Characterization of Organic Anion Transporter 2 (SLC22A7): A Highly Efficient Transporter for Creatinine and Species-Dependent Renal Tubular Expression

Discrepancies among equations to estimate the glomerular filtration rate for drug dosing decision making in aged patients: a cross sectional study

BACKGROUND

Patients often require adjustments to drug doses due to impaired renal function. Glomerular filtration rate (GFR) estimation using various equations can result in discrepancies, potentially leading to different dose adjustment recommendations.

AIM

To determine the clinical significance of discrepancies observed between different equations used to estimate GFR for drug dose adjustments in a real-world group of patients over 65 years in primary care.

METHOD

The Cockcroft-Gault (CG), Modification of Diet in Renal Disease (MDRD), Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), and Berlin Initiative Study 1 equations were applied to estimate GFR in a group of patients over 65 years old attending a primary care center. Results were compared using Bland-Altman plots, and limits of agreement (LoA) and overall bias were calculated. Regression analyses were conducted to identify the null difference GFR and the slope of differences for each pairwise comparison.

RESULTS

A total of 1886 patients were analyzed. Differences between patient-adjusted and body surface area (BSA)-normalized versions of the equations were not clinically relevant for dose adjustments, with LoAs below 20 mL/min. However, discrepancies among the original versions of several equations presented LoAs over 30 mL/min. Greater differences were found between CG and MDRD or CKD-EPI equations.

CONCLUSION

Clinically relevant differences in GFR estimation were observed among different equations, potentially impacting drug dose adjustments. However, discrepancies were not considered significant when comparing patient-adjusted and BSA-normalized versions of the equations, particularly for patients with BSA close to the average.

Genome-wide association study of serum magnesium in type 2 diabetes

People with type 2 diabetes have a tenfold higher prevalence of hypomagnesemia, which is suggested to be caused by low dietary magnesium intake, medication use, and genetics. This study aims to identify the genetic loci that influence serum magnesium concentration in 3466 people with type 2 diabetes. The GWAS models were adjusted for age, sex, eGFR, and HbA1c. Associated traits were identified using publicly available data from GTEx consortium, a human kidney eQTL atlas, and the Open GWAS database. The GWAS identified a genome-wide significant locus in TAF3 (p = 2.9 × 10-9) in people with type 2 diabetes. In skeletal muscle, loci located in TAF3 demonstrate an eQTL link to ATP5F1C, a gene that is involved in the formation of Mg2+-ATP. Serum Mg2+ levels were associated with MUC1/TRIM46 (p = 2.9 × 10-7), SHROOM3 (p = 4.0 × 10-7), and SLC22A7 (p = 1.0 × 10-6) at nominal significance, which is in combination with the eQTL data suggesting that they are possible candidates for renal failure. Several genetic loci were in agreement with previous genomic studies which identified MUC1/TRIM46 (Pmeta = 6.9 × 10-29, PQ = 0.81) and SHROOM3 (Pmeta = 2.9 × 10-27, PQ = 0.04) to be associated with serum Mg2+ in the general population. In conclusion, serum magnesium concentrations are associated with genetic variability around the regions of TAF3, MUC1/TRIM46, SHROOM3, and SLC22A7 in type 2 diabetes.

Significance of Organic Anion Transporter 2 and Organic Cation Transporter 2 in Creatinine Clearance: Mechanistic Evaluation Using Freshly Prepared Human Primary Renal Proximal Tubule Cells

Creatinine, a clinical marker for kidney function, is predominantly cleared by glomerular filtration, with active tubular secretion contributing to about 30% of its renal clearance. Recent studies suggested the potential involvement of organic anion transporter (OAT)2, in addition to the previously known organic cation transporter (OCT)2-mediated basolateral uptake, in creatinine active secretion. Here we characterized the transport mechanisms of creatinine using transfected human embryonic kidney (HEK)293 cells and freshly prepared human primary renal proximal tubule epithelial cells (hPTCs). Creatinine showed transport by OAT2 in transfected HEK293 cells. In addition, both creatinine and metformin showed transport by OCT2 and multidrug and toxin extrusion pump (MATE)1 and MATE2K, while penciclovir was selective for OAT2. Time-dependent cell accumulation was observed for creatinine and metformin in hPTCs. Their accumulation was increased by pyrimethamine but inhibited by decynium-22, likely due to differential inhibition of OCT2 versus MATEs. Additionally, indomethacin (an OAT2 inhibitor) reduced penciclovir uptake (∼75%) in hPTCs illustrating functional OAT2 activity. However, no modulation of creatinine and metformin cell accumulation was apparent with indomethacin. Creatinine transport characteristics in the presence of inhibitors approached those of metformin, an OCT2/MATE substrate, but were distinct from those of penciclovir, an OAT2-selective substrate. Moreover, indomethacin showed no significant effect on the basolateral-to-apical transport and net secretion of creatinine across hPTC monolayers. Collectively, the functional studies suggest OCT2 as the primary basolateral uptake mechanism and that OAT2 has a minimal role, in creatinine renal secretion. Our results highlight the utility of hPTCs to enable the functional assessment of renal transport mechanisms. SIGNIFICANCE STATEMENT: Our results obtained with primary hPTCs indicate that OCT2/MATE (vs. OAT2) play a major role in the active renal secretion of creatinine. Quantitative pharmacokinetic models should therefore focus on OCT2/MATE when describing serum creatinine and creatinine clearance modulation by inhibitor drugs and genotype- or disease-related activity changes. The present study highlights the utility of freshly isolated hPTCs to support solute carrier phenotyping to enable the functional assessment of renal transport mechanisms.

Emerging role of the crosstalk between gut microbiota and liver metabolome of subterranean herbivores in response to toxic plants

Plant secondary metabolites (PSMs) are a defense mechanism against herbivores, which in turn use detoxification metabolism to process ingested and absorbed PSMs. The feeding environment can cause changes in liver metabolism patterns and the gut microbiota. Here, we compared gut microbiota and liver metabolome to investigate the response mechanism of plateau zokors (Eospalax baileyi) to toxic plant Stellera chamaejasme (SC) in non-SC and SC grassland (-SCG and +SCG). Our results indicated that exposure to SC in the -SCG population increased liver inflammatory markers including prostaglandin (PG) in the Arachidonic acid pathway, while exposure to SC in the +SCG population exhibited a significant downregulation of PGs. Secondary bile acids were significantly downregulated in +SCG plateau zokors after SC treatment. Of note, the microbial taxa Veillonella in the -SCG group was significantly correlated with liver inflammation markers, while Clostridium innocum in the +SCG group had a significant positive correlation with secondary bile acids. The increase in bile acids and PGs can lead to liver inflammatory reactions, suggesting that +SCG plateau zokors may mitigate the toxicity of SC plants by reducing liver inflammatory markers including PGs and secondary bile acids, thereby avoiding liver damage. This provides new insight into mechanisms of toxicity by PSMs and counter-mechanisms for toxin tolerance by herbivores.

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

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

Biological Distribution after Oral Administration of Radioiodine-Labeled Acetaminophen to Estimate Gastrointestinal Absorption Function via OATPs, OATs, and/or MRPs

We evaluated the whole-body distribution of orally-administered radioiodine-125 labeled acetaminophen (¹²⁵I-AP) to estimate gastrointestinal absorption of anionic drugs. ¹²⁵I-AP was added to human embryonic kidney (HEK)293 and Flp293 cells expressing human organic anion transporting polypeptide (OATP)1B1/3, OATP2B1, organic anion transporter (OAT)1/2/3, or carnitine/organic cation transporter (OCTN)2, with and without bromosulfalein (OATP and multidrug resistance-associated protein (MRP) inhibitor) and probenecid (OAT and MRP inhibitor). The biological distribution in mice was determined by oral administration of ¹²⁵I-AP with and without bromosulfalein and by intravenous administration of ¹²⁵I-AP. The uptake of ¹²⁵I-AP was significantly higher in HEK293/OATP1B1, OATP1B3, OATP2B1, OAT1, and OAT2 cells than that in mock cells. Bromosulfalein and probenecid inhibited OATP- and OAT-mediated uptake, respectively. Moreover, ¹²⁵I-AP was easily excreted in the urine when administered intravenously. The accumulation of ¹²⁵I-AP was significantly lower in the blood and urinary bladder of mice receiving oral administration of both ¹²⁵I-AP and bromosulfalein than those receiving only ¹²⁵I-AP, but significantly higher in the small intestine due to inhibition of OATPs and/or MRPs. This study indicates that whole-body distribution after oral ¹²⁵I-AP administration can be used to estimate gastrointestinal absorption in the small intestine via OATPs, OATs, and/or MRPs by measuring radioactivity in the urinary bladder.

The Role of Solute Carrier Transporters in Efficient Anticancer Drug Delivery and Therapy

Transporter-mediated drug resistance is a major obstacle in anticancer drug delivery and a key reason for cancer drug therapy failure. Membrane solute carrier (SLC) transporters play a crucial role in the cellular uptake of drugs. The expression and function of the SLC transporters can be down-regulated in cancer cells, which limits the uptake of drugs into the tumor cells, resulting in the inefficiency of the drug therapy. In this review, we summarize the current understanding of low-SLC-transporter-expression-mediated drug resistance in different types of cancers. Recent advances in SLC-transporter-targeting strategies include the development of transporter-utilizing prodrugs and nanocarriers and the modulation of SLC transporter expression in cancer cells. These strategies will play an important role in the future development of anticancer drug therapies by enabling the efficient delivery of drugs into cancer cells.

Characterization of the renal tubular transport of creatinine by activity-based protein profiling and transport kinetics

Serum creatinine is widely used to adjust the dosing of drugs eliminated by the kidney in patients with renal dysfunction, as it is a readily accessible indicator of kidney function. However, there are many limitations for drug dosage adjustment based on serum creatinine levels, one of which is the limited understanding of creatinine's tubular transport. Thus, we aimed to complement and advance the renal tubular transport of creatinine by activity-based protein profiling (ABPP) and transporter-overexpression technology. Renal tubular transporters were not identified via ABPP due to the low-affinity interaction between transporters and creatinine. The uptake of isotopically labeled d3-creatinine was significantly increased in OCT2-overexpressing cell lines (p<0.01), and the K_m and V_max of d3-creatinine uptake mediated by OCT2 was 3.1 mM and 408 pmol/mg protein/min, respectively. In the OCT2-overexpressing cell lines, the IC50 of creatinine for d3-creatinine uptake was 10.3 mM, and that of the OCT2 inhibitor cimetidine for d3-creatinine uptake was 99.04 μM. Different dosages of creatinine did not affect the renal excretion of d3-creatinine in mice (p>0.05), while cimetidine significantly reduced the renal excretion of d3-creatinine (p<0.01) without affecting the glomerular filtration rate. Molecular docking in silico showed that the OCT2 amino acid GLN242 could form a hydrogen bond of 2.5 Å with creatinine, and there may be a π-π interaction between TYR362 and creatinine. A site mutation experiment demonstrated that TYR362 and GLN242 were important sites for the OCT2-creatinine interaction. These results demonstrate that OCT2 mediates the renal tubular secretion of creatinine with low affinity and is a minor contributor to creatinine secretion.

Immunohistochemical Localization of Alogliptin, a DPP-4 Inhibitor, in Tissues of Normal and Type 2 Diabetes Model Rat

We investigated the pharmacokinetics of alogliptin (AG) at the cell and tissue level in healthy Wistar rats and a type 2 diabetic Goto-Kakizaki (GK) rat model. Immunohistochemistry of the renal tissue in these rats, post 1 hr of AG administration, showed that the signal was observed in the glomeruli, proximal tubule S3 segments, distal tubules, collecting ducts, and only in the brush border of the epithelial cells of the proximal tubule S1, S2 segments. After 6 hr of AG administration, the staining intensity of the regions other than the S3 segments was considerably reduced in Wistar rats, with no change observed in GK rats. At 24 hr, the staining intensity was considerably reduced, even in GK rats; however, the staining of the S3 segment remained unaltered in both. Hepatocytes in zone III of the hepatic lobule were more intensely stained than those in zone I in Wistar rats at 1 hr. However, almost no staining was observed in the hepatocytes of GK rats at 1 hr. Complete loss of signal was observed in the hepatocytes of the Wistar rats after 6 hr. This study revealed that the pharmacokinetics of AG in GK rats are different from those in Wistar rats.

Time to Rethink the Current Paradigm for Assessing Kidney Function in Drug Development and Beyond

Chronic kidney disease (CKD) is an important health issue that affects ~ 9.1% of the world adult population. Serum creatinine is the most commonly used biomarker for assessing kidney function and is utilized in different equations for estimating creatinine clearance or glomerular filtration rate (GFR). The Cockcroft-Gault formula for adults and "original" Schwartz formula for children have been the most commonly used equations for estimating kidney function during the last 3-4 decades. Introduction of standardized serum creatinine bioanalytical methodology has reduced interlaboratory variability but is not intended to be used with Cockcroft-Gault or original Schwartz equations. More accurate equations (for instance, Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) for adults and bedside Schwartz or Chronic Kidney Disease in Children Schwartz equation for children) based on standardized serum creatinine values (and another biomarker-cystatin C) have been introduced and validated in recent years. Recently, the CKD-EPI equation refitted without a race variable was introduced. Clinical practice guidance in nephrology advocates a shift to these equations for managing health care of patients with CKD. The guidance also recommends use of albuminuria in addition to GFR for CKD diagnosis and management. Significant research with large data sets would be necessary to evaluate whether this paradigm would also be valuable in drug dose adjustments. This article attempts to highlight some important advancements in the field from a clinical pharmacology perspective and is a call to action to industry, regulators, and academia to rethink the current paradigm for assessing kidney function to enable dose recommendation in patients with CKD.

Identification of Organic Anion Transporter 2 Inhibitors: Screening, Structure-Based Analysis, and Clinical Drug Interaction Risk Assessment

Organic anion transporter 2 (OAT2 or SLC22A7) plays an important role in the hepatic uptake and renal secretion of several endogenous compounds and drugs. The goal of this work is to understand the structure activity of OAT2 inhibition and assess clinical drug interaction risk. A single-point inhibition assay using OAT2-transfected HEK293 cells was employed to screen about 150 compounds; and concentration-dependent inhibition potency (IC₅₀) was measured for the identified "inhibitors". Acids represented about 65% of all inhibitors, and the frequency of bases-plus-zwitterions approximately doubled for "non-inhibitors". Interestingly, 9 of 10 most potent inhibitors (low IC₅₀) are acids (pK_a ∼ 3-5). Additionally, inhibitors are significantly larger and lipophilic than non-inhibitors. In silico (binary) models were developed to identify inhibitors and non-inhibitors. Finally, in vivo risk assessed via static drug-drug interaction models identified several inhibitors with potential for renal and hepatic OAT2 inhibition at clinical doses. This is the first study assessing the global pattern of OAT2-ligand interactions.

Molecular Basis Underlying Hepatobiliary and Renal Excretion of Phenolic Acids of Salvia miltiorrhiza Roots (Danshen)

Phenolic acids are cardiovascular constituents (originating from the Chinese medicinal herb Salvia miltiorrhiza root/Danshen) of DanHong and many other Danshen-containing injections. Our earlier pharmacokinetic investigation of DanHong suggested that hepatic and/or renal uptake of the Danshen compounds was the crucial steps in their systemic elimination. This investigation was designed to survey the molecular basis underlying hepatobiliary and renal excretion of the Danshen compounds, i.e., protocatechuic acid, tanshinol, rosmarinic acid, salvianolic acid D, salvianolic acid A, lithospermic acid, and salvianolic acid B. A large battery of human hepatic and renal transporters were screened for transporting the Danshen compounds and then characterized for the uptake kinetics and also compared with associated rat transporters. The samples were analyzed by liquid chromatography/mass spectrometry. Because the Danshen phenolic acids are of poor or fairly good membrane permeability, their elimination via the liver or kidneys necessitates transporter-mediated hepatic or renal uptake from blood. Several human transporters were found to mediate hepatic and/or renal uptake of the Danshen compounds in a compound-molecular-mass-related manner. Lithospermic acid and salvianolic acid B (both >500 Da) underwent systemic elimination, initiated by organic anion-transporting polypeptide (OATP)1B1/OATP1B3-mediated hepatic uptake. Rosmarinic acid and salvianolic acids D (350–450 Da) underwent systemic elimination, initiated by OATP1B1/OATP1B3/organic anion transporter (OAT)2-mediated hepatic uptake and by OAT1/OAT2-mediated renal uptake. Protocatechuic acid and tanshinol (both <200 Da) underwent systemic elimination, initiated by OAT1/OAT2-mediated renal uptake and OAT2-mediated hepatic uptake. A similar scenario was observed with the rat orthologs. The investigation findings advance our understanding of the disposition of the Danshen phenolic acids and could facilitate pharmacokinetic research on other Danshen-containing injections.

Transporters in drug development: International transporter consortium update on emerging transporters of clinical importance

During its 4^th transporter workshop in 2021, the International Transporter Consortium (ITC) provided updates on emerging clinically relevant transporters for drug development. Previously highlighted and new transporters were considered based on up-to-date clinical evidence of their importance in drug-drug interactions and potential for altered drug efficacy and safety, including drug-nutrient interactions leading to nutrient deficiencies. For the first time, folate transport pathways (PCFT, RFC, and FRα) were examined in-depth as a potential mechanism of drug-induced folate deficiency and related toxicities (e.g., neural tube defects, megaloblastic anemia). However, routine toxicology studies conducted in support of drug development appear sufficient to flag such folate deficiency toxicities, while prospective prediction from in vitro folate metabolism and transport inhibition is not well enough established to inform drug development. Previous suggestion of retrospective study of intestinal OATP2B1 inhibition to explain unexpected decreases in drug exposure were updated. Furthermore, when the absorption of a new molecular entity is more rapid and extensive than can be explained by passive permeability, evaluation of OATP2B1 transport may be considered. Emerging research on hepatic and renal OAT2 is summarized, but current understanding of the importance of OAT2 was deemed insufficient to justify specific consideration for drug development. Hepatic, renal, and intestinal MRPs (MRP2, MRP3, MRP4) were revisited. MRPs may be considered when they are suspected to be the major determinant of drug disposition (e.g., direct glucuronide conjugates); MRP2 inhibition as a mechanistic explanation for drug-induced hyperbilirubinemia remains justified. There were no major changes in recommendations from previous ITC whitepapers.

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.

Inhibitory effects of indoxyl sulfate and creatinine on the renal transport of meropenem and biapenem in rats

Carbapenem antibiotics are excreted preferentially in the urine after intravenous administration, with organic anion transporters (OATs) known to be involved in the renal tubular secretion of carbapenem antibiotics. Various uremic toxins (UTs) accumulate in the blood of patients with end-stage renal failure, and some UTs such as indoxyl sulfate (IS) and creatinine (Cr) are excreted in the urine via OATs. However, information about the possible interactions between these UTs and carbapenems in the renal secretion remains limited. In this study, we investigated the effects of IS and Cr on the renal transport of anionic meropenem and zwitterionic biapenem by using rat renal cortical slices. The uptake of meropenem and biapenem in the renal cortical slices was significantly decreased in the presence of 0.1 mM IS or 1 mM Cr. When biapenem and Cr were co-administered to rats intravenously, biapenem clearance from the plasma was clearly retarded, reflecting the current in vitro results. However, IS and Cr exerted no inhibitory effect on the uptake of metformin, a substrate of renal organic cation transporter (OCT) 2, in the renal cortical slices. Thus, our findings indicate that IS and Cr interfere with the renal secretion of carbapenem antibiotics by preferentially inhibiting OATs.

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

Endogenous biomarkers are emerging to advance clinical drug‐drug interaction (DDI) risk assessment in drug development. Twelve healthy subjects received a multidrug and toxin exclusion protein (MATE) inhibitor (pyrimethamine, 10, 25, and 75 mg) in a crossover fashion to identify an appropriate endogenous biomarker to assess MATE1/2‐K‐mediated DDI in the kidneys. Metformin (500 mg) was also given as reference probe drug for MATE1/2‐K. In addition to the previously reported endogenous biomarker candidates (creatinine and N¹‐methylnicotinamide (1‐NMN)), N¹‐methyladenosine (m¹A) was included as novel biomarkers. 1‐NMN and m¹A presented as superior MATE1/2‐K biomarkers since changes in their renal clearance (CL_r) along with pyrimethamine dose were well‐correlated with metformin CL_r changes. The CL_r of creatinine was reduced by pyrimethamine, however, its changes poorly correlated with metformin CL_r changes. Nonlinear regression analysis (CL_r vs. mean total concentration of pyrimethamine in plasma) yielded an estimate of the inhibition constant (K_i) of pyrimethamine and the fraction of the clearance pathway sensitive to pyrimethamine. The in vivoK_i value thus obtained was further converted to unbound Ki using plasma unbound fraction of pyrimethamine, which was comparable to the in vitroK_i for MATE1 (1‐NMN) and MATE2‐K (1‐NMN and m¹A). It is concluded that 1‐NMN and m¹A CL_r can be leveraged as quantitative MATE1/2‐K biomarkers for DDI risk assessment in healthy volunteers.

Current progress in identifying endogenous biomarker candidates for drug transporter phenotyping and their potential application to drug development

Drug transporters play important roles in the elimination of various compounds from the blood. Genetic variation and drug-drug interactions underlie the pharmacokinetic differences for the substrates of drug transporters. Some endogenous substrates of drug transporters have emerged as biomarkers to assess differences in drug transporter activity-not only in animals, but also in humans. Metabolomic analysis is a promising approach for identifying such endogenous substrates through their metabolites. The appropriateness of metabolites is supported by studies in vitro and in vivo, both in animals and through pharmacogenomic or drug-drug interaction studies in humans. This review summarizes current progress in identifying such endogenous biomarkers and applying them to drug transporter phenotyping.

Lack of Drug-Drug Interaction Between Cimetidine, a Renal Transporter Inhibitor, and Imeglimin, a Novel Oral Antidiabetic Drug, in Healthy Volunteers

BACKGROUND AND OBJECTIVE: Imeglimin is a novel oral antidiabetic drug to treat type 2 diabetes, targeting the mitochondrial bioenergetics. In vitro, imeglimin was shown to be a substrate of human multidrug and toxic extrusion transporters MATE1 and MATE2-K and organic cation transporters OCT1 and OCT2. The objective of the study was to assess the potential drug-drug interaction between imeglimin and cimetidine, a reference inhibitor of these transporters.

METHODS

A phase 1 study was carried out in 16 subjects who received a single dose of 1500 mg imeglimin alone on day 1 followed by a 6-day treatment (day 5 to day 10) with cimetidine 400 mg twice daily. On day 8, a single dose of imeglimin was co-administered with cimetidine. Blood and urine samples were collected up to 72 h after each imeglimin administration. Pharmacokinetic parameters were determined using non-compartmental methods.

RESULTS

Imeglimin maximum plasma concentration (C_max) and area under the plasma concentration-time curve (AUC) were 1.3-fold [90% CI (1.12-1.62) and (1.10-1.46) for C_max and AUC_0-last, respectively] higher when imeglimin was co-administered with cimetidine but this increase was not considered clinically relevant. This increase could be mainly explained by a reduction in renal elimination, mediated through the cimetidine inhibition of renal MATE1 transporter. Imeglimin taken alone or with cimetidine was safe and well tolerated in all subjects.

CONCLUSIONS

No clinically significant drug-drug interaction exists between imeglimin and cimetidine, a reference inhibitor of MATE1, MATE2-K, OCT1 and OCT2 transporters.

CLINICAL TRIAL REGISTRATION

EudraCT 2018-001103-36.

Potent Therapy and Transcriptional Profile of Combined Erythropoietin-Derived Peptide Cyclic Helix B Surface Peptide and Caspase-3 siRNA against Kidney Ischemia/Reperfusion Injury in Mice

Cause-specific treatment and timely diagnosis are still not available for acute kidney injury (AKI) apart from supportive therapy and serum creatinine measurement. A novel erythropoietin-derived cyclic helix B surface peptide (CHBP) protects kidneys against AKI with different causes, but the underlying mechanism is not fully defined. Herein, we investigated the transcriptional profile of renoprotection induced by CHBP and its potential synergistic effects with siRNA targeting caspase-3, an executing enzyme of apoptosis and inflammation (CASP3siRNA), on ischemia/reperfusion (IR)-induced AKI. Utilizing a mouse model with 30-minute renal bilateral ischemia and 48-hour reperfusion, the renoprotection of CHBP or CASP3siRNA was demonstrated in renal function and structure, active caspase-3 and HMGB1 expression. Combined treatment of CHBP and CASP3siRNA further preserved kidney structure and reduced active caspase-3 and HMGB1. Furthermore, differentially expressed genes (DEGs) were identified with fold change >1.414 and P < 0.05. In IR kidneys, 281 DEGs induced by CHBP were mainly involved in promoting cell division and improving cellular function and metabolism (upregulated signal transducer and activator of transcription 5B and solute carrier family 22 member 7). The additional administration of CASP3siRNA caused 504 and 418 DEGs in IR + CHBP kidneys with or without negative control small-interfering RNA, with 37 genes in common. These DEGs were associated with modulated apoptosis and inflammation (upregulated BCL6, SLPI, and SERPINA3M) as well as immunity, injury, and microvascular homeostasis (upregulated complement factor H and GREM1 and downregulated ANGPTL2). This proof-of-effect study indicated the potent renoprotection of CASP3siRNA upon CHBP at the early stage of IR-induced AKI. Underlying genes, BCL6, SLPI, SERPINA3M, GREM1, and ANGPTL2, might be potential new biomarkers for clinical applications. SIGNIFICANCE STATEMENT: It is imperative to explore new strategies of cause-specific treatment and timely diagnosis for acute kidney injury (AKI). CHBP and CASP3siRNA synergistically protected kidney structure after 48-hour ischemia/reperfusion-induced AKI with reduced injury mediators CASP3 and high mobility group box 1. CHBP upregulated cell division-, function-, and metabolism-related genes, whereas CASP3siRNA further regulated immune response- and tissue homeostasis-associated genes. Combined CHBP and CASP3siRNA might be a potent and specific treatment for AKI, and certain dysregulated genes secretory leukocyte peptidase inhibitor and SERPINA3M could facilitate timely diagnosis.

Kidney Transporters and Drug-Induced Injury in Drug Development

Influx and efflux kidney tubular transporters are major determinants of the disposition of xenobiotics, including pharmaceutical drugs. On the basolateral membrane of proximal tubular cells, there are influx transporters, such as organic cation transporters. On the apical membrane of proximal tubular cells, there are efflux transporters, such as multidrug and toxin extrusion proteins. The secretion process across the apical membrane into the lumen occurs via efflux transporters which plays an important role in serum creatinine (sCr) elimination in urine. The interference of a pharmaceutical drug with transporters can lead to changes in sCr with no alterations in biomarkers or light microscopic evidence indicative of renal injury. Identification of transporters that influence drug disposition, toxicity, and overall nonclinical safety assessment is important in drug discovery and development programs. This mini review describes some key aspects of kidney tubular transporters and drug-induced renal toxicities in safety risk assessment and drug development.

Novel insights into the organic solute transporter alpha/beta, OSTα/β: From the bench to the bedside

Organic solute transporter alpha/beta (OSTα/β) is a heteromeric solute carrier protein that transports bile acids, steroid metabolites and drugs into and out of cells. OSTα/β protein is expressed in various tissues, but its expression is highest in the gastrointestinal tract where it facilitates the recirculation of bile acids from the gut to the liver. Previous studies established that OSTα/β is upregulated in liver tissue of patients with extrahepatic cholestasis, obstructive cholestasis, and primary biliary cholangitis (PBC), conditions that are characterized by elevated bile acid concentrations in the liver and/or systemic circulation. The discovery that OSTα/β is highly upregulated in the liver of patients with nonalcoholic steatohepatitis (NASH) further highlights the clinical relevance of this transporter because the incidence of NASH is increasing at an alarming rate with the obesity epidemic. Since OSTα/β is closely linked to the homeostasis of bile acids, and tightly regulated by the nuclear receptor farnesoid X receptor, OSTα/β is a potential drug target for treatment of cholestatic liver disease, and other bile acid-related metabolic disorders such as obesity and diabetes. Obeticholic acid, a semi-synthetic bile acid used to treat PBC, under review for the treatment of NASH, and in development for the treatment of other metabolic disorders, induces OSTα/β. Some drugs associated with hepatotoxicity inhibit OSTα/β, suggesting a possible role for OSTα/β in drug-induced liver injury (DILI). Furthermore, clinical cases of homozygous genetic defects in both OSTα/β subunits resulting in diarrhea and features of cholestasis have been reported. This review article has been compiled to comprehensively summarize the recent data emerging on OSTα/β, recapitulating the available literature on the structure-function and expression-function relationships of OSTα/β, the regulation of this important transporter, the interaction of drugs and other compounds with OSTα/β, and the comparison of OSTα/β with other solute carrier transporters as well as adenosine triphosphate-binding cassette transporters. Findings from basic to more clinically focused research efforts are described and discussed.

A Novel Physiologically Based Model of Creatinine Renal Disposition to Integrate Current Knowledge of Systems Parameters and Clinical Observations

Creatinine is the most common clinical biomarker of renal function. As a substrate for renal transporters, its secretion is susceptible to inhibition by drugs, resulting in transient increase in serum creatinine and false impression of damage to kidney. Novel physiologically based models for creatinine were developed here and (dis)qualified in a stepwise manner until consistency with clinical data. Data from a matrix of studies were integrated, including systems data (common to all models), proteomics-informed in vitro-in vivo extrapolation of all relevant transporter clearances, exogenous administration of creatinine (to estimate endogenous synthesis rate), and inhibition of different renal transporters (11 perpetrator drugs considered for qualification during creatinine model development and verification on independent data sets). The proteomics-informed bottom-up approach resulted in the underprediction of creatinine renal secretion. Subsequently, creatinine-trimethoprim clinical data were used to inform key model parameters in a reverse translation manner, highlighting best practices and challenges for middle-out optimization of mechanistic models.

Systems Biology Analysis Reveals Eight SLC22 Transporter Subgroups, Including OATs, OCTs, and OCTNs

The SLC22 family of OATs, OCTs, and OCTNs is emerging as a central hub of endogenous physiology. Despite often being referred to as “drug” transporters, they facilitate the movement of metabolites and key signaling molecules. An in-depth reanalysis supports a reassignment of these proteins into eight functional subgroups, with four new subgroups arising from the previously defined OAT subclade: OATS1 (SLC22A6, SLC22A8, and SLC22A20), OATS2 (SLC22A7), OATS3 (SLC22A11, SLC22A12, and Slc22a22), and OATS4 (SLC22A9, SLC22A10, SLC22A24, and SLC22A25). We propose merging the OCTN (SLC22A4, SLC22A5, and Slc22a21) and OCT-related (SLC22A15 and SLC22A16) subclades into the OCTN/OCTN-related subgroup. Using data from GWAS, in vivo models, and in vitro assays, we developed an SLC22 transporter-metabolite network and similar subgroup networks, which suggest how multiple SLC22 transporters with mono-, oligo-, and multi-specific substrate specificity interact to regulate metabolites. Subgroup associations include: OATS1 with signaling molecules, uremic toxins, and odorants, OATS2 with cyclic nucleotides, OATS3 with uric acid, OATS4 with conjugated sex hormones, particularly etiocholanolone glucuronide, OCT with neurotransmitters, and OCTN/OCTN-related with ergothioneine and carnitine derivatives. Our data suggest that the SLC22 family can work among itself, as well as with other ADME genes, to optimize levels of numerous metabolites and signaling molecules, involved in organ crosstalk and inter-organismal communication, as proposed by the remote sensing and signaling theory.

A pharmaceutical industry perspective on microphysiological kidney systems for evaluation of safety for new therapies

The human kidney contains approximately one million nephrons. As the functional unit of the kidney, the nephron affords an opportunity to approximate the kidney at a microphysiological scale. Recent emergence of physiologically accurate human tissue models has radically advanced the possibilities of mimicking organ biology and multi-organ combinations in vitro. Anatomically, the nephron is one of the most complex, sequentially integrated microfluidic units in the body making the miniaturized microfluidic systems excellent candidates for capturing the kidney biology in vitro. While these models are promising, there are a number of considerations for practical implementation into a drug development paradigm. Opportunities for pharmaceutical industry applications of new MPS models often start with drug safety testing. As such, the intent of this article is to focus on safety and ADME applications. This article reviews biological functions of the kidney and options for characterizing known roles in nephrotoxicity. The concept of "context-of-use" is introduced as a framework for describing and verifying the specific features of an MPS platform for use in drug development. Overall, we present a perspective on key attributes of microphysiological kidney models, which the pharmaceutical industry could leverage to improve confident safety and ADME evaluations of experimental therapies.

The Importance of Tubular Function in Chronic Kidney Disease

Glomerular filtration rate (GFR) and proteinuria-albuminuria are the renal functional parameters currently used to evaluate chronic kidney disease (CKD) severity. However, tubular secretion is another important renal functional parameter to be taken into account since proximal tubule (PT) secretion, in particular, is a crucial renal mechanism for endogenous organic cations, anions and drug elimination. The residual diuresis is a relevant survival predictor in patients on dialysis, since their urine is produced by the glomerular and tubular functions. It has been hypothesized that drugs which up-regulate some renal tubular transporters could contribute to uremic toxin excretion, and nephroprevention. However, if tubular transporters' down-regulation observed in CKD patients and experimental models is a PT adaptation to avoid intracellular accumulation and damage from uremic toxins, consequently the increase of toxin removal by inducing tubular transporters' up-regulation could be deleterious to the kidney. Therefore, a deeper understanding of this phenomenon is currently needed. In conclusion, tubular function has an important role for endogenous organic cations, anions and drug excretion in CKD patients, and a deeper understanding of its multiple mechanisms could provide new therapeutic alternatives in this population.

FOXM1 drives proximal tubule proliferation during repair from acute ischemic kidney injury

The proximal tubule has a remarkable capacity for repair after acute injury, but the cellular lineage and molecular mechanisms underlying this repair response are incompletely understood. Here, we developed a Kim1-GFPCreERt2 knockin mouse line (Kim1-GCE) in order to perform genetic lineage tracing of dedifferentiated cells while measuring the cellular transcriptome of proximal tubule during repair. Acutely injured genetically labeled clones coexpressed KIM1, VIMENTIN, SOX9, and KI67, indicating a dedifferentiated and proliferative state. Clonal analysis revealed clonal expansion of Kim1+ cells, indicating that acutely injured, dedifferentiated proximal tubule cells, rather than fixed tubular progenitor cells, account for repair. Translational profiling during injury and repair revealed signatures of both successful and unsuccessful maladaptive repair. The transcription factor Foxm1 was induced early in injury, was required for epithelial proliferation in vitro, and was dependent on epidermal growth factor receptor (EGFR) stimulation. In conclusion, dedifferentiated proximal tubule cells effect proximal tubule repair, and we reveal an EGFR/FOXM1-dependent signaling pathway that drives proliferative repair after injury.

Renal Excretion of Dabigatran: The Potential Role of Multidrug and Toxin Extrusion (MATE) Proteins

Following oral administration, dabigatran etexilate (DABE) is rapidly hydrolyzed to its active form, dabigatran. DABE, but not dabigatran, presents as a P-glycoprotein (P-gp) substrate and has increasingly been used as a probe drug. Therefore, although dosed as DABE, a P-gp drug-drug interaction (DDI) is reported as a dabigatran plasma concentration ratio (perpetrator versus placebo). Because the majority of a DABE dose (80 to 85%) is recovered in urine as unchanged dabigatran (renal active secretion is ∼25% of total clearance), dabigatran was evaluated in vitro as a substrate of various human renal transporters. Active (pyrimethamine-sensitive) dabigatran uptake was observed with human embryonic kidney (HEK) 293 cells expressing multidrug and toxin extrusion protein 1 (MATE1) and 2K (MATE2K), with Michaelis-Menten constant (K_m) values of 4.0 and 8.0 μM, respectively. By comparison, no uptake of 2 μM dabigatran (versus mock-transfected HEK293 cells) was evident with HEK293 cells transfected with organic cation transporters (OCT1 and OCT2) and organic anion transporters (OAT1, 2, 3, and 4). The efflux ratios of dabigatran across P-gp- and BCRP (breast cancer resistance protein)-MDCK (Madin-Darby canine kidney) cell monolayers were 1.5 and 2.0 (versus mock-MDCK cell monolayers), suggesting dabigatran is a relatively poor P-gp and BCRP substrate. Three of five drugs (verapamil, ketoconazole, and quinidine) known to interact clinically with dabigatran, as P-gp inhibitors, presented as MATE inhibitors in vitro (IC₅₀ = 1.0 to 25.2 μM). Taken together, although no basolateral transporter was identified for dabigatran, the results suggest that apical MATE1 and MATE2K could play an important role in its renal clearance. MATE-mediated renal secretion of dabigatran needs to be considered when interpreting the results of P-gp DDI studies following DABE administration.

Saliva for assessing creatinine, uric acid, and potassium in nephropathic patients

Background

Lab tests on saliva could be useful because of low invasivity. Previous reports indicated that creatinine, uric acid, and potassium are measurable in saliva. For these analytes the study investigated methodology of saliva tests and correlations between plasma and saliva levels.

Methods

The study enrolled 15 healthy volunteers for methodological analyses and 42 nephropathic patients for plasma-saliva correlations (35 non-dialysis and 7 dialysis). Saliva was collected by synthetic swap right after venipuncture for blood withdrawal. Blood and saliva, unless otherwise indicated, were collected early in the morning after overnight fast and lab tests were performed in fresh samples by automated biochemistry (standard). Methodological analyses included blind duplicates, different collection mouth sites, day-to-day variability, different collection times, and freezing-thawing effects. Analyses on plasma-saliva correlations included post-dialysis changes.

Results

For saliva lab tests of all analytes, blind duplicates, samples from different mouth sites or of different days were not significantly different but were significantly correlated (differences ≤14.4%; R ≥ 0.620, P ≤ 0.01). For all analytes, mid-morning saliva had lower levels than but correlated with standard saliva (differences ≥15.8%; R ≥ 0.728, P ≤ 0.01). Frozen-thawed saliva had lower levels than fresh saliva for uric acid only (− 17.2%, P < 0.001). Frozen-thawed saliva correlated with fresh saliva for all analytes (R ≥ 0.818, P ≤ 0.001). Saliva and plasma levels differed but correlated with plasma for creatinine (R = 0.874, P < 0.001), uric acid (R = 0.821, P < 0.001) and potassium (R = 0.767, P < 0.001). Post-dialysis changes in saliva paralleled post-dialysis changes in plasma.

Conclusion

Saliva levels of creatinine, uric acid, and potassium are measurable and correlated with their plasma levels. Early morning fasting fresh saliva samples are advisable because later collection times or freezing lower the saliva levels of these analytes.

The SLC22 Transporter Family: A Paradigm for the Impact of Drug Transporters on Metabolic Pathways, Signaling, and Disease

The SLC22 transporter family consists of more than two dozen members, which are expressed in the kidney, the liver, and other tissues. Evolutionary analysis indicates that SLC22 transporters fall into at least six subfamilies: OAT (organic anion transporter), OAT-like, OAT-related, OCT (organic cation transporter), OCTN (organic cation/carnitine transporter), and OCT/OCTN-related. Some-including OAT1 [SLC22A6 or NKT (novel kidney transporter)] and OAT3 (SLC22A8), as well as OCT1 (SLC22A1) and OCT2 (SLC22A2)-are widely studied drug transporters. Nevertheless, analyses of knockout mice and other data indicate that SLC22 transporters regulate key metabolic pathways and levels of signaling molecules (e.g., gut microbiome products, bile acids, tricarboxylic acid cycle intermediates, dietary flavonoids and other nutrients, prostaglandins, vitamins, short-chain fatty acids, urate, and ergothioneine), as well as uremic toxins associated with chronic kidney disease. Certain SLC22 transporters-such as URAT1 (SLC22A12) and OCTN2 (SLC22A5)-are mutated in inherited metabolic diseases. A new systems biology view of transporters is emerging. As proposed in the remote sensing and signaling hypothesis, SLC22 transporters, together with other SLC and ABC transporters, have key roles in interorgan and interorganism small-molecule communication and, together with the neuroendocrine, growth factor-cytokine, and other homeostatic systems, regulate local and whole-body homeostasis.

Physiologically-Based Pharmacokinetic Modeling Analysis for Quantitative Prediction of Renal Transporter-Mediated Interactions Between Metformin and Cimetidine

Metformin is an important antidiabetic drug and often used as a probe for drug–drug interactions (DDIs) mediated by renal transporters. Despite evidence supporting the inhibition of multidrug and toxin extrusion proteins as the likely DDI mechanism, the previously reported physiologically‐based pharmacokinetic (PBPK) model required the substantial lowering of the inhibition constant values of cimetidine for multidrug and toxin extrusion proteins from those obtained in vitro to capture the clinical DDI data between metformin and cimetidine.¹ We constructed new PBPK models in which the transporter‐mediated uptake of metformin is driven by a constant membrane potential. Our models successfully captured the clinical DDI data using in vitro inhibition constant values and supported the inhibition of multidrug and toxin extrusion proteins by cimetidine as the DDI mechanism upon sensitivity analysis and data fitting. Our refined PBPK models may facilitate prediction approaches for DDI involving metformin using in vitro inhibition constant values.

Emerging Models of Drug Metabolism, Transporters, and Toxicity

This commentary summarizes expert mini-reviews and original research articles that have been assembled in a special issue on novel models of drug metabolism and disposition. The special issue consists of research articles or reviews on novel static or micro-flow based models of the intestine, liver, eye, and kidney. This issue reviews static intestinal systems like mucosal scrapings and cryopreserved intestinal enterocytes, as well as novel bioengineered or chemically engineered intestinal models derived from primary human tissue, iPSCs, enteroids, and crypts. Experts have reviewed hepatic systems like cryopermeabilized Metmax hepatocytes and longer term, hepatocyte coculture system from HµREL, yielding in vivo-like primary and secondary drug metabolite profiles. Additional liver models, including micropattern hepatocyte coculture, 3D liver spheroids, and microflow systems, applicable to the study of drug disposition and toxicology have also been reviewed. In this commentary, we have outlined expert opinions and current efforts on hepatic- and nephrotoxicity models. Ocular disposition models including corneal permeability models have been included within this special issue. This commentary provides a summary of in vivo mini-reviews of the issue, which have discussed the applications and drawbacks of pig and humanized mice models of P450, UGT, and rat organic anionic transporting polypeptide 1a4. While not extensively reviewed, novel positron emissions tomography imaging-based approaches to study the distribution of xenobiotics have been highlighted. This commentary also outlines in vitro and in vivo models of drug metabolism derived from breakthrough genetic, chromosomal, and tissue engineering techniques. The commentary concludes by providing a futuristic view of the novel models discussed in this issue.

Investigating Aspects of Renal Physiology and Pharmacology in Organ and Organoid Culture

Some aspects of renal physiology, in particular transport across tubular epithelia, are highly relevant to pharmacokinetics and to drug toxicity. The use of animals to model human renal physiology is limited, but human-derived renal organoids offer an alternative, relevant system in culture. Here, we explain how the activity of specific transport systems can be assessed in renal organoid and organ culture, using a system illustrated mainly for mouse but that can be extended to human organoids.

Biomarkers for In Vivo Assessment of Transporter Function

Drug-drug interactions are a major concern not only during clinical practice, but also in drug development. Due to limitations of in vitro-in vivo predictions of transporter-mediated drug-drug interactions, multiple clinical Phase I drug-drug interaction studies may become necessary for a new molecular entity to assess potential drug interaction liabilities. This is a resource-intensive process and exposes study participants, who frequently are healthy volunteers without benefit from study treatment, to the potential risks of a new drug in development. Therefore, there is currently a major interest in new approaches for better prediction of transporter-mediated drug-drug interactions. In particular, researchers in the field attempt to identify endogenous compounds as biomarkers for transporter function, such as hexadecanedioate, tetradecanedioate, coproporphyrins I and III, or glycochenodeoxycholate sulfate for hepatic uptake via organic anion transporting polypeptide 1B or N¹-methylnicotinamide for multidrug and toxin extrusion protein-mediated renal secretion. We summarize in this review the currently proposed biomarkers and potential limitations of the substances identified to date. Moreover, we suggest criteria based on current experiences, which may be used to assess the suitability of a biomarker for transporter function. Finally, further alternatives and supplemental approaches to classic drug-drug interaction studies are discussed.

Liver Receptor Homolog-1 Regulates Organic Anion Transporter 2 and Docetaxel Pharmacokinetics

Organic anion transporter 2 (OAT2/SLC22A7) is an uptake transporter that plays an important role in drug disposition. Here, we investigate a potential role of liver receptor homolog-1 (Lrh-1) in regulation of Oat2 and docetaxel pharmacokinetics. Hepatoma cells (Hepa1-6 and HepG2 cells) were transfected with Lrh-1/LRH-1 expression vector or siRNA. The relative mRNA and protein levels of Oat2/OAT2 in the cells or livers of Lrh-1^hep-/- mice were determined by qPCR and Western blotting, respectively. Transcriptional regulation of Oat2/OAT2 by Lrh-1/LRH-1 was investigated using luciferase reporter, mobility shift, and chromatin immunoprecipitation (ChIP) assays. Pharmacokinetic studies were performed with wild-type (Lrh-1^fl/fl) and Lrh-1^hep-/- mice after intraperitoneal injection of docetaxel. Overexpression of Lrh-1 in Hepa1-6 cells led to significant increases in Oat2 mRNA and protein. Consistently, Lrh-1 knockdown caused decreases in Oat2 mRNA and protein, as well as reduced cellular uptake of PGE2, a prototypical substrate of Oat2. Similarly, an activation effect of LRH-1 on OAT2 expression was observed in HepG2 cells. In addition, the levels of Oat2 mRNA and protein were markedly reduced in Lrh-1^hep-/- mice. Lrh-1/LRH-1 induced the transcription of Oat2/OAT2 in luciferase reporter assays. Truncation analysis revealed a potential Lrh-1 response element (-716- to -702-bp) in Oat2 promoter. Direct binding of Lrh-1 to this response element was confirmed by mobility shift and ChIP assays. Furthermore, systemic exposure of docetaxel was upregulated in Lrh-1^hep-/- mice due to reduced hepatic uptake. In conclusion, Lrh-1 transcriptionally regulates Oat2, thereby impacting tissue uptake and pharmacokinetics of Oat2 substrates.

The role of OAT2 (SLC22A7) in the cyclic nucleotide biokinetics of human erythrocytes

The present study was conducted to characterise the transporter(s) responsible for the uptake of cyclic nucleotides to human erythrocytes. Western blotting showed that hRBC expressed OAT2 (SLC22A7), but detection of OAT1 (SLC22A6), or OAT3 (SLC22A8) was not possible. Intact hRBC were employed to clarify the simultaneous cyclic nucleotide egression and uptake. Both these opposing processes were studied. The K_m‐values for high affinity efflux was 3.5 ± 0.1 and 39.4 ± 5.7 μM for cGMP and cAMP, respectively. The respective values for low affinity efflux were 212 ± 11 and 339 ± 42 μM. The uptake was characterised with apparently low affinity and similar K_m‐values for cGMP (2.2 mM) and cAMP (0.89 mM). Using an iterative approach in order to balance uptake with efflux, the predicted real K_m‐values for uptake were 100–200 μM for cGMP and 50–150 μM for cAMP. The established OAT2‐substrate indomethacin showed a competitive interaction with cyclic nucleotide uptake. Creatinine, also an OAT2 substrate, showed saturable uptake with a K_m of 854 ± 98 μM. Unexpectedly, co‐incubation with cyclic nucleotides showed an uncompetitive inhibition. The observed K_m‐values were 399 ± 44 and 259 ± 30 μM for creatinine, in the presence of cGMP and cAMP, respectively. Finally, the OAT1‐substrate para‐aminohippurate (PAH) showed some uptake (K_m‐value of 2.0 ± 0.4 mM) but did not interact with cyclic nucleotide or indomethacin transport.

The urinary excretion of metformin, ceftizoxime and ofloxacin in high serum creatinine rats: Can creatinine predict renal tubular elimination?

The renal excretion of creatinine and most drugs are the net result of glomerular filtration and tubular secretion, and their tubular secretions are mediated by individual transporters. Thus, we hypothesized that the increase of serum creatinine (SCr) levels attributing to inhibiting tubular transporters but not glomerular filtration rate (GFR) could be used to evaluate the tubular excretion of drugs mediated by identical or partial overlap transporter with creatinine. In this work, we firstly developed the creatinine excretion inhibition model with normal GFR by competitively inhibiting tubular transporters, and investigated the renal excretion of metformin, ceftizoxime and ofloxacin in vivo and in vitro. The results showed that the 24-hour urinary excretion of metformin and ceftizoxime in model rats were decreased by 25% and 17% compared to that in control rats, respectively. The uptake amount and urinary excretion of metformin and ceftizoxime could be inhibited by creatinine in renal cortical slices and isolated kidney perfusion. However, the urinary excretion of ofloxacin was not affected by high SCr. These results showed that the inhibition of tubular creatinine transporters by high SCr resulted to the decrease of urinary excretion of metformin and ceftizoxime, but not ofloxacin, which implied that the increase of SCr could also be used to evaluate the tubular excretion of drugs mediated by identical or partial overlap transporter with creatinine in normal GFR rats.

Quantitative analysis of elevation of serum creatinine via renal transporter inhibition by trimethoprim in healthy subjects using physiologically-based pharmacokinetic model

Serum creatinine (SCr) levels rise during trimethoprim therapy for infectious diseases. This study aimed to investigate whether the elevation of SCr can be quantitatively explained using a physiologically-based pharmacokinetic (PBPK) model incorporating inhibition by trimethoprim on tubular secretion of creatinine via renal transporters such as organic cation transporter 2 (OCT2), OCT3, multidrug and toxin extrusion protein 1 (MATE1), and MATE2-K. Firstly, pharmacokinetic parameters in the PBPK model of trimethoprim were determined to reproduce the blood concentration profile after a single intravenous and oral administration of trimethoprim in healthy subjects. The model was verified with datasets of both cumulative urinary excretions after a single administration and the blood concentration profile after repeated oral administration. The pharmacokinetic model of creatinine consisted of the creatinine synthesis rate, distribution volume, and creatinine clearance (CL_cre), including tubular secretion via each transporter. When combining the models for trimethoprim and creatinine, the predicted increments in SCr from baseline were 29.0%, 39.5%, and 25.8% at trimethoprim dosages of 5 mg/kg (b.i.d.), 5 mg/kg (q.i.d.), and 200 mg (b.i.d.), respectively, which were comparable with the observed values. The present model analysis enabled us to quantitatively explain increments in SCr during trimethoprim treatment by its inhibition of renal transporters.

In vitro characterization of zebrafish (Danio rerio) organic anion transporters Oat2a-e

OATS/Oats are transmembrane proteins that transport a variety of drugs, environmental toxins and endogenous metabolites into the cell. Zebrafish (Danio rerio) has seven OAT orthologs: Oat1, Oat2a-e and Oat3. In this study we specifically address Oat2 (Slc22a7) family. Conserved synteny analysis showed localization of zebrafish oat2 genes on two chromosomes, 11 and 17. All five zebrafish Oats were localized by live cell imaging in membranes of transiently transfected HEK293-T cells, and Oat2a, b, d, and e were confirmed using western blot analysis. Functional studies using the HEK293T cells overexpressing zebrafish Oats revealed two model fluorescent substrates of three Oats: Lucifer yellow for Oat2a and Oat2d (Km 122, and 49.7μM), and 6-carboxyfluorescein for Oat2b and Oat2d (Km 199.7, and 266.9μM). The initial screening of a series of diverse endo- and xenobiotics showed interaction with a number of compounds, including cGMP and diclofenac (IC50 27.74, and 19.14μM) with Oat2a; estrone-3-sulfate and diclofenac (IC50 30.96, and 12.6μM) with Oat2b; and fumarate and indomethacin (IC50 68.24, and 20.41μM) with Oat2d. This study provides the first comprehensive data set on Oat2 in zebrafish and offers an important basis for more detailed molecular and (eco)toxicological characterizations of these transporters.

Beyond drug-drug interactions: effects of transporter inhibition on endobiotics, nutrients and toxins

INTRODUCTION

Membrane transport proteins play a central role in regulating the disposition of endobiotics, dietary nutrients and environmental toxins. The inhibition of transporters by drugs has potential physiologic consequences. The full extent of the effect of drugs on the function of transporters is poorly understood because only a small subset of the hundreds of transporters expressed in humans - primarily those mediating the rate-determining step in the elimination of specific drugs - are assessed during clinical development. Areas covered: We provide a comprehensive overview of literature reports implicating the inhibition of transporters as the mechanism for off-target effects of drugs. Expert opinion: Transporter inhibition, the mechanism of action of many marketed drugs, appears to play an underappreciated role in a number of side effects including vitamin deficiency, edema, dyslipidemia, cholestasis and gout. Cell systems more broadly expressing transporter networks and methods like unbiased metabolomics should be incorporated into the screening paradigm to expand our understanding of the impact of drugs on the physiologic function of transporters and to allow for these effects to be taken into account in drug discovery and clinical practice.