Expression and functional relevance of UGT1A4 in a cohort of human drug-resistant epileptic brains

The potential impact of CYP and UGT drug-metabolizing enzymes on brain target site drug exposure

Drug metabolism is one of the critical determinants of drug disposition throughout the body. While traditionally associated with the liver, recent research has unveiled the presence and functional significance of drug-metabolizing enzymes (DMEs) within the brain. Specifically, cytochrome P-450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) enzymes have emerged as key players in drug biotransformation within the central nervous system (CNS). This comprehensive review explores the cellular and subcellular distribution of CYPs and UGTs within the CNS, emphasizing regional expression and contrasting profiles between the liver and brain, humans and rats. Moreover, we discuss the impact of species and sex differences on CYPs and UGTs within the CNS. This review also provides an overview of methodologies for identifying and quantifying enzyme activities in the brain. Additionally, we present factors influencing CYPs and UGTs activities in the brain, including genetic polymorphisms, physiological variables, pathophysiological conditions, and environmental factors. Examples of CYP- and UGT-mediated drug metabolism within the brain are presented at the end, illustrating the pivotal role of these enzymes in drug therapy and potential toxicity. In conclusion, this review enhances our understanding of drug metabolism's significance in the brain, with a specific focus on CYPs and UGTs. Insights into the expression, activity, and influential factors of these enzymes within the CNS have crucial implications for drug development, the design of safe drug treatment strategies, and the comprehension of drug actions within the CNS. To that end, CNS pharmacokinetic (PK) models can be improved to further advance drug development and personalized therapy.

Association of ABCB1 Polymorphisms with Efficacy and Adverse Drug Reactions of Valproic Acid in Children with Epilepsy

Genetic polymorphisms in ATP-binding cassette subfamily B member 1 (ABCB1, also known as MDR1) have been reported to be possibly associated with the regulation of response to antiseizure medications. The aim of this study was to investigate the association of ABCB1 polymorphisms with the efficacy of and adverse drug reactions to valproic acid among Chinese children with epilepsy. A total of 170 children from southern China with epilepsy treated with valproic acid for more than one year were recruited, including 61 patients with persistent seizures and 109 patients who were seizure-free. Two single nucleotide polymorphisms of ABCB1, rs1128503 and rs3789243, were genotyped using the Sequenom MassArray system. The two single nucleotide polymorphisms of ABCB1 were found to be significantly associated with treatment outcomes of valproic acid in children with epilepsy. Carriers with the TT genotype of ABCB1 rs1128503 were more inclined to exhibit persistent seizures after treatment with valproic acid (p = 0.013). The CC genotype of rs3789243 was observed to be a potential protective factor for valproic acid-induced gastrointestinal adverse drug reactions (p = 0.018), but possibly increased the risk of valproic acid-induced cutaneous adverse drug reactions (p = 0.011). In contrast, the CT genotype of rs3789243 was associated with a lower risk of valproic acid-induced cutaneous adverse drug reactions (p = 0.011). Haplotype analysis showed that CC haplotype carriers tended to respond better to valproic acid treatment (p = 0.009). Additionally, no significant association was found between ABCB1 polymorphisms and serum concentrations of valproic acid. This study revealed that the polymorphisms and haplotypes of the ABCB1 gene might be associated with the treatment outcomes of valproic acid in Chinese children with epilepsy.

Implications of BCRP modulation on PTZ-induced seizures in mice: Role of ko143 and metformin as adjuvants to lamotrigine

Blood-brain barrier (BBB) efflux transporters' overexpression hinders antiepileptic drug brain entry. Breast cancer resistance protein (BCRP) is a major BBB efflux transporter. In the present work, BCRP's role as a mechanism that might contribute to drug-resistant epilepsy (DRE) in a mouse model of acute seizures was studied with further assessment of the effect of its inhibition by ko143 and metformin (MET) on lamotrigine (LTG) bioavailability and efficacy. 42 male mice divided into 6 groups: G1: Normal control, G2: LTG-injected healthy mice: LTG 20 mg/kg i.p., G3: Acute seizures (A.S) mice: Pentylenetetrazole (PTZ) 50 mg/kg i.p., G4: LTG-treated A.S mice: LTG 20 mg/kg + PTZ 50 mg/kg i.p., G5: Ko143 + LTG treated A.S mice: Ko143 15 mg/kg i.p. before LTG + PTZ, G6: MET + LTG treated A.S mice: MET 200 mg/kg i.p. before LTG + PTZ. Seizures severity, serum, brain LTG, and brain BCRP were assessed. PTZ group experienced the highest seizure frequency and brain BCRP expression. Ko143 and MET groups showed a significant decrease in brain BCRP with subsequent improvement in brain LTG level and better seizure control. BCRP has a significant role in epilepsy resistance and its inhibition with ko143 or MET adds value to DRE management.

Xenobiotic transport and metabolism in the human brain

Organisms have metabolic pathways responsible for eliminating endogenous and exogenous toxicants. Generally, we associate the liver par excellence as the organ in charge of detoxifying the body; however, this process occurs in all tissues, including the brain. Due to the presence of the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB), the Central Nervous System (CNS) is considered a partially isolated organ, but similar to other organs, the CNS possess xenobiotic transporters and metabolic pathways associated with the elimination of xenobiotic agents. In this review, we describe the different systems related to the detoxification of xenobiotics in the CNS, providing examples in which their association with neurodegenerative processes is suspected. The CNS detoxifying systems include carrier-mediated, active efflux and receptor-mediated transport, and detoxifying systems that include phase I and phase II enzymes, as well as those enzymes in charge of neutralizing compounds such as electrophilic agents, reactive oxygen species (ROS), and free radicals, which are products of the bioactivation of xenobiotics. Moreover, we discuss the differential expression of these systems in different regions of the CNS, showing the different detoxifying needs and the composition of each region in terms of the cell type, neurotransmitter content, and the accumulation of xenobiotics and/or reactive compounds.

Drug-resistant epilepsy: From multiple hypotheses to an integral explanation using preclinical resources

Drug-resistant epilepsy affects approximately one-third of the patients with epilepsy. The pharmacoresistant condition in epilepsy is mainly explained by six hypotheses. In addition, several experimental models have been used to understand the mechanisms involved in pharmacoresistant epilepsy and to identify novel therapies to control this condition. However, the global prevalence of this disease persists without changes. Several factors can explain this situation. First of all, the pharmacoresistant epilepsy is explained by different and independent hypotheses. Each hypothesis indicates specific mechanisms to explain the drug-resistant condition in epilepsy. However, there are different findings suggesting common mechanisms between the different hypotheses. Other important situation is that the experimental models designed for the screening of drugs with potential anticonvulsant effect do not consider factors such as age, gender, type of epilepsy, and comorbid disorders. The present review focuses on indicating the limitations for each hypothesis and the relationships among them. The relevance to consider central and peripheral phenomena associated with the drug-resistant condition in different types of epilepsy is also indicated. The necessity to establish a global hypothesis that integrates all the phenomena associated with the pharmacoresistant epilepsy is proposed. This article is part of the Special Issue "NEWroscience 2018".

Alterations of Cytochrome P450s and UDP-Glucuronosyltransferases in Brain Under Diseases and Their Clinical Significances

Cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs) are both greatly important metabolic enzymes in various tissues, including brain. Although expressions of brain CYPs and UGTs and their contributions to drug disposition are much less than liver, both CYPs and UGTs also mediate metabolism of endogenous substances including dopamine and serotonin as well as some drugs such as morphine in brain, demonstrating their important roles in maintenance of brain homeostasis or pharmacological activity of drugs. Some diseases such as epilepsy, Parkinson's disease and Alzheimer's disease are often associated with the alterations of CYPs and UGTs in brain, which may be involved in processes of these diseases via disturbing metabolism of endogenous substances or resisting drugs. This article reviewed the alterations of CYPs and UGTs in brain, the effects on endogenous substances and drugs and their clinical significances. Understanding the roles of CYPs and UGTs in brain provides some new strategies for the treatment of central nervous system diseases.

In vitro Models of the Blood-Brain Barrier: Tools in Translational Medicine

Medical progress has historically depended on scientific discoveries. Until recently, science was driven by technological advancements that, once translated to the clinic, fostered new treatments and interventions. More recently, technology-driven medical progress has often outpaced laboratory research. For example, intravascular devices, pacemakers for the heart and brain, spinal cord stimulators, and surgical robots are used routinely to treat a variety of diseases. The rapid expansion of science into ever more advanced molecular and genetic mechanisms of disease has often distanced laboratory-based research from day-to-day clinical realities that remain based on evidence and outcomes. A recognized reason for this hiatus is the lack of laboratory tools that recapitulate the clinical reality faced by physicians and surgeons. To overcome this, the NIH and FDA have in the recent past joined forces to support the development of a "human-on-a-chip" that will allow research scientists to perform experiments on a realistic replica when testing the effectiveness of novel experimental therapies. The development of a "human-on-a-chip" rests on the capacity to grow in vitro various organs-on-a-chip, connected with appropriate vascular supplies and nerves, and our ability to measure and perform experiments on these virtually invisible organs. One of the tissue structures to be scaled down on a chip is the human blood-brain barrier. This review gives a historical perspective on in vitro models of the BBB and summarizes the most recent 3D models that attempt to fill the gap between research modeling and patient care. We also present a summary of how these in vitro models of the BBB can be applied to study human brain diseases and their treatments. We have chosen NeuroAIDS, COVID-19, multiple sclerosis, and Alzheimer's disease as examples of in vitro model application to neurological disorders. Major insight pertaining to these illnesses as a consequence of more profound understanding of the BBB can reveal new avenues for the development of diagnostics, more efficient therapies, and definitive clarity of disease etiology and pathological progression.

How Can Drug Metabolism and Transporter Genetics Inform Psychotropic Prescribing?

Many genetic variants in drug metabolizing enzymes and transporters have been shown to be relevant for treating psychiatric disorders. Associations are strong enough to feature on drug labels and for prescribing guidelines based on such data. A range of commercial tests are available; however, there is variability in included genetic variants, methodology, and interpretation. We herein provide relevant background for understanding clinical associations with specific variants, other factors that are relevant to consider when interpreting such data (such as age, gender, drug-drug interactions), and summarize the data relevant to clinical utility of pharmacogenetic testing in psychiatry and the available prescribing guidelines. We also highlight areas for future research focus in this field.

Association of UGT2B7 and CaMK4 with response of valproic acid in Chinese children with epilepsy

AIM OF THE STUDY

Valproic acid (VPA) is a widely used antiepileptic drug for epilepsy. However, approximately 30% of patients with epilepsy do not respond to this therapy even when it was appropriately used. In order to explore the potential genetic factors related to the VPA response, this pharmacogenetics study was conducted.

METHODS

A total of one hundred and fifty-seven Chinese children with epilepsy who were administered with by VPA for at least one year were enrolled. Thirteen single-nucleotide polymorphisms (SNPs) located in eight genes involving targets and metabolic enzymes of VPA were genotyped. The frequencies of these polymorphisms and the effect of genotypes on the efficacy of VPA were analyzed.

RESULTS

The frequencies of two SNPs, rs7668258 (uridine diphosphate glucuronosyltransferase-2B7, UGT2B7) and rs306104 (calmodulin-kinase 4, CaMK4) were associated with VPA responses. However, no association was found for the other SNPs. Furthermore, the polymorphism of UGT2B7 influenced the adjusted concentration (AC) in the responders rather than in the non-responders.

CONCLUSION

Two SNPs (UGT2B7 and CaMK4) were associated with VPA response, which may explain the pharmacological mechanism of VPA resistance to some extent.

Emerging Roles of Aryl Hydrocarbon Receptors in the Altered Clearance of Drugs during Chronic Kidney Disease

Chronic kidney disease (CKD) is a major public health problem, since 300,000,000 people in the world display a glomerular filtration rate (GFR) below 60 mL/min/1.73m². Patients with CKD have high rates of complications and comorbidities. Thus, they require the prescription of numerous medications, making the management of patients very complex. The prescription of numerous drugs associated with an altered renal- and non-renal clearance makes dose adjustment challenging in these patients, with frequent drug-related adverse events. However, the mechanisms involved in this abnormal drug clearance during CKD are not still well identified. We propose here that the transcription factor, aryl hydrocarbon receptor, which is the cellular receptor for indolic uremic toxins, could worsen the metabolism and the excretion of drugs in CKD patients.

Expression and Functional Relevance of Death-Associated Protein Kinase in Human Drug-Resistant Epileptic Brain: Focusing on the Neurovascular Interface

Death-associated protein kinase (DAPK) is a key player in various cell death signaling pathways. Prolonged seizures induce neuronal stress; thus, we studied DAPK expression in resected brain tissues from patients with refractory epilepsy and the pathophysiological relevance of neurovascular DAPK. We used brain resections from temporal lobe epilepsy (TLE), tumor (BT), arteriovenous malformation (AVM), and autopsy, and isolated human endothelial cells (EPI-ECs) and glial cells (EPI-Astro) from epileptic brains compared to control brain endothelial cells (HBMECs) and astrocytes. DAPK and phosphorylated DAPK (p-DAPK) expression was evaluated by immunohistochemistry and western blot. Subcellular localization of DAPK in epileptic brain was explored; DAPK mRNA/protein levels in EPI-ECs/EPI-Astro were evaluated. We assessed DAPK localization with hypoxic inducible factor (HIF-1α) and vascular endothelial growth factor (VEGF) in epilepsy, BT, and AVM. We found DAPK overexpression across neurons, microcapillaries, and astrocytes in TLE vs controls; DAPK and p-DAPK levels significantly increased only in microsomal fractions of epileptic brain. DAPK mRNA remained unchanged, although increased DAPK and p-DAPK protein expression was observed in EPI-ECs. DAPK inhibition reduced p-DAPK, HIF-1α, and VEGF expression, but increased cytotoxicity and decreased cell viability in EPI-ECs and EPI-astro vs. controls. DAPK staining in TLE resembled BT and AVM, with predominant DAPK/p-DAPK expression in neurons and vasculature. Taken together, these findings suggest DAPK could be a potential molecular target in neuronal death and vascular changes in epilepsy. Increased brain endothelial and astrocytic DAPK in epilepsy, identified for the first time, may have relevance to angiogenesis, hypoxia, and cell survival in pathological conditions.

Modulation of glucocorticoid receptor in human epileptic endothelial cells impacts drug biotransformation in an in vitro blood-brain barrier model

Objective

Nuclear receptors and cytochrome P450 (CYP) regulate hepatic metabolism of several drugs. Nuclear receptors are expressed at the neurovascular unit of patients with drug‐resistant epilepsy. We studied whether glucocorticoid receptor (GR) silencing or inhibition in human epileptic brain endothelial cells (EPI‐ECs) functionally impacts drug bioavailability across an in vitro model of the blood–brain barrier (BBB) by CYP‐multidrug transporter (multidrug resistance protein 1, MDR1) mechanisms.

Methods

Surgically resected brain specimens from patients with drug‐resistant epilepsy, primary EPI‐ECs, and control human brain microvascular endothelial cells (HBMECs) were used. Expression of GR, pregnane X receptor, CYP3A4, and MDR1 was analyzed pre‐ and post‐GR silencing in EPI‐ECs. Endothelial cells were co‐cultured with astrocytes and seeded in an in vitro flow‐based BBB model (DIV‐BBB). Alternatively, the GR inhibitor mifepristone was added to the EPI‐EC DIV‐BBB. Integrity of the BBB was monitored by measuring transendothelial electrical resistance. Cell viability was assessed by glucose‐lactate levels. Permeability of [³H]sucrose and [¹⁴C]phenytoin was quantified. CYP function was determined by measuring resorufin formation and oxcarbazepine (OXC) metabolism.

Results

Silencing and inhibition of GR in EPI‐ECs resulted in decreased pregnane X receptor, CYP3A4, and MDR1 expression. GR silencing or inhibition did not affect BBB properties in vitro, as transendothelial electrical resistance and P_sucrose were unaltered, and glucose metabolism was maintained. GR EPI‐EC silencing or inhibition led to (1) increased P_phenytoinBBB permeability as compared to control; (2) decreased CYP function, indirectly evaluated by resorufin formation; (3) improved OXC bioavailability with increased abluminal (brain‐side) OXC levels as compared to control.

Significance

Our results suggest that modulating GR expression in EPI‐ECs at the BBB modifies drug metabolism and penetration by a mechanism encompassing P450 and efflux transporters. The latter could be exploited for future drug design and to overcome pharmacoresistance.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Zebrafish: A promising in vivo model for assessing the delivery of natural products, fluorescence dyes and drugs across the blood-brain barrier

The blood brain barrier (BBB) is the network of capillaries that controls the passage of substances from the blood into the brain and other parts of the central nervous system (CNS). As this barrier is the major obstacle for drug delivery into CNS, a credible BBB model is very necessary to assess the BBB permeability of novel neuroactive compounds including thousands of bioactive compounds which have been extracted from medicinal plants and have the potential for the treatment of CNS diseases. Increasing reports indicated that zebrafish has emerged as a timely, reproducible model for BBB permeability assessment. In this review, the development and functions of the BBB in zebrafish, such as its anatomical morphology, tight junctions, drug transporters and enzyme expression, are compared with those in mammals. The studies outlined in this review describe the utilization of the zebrafish as a BBB model to investigate the permeability and distribution of fluorescent dyes and drugs. Particularly, this review focuses on the use of zebrafish to evaluate the delivery of natural products and nanosized drug delivery systems across the BBB. Due to the highly conserved nature of both the structure and function of the BBB between zebrafish and mammals, zebrafish has the potential to be developed as a model for assessing and predicting the permeability of BBB to novel compounds.

P-gp Protein Expression and Transport Activity in Rodent Seizure Models and Human Epilepsy

A cure for epilepsy is currently not available, and seizure genesis, seizure recurrence, and resistance to antiseizure drugs remain serious clinical problems. Studies show that the blood-brain barrier is altered in animal models of epilepsy and in epileptic patients. In this regard, seizures increase expression of blood-brain barrier efflux transporters such as P-glycoprotein (P-gp), which is thought to reduce brain uptake of antiseizure drugs, and thus, contribute to antiseizure drug resistance. The goal of the current study was to assess the viability of combining in vivo and ex vivo preparations of isolated brain capillaries from animal models of seizures and epilepsy as well as from patients with epilepsy to study P-gp at the blood-brain barrier. Exposing isolated rat brain capillaries to glutamate ex vivo upregulated P-gp expression to levels that were similar to those in capillaries isolated from rats that had status epilepticus or chronic epilepsy. Moreover, the fold-increase in P-gp protein expression seen in animal models is consistent with the fold-increase in P-gp observed in human brain capillaries isolated from patients with epilepsy compared to age-matched control individuals. Overall, the in vivo/ex vivo approach presented here allows detailed analysis of the mechanisms underlying seizure-induced changes of P-gp expression and transport activity at the blood-brain barrier. This approach can be extended to other blood-brain barrier proteins that might contribute to drug-resistant epilepsy or other CNS disorders as well.

Upregulation of breast cancer resistance protein and major vault protein in drug resistant epilepsy

PURPOSE

Identifying factors involved in the development of drug resistant epilepsy (DRE) remains a challenge. Candidate gene studies have shown modulation of resistance to drugs by various multidrug resistance proteins in DRE. However the resistance to drugs in DRE could be more complex and multifactorial involving molecules in different pharmacokinetic processes. In this study for the first time we have analyzed the relative expression of four molecules with different drug resistance mechanisms in two most common DRE pathologies, mesial temporal lobe epilepsy (MTLE) and focal cortical dysplasia (FCD) with respect to each other and also with different non-epileptic controls.

METHODS

Brain tissues resected from MTLE (n=16) and FCD type I and II (n=12) patients who had undergone surgery were analysed for mRNA levels of multidrug resistance-associated protein 1(MRP1), major vault protein (MVP), breast cancer resistance protein (BCRP), and one drug metabolising enzyme (UGT1A4) as compared to non-epileptic controls which were tissues resected from tumor periphery (n=6) and autopsy tissues (n=4) by quantitative PCR.

RESULTS

We found significant upregulation of MVP and BCRP whereas MRP1 and UGT1A4 were unaltered in both pathologies. While upregulation of BCRP was significantly higher in MTLE (9.34±0.45; p<0.05), upregulation of MVP was significantly higher in FCD (2.94±0.65; p<0.01).

CONCLUSION

We propose that upregulation of BCRP and MVP is associated with MTLE and FCD and these molecules not only may have the potential to predict pathology specific phenotypes but may also have therapeutic potential as adjunct treatment in these pathologies.

Overexpression of pregnane X and glucocorticoid receptors and the regulation of cytochrome P450 in human epileptic brain endothelial cells

Objective

Recent evidence suggests a metabolic contribution of cytochrome P450 enzymes (CYPs) to the drug‐resistant phenotype in human epilepsy. However, the upstream molecular regulators of CYP in the epileptic brain remain understudied. We therefore investigated the expression and function of pregnane xenobiotic (PXR) and glucocorticoid (GR) nuclear receptors in endothelial cells established from post‐epilepsy surgery brain samples.

Methods

PXR/GR localization was evaluated by immunohistochemistry in specimens from subjects who underwent temporal lobe resections to relieve drug‐resistant seizures. We used primary cultures of endothelial cells obtained from epileptic brain tissues (EPI‐ECs; n = 8), commercially available human brain microvascular endothelial cells (HBMECs; n = 8), and human hepatocytes (n = 3). PXR/GR messenger RNA (mRNA) levels in brain ECs was initially determined by complementary DNA (cDNA) microarrays. The expression of PXR/GR proteins was quantified by Western blot. PXR and GR silencing was performed in EPI‐ECs (n = 4), and the impact on downstream CYP expression was determined.

Results

PXR/GR expression was detected by immunofluorescence in ECs and neurons in the human temporal lobe samples analyzed. Elevated mRNA and protein levels of PXR and GR were found in EPI‐ECs versus control HBMECs. Hepatocytes, used as a positive control, displayed the highest levels of PXR/GR expression. We confirmed expression of PXR/GR in cytoplasmic‐nuclear subcellular fractions, with a significant increase of PXR/GR in EPI‐ECs versus controls. CYP3A4, CYP2C9, and CYP2E1 were overexpressed in EPI‐ECs versus control, whereas CYP2D6 and CYP2C19 were downregulated or absent in EPI‐ECs. GR silencing in EPI‐ECs led to decreased CYP3A4, CYP2C9, and PXR expression. PXR silencing in EPI‐ECs resulted in the specific downregulation of CYP3A4 expression.

Significance

Our results indicate increased PXR and GR in primary ECs derived from human epileptic brains. PXR or GR may be responsible for a local drug brain metabolism sustained by abnormal CYP regulation.

In vitro and in vivo evaluation of functionalized chitosan-Pluronic micelles loaded with myricetin on glioblastoma cancer

This study aimed to develop a novel polymeric carrier based on chitosan-functionalized Pluronic P123/F68 micelles loaded with myricetin (MYR) to improve the therapeutic index of chemotherapy for glioblastoma cancer. Following characterization and assessment of the cellular uptake and antitumor effects of MYR-loaded micelles (MYR-MCs) in vitro, the acute toxicity, blood-brain barrier (BBB) translocation, brain uptake and biodistribution in vivo were assessed. The results demonstrated that MYR-MCs exhibited improved cellular uptake and antitumor activity compared to free MYR in vitro, with a significantly enhanced anticancer effect in vivo following efficient transport across the BBB. However, MYR-MCs did not affect the brain endothelial, barrier function, the liver, heart or kidneys. Furthermore, MYR-MCs altered the expression of apoptotic proteins, such as Bcl-2, BAD and BAX, in mice. In conclusion, MYR-MCs may be considered an effective and promising drug delivery system for glioblastoma treatment.

The role of glucuronidation in drug resistance

The final therapeutic effect of a drug candidate, which is directed to a specific molecular target strongly depends on its absorption, distribution, metabolism and excretion (ADME). The disruption of at least one element of ADME may result in serious drug resistance. In this work we described the role of one element of this resistance: phase II metabolism with UDP-glucuronosyltransferases (UGTs). UGT function is the transformation of their substrates into more polar metabolites, which are better substrates for the ABC transporters, MDR1, MRP and BCRP, than the native drug. UGT-mediated drug resistance can be associated with (i) inherent overexpression of the enzyme, named intrinsic drug resistance or (ii) induced expression of the enzyme, named acquired drug resistance observed when enzyme expression is induced by the drug or other factors, as food-derived compounds. Very often this induction occurs via ligand binding receptors including AhR (aryl hydrocarbon receptor) PXR (pregnane X receptor), or other transcription factors. The effect of UGT dependent resistance is strengthened by coordinate action and also a coordinate regulation of the expression of UGTs and ABC transporters. This coupling of UGT and multidrug resistance proteins has been intensively studied, particularly in the case of antitumor treatment, when this resistance is "improved" by differences in UGT expression between tumor and healthy tissue. Multidrug resistance coordinated with glucuronidation has also been described here for drugs used in the management of epilepsy, psychiatric diseases, HIV infections, hypertension and hypercholesterolemia. Proposals to reverse UGT-mediated drug resistance should consider the endogenous functions of UGT.

Blood-brain barrier, bulk flow, and interstitial clearance in epilepsy

Understanding the pathophysiology of epilepsy implies elucidating the neurovascular modifications occurring before or at time of seizures. Cerebrovascular dysfunction provokes or sustains seizures and loss of selective blood-brain barrier (BBB) permeability is a modulator of seizure threshold. However, cerebrovascular pathology in epilepsy extends beyond BBB "leakage" to encompass vascular and parenchymal events. Whenever abnormal accumulation of protein is observed surrounding brain blood vessels, BBB disruption (BBBD) was invoked. Recent clinical and laboratory findings challenged an exclusive role of BBBD in perivascular accumulation of serum-derived products. The circulation of interstitial fluid (ISF) and its bulk flow have emerged as candidate mechanisms which play a role in clearance of CNS waste. Although controversy exists, changes of ISF flow may contribute to CNS disorders through a mechanism encompassing incomplete parenchymal clearance and accompanying accumulation of toxic byproducts. We summarize the evidence in favor and against ISF bulk flow and propose a scenario where abnormal ISF in the epileptic brain allows accumulation of brain protein, sustaining pathophysiology and altering the pharmacology of antiepileptic drugs. We also describe the methods routinely used to dissect out the contribution of BBB-dependent, vascular or paracellular mechanisms to altered neuronal excitability.

Sertraline-induced potentiation of the CYP3A4-dependent neurotoxicity of carbamazepine: an in vitro study

OBJECTIVE

Drug toxicity is a hurdle to drug development and to clinical translation of basic research. Antiepileptic drugs such as carbamazepine (CBZ) and selective serotonin reuptake inhibitors such as sertraline (SRT) are commonly co-prescribed to patients with epilepsy and comorbid depression. Because SRT may interfere with cytochrome P450 (CYP) enzyme activity and CYPs have been implicated in the conversion of CBZ to reactive cytotoxic metabolites, we investigated in vitro models to determine whether SRT affects the neurotoxic potential of CBZ and the mechanisms involved.

METHODS

Human fetal brain-derived dopaminergic neurons, human brain microvascular endothelial cells (HBMECs), and embryonic kidney (HEK) cells were used to evaluate cytotoxicity of CBZ and SRT individually and in combination. Nitrite and glutathione (GSH) levels were measured with drug exposure. To validate the role of CYP3A4 in causing neurotoxicity, drug metabolism was compared to cell death in HEK CYP3A4 overexpressed and cells pretreated with the CYP3A4 inhibitor ketoconazole.

RESULTS

In all cellular systems tested, exposure to CBZ (127 μM) or SRT (5 μM) alone caused negligible cytotoxicity. By contrast CBZ, tested at a much lower concentration (17 μM) in combination with SRT (5 μM), produced prominent cytotoxicity within 15 min exposure. In neurons and HBMECs, cytotoxicity was associated with increased nitrite levels, suggesting involvement of free radicals as a pathogenetic mechanism. Pretreatment of HBMECs with reduced GSH or with the GSH precursor N-acetyl-L-cysteine prevented cytotoxic response. In HEK cells, the cytotoxic response to the CBZ + SRT combination correlated with the rate of CBZ biotransformation and production of 2-hydroxy CBZ, further suggesting a causative role of reactive metabolites. In the same system, cytotoxicity was potentiated by overexpression of CYP3A4, and prevented by CYP3A4 inhibitor.

SIGNIFICANCE

These results demonstrate an unexpected neurotoxic interaction between CBZ and SRT, apparently related to increased CYP3A4-mediated production of reactive CBZ metabolites. The potential clinical implications of these findings are discussed.

Frequencies of UGT1A4*2 (P24T) and *3 (L48V) and their effects on serum concentrations of lamotrigine

The gene encoding uridine diphosphate glucuronosyltransferase (UGT) 1A4 shows considerable polymorphism. Several common drugs are metabolised by UGT1A4, among them lamotrigine (LTG). Experimental and clinical studies suggest that certain variants of UGT1A4 are associated with altered enzyme activity. However, results are conflicting. This clinical study aimed to investigate the frequencies of two common UGT1A4 variants, *2 (P24T) and *3 (L48V), and their potential effects on serum concentrations of LTG. The *2 variant was associated with a trend towards higher serum concentrations, while the *3 variant was associated with significantly lower serum concentrations of LTG. The calculated allele frequencies were in the same range as in earlier studies on Caucasian populations. To our knowledge, this is the first study suggesting a clinical effect of UGT1A4*2. Further study is needed to confirm this finding.

In vitro cerebrovascular modeling in the 21st century: current and prospective technologies

The blood-brain barrier (BBB) maintains the brain homeostasis and dynamically responds to events associated with systemic and/or rheological impairments (e.g., inflammation, ischemia) including the exposure to harmful xenobiotics. Thus, understanding the BBB physiology is crucial for the resolution of major central nervous system CNS) disorders challenging both health care providers and the pharmaceutical industry. These challenges include drug delivery to the brain, neurological disorders, toxicological studies, and biodefense. Studies aimed at advancing our understanding of CNS diseases and promoting the development of more effective therapeutics are primarily performed in laboratory animals. However, there are major hindering factors inherent to in vivo studies such as cost, limited throughput and translational significance to humans. These factors promoted the development of alternative in vitro strategies for studying the physiology and pathophysiology of the BBB in relation to brain disorders as well as screening tools to aid in the development of novel CNS drugs. Herein, we provide a detailed review including pros and cons of current and prospective technologies for modelling the BBB in vitro including ex situ, cell based and computational (in silico) models. A special section is dedicated to microfluidic systems including micro-BBB, BBB-on-a-chip, Neurovascular Unit-on-a-Chip and Synthetic Microvasculature Blood-brain Barrier.

The UDP-glucuronosyltransferases of the blood-brain barrier: their role in drug metabolism and detoxication

UDP-glucuronosyltransferases (UGTs) form a multigenic family of membrane-bound enzymes expressed in various tissues, including brain. They catalyze the formation of β-D-glucuronides from structurally unrelated substances (drugs, other xenobiotics, as well as endogenous compounds) by the linkage of glucuronic acid from the high energy donor, UDP-α-D-glucuronic acid. In brain, UGTs actively participate to the overall protection of the tissue against the intrusion of potentially harmful lipophilic substances that are metabolized as hydrophilic glucuronides. These metabolites are generally inactive, except for important pharmacologically glucuronides such as morphine-6-glucuronide. UGTs are mainly expressed in endothelial cells and astrocytes of the blood brain barrier (BBB). They are also associated to brain interfaces devoid of BBB, such as circumventricular organ, pineal gland, pituitary gland and neuro-olfactory tissues. Beside their key-role as a detoxication barrier, UGTs play a role in the steady-state of endogenous compounds, like steroids or dopamine (DA) that participate to the function of the brain. UGT isoforms of family 1A, 2A, 2B and 3A are expressed in brain tissues to various levels and are known to present distinct but overlapping substrate specificity. The importance of these enzyme species with regard to the formation of toxic, pharmacologically or physiologically relevant glucuronides in the brain will be discussed.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.