Effects of renal failure on drug transport and metabolism

Renal organic anion transporters (SLC22 family): expression, regulation, roles in toxicity, and impact on injury and disease

Organic solute flux across the basolateral and apical membranes of renal proximal tubule cells is a key process for maintaining systemic homeostasis. It represents an important route for the elimination of metabolic waste products and xenobiotics, as well as for the reclamation of essential compounds. Members of the organic anion transporter (OAT, SLC22) family expressed in proximal tubules comprise one pathway mediating the active renal secretion and reabsorption of organic anions. Many drugs, pesticides, hormones, heavy metal conjugates, components of phytomedicines, and toxins are OAT substrates. Thus, through transporter activity, the kidney can be a target organ for their beneficial or detrimental effects. Detailed knowledge of the OATs expressed in the kidney, their membrane targeting, substrate specificity, and mechanisms of action is essential to understanding organ function and dysfunction. The intracellular processes controlling OAT expression and function, and that can thus modulate kidney transport capacity, are also critical to this understanding. Such knowledge is also providing insight to new areas such as renal transplant research. This review will provide an overview of the OATs for which transport activity has been demonstrated and expression/function in the kidney observed. Examples establishing a role for renal OATs in drug clearance, food/drug-drug interactions, and renal injury and pathology are presented. An update of the current information regarding the regulation of OAT expression is also provided.

Introduction to drug pharmacokinetics in the critically ill patient

Despite regular use of drugs for critically ill patients, overall data are limited regarding the impact of critical illness on pharmacokinetics (PK). Designing safe and effective drug regimens for patients with critical illness requires an understanding of PK. This article reviews general principles of PK, including absorption, distribution, metabolism, and elimination, and how critical illness can influence these parameters. In the area of drug absorption, we discuss the impact of vasopressor use, delayed gastric emptying and feeding tubes, and nutrient interactions. On the topic of drug distribution, we review fluid resuscitation, alterations in plasma protein binding, and tissue perfusion. With drug metabolism, we discuss hepatic enzyme activity, protein binding, and hepatic blood flow. Finally, we review drug elimination in the critically ill patient and discuss the impact of augmented renal clearance and acute kidney injury on drug therapies. In each section, we highlight select literature reviewing the PK impact of these conditions on a drug PK profile and, where appropriate, provide general suggestions for clinicians on how to modify drug regimens to manage PK challenges.

Current understanding of drug disposition in kidney disease

Patients with chronic kidney disease (CKD) represent 13% of the American population. CKD has been shown to significantly alter drug disposition of nonrenally eliminated drugs. Indeed, modifications in the expression and function of intestinal and hepatic drug metabolism enzymes and uptake and efflux transporters have been reported. Uremic toxins, inflammatory cytokines, and parathyroid hormone have been implicated as causes. These changes can have an important clinical impact on drug disposition and lead to unintended toxicity if they are administered without dose adjustment in patients with impaired kidney function. This review summarizes recent preclinical and clinical studies and presents the current understanding of the effect of CKD on drug absorption, distribution, metabolism, and excretion.

Effects of chronic renal failure on brain drug transporters in rats

Studies demonstrated that chronic renal failure (CRF) affects the expression and activity of intestinal, hepatic, and renal drug transporters. Such drug transporters are expressed in brain cells and at the blood-brain barrier (BBB), where they limit the entry and distribution of drugs in the brain. Perturbations in brain drug transporter equilibrium by CRF could lead to central drug toxicity. This study evaluates how CRF affects BBB drug transporters using a 5/6 nephrectomized rat model. Protein and mRNA expression of influx transporters [organic anion-transporting polypeptide (Oatp), organic anion transporter (Oat)] and efflux transporters [P-glycoprotein (P-gp), multidrug resistance-related protein (Mrp), breast cancer resistance protein (Bcrp)] were measured in CRF and control rat brain. Intracerebral accumulation of radiolabeled benzylpenicillin, digoxin, doxorubicin, and verapamil was used to evaluate BBB drug permeability. Protein expression of the transporters was evaluated in rat brain endothelial cells (RBECs) and astrocytes incubated with control and CRF rat serum. We demonstrated significant decreases (30-50%) in protein and mRNA levels of Bcrp, Mrp2 to -4, Oat3, Oatp2 and -3, and P-gp in CRF rat brain biopsies, as well as in astrocytes and RBECs incubated with CRF serum. These decreases did not correlate with in vivo changes because BBB permeability of benzylpenicillin was decreased by 30% in CRF rats, whereas digoxin, doxorubicin, and verapamil permeabilities were unchanged. It thus seems that even with decreased drug transporters, BBB integrity and function is conserved in CRF.

The effects of media on pharmaceutically relevant transporters in the human HT-29 adenocarcinoma cell line: does culture media need to be controlled?

The HT-29 cell line forms a confluent monolayer with tight junctions, but displays different phenotypes when cultured for 21 days in galactose-supplemented media (differentiated) versus glucose-supplemented media (dedifferentiated). This study is aimed at elucidating how media differences might affect selected drug transporter expression and peptide-based substrate transport toward reducing this variability. A vial of HT-29 cells was amplified and cultured over several passages in four different mediums (American Type Culture Collection recommended McCoy's 5A versus Dulbecco's modified Eagle's media containing glucose, galactose, or neither carbohydrate) with normal supplementation. Transporter mRNA expression was characterized at days 5 and 21 postseeding utilizing SABiosciences quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) drug transporter arrays. Transport studies using [H]histidine, [(3) H]glycylsarcosine, [(3) H]valacyclovir, and [(3) H]carnosine were performed to assess the functional effects of oligopeptide transporter expression changes in HT-29 cells grown in each media. qRT-PCR arrays illustrated variable, media-dependent transporter expression between both the initial and differentiated time points. Permeability studies illustrated considerable media-dependent differences in both paracellular and transcellular substrate fluxes. The results demonstrate that these cells exhibit differing monolayer characteristics and genotypic/phenotypic profile properties when cultured under different media. The results suggest a need for standardization of culture methodologies for reducing inter- and intralaboratory variability.

Nitric oxide-induced regulation of renal organic cation transport after renal ischemia-reperfusion injury

Renal organic cation transporters are downregulated by nitric oxide (NO) in rat endotoxemia. NO generated by inducible NO synthase (iNOS) is substantially increased in the renal cortex after renal ischemia-reperfusion (I/R) injury. Therefore, we investigated the effects of iNOS-specific NO inhibition on the expression of the organic cation transporters rOct1 and rOct2 (Slc22a1 and Slc22a2, respectively) after I/R injury both in vivo and in vitro. In vivo, N6-(1-iminoethyl)-l-lysine (l-NIL) completely inhibited NO generation after I/R injury. Moreover, l-NIL abolished the ischemia-induced downregulation of rOct1 and rOct2 as determined by qPCR and Western blotting. Functional evidence was obtained by measuring the fractional excretion (FE) of the endogenous organic cation serotonin. Concordant with the expression of the rate-limiting organic cation transporter, the FE of serotonin decreased after I/R injury and was totally abolished by l-NIL. In vitro, ischemia downregulated both rOct1 and rOct2, which were also abolished by l-NIL; the same was true for the uptake of the organic cation MPP. We showed that renal I/R injury downregulates rOct1 and rOct2, which is most probably mediated via NO. In principle, this may be an autocrine effect of proximal tubular epithelial cells. We conclude that rOct1, or rOct1 and rOct2 limit the rate of the renal excretion of serotonin.

Effects of chronic renal failure on kidney drug transporters and cytochrome P450 in rats

Chronic renal failure (CRF) leads to decreased drug renal clearance due to a reduction in the glomerular filtration rate. However, little is known about how renal failure affects renal metabolism and elimination of drugs. Because both depend on the activity of uptake and efflux by renal transporters as well as enzymes in tubular cells, the purpose of this study was to investigate the effects of CRF on the expression and activity of select renal drug transporters and cytochrome P450. Two groups of rats were studied: control and CRF (induced by 5/6 nephrectomy). Compared with control rats, we observed reductions in the expression of both protein and mRNA of Cyp1a, sodium-dependent phosphate transport protein 1, organic anion transporter (Oat)1, 2, and 3, OatK1/K2, organic anion-transporting polypeptide (Oatp)1 and 4c1, P-glycoprotein, and urate transporter 1, whereas an induction in the protein and mRNA expression of Mrp2, 3, and 4 and Oatp2 and 3 was observed. Cyp3a expression remained unchanged. Similar results were obtained by incubating a human proximal tubule cell line (human kidney-2) with sera from CRF rats, suggesting the presence of uremic modulators. Finally, the renal elimination of [(3)H]digoxin and [(14)C]benzylpenicillin was decreased in CRF rats, compared with controls, as shown by a 4- and 9-fold accumulation, respectively, of these drugs in kidneys of rats in CRF. Our results demonstrate that CRF affects the expression and activity of several kidney drug transporters leading to the intrarenal accumulation of drugs and reduced renal clearance that could, at least partially, explain the tubular toxicity of many drugs.

VEGF inhibitor (Iressa) arrests histone deacetylase expression: single-cell cotransfection imaging cytometry for multi-target-multi-drug analysis

Multi-target-multi-drug approaches are needed to accelerate the process of drug discovery screening and to design efficient therapeutic strategies against diseases that involve alterations in multiple cellular targets. Herein we report single-cell cotransfection imaging cytometry to quantitatively screen drug-induced off-target effects. Vascular endothelial growth factor (VEGF) and histone deacetylase (HDAC) genes amplified from the genomic DNA were cloned in fluorescently tagged gene constructs (RFP-HDAC/YFP-VEGF). These gene constructs were cotransfected in HEK-293 cells to explore the possibility of off-target effects of 4-phenylbutyrate and Iressa on the expression of VEGF and HDAC through single-cell imaging cytometry. Iressa (10 µM) treatment at the time of cotransfection or 48 h after cotransfection of RFP-HDAC/YFP-VEGF plasmids in HEK-293 cells resulted in off-target effects on HDAC expression. These results suggest possible applications of Iressa in the treatment of diseases in which expression of both HDAC and VEGF should be inhibited. 4-Phenylbutyrate (2.0 mM) did not show any off-target effects on VEGF expression. The developed quantitative multicolor live single-cell cotransfection imaging can be employed to select better drug combinations for faster screening and greater accuracy in multi-target-multi-drug analysis by increasing the on-target/desired off-target effects and eliminating the undesirable off-target effects.

Remote communication through solute carriers and ATP binding cassette drug transporter pathways: an update on the remote sensing and signaling hypothesis

Recent data from knockouts, human disease, and transport studies suggest that solute carrier (SLC) and ATP binding cassette (ABC) multispecific "drug" transporters maintain effective organ and body fluid concentrations of key nutrients, signaling molecules, and antioxidants. These processes involve transcellular movement of solutes across epithelial barriers and fluid compartments (e.g., blood, cerebrospinal fluid, urine, bile) via "matching" or homologous sets of SLC (e.g., SLC21, SLC22, SLC47) and ABC transporters. As described in the "Remote Sensing and Signaling Hypothesis" (Biochem Biophys Res Commun 323:429-436, 2004; Biochem Biophys Res Commun 351:872-876, 2006; J Biol Chem 282:23841-23853, 2007; Nat Clin Pract Nephrol 3:443-448, 2007; Mol Pharmacol 76:481-490, 2009), highly regulated transporter networks with overlapping substrate preferences are involved in sensing and signaling to maintain homeostasis in response to environmental changes (e.g., substrate imbalance and injury). They function in parallel with (and interact with) the endocrine and autonomic systems. Uric acid (urate), carnitine, prostaglandins, conjugated sex steroids, cGMP, odorants, and enterobiome metabolites are discussed here as examples. Xenobiotics hitchhike on endogenous carrier systems, sometimes leading to toxicity and side effects. By regulation of the expression and/or function of various remote organ multispecific transporters after injury, the overall transport capacity of the remote organ to handle endogenous toxins, metabolites, and signaling molecules may change, aiding in recovery. Moreover, these transporters may play a role in communication between organisms. The specific cellular components involved in sensing and altering transporter abundance or functionality depend upon the metabolite in question and probably involve different types of sensors as well as epigenetic regulation.

Population pharmacokinetic analysis of letrozole in Japanese postmenopausal women

PURPOSE

Letrozole is an orally active aromatase inhibitor for the treatment of breast cancer. The objectives of this study were to examine the pharmacokinetic profile of letrozole in Japanese subjects and to identify factors that influence variability in the pharmacokinetics of letrozole using population pharmacokinetic (PPK) analysis.

METHODS

Twenty-five healthy postmenopausal Japanese women were enrolled in the study and received 2.5 mg letrozole once daily for 14 or 28 days. A PPK model was developed using NONMEM software. Age, body weight (WT), AST, ALT, total bilirubin, serum creatinine (CRE), and genotype of CYP2A6 were studied as covariates. Estrone, estrone sulfate, and estradiol in plasma were measured as pharmacodynamic markers.

RESULTS

CYP2A6 genotype, CRE, and AST were significant covariates for apparent systemic clearance (CL/F), and WT was a significant covariate for apparent distribution volume (Vd/F). Population mean estimates of CL/F and Vd/F in subjects without CYP2A6 mutation were 1.03 × (CRE/0.70)(-1.27) × (AST/17.5)(-0.793) L/h and 94.2 × (WT/51.1)(1.12) L respectively. CL/F in subjects possessing 1 and 2 CYP2A6 mutation alleles were 84.3% and 44.8% of the value in the subjects without mutation respectively. Estrogen levels fell to below detection limits in most subjects after letrozole administration. Three mild and transient adverse events (upper respiratory tract inflammation, arthralgia, and vomiting) were reported in the study.

CONCLUSIONS

CYP2A6 genotype largely influences CL/F of letrozole. Genetic polymorphism of CYP2A6 and body weight will be causes of ethnic difference in PK. However, dose adjustment is not necessary, because of the wide therapeutic range.

Chronic kidney disease: pharmacological considerations for the dentist

BACKGROUND

Patients with chronic kidney disease (CKD) represent a challenge for the dentist seeking to prescribe medications. Understanding the medical management of renal insufficiency and the pharmacokinetics of common dental drugs will aid clinicians in safely treating these patients.

TYPES OF STUDIES REVIEWED

The authors reviewed the literature concerning the medical and pharmacological management of CKD. They reviewed the pharmacokinetic effects of drugs described in case reports and research articles and obtained from them recommendations regarding the use of drugs and adjustment of dosages.

CLINICAL IMPLICATIONS

Because CKD is progressive, patients have varying levels of renal function but do not yet have end-stage renal disease. Some drugs that dentists prescribe commonly may worsen a patient's renal function, lead to drug toxicity or both. Managing the care of patients and prescribing medications tailored to their needs begin with a recognition of the patient with renal disease at risk of developing adverse effects. Clinicians can identify these patients from information obtained in their medical histories and from the drugs they may be taking.

CONCLUSIONS

To treat patients with kidney disease, clinicians must recognize those at risk, have knowledge of the pharmacokinetic changes that occur and recognize that adjustment of drug dosages often is needed.

Elimination of endogenous toxin, creatinine from blood plasma depends on albumin conformation: site specific uremic toxicity & impaired drug binding

Uremic syndrome results from malfunctioning of various organ systems due to the retention of uremic toxins which, under normal conditions, would be excreted into the urine and/or metabolized by the kidneys. The aim of this study was to elucidate the mechanisms underlying the renal elimination of uremic toxin creatinine that accumulate in chronic renal failure. Quantitative investigation of the plausible correlations was performed by spectroscopy, calorimetry, molecular docking and accessibility of surface area. Alkalinization of normal plasma from pH 7.0 to 9.0 modifies the distribution of toxin in the body and therefore may affect both the accumulation and the rate of toxin elimination. The ligand loading of HSA with uremic toxin predicts several key side chain interactions of site I that presumably have the potential to impact the specificity and impaired drug binding. These findings provide useful information for elucidating the complicated mechanism of toxin disposition in renal disease state.

Antibiotic dosing in critically ill patients with acute kidney injury

A common cause of acute kidney injury (AKI) is sepsis, which makes appropriate dosing of antibiotics in these patients essential. Drug dosing in critically ill patients with AKI, however, can be complicated. Critical illness and AKI can both substantially alter pharmacokinetic parameters as compared with healthy individuals or patients with end-stage renal disease. Furthermore, drug pharmacokinetic parameters are highly variable within the critically ill population. The volume of distribution of hydrophilic agents can increase as a result of fluid overload and decreased binding of the drug to serum proteins, and antibiotic loading doses must be adjusted upwards to account for these changes. Although renal elimination of drugs is decreased in patients with AKI, residual renal function in conjunction with renal replacement therapies (RRTs) result in enhanced drug clearance, and maintenance doses must reflect this situation. Antibiotic dosing decisions should be individualized to take into account patient-related, RRT-related, and drug-related factors. Efforts must also be made to optimize the attainment of antibiotic pharmacodynamic goals in this population.

Delayed elimination of SN-38 in cancer patients with severe renal failure

This prospective study is designed to examine the effects of severe renal failure on the pharmacokinetics of irinotecan. The pharmacokinetics of irinotecan, 7-ethyl-10-hydroxycamptothecin (SN-38), and SN-38 glucuronide (SN-38G) in three cancer patients with severe renal failure [creatinine clearance (Ccr) ≤ 20 ml/min] who were undergoing dialysis and received 100 mg/m(2) irinotecan as monotherapy were prospectively compared with those in five cancer patients with normal renal function (Ccr ≥ 60 ml/min). To ensure that the subjects had similar genetic backgrounds of UDP-glucuronosyltransferase (UGT) 1A1, patients with UGT1A1*1/*1, *1/*6, or *1/*28 were enrolled. The estimated terminal elimination rate constant of SN-38 in patients undergoing dialysis was approximately one tenth of that in patients with normal renal function (P = 0.025). Approximately 50% of SN-38 was dialyzed with a 2.1-m(2) dialysis membrane, whereas 27% was dialyzed with a 1.5-m(2) membrane. Our results showed that the elimination of SN-38 was significantly delayed in patients with severe renal failure compared with patients with normal renal function. We demonstrated that SN-38 was partly dialyzed.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Effects of synthetic androgens on liver function using the rabbit as a model

The objective of this study was to determine whether the rabbit was a suitable model to test new synthetic androgens for potential liver toxicity within a short dosing interval. Adult male rabbits were dosed orally daily on days 0-13 with 17α-methyltestosterone (MT) as a positive control and testosterone (T) as a negative control to validate this model. Synthetic androgens tested were: 7α-methyl-19-nortestosterone (MENT), dimethandrolone-undecanoate (DMAU), and 11β-methyl-19-nortestosterone-17β-dodecylcarbonate (11β-MNTDC). Serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma glutamyl transpeptidase (GGT), and sorbitol dehydrogenase (SDH), as well as clearance of intravenous injected bromsulfonphthalein (BSP) from serum on days 0, 7, and 14, were determined. As expected, T (10 mg/kg/d) did not adversely affect BSP retention or serum liver enzymes. MT (10 mg/kg/d) increased BSP retention, and AST, ALT, GGT, and SDH levels, indicating that this model could detect androgens known to be hepatotoxic. DMAU and MENT (10 mg/kg/d) increased BSP retention and all 4 serum liver enzymes as well, but the effects were less than those observed with MT at the same dose. All parameters returned to baseline 2 weeks after cessation of dosing. 11β-MNTDC at 10 mg/kg/d did not have an effect on BSP retention or liver enzymes, but a slight increase in serum GGT levels was observed in rabbits treated with 25 mg/kg/d. For the androgens that exhibited liver toxicity at 10 mg/kg/d (MT, DMAU, and MENT), a no-observed-effect level of 1 mg/kg/d was established. Overall ranking of the synthetic androgens from most to least hepatotoxic on the basis of percent BSP retention was: MT & DMAU > MENT > 11β-MNTDC. Hence, the rabbit appears to be a promising model for detection of potential liver toxicity by synthetic androgens using BSP clearance and serum liver enzyme levels as early indicators of injury.

Down-regulation of liver drug-metabolizing enzymes in a murine model of chronic renal failure

Drug metabolism could be altered in patients with chronic renal failure (CRF). In rats, this phenomenon is related to a decrease in liver cytochrome P450 (P450) and phase II enzymes, particularly N-acetyltransferase 2 (NAT2). This study attempted to determine the effects of CRF on liver P450 isoforms and NAT2 expressions by using a CRF mouse model. Two groups of mice were studied: CRF induced by 3/4 nephrectomy and control. Liver protein expression and mRNA levels of the major P450 isoforms involved in drug metabolism (CYP1A2, 2C29, 2D, 2E1, and 3A11) and NAT2 were measured by Western blot and real-time polymerase chain reaction (PCR), respectively. CYP3A activity was also assessed by the N-demethylation of erythromycin. Results showed a significant reduction in the protein expression of CYP1A2 (56%), 2C29 (31%), and 3A11 (37%) in CRF mice compared with control animals. Real-time PCR revealed a similar reduction in mRNA levels of CYP1A2, 2C29, and 3A11 (59, 56, and 37%, respectively), in CRF mice. There was no significant modification in protein expression and mRNA of CYP2D and 2E1. Compared with control animals, CRF mice displayed a 25% reduction in N-demethylation of erythromycin. For NAT2, protein expression decreased by 33% and mRNA levels decreased by 23%. In conclusion, this study demonstrates that protein expression of liver CYP1A2, CYP2C29, and CYP3A11 is down-regulated in CRF mice, secondary to reduced gene expression. Phase II enzymes are similarly affected by CRF. Our results will allow the use of knockout mice to determine the mechanism underlying CRF-induced down-regulation of liver drug-metabolizing enzymes.

Hepatic clearance, but not gut availability, of erythromycin is altered in patients with end-stage renal disease

Nonrenal clearance of drugs can be significantly lower in patients with end-stage renal disease (ESRD) than in those with normal renal function. Using erythromycin (ER) as a probe compound, we investigated whether this decrease in nonrenal clearance is due to reduced hepatic clearance (CL(H)) and/or gut metabolism. We also examined the potential effects of the uremic toxins 3-carboxy-4-methyl-5-propyl-2-furan propanoic acid (CMPF) and indoxyl sulfate (Indox) on ER disposition. Route-randomized, two-way crossover pharmacokinetic studies of ER were conducted in 12 ESRD patients and 12 healthy controls after oral (250 mg) and intravenous (125 mg) dosing with ER. In patients with ESRD, CL(H) decreased 31% relative to baseline values (0.35 +/- 0.14 l/h/kg vs. 0.51 +/- 0.13 l/h/kg, P = 0.01), with no change in steady-state volume of distribution. With oral dosing, the bioavailability of ER increased 36% in patients with ESRD, and this increase was not related to changes in gut availability. As expected, plasma levels of CMPF and Indox were significantly higher in the patients than in the healthy controls. However, no correlation was observed between CL(H) of ER and the levels of uremic toxins.

Pharmacokinetics in special populations

Pharmacokinetics are typically dependent on a variety of physiological variables (e.g., age, ethnicity, or pregnancy) or pathological conditions (e.g., renal and hepatic insufficiency, cardiac dysfunction, obesity, etc.). The influence of some of these conditions has not always been thoroughly assessed in the clinical studies of antiallergic drugs. However, the knowledge of the physiological grounds of the pharmacokinetics can provide some insight for predicting the potential alterations and guiding the initial prescription strategies. It is important to recognize that both pharmacokinetic and pharmacodynamic differences between populations should be considered. The available information on drugs used for the therapy of allergic diseases is reviewed in this chapter.

ESRD impairs nonrenal clearance of fexofenadine but not midazolam

ESRD can affect the pharmacokinetic disposition of drugs subject to nonrenal clearance. Cytochrome P450 (CYP) enzymes, including CYP3A, and multiple intestinal and hepatic drug transporters are thought to mediate this process, but the extent to which kidney disease alters the function of these proteins in humans is unknown. We used midazolam and fexofenadine to assess CYP3A (intestinal and hepatic) and drug transport, respectively, in patients with ESRD and healthy control subjects. We evaluated the effect of uremia on CYP3A and transporter expression in vitro by incubating normal rat hepatocytes and enterocytes with serum drawn from study participants. ESRD dramatically reduced nonrenal transporter function, evidenced by a 63% decrease in clearance (P < 0.001) and a 2.8-fold increase in area under the plasma concentration-time curve for fexofenadine (P = 0.002), compared with control subjects. We did not observe significant differences in midazolam or 1'-hydroxymidazolam clearance or area under the curve after oral administration, suggesting that CYP3A function is not changed by ESRD. Changes in hepatocyte and enterocyte protein expression in the presence of uremic serum were consistent with in vivo results. These findings demonstrate a mechanism for altered drug disposition in kidney disease, which may partially account for the high rates of drug toxicity in this population.

mTOR inhibitor everolimus ameliorates progressive tubular dysfunction in chronic renal failure rats

Responsible factors in progressive tubular dysfunction in chronic renal failure have not been fully identified. In the present study, we hypothesized that the mammalian target of rapamycin, mTOR, was a key molecule in the degenerative and progressive tubular damage in chronic renal failure. Everolimus, an mTOR inhibitor, was administered for 14 days in 5/6 nephrectomized (Nx) rats at 2 and 8 weeks after renal ablation. Marked activation of the mTOR pathway was found at glomeruli and proximal tubules in remnant kidneys of Nx rats. The reduced expression levels of the phosphorylated S6 indicated the satisfactory pharmacological effects of treatment with everolimus for 14 days. Everolimus suppressed the accumulation of smooth muscle alpha actin, infiltration of macrophages and expression of kidney injury molecule-1 in the proximal tubules. In addition, everolimus-treatment restored the tubular reabsorption of albumin, and had a restorative effect on the expression levels of membrane transporters in the polarized proximal tubular epithelium, when its administration was started at 8 weeks after Nx. These results indicate that the constitutively activated mTOR pathway in proximal tubules has an important role in the progressive tubular dysfunction, and that mTOR inhibitors have renoprotective effects to improve the proximal tubular functions in end-stage renal disease.

Pharmacokinetics and dosage adjustment in patients with renal dysfunction

INTRODUCTION

Chronic kidney disease is a common, progressive illness that is becoming a global public health problem. In patients with kidney dysfunction, the renal excretion of parent drug and/or its metabolites will be impaired, leading to their excessive accumulation in the body. In addition, the plasma protein binding of drugs may be significantly reduced, which in turn could influence the pharmacokinetic processes of distribution and elimination. The activity of several drug-metabolizing enzymes and drug transporters has been shown to be impaired in chronic renal failure. In patients with end-stage renal disease, dialysis techniques such as hemodialysis and continuous ambulatory peritoneal dialysis may remove drugs from the body, necessitating dosage adjustment.

METHODS

Inappropriate dosing in patients with renal dysfunction can cause toxicity or ineffective therapy. Therefore, the normal dosage regimen of a drug may have to be adjusted in a patient with renal dysfunction. Dosage adjustment is based on the remaining kidney function, most often estimated on the basis of the patient's glomerular filtration rate (GFR) estimated by the Cockroft-Gault formula. Net renal excretion of drug is a combination of three processes: glomerular filtration, tubular secretion and tubular reabsorption. Therefore, dosage adjustment based on GFR may not always be appropriate and a re-evaluation of markers of renal function may be required.

DISCUSSION

According to EMEA and FDA guidelines, a pharmacokinetic study should be carried out during the development phase of a new drug that is likely to be used in patients with renal dysfunction and whose pharmacokinetics are likely to be significantly altered in these patients. This study should be carried out in carefully selected subjects with varying degrees of renal dysfunction. In addition to this two-stage pharmacokinetic approach, a population PK/PD study in patients participating in phase II/phase III clinical trials can also be used to assess the impact of renal dysfunction on the drug's pharmacokinetics and pharmacodynamics.

CONCLUSION

In conclusion, renal dysfunction affects more that just the renal handling of drugs and/or active drug metabolites. Even when the dosage adjustment recommended for patients with renal dysfunction are carefully followed, adverse drug reactions remain common.

Improved pharmacokinetic and bioavailability support of drug discovery using serial blood sampling in mice

Pharmacokinetic studies in mice traditionally require one animal per time point, resulting in dosing and euthanizing a large number of animals and producing suboptimal quality of pharmacokinetic data due to inter-animal variability and dosing error. These studies are time-consuming and labor-intensive. To improve the throughput and quality of pharmacokinetic evaluation in mice, we have developed a serial blood sampling methodology using the lateral saphenous vein puncture technique. Two marketed drugs, indinavir and rosuvastatin, were selected for this validation study because of their distinctly different physicochemical and pharmacokinetic properties. Each compound was dosed orally and intravenously in mice using both discrete and serial blood sampling methods. The pharmacokinetic results from serial bleeding are in excellent agreement with those from discrete sampling for both compounds. Compared to the discrete sampling, the serial sampling procedure is a more humane method, allowing for rapid and repeated sampling from the same site without the need for anesthesia. The application of this new method has led to a remarkable reduction in animal and compound usage, a significant increase in throughput and speed, and a drastic improvement in pharmacokinetic data quality. This approach is especially useful for the first-tier in vivo pharmacokinetic screening of discovery compounds.

Nonnucleoside reverse transcriptase inhibitor pharmacokinetics in a large unselected cohort of HIV-infected women

BACKGROUND

Small intensive pharmacokinetic (PK) studies of medications in early-phase trials cannot identify the range of factors that influence drug exposure in heterogenous populations. We performed PK studies in large numbers of HIV-infected women on nonnucleoside reverse transcriptase inhibitors (NNRTIs) under conditions of actual use to assess patient characteristics that influence exposure and evaluated the relationship between exposure and response.

METHODS

Two hundred twenty-five women on NNRTI-based antiretroviral regimens from the Women's Interagency HIV Study were enrolled into 12-hour or 24-hour PK studies. Extensive demographic, laboratory, and medication covariate data were collected before and during the visit to be used in multivariate models. Total NNRTI drug exposure was estimated by area under the concentration-time curves.

RESULTS

Hepatic inflammation and renal insufficiency were independently associated with increased nevirapine exposure in multivariate analysis: crack cocaine, high fat diets, and amenorrhea were associated with decreased levels (n = 106). Higher efavirenz exposure was seen with increased transaminase, albumin levels, and orange juice consumption; tenofovir use, increased weight, being African American, and amenorrhea were associated with decreased exposure (n = 119). With every 10-fold increase in nevirapine or efavirenz exposure, participants were 3.3 and 3.6 times likely to exhibit virologic suppression, respectively. Patients with higher drug exposure were also more likely to report side effects on therapy.

CONCLUSIONS

Our study identifies and quantitates previously unrecognized factors modifying NNRTI exposure in the "real-world" setting. Comprehensive PK studies in representative populations are feasible and may ultimately lead to dose optimization strategies in patients at risk for failure or adverse events.

Psychotropic drugs and renal failure: translating the evidence for clinical practice

INTRODUCTION

The kidney is a primary route of drug elimination; abnormal kidney function is predicted to alter the pharmacokinetics of agents metabolized and/or excreted predominantly through this route. The high prevalence of mental disorders associated with psychotropic drug use in individuals with deteriorating renal function suggests there is a need to investigate the effects of renal failure on psychotropic pharmacokinetics. The aim of this review is to provide a clinically accessible overview of the effect of chronic renal failure on the pharmacokinetics for each of the major classes of prescribed psychotropic agents.

METHODS

All English language articles published between 1977 and 2008 were searched through PubMed, using the following keywords: "renal," "kidney," "pharmacokinetics," "renal impairment," "renal insufficiency," and "renal failure." Each of these search words was cross-referenced with the non-proprietary name of each psychotropic agent. The manufacturer's product insert was also reviewed for some agents for updated dosing. Owing to the lack of adequately powered studies, an inclusive manner was used.

RESULTS

Chronic renal failure variably affects the pharmacokinetic parameters of psychotropic drugs. A review of each psychotropic drug is provided, with an emphasis on the individual pharmacokinetic parameters and recommended dosing.

CONCLUSIONS

The adjudication of safe and effective doses for any psychotropic agent needs to be individualized. Tactics including dosage adjustment, slow titration, and careful monitoring for serious adverse events should be incorporated into practice.

Metabolism and renal elimination of gaboxadol in humans: role of UDP-glucuronosyltransferases and transporters

PURPOSE

Gaboxadol, a selective extrasynaptic agonist of the delta-containing gamma-aminobutyric acid type A (GABAA) receptor, is excreted in humans into the urine as parent drug and glucuronide conjugate. The goal of this study was to identify the UDP-Glucuronosyltransferase (UGT) enzymes and the transporters involved in the metabolism and active renal secretion of gaboxadol and its metabolite in humans.Methods. The structure of the glucuronide conjugate of gaboxadol in human urine was identified by LC/MS/MS. Human recombinant UGT isoforms were used to identify the enzymes responsible for the glucuronidation of gaboxadol. Transport of gaboxadol and its glucuronide was evaluated using cell lines and membrane vesicles expressing human organic anion transporters hOAT1 and hOAT3, organic cation transporter hOCT2, and the multidrug resistance proteins MRP2 and MRP4.Results. Our study indicated that the gaboxadol-O-glucuronide was the major metabolite excreted in human urine. UGT1A9, and to a lesser extent UGT1A6, UGT1A7 and UGT1A8, catalyzed the O-glucuronidation of gaboxadol in vitro. Gaboxadol was transported by hOAT1, but not by hOCT2, hOAT3, MRP2, and MRP4. Gaboxadol-O-glucuronide was transported by MRP4, but not MRP2.Conlusion. Gaboxadol could be taken up into the kidney by hOAT1 followed by glucuronidation and efflux of the conjugate into urine via MRP4.

Clinical review: Drug metabolism and nonrenal clearance in acute kidney injury

Decreased renal drug clearance is an obvious consequence of acute kidney injury (AKI). However, there is growing evidence to suggest that nonrenal drug clearance is also affected. Data derived from human and animal studies suggest that hepatic drug metabolism and transporter function are components of nonrenal clearance affected by AKI. Acute kidney injury may also impair the clearance of formed metabolites. The fact that AKI does not solely influence kidney function may have important implications for drug dosing, not only of renally eliminated drugs but also of those that are hepatically cleared. A review of the literature addressing the topic of drug metabolism and clearance alterations in AKI reveals that changes in nonrenal clearance are highly complicated and poorly studied, but they may be quite common. At present, our understanding of how AKI affects drug metabolism and nonrenal clearance is limited. However, based on the available evidence, clinicians should be cognizant that even hepatically eliminated drugs and formed drug metabolites may accumulate during AKI, and renal replacement therapy may affect nonrenal clearance as well as drug metabolite clearance.

Effect of hemodialysis on hepatic cytochrome P450 functional expression

Cytochrome P450 (CYP) functional expression is reduced in uremia and normalized after restoration of kidney function via transplantation. The aim of this study was to evaluate the effect of conventional hemodialysis on the functional expression of CYP1A, 2C, and 3A. We also investigated the role of nuclear factor-kappaB (NF-kappaB) in CYP regulation during uremia. Primary cultures of normal rat hepatocytes were incubated with serum obtained from end-stage renal disease patients pre- and post-hemodialysis and healthy control subjects, in the presence and absence of the NF-kappaB inhibitor andrographolide. Uremic pre-hemodialysis serum caused significant reductions (P<0.01) in CYP1A (44%), 2C (27%), and 3A (35%) protein expression compared to control serum, while dialyzed serum (i.e., obtained immediately post-hemodialysis) had no effect. CYP1A2, 2C11, and 3A2 mRNA expression, as well as CYP3A activity, were similarly impacted by uremic serum and were improved to >80% of control values after hemodialysis. NF-kappaB inhibition nearly eliminated the effect of uremic serum on CYP functional expression. This is the first study to demonstrate that conventional hemodialysis acutely improves altered CYP functional expression observed in rat hepatocytes incubated with uremic human serum.

The effect of chronic renal failure on drug metabolism and transport

BACKGROUND

Chronic renal failure (CRF) has been shown to significantly reduce the nonrenal clearance and alter bioavailability of drugs predominantly metabolized by the liver and intestine.

OBJECTIVES

The purpose of this article is to review all significant animal and clinical studies dealing with the effect of CRF on drug metabolism and transport.

METHODS

A search of the National Library of Medicine PubMed was done with terms such as chronic renal failure, cytochrome P450 [CYP], liver metabolism, efflux drug transport and uptake transport, including relevant articles back to 1969.

RESULTS

Animal studies in CRF have shown a significant downregulation (40-85%) of hepatic and intestinal CYP metabolism. High levels of parathyroid hormone, cytokines and uremic toxins have been shown to reduce CYP activity. Phase II reactions and drug transporters such as P-glycoprotein and organic anion transporting polypeptide are also affected.

CONCLUSION

CRF alters intestinal, renal and hepatic drug metabolism and transport producing a clinically significant impact on drug disposition and increasing the risk for adverse drug reactions.

Downregulation of hepatic acetylation of drugs in chronic renal failure

Drug metabolism can be affected by chronic renal failure (CRF). Although it is known that several drugs that are known to be acetylated accumulate in CRF, the effect of CRF on N-acetyltransferase (NAT), the enzyme responsible for this acetylation, is unknown. Herein is reported that protein and gene expression of both Nat isoforms in the liver was reduced by >30% and Nat2 activity was reduced by 50% in rats with CRF compared with control rats. Incubation of hepatocytes with serum from rats with CRF suggested that a circulating factor is responsible for the decrease in protein and gene expression. For testing the hypothesis that parathyroid hormone may be this factor, CRF was induced in parathyroidectomized rats; downregulation of Nat expression and activity was not observed in these rats. Furthermore, addition of parathyroid hormone to cultured hepatocytes induced a decrease in Nat2 protein and gene expression. In conclusion, liver acetylation of drugs in a rat model of CRF is reduced by a downregulation of Nat1 and Nat2 isoforms, secondary to decreased gene expression. Parathyroid hormone seems to be an important mediator of this phenomenon.

Emerging evidence of the impact of kidney disease on drug metabolism and transport

Several lines of emerging evidence indicate that kidney disease differentially affects uptake and efflux transporters and metabolic enzymes in the liver and gastrointestinal (GI) tract, and uremic toxins have been implicated as the cause. In patients with kidney disease, even drugs that are eliminated by nonrenal transport and metabolism could lead to important unintended consequences if they are administered without dose adjustment for reduced renal function. This is particularly so in the case of drugs with narrow therapeutic windows and may translate into clinically significant variations in exposure and response.