The Effect of Uremic Solutes on the Organic Cation Transporter 2

Predicting OCT2/MATEs-Mediated Drug Interactions in Healthy Volunteers and Patients with Chronic Kidney Disease: Insights from Extended Clearance Concept, Endogenous Biomarkers, and In Vitro Inhibition Studies (Perspectives from the International Transporter Consortium)

Organic cation transporter (OCT) 2 and multidrug and toxin extrusion (MATE) transporters play significant roles in the renal secretion of organic cations and drug-drug interactions (DDIs). Recent in vitro studies indicate that the Ki values for OCT2 exhibit substrate dependency and increase in potency with pre-incubation. However, consensus is lacking on whether these factors should be considered in predicting in vivo inhibition. Physiologically based pharmacokinetic models, combined with the extended clearance concept, have been used and are discussed here for OCT2/MATEs probes. In addition to modeling, early clinical studies use endogenous biomarkers to evaluate transporter-mediated DDI risk, with the aim of avoiding unnecessary clinical DDI studies. Identified biomarkers for OCT2/MATEs, such as creatinine, N1-methylnicotinamide, and N1-methyladenosine, have proven useful in confirming clinically relevant OCT2/MATEs-mediated DDIs when renal clearance (CLr) is used as an endpoint; their application is discussed further. From a clinical perspective, the intact nephron hypothesis (INH), which postulates that the decrease in CLr in chronic kidney disease (CKD) is proportional to that in nephron numbers, has been proposed. However, reports suggest that the secretion clearance of creatinine and substrates of organic anion transporters (OATs) does not follow this proportionality in patients with CKD. This state-of-the-art review highlights key developments in predicting OCT2/MATEs-mediated DDIs in healthy volunteers and explores the prediction of clinical OCT2/MATEs DDI risk in patients with CKD by comparing substrate-dependent changes in secretion clearance for substrates of OCT2/MATEs and OATs. Recommendations for the prediction of OCT2/MATEs-mediated DDI risk, together with the current knowledge gaps and future directions, are discussed.

Contribution and expression of renal drug transporters in renal cell carcinoma

Renal cell carcinoma (RCC) is a common substantive tumor. According to incomplete statistics, RCC incidence accounts for approximately 90% of renal malignant tumors, and is the second most prevalent major malignant tumor in the genitourinary system, following bladder cancer. Only 10%-15% of chemotherapy regimens for metastatic renal cell carcinoma (mRCC) are effective, and mRCC has a high mortality. Drug transporters are proteins located on the cell membrane that are responsible for the absorption, distribution, and excretion of drugs. Lots of drug transporters are expressed in the kidneys. Changes in carrier function weaken balance, cause disease, or modify the effectiveness of drug treatment. The changes in expression of these transporters during cancer pathology results in multi-drug resistance to cancer chemotherapy. In the treatment of RCC, the study of drug transporters helps to optimize treatment regimens, improve therapeutic effects, and reduce drug side effects. In this review, we summarize advances in the role of renal drug transporters in the genesis, progression, and treatment of RCC.

Beyond SGLT2: proximal tubule transporters as potential drug targets for chronic kidney disease

The kidneys produce daily about 180 liters of urine but only about 2 liters are excreted. The proximal tubule plays an important role in reabsorbing the majority of filtered urine and many metabolites such as sugars, amino acids, salts or phosphate that are contained in this large volume. Reabsorption of these important metabolites is mediated by a diverse group of highly specialized transport proteins. Another group of transport proteins in the proximal tubule is responsible for the active secretion of metabolic waste products or toxins and drugs into urine. All these transporters have in common that they are directly linked to kidney metabolism and indirectly to whole-body metabolism and functions. In recent years, it has become evident that modulation of these transporters may influence the onset, progression and consequences of kidney disease. This review summarizes recent developments in this field and discusses some examples of drugs already in clinical use or in development. The examples include inhibitors of sugar transporters (SGLT2 inhibitors) that are successfully used in patients with kidney disease, diabetes or heart failure. Likewise, indirect inhibitors (acetazolamide) of an transporter absorbing sodium in exchange for protons (NHE3) are used mostly in patients with heart failure or for prevention of high altitude disease, while direct inhibitors show promise in preclinical studies to reduce damage in episodes of acute kidney disease or high blood pressure. Modulators of transporters mediating the excretion of urate have been used in patients with gout and are also discussed to prevent kidney disease. Novel drugs in development target transporters for phosphate, amino acids, or toxin and drug excretion and may be helpful for specific conditions associated with kidney disease. The advantages and challenges associated with these (novel) drugs targeting proximal tubule transport are discussed.

ABSTRACT

The proximal tubule is responsible for reabsorbing about 60% of filtered solutes and water and is critical for the secretion of metabolic waste products, drugs and toxins. A large number of highly specialized ion channels and transport proteins belonging to the SLC and ABC transporter families are involved. Their activity is directly or indirectly linked to ATP consumption and requires large quantities of energy and oxygen supply. Moreover, the activity of these transporters is often coupled to the movement of Na+ ions thus influencing also salt and water balance, as well as transport and regulatory processes in downstream segments. Because of their relevance for systemic ion balance, for renal metabolism or for affecting regulatory processes, proximal tubule transporters are attractive targets for existing drug and for novel strategies to reduce kidney disease progression or to alleviate the consequences of decreased kidney function. In this review, the relevance of some major proximal tubule transport systems as drug targets in individuals with chronic kidney disease (CKD) is discussed. Inhibitors of the sodium-glucose cotransporter 2, SGLT2, are now part of standard therapy in patients with CKD and/or heart failure. Also, indirect inhibition of Na+/H+-exchangers by carbonic anhydrase inhibitors and uricosuric drugs have been used for decades. Inhibition of phosphate and amino acid transporters have recently been proposed as novel principles to remove excess phosphate or to protect the proximal tubule metabolically, respectively. In addition, organic cation and anion transporters involved in drug and toxin excretion may serve as targets of new drugs. The advantages and challenges associated with (novel) drugs targeting proximal tubule transport are discussed.

Molecular basis of polyspecific drug and xenobiotic recognition by OCT1 and OCT2

A wide range of endogenous and xenobiotic organic ions require facilitated transport systems to cross the plasma membrane for their disposition. In mammals, organic cation transporter (OCT) subtypes 1 and 2 (OCT1 and OCT2, also known as SLC22A1 and SLC22A2, respectively) are polyspecific transporters responsible for the uptake and clearance of structurally diverse cationic compounds in the liver and kidneys, respectively. Notably, it is well established that human OCT1 and OCT2 play central roles in the pharmacokinetics and drug-drug interactions of many prescription medications, including metformin. Despite their importance, the basis of polyspecific cationic drug recognition and the alternating access mechanism for OCTs have remained a mystery. Here we present four cryo-electron microscopy structures of apo, substrate-bound and drug-bound OCT1 and OCT2 consensus variants, in outward-facing and outward-occluded states. Together with functional experiments, in silico docking and molecular dynamics simulations, these structures uncover general principles of organic cation recognition by OCTs and provide insights into extracellular gate occlusion. Our findings set the stage for a comprehensive structure-based understanding of OCT-mediated drug-drug interactions, which will prove critical in the preclinical evaluation of emerging therapeutics.

Molecular basis of polyspecific drug binding and transport by OCT1 and OCT2

A wide range of endogenous and xenobiotic organic ions require facilitated transport systems to cross the plasma membrane for their disposition 1, 2 . In mammals, organic cation transporter subtypes 1 and 2 (OCT1 and OCT2, also known as SLC22A1 and SLC22A2, respectively) are polyspecific transporters responsible for the uptake and clearance of structurally diverse cationic compounds in the liver and kidneys, respectively 3, 4 . Notably, it is well established that human OCT1 and OCT2 play central roles in the pharmacokinetics, pharmacodynamics, and drug-drug interactions (DDI) of many prescription medications, including metformin 5, 6 . Despite their importance, the basis of polyspecific cationic drug recognition and the alternating access mechanism for OCTs have remained a mystery. Here, we present four cryo-EM structures of apo, substrate-bound, and drug-bound OCT1 and OCT2 in outward-facing and outward-occluded states. Together with functional experiments, in silico docking, and molecular dynamics simulations, these structures uncover general principles of organic cation recognition by OCTs and illuminate unexpected features of the OCT alternating access mechanism. Our findings set the stage for a comprehensive structure-based understanding of OCT-mediated DDI, which will prove critical in the preclinical evaluation of emerging therapeutics.

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.

What If Not All Metabolites from the Uremic Toxin Generating Pathways Are Toxic? A Hypothesis

The topic of uremic toxicity has received broad attention from the nephrological community over the past few decades. An aspect that is much less often considered is the possibility that the metabolic pathways that generate uremic toxins also may produce molecules that benefit body functions. Here, we discuss this dualism based on the example of tryptophan-derived metabolites, which comprise elements that are mainly toxic, such as indoxyl sulfate, kynurenine and kynurenic acid, but also beneficial compounds, such as indole, melatonin and indole-3-propionic acid, and ambivalent (beneficial for some aspects and harmful for others) compounds such as serotonin. This dualism can also be perceived at the level of the main receptor of the tryptophan-derived metabolites, the aryl hydrocarbon receptor (AHR), which has also been linked to both harm and benefit. We hypothesize that these beneficial effects are the reason why uremic toxin generation remained preserved throughout evolution. This duality is also not unique for the tryptophan-derived metabolites, and in this broader context we discuss the remote sensing and signaling theory (RSST). The RSST proposes that transporters (e.g., organic anion transporter 1—OAT1; ATP-binding cassette transporter G—ABCG2) and drug metabolizing enzymes form a large network of proteins interacting to promote small molecule remote communication at the inter-organ (e.g., gut–liver–heart–brain–kidney) and inter-organismal (e.g., gut microbe–host) levels. These small molecules include gut microbe-derived uremic toxins as well as beneficial molecules such as those discussed here. We emphasize that this positive side of uremic metabolite production needs more attention, and that this dualism especially needs to be considered when assessing and conceiving of therapeutic interventions. These homeostatic considerations are central to the RSST and suggest that interventions be aimed at preserving or restoring the balance between positive and negative components rather than eliminating them all without distinction.

The Interplay between Uremic Toxins and Albumin, Membrane Transporters and Drug Interaction

Uremic toxins are a heterogeneous group of molecules that accumulate in the body due to the progression of chronic kidney disease (CKD). These toxins are associated with kidney dysfunction and the development of comorbidities in patients with CKD, being only partially eliminated by dialysis therapies. Importantly, drugs used in clinical treatments may affect the levels of uremic toxins, their tissue disposition, and even their elimination through the interaction of both with proteins such as albumin and cell membrane transporters. In this context, protein-bound uremic toxins (PBUTs) are highlighted for their high affinity for albumin, the most abundant serum protein with multiple binding sites and an ability to interact with drugs. Membrane transporters mediate the cellular influx and efflux of various uremic toxins, which may also compete with drugs as substrates, and both may alter transporter activity or expression. Therefore, this review explores the interaction mechanisms between uremic toxins and albumin, as well as membrane transporters, considering their potential relationship with drugs used in clinical practice.

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.

A Physiological Approach to Pharmacokinetics in Chronic Kidney Disease

The conventional approach to approximating the pharmacokinetics of drugs in patients with chronic kidney disease (CKD) only accounts for changes in the estimated glomerular filtration rate. However, CKD is a systemic and multifaceted disease that alters many body systems. Therefore, the objective of this exercise was to develop and evaluate a whole-body mechanistic approach to predicting pharmacokinetics in patients with CKD. Physiologically based pharmacokinetic models were developed in PK-Sim v8.0 (www.open-systems-pharmacology.org) to mechanistically represent the disposition of 7 compounds in healthy human adults. The 7 compounds selected were eliminated by glomerular filtration and active tubular secretion by the organic cation transport system to varying degrees. After a literature search, the healthy adult models were adapted to patients with CKD by numerically accounting for changes in glomerular filtration rate, kidney volume, renal perfusion, hematocrit, plasma protein concentrations, and gastrointestinal transit. Literature-informed interindividual variability was applied to the physiological parameters to facilitate a population approach. Model performance in CKD was evaluated against pharmacokinetic data from 8 clinical trials in the literature. Overall, integration of the CKD parameterization enabled exposure predictions that were within 1.5-fold error across all compounds and patients with varying stages of renal impairment. Notable improvement was observed over the conventional approach to scaling exposure, which failed in all but 1 scenario in patients with advanced CKD. Further research is required to qualify its use for first-in-CKD dose selection and clinical trial planning for a wider selection of renally eliminated compounds, including those subject to anion transport.

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.

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.

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.

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.

Urinary Dimethylamine (DMA) and Its Precursor Asymmetric Dimethylarginine (ADMA) in Clinical Medicine, in the Context of Nitric Oxide (NO) and Beyond

Asymmetric protein-arginine dimethylation is a major post-translational modification (PTM) catalyzed by protein-arginine methyltransferase (PRMT). Regular proteolysis releases asymmetric dimethylarginine (ADMA). Of the daily produced ADMA, about 10% are excreted unchanged in the urine. The remaining 90% are hydrolyzed by dimethylarginine dimethylaminohydrolase (DDAH) to L-citrulline and dimethylamine (DMA), which is readily excreted in the urine. The PRMT/DDAH pathway is almost the exclusive origin of urinary ADMA and the major source of urinary DMA. Dietary fish and seafood represent additional abundant sources of urinary DMA. The present article provides an overview of urinary ADMA and DMA reported thus far in epidemiological, clinical and pharmacological studies, in connection with the L-arginine/nitric oxide (NO) pathway and beyond, in neonates, children and adolescents, young and elderly subjects, males and females. Discussed diseases mainly include those relating to the renal and cardiovascular systems such as peripheral arterial occlusive disease, coronary artery disease, chronic kidney disease, rheumatoid arthritis, Becker muscular disease, Duchenne muscular disease (DMD), attention deficit hyperactivity disorder (ADHD), and type I diabetes. Under standardized conditions involving the abstinence of DMA-rich fresh and canned fish and seafood, urinary DMA and ADMA are useful as measures of whole-body asymmetric arginine-dimethylation in health and disease. The creatinine-corrected excretion rates of DMA range from 10 to 80 µmol/mmol in adults and up to 400 µmol/mmol in children and adolescents. The creatinine-corrected excretion rates of ADMA are on average 10 times lower. In general, diseases are associated with higher urinary DMA and ADMA excretion rates, and pharmacological treatment, such as with steroids and creatine (in DMD), decreases their excretion rates, which may be accompanied by a decreased urinary excretion of nitrate, the major metabolite of NO. In healthy subjects and in rheumatoid arthritis patients, the urinary excretion rate of DMA correlates positively with the excretion rate of dihydroxyphenylglycol (DHPG), the major urinary catecholamines metabolite, suggesting a potential interplay in the PRMT/DDAH/NO pathway.

Evaluation of kidney function in HIV-infected patients receiving an antiretroviral regimen containing one or two inhibitors of the tubular secretion of creatinine

OBJECTIVES

The aim of this study was to determine the evolution of renal function in patients receiving one or two inhibitors, according to different baseline factors. Some antiretroviral drugs such as rilpivirine (RPV), dolutegravir (DTG), or cobicistat (COBI), interact with the tubular secretion of creatinine, but there are no data about their impact in renal function evaluation in patients with renal disease or when these drugs are used concomitantly.

METHODS

A prospective cohort study was carried out in HIV-infected patients who switched to a dual regimen including DTG, RPV or darunavir/COBI, separately or in combination. The primary endpoint was the evolution of the serum creatinine-based estimated glomerular filtration rate (eGFR-scr). A control group not receiving any transporter inhibitor was included.

RESULTS

A total of 288 patients on different dual regimens were included (DTG + RPV, 92; DTG + darunavir/COBI, 23; DTG, 26; COBI, 19; control group, 128). In patients receiving two transporter inhibitors, eGFR-scr decreased by a mean of -8.4 mL/min/1.73 m² , similar to that observed with the separate use of DTG or COBI (mean of both groups, -8.6 mL/min/1.73 m² ), while eGFR-scr improved in the control group. Similar evolution of proteinuria and tubular dysfunction was observed in all the groups, and there were no significant changes in the cystatin C-based eGFR. Mean eGFR-scr change inversely correlated with baseline eGFR-scr value (r = -0.39; P < 0.01), with a lower eGFR-scr decrease in patients with chronic kidney disease.

CONCLUSIONS

Similar eGFR-scr decreases were observed in patients using different antiretroviral drugs inhibiting the tubular transport of creatinine, separately or in combination, with no alterations in proteinuria or cystatin C-based eGFR. The lack of additional changes when the drugs were used in combination, and the lower impact in cases of previous chronic kidney disease, suggest that there are compensatory mechanisms for creatinine secretion.

A swan song for Kt/Vurea

Dialyzer clearance of urea multiplied by dialysis time and normalized for urea distribution volume (Kt/V_urea or simply Kt/V) has been used as an index of dialysis adequacy since more than 30 years. This article reviews the flaws of Kt/V, starting with a lack of proof of concept in three randomized controlled hard outcome trials (RCTs), and continuing with a long list of conditions where the concept of Kt/V was shown to be flawed. This information leaves little room for any conclusion other than that Kt/V, as an indicator of dialysis adequacy, is obsolete. The dialysis patient might benefit more if, instead, the nephrology community concentrates in the future on pursuing the optimal dialysis dose that conforms with adequate quality of life and on factors that are likely to affect outcomes more than Kt/V. These include residual renal function, volume status, dialysis length, ultrafiltration rate, the number of intra-dialytic hypotensive episodes, interdialytic blood pressure, serum potassium and phosphate, serum albumin, and C reactive protein.

Contributory Role of Gut Microbiota and Their Metabolites Toward Cardiovascular Complications in Chronic Kidney Disease

The gut microbiome recently has emerged as a novel risk factor that impacts health and disease. Our gut microbiota can function as an endocrine organ through its unique ability to metabolize various dietary precursors, and can fuel the systemic inflammation observed in chronic disease. This is especially important in the setting of chronic kidney disease, in which microbial metabolism can contribute directly to accumulation of circulating toxins that then can alter and shift the balance of microbiota composition and downstream functions. To study this process, advances in -omics technologies are providing opportunities to understand not only the taxonomy, but also the functional diversity of our microbiome. We also reliably can quantify en masse a wide range of uremic byproducts of microbial metabolism. Herein, we examine the bidirectional relationship between the gut microbiome and the failing kidneys. We describe potential approaches targeting gut microbiota for cardiovascular risk reduction in chronic kidney disease using an illustrative example of a novel gut-generated metabolite, trimethylamine N-oxide.

Disease-Associated Changes in Drug Transporters May Impact the Pharmacokinetics and/or Toxicity of Drugs: A White Paper From the International Transporter Consortium

Drug transporters are critically important for the absorption, distribution, metabolism, and excretion (ADME) of many drugs and endogenous compounds. Therefore, disruption of these pathways by inhibition, induction, genetic polymorphisms, or disease can have profound effects on overall physiology, drug pharmacokinetics, drug efficacy, and toxicity. This white paper provides a review of changes in transporter function associated with acute and chronic disease states, describes regulatory pathways affecting transporter expression, and identifies opportunities to advance the field.

Residual Function Effectively Controls Plasma Concentrations of Secreted Solutes in Patients on Twice Weekly Hemodialysis

Background Most patients on hemodialysis are treated thrice weekly even if they have residual kidney function, in part because uncertainty remains as to how residual function should be valued and incorporated into the dialysis prescription. Recent guidelines, however, have increased the weight assigned to residual function and thus reduced the treatment time required when it is present. Increasing the weight assigned to residual function may be justified by knowledge that the native kidney performs functions not replicated by dialysis, including solute removal by secretion. This study tested whether plasma concentrations of secreted solutes are as well controlled in patients with residual function on twice weekly hemodialysis as in anuric patients on thrice weekly hemodialysis.Methods We measured the plasma concentration and residual clearance, dialytic clearance, and removal rates for urea and the secreted solutes hippurate, phenylacetylglutamine, indoxyl sulfate, and p-cresol sulfate in nine patients on twice weekly hemodialysis and nine patients on thrice weekly hemodialysis.Results Compared with anuric patients on thrice weekly dialysis with the same standard Kt/V_urea, patients on twice weekly hemodialysis had lower hippurate and phenylacetylglutamine concentrations and similar indoxyl sulfate and p-cresol sulfate concentrations. Mathematical modeling revealed that residual secretory function accounted for the observed pattern of solute concentrations.Conclusions Plasma concentrations of secreted solutes can be well controlled by twice weekly hemodialysis in patients with residual kidney function. This result supports further study of residual kidney function value and the inclusion of this function in dialysis adequacy measures.

Biochemical and Clinical Impact of Organic Uremic Retention Solutes: A Comprehensive Update

In this narrative review, the biological/biochemical impact (toxicity) of a large array of known individual uremic retention solutes and groups of solutes is summarized. We classified these compounds along their physico-chemical characteristics as small water-soluble compounds or groups, protein bound compounds and middle molecules. All but one solute (glomerulopressin) affected at least one mechanism with the potential to contribute to the uremic syndrome. In general, several mechanisms were influenced for each individual solute or group of solutes, with some impacting up to 7 different biological systems of the 11 considered. The inflammatory, cardio-vascular and fibrogenic systems were those most frequently affected and they are one by one major actors in the high morbidity and mortality of CKD but also the mechanisms that have most frequently been studied. A scoring system was built with the intention to classify the reviewed compounds according to the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence. Among the highest globally scoring solutes were 3 small water-soluble compounds [asymmetric dimethylarginine (ADMA); trimethylamine-N-oxide (TMAO); uric acid], 6 protein bound compounds or groups of protein bound compounds [advanced glycation end products (AGEs); p-cresyl sulfate; indoxyl sulfate; indole acetic acid; the kynurenines; phenyl acetic acid;] and 3 middle molecules [β₂-microglobulin; ghrelin; parathyroid hormone). In general, more experimental data were provided for the protein bound molecules but for almost half of them clinical evidence was missing in spite of robust experimental data. The picture emanating is one of a complex disorder, where multiple factors contribute to a multisystem complication profile, so that it seems of not much use to pursue a decrease of concentration of a single compound.

The Role of the Kidney in Drug Elimination: Transport, Metabolism, and the Impact of Kidney Disease on Drug Clearance

Recent advances in the identification and characterization of renal drug transporters and drug-metabolizing enzymes has led to greater understanding of their roles in drug and chemical elimination and in modulation of the intrarenal exposure and response to drugs, nephrotoxic compounds, and physiological mediators. Furthermore, there is increasing awareness of the potential importance of drug-drug interactions (DDIs) arising from inhibition of renal transporters, and regulatory agencies now provide recommendations for the evaluation of transporter-mediated DDIs. Apart from the well-recognized effects of kidney disease on renal drug clearance, there is a growing body of evidence demonstrating that the nonrenal clearances of drugs eliminated by certain transporters and drug-metabolizing enzymes are decreased in patients with chronic kidney disease (CKD). Based on these observations, renal impairment guidance documents of regulatory agencies recommend pharmacokinetic characterization of both renally cleared and nonrenally cleared drugs in CKD patients to inform possible dosage adjustment.