and OAT3. What's more, AD extract also exerted strongly inhibition to human transporters OATP1B1 and OATP1B3. It was also detected that IPT, ISP, and AD extract significantly downregulated the expression of Oatp1a1, Oatp1a4, and Oatp1b2 of liver in mice. The in vivo results show that the concentration of TDE in liver and kidney significantly decreased, while the TDE concentration in blood and brain were both significantly enhanced in the presence of IPT, ISP, and AD extract. These results suggest that the ingredients in AD with pungent taste could enhance the exposure of TDE in blood and brain by inhibiting the uptake of TDE in liver and kidney. That is to say, TDE with bitter taste could "flood up" into the central nervous system to play its therapeutic effect by the cut-off of that into liver and kidney in the presence of ingredients within AD. This paper not only proves the meridian distribution of CYH in liver and kidney with the role of OAT3, OATP1B1, and OATP1B3, but also illustrates how to improve the efficacy of CYH by reasonable compatibility with AD. This study may offer a valuable clue to illustrate the mechanism of compatibility theory.

Impaired Tubular Secretion of Organic Solutes in Acute Kidney Injury

BACKGROUND

Impairment of kidney function is routinely assessed by measuring the accumulation of creatinine, an organic solute cleared largely by glomerular filtration. We tested whether the clearance of solutes that undergo tubular secretion is reduced in proportion to the clearance of creatinine in humans with AKI.

METHODS

Four endogenously produced organic solutes (phenylacetylglutamine [PAG], hippurate [HIPP], indoxyl sulfate [IS], and p-cresol sulfate [PCS]) were measured in spot urine and plasma samples from ten patients with AKI and 17 controls. Fractional clearance relative to creatinine was calculated to assess tubular secretion. Fractional clearance values were calculated in terms of the free, unbound levels of HIPP, IS, and PCS that bind to plasma proteins.

RESULTS

Fractional clearance values for PAG, HIPP, IS, and PCS were >1.0 in patients with AKI as well as controls, indicating that these solutes were still secreted by the tubules of the injured kidneys. Fractional clearance values were, however, significantly lower in patients with AKI than controls, indicating that kidney injury reduced tubular secretion more than glomerular filtration (AKI versus control: PAG, 2.1±0.7 versus 4.6±1.4, P<0.001; HIPP, 10±5 versus 15±7, P=0.02; IS, 10±6 versus 28±7, P<0.001; PCS, 3.3±1.8 versus 10±3, P<0.001). Free plasma levels rose out of proportion to total plasma levels for each of the bound solutes in AKI, so that calculating their fractional clearance in terms of their total plasma levels failed to reveal their impaired secretion.

CONCLUSIONS

Tubular secretion of organic solutes can be reduced out of proportion to glomerular filtration in AKI. Impaired secretion of protein-bound solutes may be more reliably detected when clearances are expressed in terms of their free, unbound levels in the plasma.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

A Microdose Cocktail to Evaluate Drug Interactions in Patients with Renal Impairment

Renal impairment (RI) is known to influence the pharmacokinetics of nonrenally eliminated drugs, although the mechanism and clinical impact is poorly understood. We assessed the impact of RI and single dose oral rifampin (RIF) on the pharmacokinetics of CYP3A, OATP1B, P-gp, and BCRP substrates using a microdose cocktail and OATP1B endogenous biomarkers. RI alone had no impact on midazolam (MDZ), maximum plasma concentration (C_max ), and area under the curve (AUC), but a progressive increase in AUC with RI severity for dabigatran (DABI), and up to ~2-fold higher AUC for pitavastatin (PTV), rosuvastatin (RSV), and atorvastatin (ATV) for all degrees of RI was observed. RIF did not impact MDZ, had a progressively smaller DABI drug-drug interaction (DDI) with increasing RI severity, a similar 3.1-fold to 4.4-fold increase in PTV and RSV AUC in healthy volunteers and patients with RI, and a diminishing DDI with RI severity from 6.1-fold to 4.7-fold for ATV. Endogenous biomarkers of OATP1B (bilirubin, coproporphyrin I/III, and sulfated bile salts) were generally not impacted by RI, and RIF effects on these biomarkers in RI were comparable or larger than those in healthy volunteers. The lack of a trend with RI severity of PTV and several OATP1B biomarkers, suggests that mechanisms beyond RI directly impacting OATP1B activity could also be considered. The DABI, RSV, and ATV data suggest an impact of RI on intestinal P-gp, and potentially BCRP activity. Therefore, DDI data from healthy volunteers may represent a worst-case scenario for clinically derisking P-gp and BCRP substrates in the setting of RI.

Pharmacokinetics in children with chronic kidney disease

In children, the main causes of chronic kidney disease (CKD) are congenital diseases and glomerular disorders. CKD is associated with multiple physiological changes and may therefore influence various pharmacokinetic (PK) parameters. A well-known consequence of CKD on pharmacokinetics is a reduction in renal clearance due to a decrease in the glomerular filtration rate. The impact of renal impairment on pharmacokinetics is, however, not limited to a decreased elimination of drugs excreted by the kidney. In fact, renal dysfunction may lead to modifications in absorption, distribution, transport, and metabolism as well. Currently, insufficient evidence is available to guide dosing decisions on many commonly used drugs. Moreover, the impact of maturation on drug disposition and action should be taken into account when selecting and dosing drugs in the pediatric population. Clinicians should take PK changes into consideration when selecting and dosing drugs in pediatric CKD patients in order to avoid toxicity and increase efficiency of drugs in this population. The aim of this review is to summarize known PK changes in relation to CKD and to extrapolate available knowledge to the pediatric CKD population to provide guidance for clinical practice.

Interaction of Organic Anion Transporter 3-Mediated Uptake of Steviol Acyl Glucuronide, a Major Metabolite of Rebaudioside A, with Selected Drugs

Organic anion transporter 3 (OAT3) plays a critical role in the renal excretion of many xenobiotics. Because steviol acyl glucuronide (SVAG), an OAT3 substrate, is the major circulating metabolite after oral ingestion of steviol glycosides and is excreted into the urine, inhibition of OAT3 activity may alter pharmacokinetic profiles of SVAG. The present study showed that drugs such as probenecid and glimepiride displayed potent inhibition toward the OAT3-mediated SVAG transport, with IC₅₀ values of 4.9 and 0.8 μM, respectively. No species differences were observed. Probenecid and glimepiride could significantly elevate plasma concentrations of SVAG after oral administration of rebaudioside A, with significant increases in plasma maximum (C_max) and area under the plasma time-concentration curve values. The inhibitory effect on the OAT3-mediated SVAG transport exemplified a unique case between drugs and the metabolite of a food additive. Our data suggest that caution should be exercised when giving steviol glycoside products to human subjects with compromised renal function.

Molecular and Cellular Mechanisms that Induce Arterial Calcification by Indoxyl Sulfate and P-Cresyl Sulfate

The protein-bound uremic toxins, indoxyl sulfate (IS) and p-cresyl sulfate (PCS), are considered to be harmful vascular toxins. Arterial media calcification, or the deposition of calcium phosphate crystals in the arteries, contributes significantly to cardiovascular complications, including left ventricular hypertrophy, hypertension, and impaired coronary perfusion in the elderly and patients with chronic kidney disease (CKD) and diabetes. Recently, we reported that both IS and PCS trigger moderate to severe calcification in the aorta and peripheral vessels of CKD rats. This review describes the molecular and cellular mechanisms by which these uremic toxins induce arterial media calcification. A complex interplay between inflammation, coagulation, and lipid metabolism pathways, influenced by epigenetic factors, is crucial in IS/PCS-induced arterial media calcification. High levels of glucose are linked to these events, suggesting that a good balance between glucose and lipid levels might be important. On the cellular level, effects on endothelial cells, which act as the primary sensors of circulating pathological triggers, might be as important as those on vascular smooth muscle cells. Endothelial dysfunction, provoked by IS and PCS triggered oxidative stress, may be considered a key event in the onset and development of arterial media calcification. In this review a number of important outstanding questions such as the role of miRNA’s, phenotypic switching of both endothelial and vascular smooth muscle cells and new types of programmed cell death in arterial media calcification related to protein-bound uremic toxins are put forward and discussed.

Protein-bound uremic toxins are associated with cognitive function among patients undergoing maintenance hemodialysis

Patients with chronic kidney disease have a greater risk of cognitive impairment. Cerebral uremic solute accumulation causes uremic encephalopathy; however, the association of protein-bound uremic toxins on cognitive function remains unclear. The present study aimed to investigate the association of two protein-bound uremic toxins, namely indoxyl sulfate (IS) and p-cresyl sulfate (PCS), on cognitive function in patients receiving hemodialysis (HD) for at least 90 days. Circulating free form IS and PCS were quantified by liquid chromatography/mass spectrometry. Mini-Mental State Examination (MMSE) and Cognitive Abilities Screening Instrument (CASI) were used to evaluate cognitive function. In total, 260 HD patients were recruited with a mean age of 58.1 ± 11.3 years, of which, 53.8% were men, 40% had diabetes, and 75.4% had hypertension. The analysis revealed that both free IS and free PCS were negatively associated with the CASI score and MMSE. After controlling for confounders, circulating free IS levels persisted to be negatively associated with MMSE scores [β = −0.62, 95% confidence interval (CI): −1.16 to −0.08] and CASI scores (β = −1.97, 95% CI: −3.78 to −0.16), mainly in the CASI domains of long-term memory, mental manipulation, language ability, and spatial construction. However, there was no correlation between free PCS and total MMSE or total CASI scores after controlling for confounders. In conclusion, circulating free form IS, but not PCS is associated with lower cognitive function test scores in HD patients. Thus, a further study is needed to evaluate whether a decrease in free IS levels can slow down cognitive decline in HD patients.

Quantitative Translation of Microfluidic Transporter in Vitro Data to in Vivo Reveals Impaired Albumin-Facilitated Indoxyl Sulfate Secretion in Chronic Kidney Disease

Indoxyl sulfate (IxS), a highly albumin-bound uremic solute, accumulates in chronic kidney disease (CKD) due to reduced renal clearance. This study was designed to specifically investigate the role of human serum albumin (HSA) in IxS renal secretion via organic anion transporter 1 (OAT1) in a microfluidic system and subsequently apply quantitative translation of in vitro data to predict extent of change in IxS renal clearance in CKD stage IV relative to healthy. Conditionally immortalized human proximal tubule epithelial cells overexpressing OAT1 were incubated with IxS (5-200 μM) in the HSA-free medium or in the presence of either HSA or CKD-modified HSA. IxS uptake in the presence of HSA resulted in more than 20-fold decrease in OAT1 affinity (K_m,u) and 37-fold greater in vitro unbound intrinsic clearance (CL_int,u) versus albumin-free condition. In the presence of CKD-modified albumin, K_m,u increased four-fold and IxS CL_int,u decreased almost seven-fold relative to HSA. Fold-change in parameters exceeded differences in IxS binding between albumin conditions, indicating additional mechanism and facilitating role of albumin in IxS OAT1-mediated uptake. Quantitative translation of IxS in vitro OAT1-mediated CL_int,u predicted a 60% decrease in IxS renal elimination as a result of CKD, in agreement with the observed data (80%). The findings of the current study emphasize the role of albumin in IxS transport via OAT1 and explored the impact of modifications in albumin on renal excretion via active secretion in CKD. For the first time, this study performed quantitative translation of transporter kinetic data generated in a novel microfluidic in vitro system to a clinically relevant setting. Knowledge gaps and future directions in quantitative translation of renal drug disposition from microphysiological systems are discussed.

Cilastatin protects against imipenem-induced nephrotoxicity via inhibition of renal organic anion transporters (OATs)

Imipenem is a carbapenem antibiotic. However, Imipenem could not be marketed owing to its instability and nephrotoxicity until cilastatin, an inhibitor of renal dehydropeptidase-I (DHP-I), was developed. In present study, the potential roles of renal organic anion transporters (OATs) in alleviating the nephrotoxicity of imipenem by cilastatin were investigated in vitro and in rabbits. Our results indicated that imipenem and cilastatin were substrates of hOAT1 and hOAT3. Cilastatin inhibited hOAT1/3-mediated transport of imipenem with IC₅₀ values comparable to the clinical concentration, suggesting the potential to cause a clinical drug–drug interaction (DDI). Moreover, imipenem exhibited hOAT1/3-dependent cytotoxicity, which was alleviated by cilastatin and probenecid. Furthermore, cilastatin and probenecid ameliorated imipenem-induced rabbit acute kidney injury, and reduced the renal secretion of imipenem. Cilastatin and probenecid inhibited intracellular accumulation of imipenem and sequentially decreased the nephrocyte toxicity in rabbit primary proximal tubule cells. Renal OATs, besides DHP-I, was also the target of interaction between imipenem and cilastatin, and contributed to the nephrotoxicity of imipenem. This therefore gives in part the explanation about the mechanism by which cilastatin protected against imipenem-induced nephrotoxicity. Thus, OATs can potentially be used as a therapeutic target to avoid the renal adverse reaction of imipenem in clinic.

Dihydrophenanthrenes from Juncus effusus as Inhibitors of OAT1 and OAT3

Organic anion transporters 1 (OAT1) and 3 (OAT3) play important roles in the renal elimination of a range of substrate molecules. Little is known about natural products that can modulate OAT1 and OAT3 activities. The medullae of Juncus effusus is often used for the treatment of dysuria in traditional Chinese medicine. To study the interactions of phytochemicals in J. effusus with human OAT1 and OAT3, a bioactivity guided phytochemical investigation led to seven new phenanthrenoids along with nine known compounds, including eight phenanthrenoids and a benzophenone from the dichloromethane soluble fraction of a methanol extract of the medullae of J. effusus. The structures were established by physical data analysis, including high-resolution electrospray ionization mass spectrometry and 1D and 2D NMR. The compounds were evaluated for inhibition of OAT1 and OAT3 in vitro. Compounds 10 and 16 were inhibitors for OAT1, and compounds 1-3, 10, and 16 were inhibitors for OAT3 with IC₅₀ values less than 5.0 μM. Dihydrophenanthrene 1 markedly altered the pharmacokinetic parameters of the diuretic drug furosemide, a known substrate of both OAT1 and OAT3, in vivo.