Renal Drug Disposition Research Articles

Three-dimensional Culture of Human Proximal Tubular Epithelial Cells for an in vitro Evaluation of Drug-induced Kidney Injury

Drug-induced kidney injury (DIKI) is the major cause of acute kidney injury (AKI). Renal proximal tubular epithelial cells (RPTECs) are the primary target sites of DIKI and express transporters involved in renal drug disposition. In the present study, we focused on three-dimensionally cultured human RPTECs (3D-RPTECs) with elevated expression of drug transporters to investigate their utility in DIKI evaluation. Intracellular ATP levels in 3D-RPTECs are reduced by tenofovir and cisplatin that are substrates of an organic anion transporter 1 and an organic cation transporter 2, respectively. In addition, 3D-RPTECs were exposed to 17 and 15 drugs that are positive and negative to RPTEC toxicity, respectively, for up to 28 d. The 20 % decreasing concentration of drugs for ATP amount (EC20) was obtained, and the ratio of EC20 values and clinical maximum concentration (Cmax) ≤100 were used as cut-off value to evaluate potential of DIKI. The sensitivities of 3D-RPTECs were 82.4 % and 88.2 % after 7 d and 28 d of drug exposure, respectively, and the specificities were 100 % and 93.3 %, respectively. The predictive performance of 3D-RPTECs was higher than that of two-dimensional cultured RPTECs and the kidney cell line HK-2. In conclusion, 3D-RPTECs are useful for in vitro evaluation of RPTEC injury by measuring intracellular ATP levels.

Improvement of Protein Expression Profile in Three-Dimensional Renal Proximal Tubular Epithelial Cell Spheroids Selected Based on OAT1 Gene Expression: A Potential In Vitro Tool for Evaluating Human Renal Proximal Tubular Toxicity and Drug Disposition.

The proximal tubule plays an important role in the kidney and is a major site of drug interaction and toxicity. Analysis of kidney toxicity via in vitro assays is challenging, because only a few assays that reflect functions of drug transporters in renal proximal tubular epithelial cells (RPTECs) are available. In this study, we aimed to develop a simple and reproducible method for culturing RPTECs by monitoring organic anion transporter 1 (OAT1) as a selection marker. Culturing RPTECs in spherical cellular aggregates increased OAT1 protein expression, which was low in the conventional two-dimensional (2D) culture, to a level similar to that in human renal cortices. By proteome analysis, it was revealed that the expression of representative two proximal tubule markers was maintained and 3D spheroid culture improved the protein expression of approximately 7% of the 139 transporter proteins detected, and the expression of 2.3% of the 4,800 proteins detected increased by approximately fivefold that in human renal cortices. Furthermore, the expression levels of approximately 4,800 proteins in three-dimensional (3D) RPTEC spheroids (for 12 days) were maintained for over 20 days. Cisplatin and adefovir exhibited transporter-dependent ATP decreases in 3D RPTEC spheroids. These results indicate that the 3D RPTEC spheroids developed by monitoring OAT1 gene expression are a simple and reproducible in vitro experimental system with improved gene and protein expressions compared with 2D RPTECs and were more similar to that in human kidney cortices. Therefore, it can potentially be used for evaluating human renal proximal tubular toxicity and drug disposition. SIGNIFICANCE STATEMENT: This study developed a simple and reproducible spheroidal culture method with acceptable throughput using commercially available RPTECs by monitoring OAT1 gene expression. RPTECs cultured using this new method showed improved mRNA/protein expression profiles to those in 2D RPTECs and were more similar to those of human kidney cortices. This study provides a potential in vitro proximal tubule system for pharmacokinetic and toxicological evaluations during drug development.

Quantitative Proteomic Map of Enzymes and Transporters in the Human Kidney: Stepping Closer to Mechanistic Kidney Models to Define Local Kinetics.

The applications of translational modeling of local drug concentrations in various organs had a sharp increase over the last decade. These are part of the model-informed drug development initiative, adopted by the pharmaceutical industry and promoted by drug regulatory agencies. With respect to the kidney, the models serve as a bridge for understanding animal vs. human observations related to renal drug disposition and any consequential adverse effects. However, quantitative data on key drug-metabolizing enzymes and transporters relevant for predicting renal drug disposition are limited. Using targeted and global quantitative proteomics, we determined the abundance of multiple enzymes and transporters in 20 human kidney cortex samples. Nine enzymes and 22 transporters were quantified (8 for the first time in the kidneys). In addition, >4,000 proteins were identified and used to form an open database. CYP2B6, CYP3A5, and CYP4F2 showed comparable, but generally low expression, whereas UGT1A9 and UGT2B7 levels were the highest. Significant correlation between abundance and activity (measured by mycophenolic acid clearance) was observed for UGT1A9 (Rs=0.65, P=0.004) and UGT2B7 (Rs=0.70, P=0.023). Expression of P-gp ≈ MATE-1 and OATP4C1 transporters were high. Strong intercorrelations were observed between several transporters (P-gp/MRP4, MRP2/OAT3, and OAT3/OAT4); no correlation in expression was apparent for functionally related transporters (OCT2/MATEs). This study extends our knowledge of pharmacologically relevant proteins in the kidney cortex, with implications on more prudent use of mechanistic kidney models under the general framework of quantitative systems pharmacology and toxicology.

Divergent Regulation of OCT and MATE Drug Transporters by Cadmium Exposure

Coordinated transcellular transport by the uptake via organic cation transporters (OCTs) in concert with the efflux via multidrug and toxin extrusion proteins (MATEs) is an essential system for hepatic and renal drug disposition. Despite their clinical importance, the regulation of OCTs and MATEs remains poorly characterized. It has been reported that cadmium (Cd2+) increase the activities of OCTs while being a substrate of MATEs. Here, we found that human (h) OCT2 protein, as compared with hMATE1, was more active in trafficking between the plasma membrane and cytoplasmic storage pool. Cd2+ exposure could significantly enhance the translocation of hOCT2 and hOCT1, but not hMATE1, to the plasma membrane. We further identified that candesartan, a widely prescribed angiotensin II receptor blocker, behaved similarly toward OCT2 and MATE1 as Cd2+ did. Importantly, Cd2+ and candesartan treatments could lead to an enhanced accumulation of metformin, which is a well-characterized substrate of OCTs/MATEs, in mouse kidney and liver, respectively. Altogether, our studies have uncovered possible divergent regulation of OCTs and MATEs by certain xenobiotics, such as Cd2+ and candesartan due to the different cellular trafficking of these two families of transporter proteins, which might significantly affect drug disposition in the liver and kidney.

The Impact of Genetic Polymorphisms in Organic Cation Transporters on Renal Drug Disposition.

A considerable number of drugs and/or their metabolites are excreted by the kidneys through glomerular filtration and active renal tubule secretion via transporter proteins. Uptake transporters in the proximal tubule are part of the solute carrier (SLC) superfamily, and include the organic cation transporters (OCTs). Several studies have shown that specific genetic polymorphisms in OCTs alter drug disposition and may lead to nephrotoxicity. Multiple single nucleotide polymorphisms (SNPs) have been reported for the OCT genes (SLC22A1, SLC22A2 and SLC22A3), which can influence the proteins’ structure and expression levels and affect their transport function. A gain-in-function mutation may lead to accumulation of drugs in renal proximal tubule cells, eventually leading to nephrotoxicity. This review illustrates the impact of genetic polymorphisms in OCTs on renal drug disposition and kidney injury, the clinical significances and how to personalize therapies to minimize the risk of drug toxicity.

Cadmium exposure enhances organic cation transporter 2 trafficking to the kidney membrane and exacerbates cisplatin nephrotoxicity

Renal accumulation and exposure of cadmium originating from pollution in agricultural land and the prevalence of cigarette smoking remains an unneglectable human health concern. Whereas cadmium exposure has been correlated with increased incidence of a variety of kidney diseases, little is known pertaining to its effect on renal drug disposition and response in patients. Here, we report that cadmium exposure significantly increased the activity of organic cation transporter 2 (OCT2), a critical renal drug transporter recommended in United States Federal Drug Administration guidance for assessment during drug development. Cadmium enhanced OCT2 trafficking to the cell membrane both invitro and invivo. Mechanistically cadmium-mediated OCT2 translocation was found to involve protein-protein interaction between serine/threonine-protein kinase AKT2, calcium/calmodulin and the AKT substrate AS160 in invitro cellular studies. The formed protein complex could selectively facilitate phosphorylation of AKT2 at T309, which induced translocation of OCT2 to the plasma membrane. Moreover, cadmium exposure markedly exacerbated nephrotoxicity induced by cisplatin, an OCT2 substrate, by increasing its accumulation in the mouse kidney. Consistently, there was a significant correlation between plasma cadmium level and alteration of renal function in cervical cancer patients who underwent chemotherapy with cisplatin. Thus, our studies suggest that membrane transporter distribution induced by cadmium exposure is a previously unrecognized factor for the broad variation in renal drug disposition and response.

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

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

Optimized Renal Transporter Quantification by Using Aquaporin 1 and Aquaporin 2 as Anatomical Markers: Application in Characterizing the Ontogeny of Renal Transporters and Its Correlation with Hepatic Transporters in Paired Human Samples.

Renal transporters, which are primarily located in the proximal tubules, play an important role in secretion and nephrotoxicity of drugs. The goal of this study was to characterize the age-dependent protein abundance of human renal transporters. A total of 43 human kidneys, 26 of which were paired with livers from the same donors, were obtained and classified into three age groups: children (< 12years), adolescents (12 to < 18years), and adults (> 18years). Protein abundance of kidney-specific anatomical markers, aquaporins 1 and 2 (markers of proximal and distal/collecting tubules, respectively), and 17 transporters was quantified by LC-MS/MS proteomics. Six out of 43 kidney samples were identified as outliers (Grubbs' test) that were significantly different from the others with relatively higher aquaporin 2 to aquaporin 1 ratio, indicating that these cortex samples were likely contaminated by medulla (representing distal/collecting tubules). No significant age-related changes (age > 1year) were observed for renal transporter abundance, albeit OCT2 abundance was modestly higher (< 50%) in adolescents than that in adults. Higher protein-protein correlation between transporters was observed in the kidney but abundance of transporters between tissues was not correlated. The use of aquaporins 1 and 2 provides a method for identifying kidney cortex with significant contamination from medulla containing distal and collecting tubules. The abundance and protein-protein correlation data can be used in physiologically based pharmacokinetic (PBPK) modeling and simulation of renal drug disposition and clearance in pediatric populations.

Rat Kidney Slices for Evaluation of Apical Membrane Transporters in Proximal Tubular Cells

Kidney slice has been often used as a tool reflecting basolateral transport in renal tubular epithelial cells. Recently, we reported that several important apical reabsorptive transporters such as Octn1/2, Sglt1/2, and Pept1/2 were functional in mouse kidney slices as well as transporter activities in basolateral side, which have been well accepted. Because rats are often used for preclinical pharmacodynamic and pharmacokinetic studies as well as mice, it is important to confirm applicability of rat kidney slices for evaluation of apically expressed transporters. The present study investigates usefulness of kidney slices from rats for evaluation of apical membrane transporters for efflux (multidrug resistance 1a, mdr1a) as well as influx (Octn1/2, Sglt1/2, Pept1/2). Na+-dependent uptake of ergothioneine (Octn1), carnitine (Octn2), and methyl-α-D-glucopyranoside (Sglt1/2) by rat kidney slices was observed, and the uptake was decreased by selective inhibitors. In addition, uptake of glycyl-sarcosine (Pept1/2) showed H+-dependence and was decreased by selective inhibitor. Furthermore, accumulation of mdr1a substrate azasetron was increased in the presence of zosuquidar, an mdr1a inhibitor, while strain differences existed. In conclusion, rat kidney slices should be useful for evaluation of renal drug disposition regulated by transporters in apical as well as basolateral membranes of rat renal proximal tubule cells.

A Human Proximal Tubular Epithelial Cell Model to Explore a Knowledge Gap on Neonatal Drug Disposition.

Finding the right drug-dosage for neonates is still a challenge. Until now, neonatal doses are extrapolated from adults and children doses. However, there are differences between neonatal and adult kidney physiology that should be considered, especially when it comes to drug metabolism and/or transport. Studying renal drug disposition in neonates is limited by the lack of reliable human cell models. To illustrate the feasibility of developing an in vitro model for neonatal proximal tubule epithelial cells (nPTECs) to study renal drug disposition at this age. nPTECs were isolated from urine samples of neonates of different gestational ages and were conditionally immortalized using a temperature sensitive SV40T antigen and human telomerase hTERT. Cell clones were characterized on gene expression level for PTEC markers such as P-glycoprotein (ABCB1), aquaporin1 (AQP1), and organic cation transport protein 2 (SLC22A2), and for kidney progenitor cell and podocyte markers. In addition, protein expression and functional assessment were performed for P-gp and OCT2. We established 101 clonal cell lines of conditionally immortalized nPTECs derived from neonatal urines. Characterization of primary cells lines showed expression of genes from different cell types such as progenitors, PTECs and podocytes, however the developed conditionally immortalized nPTECs only expressed proximal tubule markers. Quantitative PCR analysis confirmed the expression of proximal tubule markers in nPTECs similar to the adult control PTECs. P-gp was expressed in nPTECs derived from the different gestational ages with a similar functionality compared with adult derived PTECs. In contrast, OCT2 functionality was significantly lower in nPTEC cell lines compared with adult PTECs. We demonstrate the feasibility of culturing proximal tubule epithelial cells with high efficiency from urine of neonates. These cells expressed PTEC-specific genes and functional drug transporters. The cell model presented is a valuable tool to study proximal tubule physiology and pharmacology in newborns. In addition, we demonstrate the physiological differences between the neonatal and adult kidney, which emphasizes the importance of studying drug disposition in neonatal models instead of extrapolating from adult data.

Delineating the Role of Various Factors in Renal Disposition of Digoxin through Application of Physiologically Based Kidney Model to Renal Impairment Populations.

Development of submodels of organs within physiologically-based pharmacokinetic (PBPK) principles and beyond simple perfusion limitations may be challenging because of underdeveloped in vitro-in vivo extrapolation approaches or lack of suitable clinical data for model refinement. However, advantage of such models in predicting clinical observations in divergent patient groups is now commonly acknowledged. Mechanistic understanding of altered renal secretion in renal impairment is one area that may benefit from such models, despite knowledge gaps in renal pathophysiology. In the current study, a PBPK kidney model was developed for digoxin, accounting for the roles of organic anion transporting peptide 4C1 (OATP4C1) and P-glycoprotein (P-gp) in its tubular secretion, with the aim to investigate the impact of age and renal impairment (moderate to severe) on renal drug disposition. Initial PBPK simulations based on changes in glomerular filtration rate (GFR) underestimated the observed reduction in digoxin renal excretion clearance (CLR) in subjects with moderately impaired renal function relative to healthy. Reduction in either proximal tubule cell number or the OATP4C1 abundance in the mechanistic kidney model successfully predicted 59% decrease in digoxin CLR, in particular when these changes were proportional to reduction in GFR. In contrast, predicted proximal tubule concentration of digoxin was only sensitive to changes in the transporter expression/ million proximal tubule cells. Based on the mechanistic modeling, reduced proximal tubule cellularity and OATP4C1 abundance, and inhibition of OATP4C1-mediated transport, are proposed as possible causes of reduced digoxin renal secretion in renally impaired patients.

Key to Opening Kidney for In Vitro-In Vivo Extrapolation Entrance in Health and Disease: Part II: Mechanistic Models and In Vitro-In Vivo Extrapolation.

It is envisaged that application of mechanistic models will improve prediction of changes in renal disposition due to drug-drug interactions, genetic polymorphism in enzymes and transporters and/or renal impairment. However, developing and validating mechanistic kidney models is challenging due to the number of processes that may occur (filtration, secretion, reabsorption and metabolism) in this complex organ. Prediction of human renal drug disposition from preclinical species may be hampered by species differences in the expression and activity of drug metabolising enzymes and transporters. A proposed solution is bottom-up prediction of pharmacokinetic parameters based on in vitro-in vivo extrapolation (IVIVE), mediated by recent advances in in vitro experimental techniques and application of relevant scaling factors. This review is a follow-up to the Part I of the report from the 2015 AAPS Annual Meeting and Exhibition (Orlando, FL; 25th-29th October 2015) which focuses on IVIVE and mechanistic prediction of renal drug disposition. It describes the various mechanistic kidney models that may be used to investigate renal drug disposition. Particular attention is given to efforts that have attempted to incorporate elements of IVIVE. In addition, the use of mechanistic models in prediction of renal drug-drug interactions and potential for application in determining suitable adjustment of dose in kidney disease are discussed. The need for suitable clinical pharmacokinetics data for the purposes of delineating mechanistic aspects of kidney models in various scenarios is highlighted.

Key to Opening Kidney for In Vitro-In Vivo Extrapolation Entrance in Health and Disease: Part I: In Vitro Systems and Physiological Data.

The programme for the 2015 AAPS Annual Meeting and Exhibition (Orlando, FL; 25-29 October 2015) included a sunrise session presenting an overview of the state-of-the-art tools for in vitro-in vivo extrapolation (IVIVE) and mechanistic prediction of renal drug disposition. These concepts are based on approaches developed for prediction of hepatic clearance, with consideration of scaling factors physiologically relevant to kidney and the unique and complex structural organisation of this organ. Physiologically relevant kidney models require a number of parameters for mechanistic description of processes, supported by quantitative information on renal physiology (system parameters) and in vitro/in silico drug-related data. This review expands upon the themes raised during the session and highlights the importance of high quality in vitro drug data generated in appropriate experimental setup and robust system-related information for successful IVIVE of renal drug disposition. The different in vitro systems available for studying renal drug metabolism and transport are summarised and recent developments involving state-of-the-art technologies highlighted. Current gaps and uncertainties associated with system parameters related to human kidney for the development of physiologically based pharmacokinetic (PBPK) model and quantitative prediction of renal drug disposition, excretion, and/or metabolism are identified.

The functional implications of common genetic variation in CYP3A5 and ABCB1 in human proximal tubule cells.

Calcineurin inhibitors (CNIs) are the primary immunosuppressive drugs used in solid organ transplantation but are associated with the development of histological lesions leading to kidney failure. CNIs are metabolized by CYP3A and excreted by not only P-glycoprotein (P-gp) (ABCB1) in the gut and liver, but also by proximal tubule cells (PTCs) in the kidney. Multiple studies have demonstrated the importance of genetic variation in CYP3A5 and ABCB1 for CNI disposition and nephrotoxicity. The present study was designed to study the functional implication of variation in these two genes in human PTCs. A technique was developed to culture cells from renal tissue obtained from renal graft recipients by routine kidney biopsy. Primary cells were immortalized, subcloned, and then characterized for specific PTC markers (AQP1, CD13, brush border morphology) and donor CYP3A5(rs776746)/ABCB1(rs1045642) genotype. We then selected specific sets of confirmed conditionally immortalized PTCs (ciPTC) according to different combinations of the aforementioned genetic variants. Quantitative real-time polymerase chain reaction, Western blot, and immunohistochemistry were performed for studying CYP3A5 and ABCB1 expression. CYP3A5 activity was assessed by differential midazolam (MDZ) hydroxylation and P-gp (ABCB1 product) activity by a calcein efflux assay. Differential drug metabolism between cell lines was assessed by tacrolimus disappearance over 24 h. Cell lines were generated from 27 out of 38 tissue samples. On the basis of genotype and PTC biomarkers, 11 subclones were selected. In vitro PTC morphology with brush border microvilli was confirmed. CYP3A5*1 carriers had increased 1-OH/4-OH MDZ formation versus homozygous *3 carriers (mean: 2.36 (95% CI:1.11-3.40) vs 0.88 (95% CI:0.48-1.27); p < 0.05). P-gp activity was confirmed by calcein accumulation (mean 38.6%; 95% CI:32.8-44.4%), which was higher in cell lines with the ABCB1 3435TT than the 3435CC/CT genotype (46.2% vs 35.5%; 95% CI:28.7-42.2%). Tacrolimus disappearance was about two-fold higher in cell lines with the combined CYP3A5*1/ABCB1 3435TT genotype versus other genotype combinations. Biopsy-derived and immortalized human PTC cell lines demonstrate functional expression of genes involved in CNI metabolism. Differences in functional expression were detected according to common genetic variants in CYP3A5 and ABCB1. The studied genetic variants had a significant impact on in vitro tacrolimus metabolism. In particular, ciPTC with the combined CYP3A5*1/ABCB1 3435TT genotype demonstrated higher tacrolimus disappearance versus ciPTCs with a different pharmacogenetic profile. This in vitro model stresses the importance of the incorporation of pharmacogenetic variation in studies involved in (renal) drug disposition.

Application of the Isolated Perfused Rat Kidney Model to Assess Gender Effects on Drug Excretion

ABSTRACTPurpose. To study the effect of gender on the renal disposition of two organic anions, p-aminohippuric acid (PAH) and furosemide (FSM) in the isolated perfused rat kidney (IPK). Methods. IPK experiments (3–4 per treatment group) were conducted using kidneys from male and female Sprague Dawley rats. PAH was administered as a continuous infusion (with loading dose, targeted steady-state concentration 10 ug/mL). FSM was added as a bolus dose (2.65 mg, targeted concentration 33 ug/mL). Urine was collected in 10-min. intervals and perfusate was sampled at the midpoint of each collection period. Control (drug naïve) perfusions were performed for both genders. PAH and FSM were measured by HPLC. Kidney viability (GFR [estimated using inulin clearance], sodium reabsorption, glucose reabsorption) was monitored continuously during each perfusion experiment (2-h duration). Results. Good kidney function was maintained across all study groups, and lower GFR estimates in female kidneys were due to differences in kidney weight. For PAH, kidney weight corrected renal clearance (0.88 ± 0.37 mL/min/g vs. 0.59 ± 0.19 mL/min/g) and excretion ratio (3.8 ± 1.7 vs. 2.2 ± 0.72) were significantly higher in male kidneys. For FSM, renal clearance was significantly lower in female (0.10 ± 0.05 mL/min/g) compared to male kidneys (0.15 ± 0.07 mL/min/g). Mass balance analysis showed that FSM cumulative urinary excretion was significantly higher and kidney accumulation was significantly lower in experiments with male kidneys. Conclusions. The study demonstrates that the IPK is a useful model to assess gender effects on renal drug disposition. The renal excretion of organic anions is reduced in female rats, possibly due to gender differences in expression and/or activity of membrane transporters (both basolateral and luminal) in the kidney.

Pharmacogenomics and renal drug disposition in the newborn

Genetic polymorphisms in the genes coding for drug metabolizing enzymes, drug transporters, and drug receptors are major determinants of an individual’s response to drugs. The potential interactions of pharmacogenomics of renal drug transporters and drug receptors with renal drug disposition and the immature kidneys are briefly reviewed. Examples of gene polymorphisms seen in the RAAS (renin angiotensin system), β-adrenergic receptors, dopamine receptors and cytochrome P450 and their potential clinical impact are discussed. The human newborn has deficient hepatic and renal drug metabolism and disposition. This immaturity in drug-handling capacity may potentially be superimposed to genetic polymorphisms determining drug metabolism and transport thereby substantially increasing interpatient variability in drug dose requirements and in drug responses in the newborn. Pharmacogenomics is a tool that can be used to individualize drug therapy in newborns to minimize adverse drug effects and to optimize efficacy.

The Isolated Perfused Rat Kidney Model: A Useful Tool for Drug Discovery and Development

Over the past three decades, the Isolated Perfused Rat Kidney (IPK) has been used to study numerous aspects of renal drug disposition. Among the available ex-vivo methods to study renal transport, the IPK allows for elucidation of the overall contributions of renal transport mechanisms on drug excretion. Therefore, IPK studies can provide a bridge between in vitro findings and in vivo disposition. This review paper begins with a detailed overview of IPK methodology (system components, surgical procedure, study design). Various applications of the IPK are then presented. These applications include characterizing renal excretion mechanisms, screening for clinically significant drug interactions, studying renal drug metabolism, and correlating renal drug disposition with drug-induced changes in kidney function. Lastly, the role of IPK studies in drug development is discussed. Demonstrated correlations between IPK data and clinical outcomes make the IPK model a potentially useful tool for drug discovery and evaluation.

Effect of protein on renal function and drug disposition.

Studies in animals have indicated that increasing dietary protein intake could increase renal blood flow, glomerular filtration rate, and kidney size and weight. More recent literature suggests that similar findings may be seen in humans with an oral protein load and continuous infusion of amino acids. This paper reviews this body of literature, and examines potential mechanisms involved in producing the observed changes in renal function seen with protein and amino acid administration and potential alterations in drug disposition.