Influence of serum in hemodialysis patients on the expression of intestinal and hepatic transporters for the excretion of pravastatin

Comparative pharmacokinetics of six bioactive components of Shen-Wu-Yi-Shen tablets in normal and chronic renal failure rats based on UPLC-TSQ-MS/MS

ETHNOPHARMACOLOGICAL RELEVANCE

Shen-Wu-Yi-Shen tablets (SWYST), a Chinese patent medicine consisting of 12 herbal medicines, was formulated by a famous TCM nephrologist, Zou Yunxiang. It is clinically used to improve the symptoms of nausea, vomiting, poor appetite, dry mouth and throat, and dry stool in patients with chronic renal failure (CRF) accompanied by qi and yin deficiency, dampness, and turbidity. SWYST can reduce urea nitrogen, blood creatinine, and urinary protein loss, and increase the endogenous creatinine clearance rate. However, little is known about its pharmacokinetics.

AIM OF STUDY

To compare the pharmacokinetics of six bioactive components after oral administration of SWYST in normal and adenine-induced CRF rats.

MATERIALS AND METHODS

A method based on ultra-performance liquid chromatography coupled with a triple-stage quadrupole mass spectrometer (UPLC-TSQ-MS/MS) was developed and validated to determine the six bioactive compounds (albiflorin, paeoniflorin, plantagoguanidinic acid, rhein, aloe-emodin, and emodin) in rat plasma. Rat plasma samples were prepared using protein precipitation. Chromatography was performed on an Agilent Eclipse Plus C18 column (3.0 × 50 mm, 1.8 μm) using gradient elution with a mobile phase composed of acetonitrile and water containing 0.1% (v/v) formic acid, while detection was achieved by electrospray ionization MS under the multiple selective reaction monitoring modes. After SWYST administration, rat plasma was collected at different time points, and the pharmacokinetic parameters of six analytes were calculated and analyzed based on the measured plasma concentrations.

RESULTS

The UPLC-TSQ-MS/MS method was fully validated for its satisfactory linearity (r ≥ 0.9913), good precisions (RSD <11.5%), and accuracy (RE: -13.4∼13.1%), as well as acceptable limits in the extraction recoveries, matrix effects, and stability (RSD <15%). In normal rats, the six analytes were rapidly absorbed (Tmax ≤ 2 h), and approximately 80% of their total exposure was eliminated within 10 h. Moreover, in normal rats, the AUC0-t and Cmax of albiflorin, plantagoguanidinic acid, and rhein exhibited linear pharmacokinetics within the dose ranges, while that of paeoniflorin is non-linear. However, in CRF rats, the six analytes exhibited reduced elimination and significantly different AUC or Cmax values. These changes may reflect a decreased renal clearance rate or inhibition of drug-metabolizing enzymes and transporters in the liver and gastrointestinal tract caused by CRF.

CONCLUSIONS

A sensitive UPLC-TSQ-MS/MS method was validated and used to investigate the pharmacokinetics of SWYST in normal and CRF rats. This is the first study to investigate the pharmacokinetics of SWYST, and our findings elucidate the causes of their different pharmacokinetic behaviors in CRF rats. Furthermore, the results provide useful information to guide further research on the pharmacokinetic-pharmacodynamic correlation and clinical application of SWYST.

Sorafenib Chemosensitization by Caryophyllane Sesquiterpenes in Liver, Biliary, and Pancreatic Cancer Cells: The Role of STAT3/ABC Transporter Axis

A combination of anticancer drugs and chemosensitizing agents has been approached as a promising strategy to potentiate chemotherapy and reduce toxicity in aggressive and chemoresistant cancers, like hepatocellular carcinoma (HCC), cholangiocarcinoma (CCA), and pancreatic ductal adenocarcinoma (PDAC). In the present study, the ability of caryophyllane sesquiterpenes to potentiate sorafenib efficacy was studied in HCC, CCA, and PDAC cell models, focusing on the modulation of STAT3 signaling and ABC transporters; tolerability studies in normal cells were also performed. Results showed that the combination of sorafenib and caryophyllane sesquiterpenes synergized the anticancer drug, especially in pancreatic Bx-PC3 adenocarcinoma cells; a similar trend, although with lower efficacy, was found for the standard ABC transporter inhibitors. Synergistic effects were associated with a modulation of MDR1 (or Pgp) and MRP transporters, both at gene and protein level; moreover, activation of STAT3 cascade and cell migration appeared significantly affected, suggesting that the STAT3/ABC-transporters axis finely regulated efficacy and chemoresistance to sorafenib, thus appearing as a suitable target to overcome drawbacks of sorafenib-based chemotherapy in hepato-biliary-pancreatic cancers. Present findings strengthen the interest in caryophyllane sesquiterpenes as chemosensitizing and chemopreventive agents and contribute to clarifying drug resistance mechanisms in HCC, CCA, and PDAC cancers and to developing possible novel therapeutic strategies.

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.

Uremic serum residue decreases SN-38 sensitivity through suppression of organic anion transporter polypeptide 2B1 in LS-180 colon cancer cells

Pharmacokinetics of SN-38 in patients with end-stage kidney disease (ESKD) is partially varied because of fluctuations in transporters expression and/or function by high protein bound-uremic toxins concentration. The fluctuations may induce variations in anticancer drugs sensitivity to cancer cells. We aimed to clarify the variations in sensitivity of SN-38 to cancer patients with ESKD and investigate this mechanism, by human colon cancer cells exposed to uremic serum residue. LS180 cells were exposed to normal or uremic serum residue (LS/NSR or LS/USR cells) for a month. IC₅₀ values of SN-38 in LS/NSR or LS/USR cells were calculated from viability of each cells treated SN-38. mRNA expression and intracellular SN-38 accumulation was evaluated by RT-PCR and HPLC-fluorescence methods, respectively. The IC₅₀ value in LS/USR cells was higher than that in LS/NSR cells. Organic anion transporter polypeptide (OATP) 2B1 mRNA expression was lower in LS/USR cells than in LS/NSR cells, and SN-38 accumulation in LS/USR cells was lower than that in LS/NSR cells. Only co-treatment baicalin, which is OATP2B1 inhibitor, almost negated the difference in SN-38 accumulation between LS/NSR and LS/USR. Anticancer effects of substrates of OATP2B1, such as SN-38, were reduced in ESKD patients at the same plasma substrate concentration.

Effects of Uremic Serum Residue on OATP1B1- and OATP1B3-Mediated Pravastatin Uptake in OATP-Expressing HEK293 Cells and Human Hepatocytes

Patients with end-stage renal disease have increased plasma concentrations of statins, which is a risk factor for rhabdomyolysis, as well as elevated levels of uremic toxins (UTs). We investigated the effects of uremic serum residue and UTs on organic anion-transporting peptide (OATP1B1)- and OATP1B3-mediated pravastatin uptake. We evaluated the effects of normal serum residue with four UTs (hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furan propionate, indole-3-acetic acid, and 3-indoxyl sulfate) and uremic serum residue on pravastatin uptake by OATP1B1- or OATP1B3-expressing HEK293 cells. Furthermore, we assessed the contribution of each transporter using cryopreserved human hepatocytes. Uremic serum residue and UTs significantly inhibited OATP1B1-mediated pravastatin uptake. Uremic serum residue accelerated OATP1B3-mediated pravastatin uptake, while UTs had no effect. There was no difference in pravastatin uptake between uremic- and normal serum residue-treated hepatocytes. The results suggest that the effects of uremic serum on pravastatin hepatic uptake may be considered negligible in end-stage renal disease patients.

Generation, clearance, toxicity, and monitoring possibilities of unaccounted uremic toxins for improved dialysis prescriptions

Current dialysis-dosing calculations provide an incomplete assessment of blood purification. They exclude clearances of protein-bound uremic toxins (PB-UTs), such as polyamines, p-cresol sulfate, and indoxyl sulfate, relying solely on the clearance of urea as a surrogate for all molecules accumulating in patients with end-stage renal disease (ESRD). PB-UTs clear differently in dialysis but also during normal renal function. The kidney clears PB toxins via the process of secretion, whereas it clears urea through filtration. Herein, we review the clearance, accumulation, and toxicity of various UTs. We also suggest possible methods for their monitoring toward the ultimate goal of a more comprehensive dialysis prescription. A more inclusive dialysis prescription would retain the kidney-filtration surrogate, urea, and consider at least one PB toxin as a surrogate for UTs cleared through cellular secretion. A more comprehensive assessment of UTs that includes both secretion and filtration is expected to result in a better understanding of ESRD toxicity and consequently, to reduce ESRD mortality.

Is 3-Carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) a Clinically Relevant Uremic Toxin in Haemodialysis Patients?

3-Carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) is a metabolite of furan fatty acid and a marker of fish oil intake. CMPF is described as a protein-bound uremic toxin and interacts with free oxygen radicals, which can induce cell damages. However, the clinical consequences of CMPF accumulation in haemodialysis patients remain poorly documented. The aims of this study are to investigate potential association between CMPF levels and (i) biochemical and nutritional parameters; (ii) cardiovascular events and (iii) mortality. Two hundred and fifty-two patients undergoing maintenance haemodialysis were included. Routine clinical biochemistry tests and assay for CMPF by HPLC technique were performed at the inclusion. Body composition parameters were measured using a bioimpedance spectroscopy method. The enrolled patients were prospectively monitored for cardiovascular events and mortality. CMPF level was positively correlated with nutritional parameters and lean mass and is significantly higher in patients without protein-energy wasting. However, the multivariate linear regression analysis indicated that CMPF level was not independently associated with albumin, prealbumin, creatinemia and body mass index. Elevated serum CMPF was not associated with mortality and cardiovascular morbidity. Our results indicate that CMPF is not a relevant uremic toxin in haemodialysis and in contrast could be a marker of healthy diet and omega 3 intakes.

Endogenous Metabolites-Mediated Communication Between OAT1/OAT3 and OATP1B1 May Explain the Association Between SLCO1B1 SNPs and Methotrexate Toxicity

Although OATP1B1 is not expressed in the kidney, polymorphisms in SLCO1B1 have been associated with methotrexate clearance or toxicity. This unexpected pharmacogenetic association may reflect remote communication between liver and kidney transporters. This study confirms the pharmacogenetic association with methotrexate toxicity in adult patients with hematological malignancies. Using a targeted urinary metabolomics approach, we identified 38 and 34 metabolites which were differentially excreted between wildtype and carriers of the c.388A>G or c.521T>C variant alleles, respectively, half of them being associated with methotrexate toxicity. These metabolites mainly consisted of fatty acid derivatives and microbiota catabolites, including glycine conjugates and other uremic toxins, all known OATs substrates. These results suggest that dysfunction of a transporter affects the excretion profile of endogenous or exogenous substrates, possibly through metabolite-mediated interactions involving other transport systems, even in distant organs. This opens the way for better comprehension of complex pharmacokinetics and transporter-mediated drug-drug or nutrient-drug interactions.

Indoxyl Sulfate Upregulates Liver P-Glycoprotein Expression and Activity through Aryl Hydrocarbon Receptor Signaling

In patients with CKD, not only renal but also, nonrenal clearance of drugs is altered. Uremic toxins could modify the expression and/or activity of drug transporters in the liver. We tested whether the uremic toxin indoxyl sulfate (IS), an endogenous ligand of the transcription factor aryl hydrocarbon receptor, could change the expression of the following liver transporters involved in drug clearance: SLC10A1, SLC22A1, SLC22A7, SLC47A1, SLCO1B1, SLCO1B3, SLCO2B1, ABCB1, ABCB11, ABCC2, ABCC3, ABCC4, ABCC6, and ABCG2 We showed that IS increases the expression and activity of the efflux transporter P-glycoprotein (P-gp) encoded by ABCB1 in human hepatoma cells (HepG2) without modifying the expression of the other transporters. This effect depended on the aryl hydrocarbon receptor pathway. Presence of human albumin at physiologic concentration in the culture medium did not abolish the effect of IS. In two mouse models of CKD, the decline in renal function associated with the accumulation of IS in serum and the specific upregulation of Abcb1a in the liver. Additionally, among 109 heart or kidney transplant recipients with CKD, those with higher serum levels of IS needed higher doses of cyclosporin, a P-gp substrate, to obtain the cyclosporin target blood concentration. This need associated with serum levels of IS independent of renal function. These findings suggest that increased activity of P-gp could be responsible for increased hepatic cyclosporin clearance. Altogether, these results suggest that uremic toxins, such as IS, through effects on drug transporters, may modify the nonrenal clearance of drugs in patients with CKD.

Microbiota-derived uremic retention solutes: perpetrators of altered nonrenal drug clearance in kidney disease

INTRODUCTION

Scientific interest in the gut microbiota is increasing due to improved understanding of its implications in human health and disease. In patients with kidney disease, gut microbiota-derived uremic toxins directly contribute to altered nonrenal drug clearance. Microbial imbalances, known as dysbiosis, potentially increase formation of microbiota-derived toxins, and diminished renal clearance leads to toxin accumulation. High concentrations of microbiota-derived toxins such as indoxyl sulfate and p-cresol sulfate perpetrate interactions with drug metabolizing enzymes and transporters, which provides a mechanistic link between increases in drug-related adverse events and dysbiosis in kidney disease. Areas covered: This review summarizes the effects of microbiota-derived uremic toxins on hepatic phase I and phase II drug metabolizing enzymes and drug transporters. Research articles that tested individual toxins were included. Therapeutic strategies to target microbial toxins are also discussed. Expert commentary: Large interindividual variability in toxin concentrations may explain some differences in nonrenal clearance of medications. Advances in human microbiome research provide unique opportunities to systematically evaluate the impact of individual and combined microbial toxins on drug metabolism and transport, and to explore microbiota-derived uremic toxins as potential therapeutic targets.

Characterization of simvastatin acid uptake by organic anion transporting polypeptide 3A1 (OATP3A1) and influence of drug-drug interaction

Human organic anion transporting polypeptide 3A1 (OATP3A1) is predominately expressed in the heart. The ability of OATP3A1 to transport statins into cardiomyocytes is unknown, although other OATPs are known to mediate the uptake of statin drugs in liver. The pleiotropic effects and uptake of simvastatin acid were analyzed in primary human cardiomyocytes and HEK293 cells transfected with the OATP3A1 gene. Treatment with simvastatin acid reduced indoxyl sulfate-mediated reactive oxygen species and modulated OATP3A1 expression in cardiomyocytes and HEK293 cells transfected with the OATP3A1 gene. We observed a pH-dependent effect on OATP3A1 uptake, with more efficient simvastatin acid uptake at pH5.5 in HEK293 cells transfected with the OATP3A1 gene. The Michaelis-Menten constant (K_m) for simvastatin acid uptake by OATP3A1 was 0.017±0.002μM and the V_max was 0.995±0.027fmol/min/10⁵ cells. Uptake of simvastatin acid was significantly increased by known (benzylpenicillin and estrone-3-sulfate) and potential (indoxyl sulfate and cyclosporine) substrates of OATP3A1. In conclusion, the presence of OATP3A1 in cardiomyocytes suggests that this transporter may modulate the exposure of cardiac tissue to simvastatin acid due to its enrichment in cardiomyocytes. Increases in uptake of simvastatin acid by OATP3A1 when combined with OATP substrates suggest the potential for drug-drug interactions that could influence clinical outcomes.

Impairment of the carnitine/organic cation transporter 1-ergothioneine axis is mediated by intestinal transporter dysfunction in chronic kidney disease

Carnitine/organic cation transporter 1 (OCTN1) is a specific transporter of the food-derived antioxidant ergothioneine. Ergothioneine is absorbed by intestinal OCTN1, distributed through the bloodstream, and incorporated into each organ by OCTN1. OCTN1 expression is upregulated in injured tissues, and promotes ergothioneine uptake to reduce further damage caused by oxidative stress. However, the role of the OCTN1-ergothioneine axis in kidney-intestine cross-talk and chronic kidney disease (CKD) progression remains unclear. Here we assessed ergothioneine uptake via intestinal OCTN1 and confirmed the expression of OCTN1. The ability of OCTN1 to absorb ergothioneine was diminished in mice with CKD. In combination with OCTN1 dysfunction, OCTN1 localization on the intestinal apical cellular membrane was disturbed in mice with CKD. Proteomic analysis, RT-PCR, Western blotting, and immunohistochemistry revealed that PDZ (PSD95, Dlg, and ZO1), a PDZK1 domain-containing protein that regulates the localization of transporters, was decreased in mice with CKD. Decreased intestinal ergothioneine uptake from food decreased ergothioneine levels in the blood of mice with CKD. Despite increased OCTN1 expression and ergothioneine uptake into the kidneys of mice with CKD, ergothioneine levels did not increase. To identify the role of the OCTN1-ergothioneine axis in CKD, we evaluated kidney damage and oxidative stress in OCTN1-knockout mice with CKD and found that kidney fibrosis worsened. Oxidative stress indicators were increased in OCTN1-knockout mice. Moreover, ergothioneine levels in the blood of patients with CKD decreased, which were restored after kidney transplantation. Thus, a novel inter-organ interaction mediated by transporters is associated with CKD progression.

Indoxyl Sulfate-Review of Toxicity and Therapeutic Strategies

Indoxyl sulfate is an extensively studied uremic solute. It is a small molecule that is more than 90% bound to plasma proteins. Indoxyl sulfate is derived from the breakdown of tryptophan by colon microbes. The kidneys achieve high clearances of indoxyl sulfate by tubular secretion, a function not replicated by hemodialysis. Clearance by hemodialysis is limited by protein binding since only the free, unbound solute can diffuse across the membrane. Since the dialytic clearance is much lower than the kidney clearance, indoxyl sulfate accumulates to relatively high plasma levels in hemodialysis patients. Indoxyl sulfate has been most frequently implicated as a contributor to renal disease progression and vascular disease. Studies have suggested that indoxyl sulfate also has adverse effects on bones and the central nervous system. The majority of studies have assessed toxicity in cultured cells and animal models. The toxicity in humans has not yet been proven, as most data have been from association studies. Such toxicity data, albeit inconclusive, have prompted efforts to lower the plasma levels of indoxyl sulfate through dialytic and non-dialytic means. The largest randomized trial showed no benefit in renal disease progression with AST-120. No trials have yet tested cardiovascular or mortality benefit. Without such trials, the toxicity of indoxyl sulfate cannot be firmly established.

Modulation of multidrug resistance-associated proteins function in erythrocytes in glycerol-induced acute renal failure rats

Objectives

Evaluation of the function of multidrug resistance-associated proteins (MRPs) expressed in erythrocytes and screening of endogenous MRPs modulator(s) in glycerol-induced acute renal failure (ARF) rats.

Methods

Concentrations of 2,4-dinitrophenyl-S-glutathione (DNP-SG), a substrate for MRPs, in erythrocytes after administration of 1-chloro-2,4-dintrobenzene (CDNB), a precursor of DNP-SG, were determined in control and ARF rats. The screening of endogenous MRPs modulator(s) was performed using washed erythrocytes and inside-out erythrocyte membrane vesicles (IOVs) in vitro.

Key findings

Accumulation of DNP-SG in erythrocytes was observed in ARF rats. Uraemic plasma components exhibited a greater inhibitory effect on DNP-SG uptake by IOVs than control plasma components and increased the DNP-SG accumulation significantly in washed erythrocytes. Several protein-bound uraemic toxins at clinically observed concentrations and bilirubin significantly inhibited DNP-SG uptake by IOVs. In washed erythrocytes, bilirubin (10 μm) and l-kynurenine (100 μm), a precursor of kynurenic acid being MRPs inhibitor, increased DNP-SG accumulation significantly.

Conclusions

Glycerol-induced ARF rats contain various MRPs inhibitors in plasma, and membrane-permeable MRP substrates/inhibitors including their precursors inhibit the MRPs function in erythrocytes cooperatively.

The Pharmacokinetics of Beraprost Sodium Following Single Oral Administration to Subjects With Impaired Kidney Function

The purpose of the present study was to evaluate the pharmacokinetics of beraprost sodium (BPS) and its active enantiomer, BPS-314d, in Japanese subjects with impaired kidney function. The plasma and urine concentrations of BPS and BPS-314d were measured following the single oral administration of 120 μg of BPS as the sustained-release tablet, TRK-100STP, under fasting conditions to 18 subjects with impaired kidney function (stage 2, 3, and 4 chronic kidney disease [CKD] as categorized by the estimated glomerular filtration rate) and to 6 age-, body weight-, and gender-matched subjects with normal kidney function (stage 1 CKD). The C_max values (mean ± SD) of BPS in stage 1, 2, 3, and 4 CKD, respectively, were 84.9 ± 22.9, 119.8 ± 36.4, 190.6 ± 137.3, and 240.2 ± 110.5 pg/mL; its AUC_0-48h were 978 ± 226, 1252 ± 427, 1862 ± 964, and 1766 ± 806 pg·h/mL, respectively, and its cumulative urinary excretion rates were 0.704 ± 0.351%, 0.638 ± 0.292%, 0.485 ± 0.294%, and 0.159 ± 0.136%. The C_max values of BPS-314d were 22.4 ± 6.4, 30.8 ± 8.5, 46.7 ± 30.6, and 54.4 ± 25.2 pg/mL, its AUC_0-48h were 155 ± 56, 226 ± 67, 341 ± 176, and 329 ± 143 pg·h/mL, and its cumulative urinary excretion rates were 0.428 ± 0.242%, 0.349 ± 0.179%, 0.356 ± 0.270%, and 0.096 ± 0.099%, respectively. Adverse events were reported in 2 subjects with stage 2 CKD and 1 subject with stage 4 CKD. The C_max and AUC_0-48h of BPS and BPS-314d were higher based on the severity of impaired kidney function. No relationship was observed between the incidence of adverse events and the severity, and tolerability was confirmed. We consider that dose adjustment is not necessary, but BPS is more carefully treated in patients with impaired kidney function.

Insights into solute carriers: physiological functions and implications in disease and pharmacokinetics

SLCs transport many endogenous and exogenous compounds including drugs; SLCs dysfunction has implications in pharmacokinetics, drug toxicity or lack of efficacy.

The intestine and the kidneys: a bad marriage can be hazardous

The concept that the intestine and chronic kidney disease influence each other, emerged only recently. The problem is multifaceted and bidirectional. On one hand, the composition of the intestinal microbiota impacts uraemic retention solute production, resulting in the generation of essentially protein-bound uraemic toxins with strong biological impact such as vascular damage and progression of kidney failure. On the other hand, the uraemic status affects the composition of intestinal microbiota, the generation of uraemic retention solutes and their precursors and causes disturbances in the protective epithelial barrier of the intestine and the translocation of intestinal microbiota into the body. All these elements together contribute to the disruption of the metabolic equilibrium and homeostasis typical to uraemia. Several measures with putative impact on intestinal status have recently been tested for their influence on the generation or concentration of uraemic toxins. These include dietary measures, prebiotics, probiotics, synbiotics and intestinal sorbents. Unfortunately, the quality and the evidence base of many of these studies are debatable, especially in uraemia, and often results within one study or among studies are contradictory. Nevertheless, intestinal uraemic metabolite generation remains an interesting target to obtain in the future as an alternative or additive to dialysis to decrease uraemic toxin generation. In the present review, we aim to summarize (i) the role of the intestine in uraemia by producing uraemic toxins and by generating pathophysiologically relevant changes, (ii) the role of uraemia in modifying intestinal physiology and (iii) the therapeutic options that could help to modify these effects and the studies that have assessed the impact of these therapies.

Consequences of renal failure on non-renal clearance of drugs

Kidney disease not only alters the renal elimination but also the non-renal disposition of drugs that are metabolized by the liver. Indeed, modifications in the expression and activity of intestinal and hepatic drug metabolism enzymes and uptake and efflux transporters have been reported. Accumulated uremic toxins, inflammatory cytokines, and parathyroid hormones may modulate these proteins either directly or by inhibiting gene expression. This can lead to important unintended variations in exposure and response when drugs are administered without dose adjustment for reduced renal function. This review summarizes our current understanding of non-renal clearance in circumstances of chronic and acute renal failure with experimental but also clinical studies. It also evaluates the clinical impact on drug disposition. Predicting the extent of the drug disposition modification is difficult first because of the complex interplay between metabolic enzymes and transport proteins but also because of the differential effects in the different organs (liver, intestines). Recommendations of the US FDA are presented as they may be potentially helpful tools to predict these modifications when no specific pharmacokinetic studies are available.

The organic anion transporter (OAT) family: a systems biology perspective

The organic anion transporter (OAT) subfamily, which constitutes roughly half of the SLC22 (solute carrier 22) transporter family, has received a great deal of attention because of its role in handling of common drugs (antibiotics, antivirals, diuretics, nonsteroidal anti-inflammatory drugs), toxins (mercury, aristolochic acid), and nutrients (vitamins, flavonoids). Oats are expressed in many tissues, including kidney, liver, choroid plexus, olfactory mucosa, brain, retina, and placenta. Recent metabolomics and microarray data from Oat1 [Slc22a6, originally identified as NKT (novel kidney transporter)] and Oat3 (Slc22a8) knockouts, as well as systems biology studies, indicate that this pathway plays a central role in the metabolism and handling of gut microbiome metabolites as well as putative uremic toxins of kidney disease. Nuclear receptors and other transcription factors, such as Hnf4α and Hnf1α, appear to regulate the expression of certain Oats in conjunction with phase I and phase II drug metabolizing enzymes. Some Oats have a strong selectivity for particular signaling molecules, including cyclic nucleotides, conjugated sex steroids, odorants, uric acid, and prostaglandins and/or their metabolites. According to the "Remote Sensing and Signaling Hypothesis," which is elaborated in detail here, Oats may function in remote interorgan communication by regulating levels of signaling molecules and key metabolites in tissues and body fluids. Oats may also play a major role in interorganismal communication (via movement of small molecules across the intestine, placental barrier, into breast milk, and volatile odorants into the urine). The role of various Oat isoforms in systems physiology appears quite complex, and their ramifications are discussed in the context of remote sensing and signaling.

Effects of chronic kidney disease on liver transport: quantitative intravital microscopy of fluorescein transport in the rat liver

Clinical studies indicate that hepatic drug transport may be altered in chronic kidney disease (CKD). Uremic solutes associated with CKD have been found to alter the expression and/or activity of hepatocyte transporters in experimental animals and in cultured cells. However, given the complexity and adaptability of hepatic transport, it is not clear whether these changes translate into significant alterations in hepatic transport in vivo. To directly measure the effect of CKD on hepatocyte transport in vivo, we conducted quantitative intravital microscopy of transport of the fluorescent organic anion fluorescein in the livers of rats following 5/6th nephrectomy, an established model of CKD. Our quantitative analysis of fluorescein transport showed that the rate of hepatocyte uptake was reduced by ∼20% in 5/6th nephrectomized rats, consistent with previous observations of Oatp downregulation. However, the overall rate of transport into bile canaliculi was unaffected, suggesting compensatory changes in Mrp2-mediated secretion. Our study suggests that uremia resulting from 5/6th nephrectomy does not significantly impact the overall hepatic clearance of an Oatp substrate.

Role of multidrug resistance-associated protein 4 in the basolateral efflux of hepatically derived enalaprilat

Hepatic uptake and efflux transporters govern the systemic and hepatic exposure of many drugs and metabolites. Enalapril is a pharmacologically inactive prodrug of enalaprilat. Following oral administration, enalapril is converted to enalaprilat in hepatocytes and undergoes translocation into the systemic circulation to exert its pharmacologic effect by inhibiting angiotensin-converting enzyme. Although the transport proteins governing hepatic uptake of enalapril and the biliary excretion of enalapril and enalaprilat are well established, it remains unknown how hepatically derived enalaprilat translocates across the basolateral membrane into the systemic circulation. In this study, the role of ATP-binding cassette transporters in the hepatic basolateral efflux of enalaprilat was investigated using membrane vesicles. ATP-dependent uptake of enalaprilat into vesicles expressing multidrug resistance-associated protein (MRP) 4 was significantly greater (∼3.8-fold) than in control vesicles. In contrast, enalaprilat was not transported to a significant extent by MRP3, and enalapril was not transported by either MRP3 or MRP4. The functional importance of MRP4 in the basolateral excretion of derived enalaprilat was evaluated using a novel basolateral efflux protocol developed in human sandwich-cultured hepatocytes. Under normal culture conditions, the mean intrinsic basolateral efflux clearance (CLint ,basolateral) of enalaprilat was 0.026 ± 0.012 µl/min; enalaprilat CLint,basolateral was significantly reduced to 0.009 ± 0.009 µl/min by pretreatment with the pan-MRP inhibitor MK-571. Results suggest that hepatically derived enalaprilat is excreted across the hepatic basolateral membrane by MRP4. Changes in MRP4-mediated basolateral efflux may alter the systemic concentrations of this active metabolite, and potentially the efficacy of enalapril.

The uremic toxicity of indoxyl sulfate and p-cresyl sulfate: a systematic review

A growing number of publications supports a biologic effect of the protein-bound uremic retention solutes indoxyl sulfate and p-cresyl sulfate. However, the use of unrealistically high free concentrations of these compounds and/or inappropriately low albumin concentrations may blur the interpretation of these results. Here, we performed a systematic review, selecting only studies in which, depending on the albumin concentration, real or extrapolated free concentrations of indoxyl sulfate and p-cresyl sulfate remained in the uremic range. The 27 studies retrieved comprised in vitro and animal studies. A quality score was developed, giving 1 point for each of the following criteria: six or more experiments, confirmation by more than one experimental approach, neutralization of the biologic effect by counteractive reagents or antibodies, use of a real-life model, and use of dose-response analyses in vitro and/or animal studies. The overall average score was 3 of 5 points, with five studies scoring 5 of 5 points and six studies scoring 4 of 5 points, highlighting the superior quality of a substantial number of the retrieved studies. In the 11 highest scoring studies, most functional deteriorations were related to uremic cardiovascular disease and kidney damage. We conclude that our systematic approach allowed the retrieval of methodologically correct studies unbiased by erroneous conditions related to albumin binding. Our data seem to confirm the toxicity of indoxyl sulfate and p-cresyl sulfate and support their roles in vascular and renal disease progression.

Effect of chronic renal failure on the hepatic, intestinal, and renal expression of bile acid transporters

Although the kidney is believed to play a minor role in bile acid (BA) excretion, chronic renal failure (CRF) has been reported to be associated with increased serum bile acid levels and alterations in BA homeostasis. The mechanisms for elevated BA levels are poorly understood in both clinical and experimental studies. This study was designed to examine the effects of naturally progressing CRF of longer duration on the hepatic and renal mRNA and protein levels of the BA-synthesizing enzyme Cyp7a1 and the BA transporters Ntcp, Bsep, Mrp3, Ost-α, and Ost-β. Sprague-Dawley rats were randomized to the CRF group (&frac56; nephrectomy) or to the sham-operated control group and were analyzed 8 wk after surgery. Results obtained in the CRF rats were compared with those obtained in rats that had undergone uninephrectomy (UNX). The CRF group exhibited significantly increased plasma cholesterol and BA concentrations. Hepatic Cyp7a1 mRNA and protein levels were almost identical in the two groups. Hepatic Mrp3, Ost-α, and Ost-β expression was increased, suggesting increased basolateral efflux of bile acids into the blood. However, no such changes in BA transporter expression were observed in the remnant kidney. In UNX rats, similar changes in plasma BA levels and in the expression of BA transporters were found. We hypothesize that the increase in plasma BA is an early event in the progression of CRF and is caused by increased efflux across the basolateral hepatocyte membrane.

Possibility of decrease in CYP1A2 function in patients with end-stage renal disease

Propranolol, the substrate of cytochrome P450 (CYP) 1A2 and CYP2D6, has been reported to be in high concentrations in end-stage renal disease (ESRD) patients. This has been thought to be due to the decrease in the nonrenal clearance of propranolol. The objective of this study is to elucidate the reason for the decrease in nonrenal clearance in ESRD patients. CYP1A2 and CYP2D6 activities were estimated by the phenacetin O-deethylation and methoprolol O-demethylation methods, respectively. Pooled normal serum and pooled uremic serum were deproteinized by methanol in order to exclude high-molecular-weight compounds. We selected as candidate inhibitors: uremic toxins such as 3-indoxyl sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indole-3-acetic acid, and hippuric acid, and xanthine derivatives such as allantoin, uric acid, and xanthine. In this study, uremic serum was found to inhibit the CYP1A2-mediated metabolism of phenacetin to acetaminophen in a concentration-dependent and competitive manner. Xanthine also inhibited the metabolism of CYP1A2. On the other hand, uremic serum and the four uremic toxins did not inhibit the CYP2D6-mediated metabolism of metoprolol to O-demethylmetoprolol. In conclusion, this study suggests that the increase of the bioavailability of propranolol in ESRD is partly induced by the inhibition of the hepatic metabolism of CYP1A2 by xanthine in the uremic serum.