Vascular colocalization of P-glycoprotein, multidrug-resistance associated protein 1, breast cancer resistance protein and major vault protein in human epileptogenic pathologies

Comprehensive insights into herbal P-glycoprotein inhibitors and nanoformulations for improving anti-retroviral therapy efficacy

The worldwide HIV cases were 39.0 million (33.1-45.7 million) in 2022. Due to genetic variations, HIV-1 is more easily transmitted than HIV-2 and favours CD4 + T cells and macrophages, producing AIDS. Conventional HIV drug therapy has many drawbacks, including adherence issues leading to resistance, side effects that lower life quality, drug interactions, high costs limiting global access, inability to eliminate viral reservoirs, chronicity requiring lifelong treatment, emerging toxicities, and a focus on managing infections. Conventional dosage forms have bioavailability issues due to intestinal P-glycoprotein (P-gp) efflux, which can reduce anti-retroviral drug efficacy and lead to resistance. Use of phyto-constituents with P-gp regulating actions has great benefits for semi-synthetic modification to create formulations with greater bioavailability and reduced toxicity, which improves drug effectiveness. Lipid-based nanocarriers, solid lipid nanoparticles, nanostructured lipid carriers, polymer-based nanocarriers, and inorganic nanoparticles may inhibit P-gp efflux. Employing potent P-gp inhibitors within nanocarriers as a Trojan horse approach can enhance the intracellular accumulation of anti-retroviral drugs (ARDs), which are substrates for efflux transporters. This technique increases oral bioavailability and offers lower-dose options, boosting HIV patient compliance and lowering costs. Molecular docking of the inhibitor with P-gp may anticipate optimum binding and function, allowing drug efflux to be minimised.

Emerging Role of ABC Transporters in Glia Cells in Health and Diseases of the Central Nervous System

ATP-binding cassette (ABC) transporters play a crucial role for the efflux of a wide range of substrates across different cellular membranes. In the central nervous system (CNS), ABC transporters have recently gathered significant attention due to their pivotal involvement in brain physiology and neurodegenerative disorders, such as Alzheimer's disease (AD). Glial cells are fundamental for normal CNS function and engage with several ABC transporters in different ways. Here, we specifically highlight ABC transporters involved in the maintenance of brain homeostasis and their implications in its metabolic regulation. We also show new aspects related to ABC transporter function found in less recognized diseases, such as Huntington's disease (HD) and experimental autoimmune encephalomyelitis (EAE), as a model for multiple sclerosis (MS). Understanding both their impact on the physiological regulation of the CNS and their roles in brain diseases holds promise for uncovering new therapeutic options. Further investigations and preclinical studies are warranted to elucidate the complex interplay between glial ABC transporters and physiological brain functions, potentially leading to effective therapeutic interventions also for rare CNS disorders.

Diagnosis and prognosis of serum Fut8 for epilepsy and refractory epilepsy in children

With adequate serum concentration of antiepileptic drugs, the epilepsy symptoms in many patients still cannot be controlled well. The alteration of glycosyltransferase has obvious influence on the pathogenesis of epilepsy. In this study, we focus on the diagnostic and prognostic value of fucosyltransferase 8 (Fut8) on epilepsy and refractory epilepsy. Serum samples of 199 patients with epilepsy, 59 patients with refractory epilepsy and 22 healthy controls who were diagnosed in Shenzhen Children's hospital from August 2018 to August 2019 were collected. The level of lectins was further analyzed by lectin chip and enzyme linked immunosorbent assay (ELISA). The diagnostic value of serum Fut8 for epilepsy and refractory epilepsy was evaluated by receiver operating characteristic curve. Finally, the difference in the recurrence rate of convulsion in patients with epilepsy or refractory epilepsy within 2 years were observed in different Fut8 expression patients. The concentration of valproic acid (VPA) were significant different between epilepsy and refractory epilepsy group. The expression of α1, 6-fucosylation and Fut8 was significantly increased in the refractory epilepsy group compared with healthy controls. The area under the curve of Fut8 as a biomarker for predicting epilepsy or refractory epilepsy was 0.620 and 0.856, respectively. There was a significant difference in the recurrence rate of convulsion within 2 years in the children with refractory epilepsy (p = 0.0493) not epilepsy (p = 0.1865) between the high and low Fut8 expression groups. Fut8 was one of the effective indicators for the diagnosis and prognosis of refractory epilepsy.

ATP-binding cassette (ABC) drug transporters in the developing blood-brain barrier: role in fetal brain protection

The blood-brain barrier (BBB) provides essential neuroprotection from environmental toxins and xenobiotics, through high expression of drug efflux transporters in endothelial cells of the cerebral capillaries. However, xenobiotic exposure, stress, and inflammatory stimuli have the potential to disrupt BBB permeability in fetal and post-natal life. Understanding the role and ability of the BBB in protecting the developing brain, particularly with respect to drug/toxin transport, is key to promoting long-term brain health. Drug transporters, particularly P-gp and BCRP are expressed in early gestation at the developing BBB and have a crucial role in developmental homeostasis and fetal brain protection. We have highlighted several factors that modulate drug transporters at the developing BBB, including synthetic glucocorticoid (sGC), cytokines, maternal infection, and growth factors. Some factors have the potential to increase expression and function of drug transporters and increase brain protection (e.g., sGC, transforming growth factor [TGF]-β). However, others inhibit drug transporters expression and function at the BBB, increasing brain exposure to xenobiotics (e.g., tumor necrosis factor [TNF], interleukin [IL]-6), negatively impacting brain development. This has implications for pregnant women and neonates, who represent a vulnerable population and may be exposed to drugs and environmental toxins, many of which are P-gp and BCRP substrates. Thus, alterations in regulated transport across the developing BBB may induce long-term changes in brain health and compromise pregnancy outcome. Furthermore, a large portion of neonatal adverse drug reactions are attributed to agents that target or access the nervous system, such as stimulants (e.g., caffeine), anesthetics (e.g., midazolam), analgesics (e.g., morphine) and antiretrovirals (e.g., Zidovudine); thus, understanding brain protection is key for the development of strategies to protect the fetal and neonatal brain.

Current Principles in the Management of Drug-Resistant Epilepsy

Drug-resistant epilepsy is associated with poor health outcomes and increased economic burden. In the last three decades, various new antiseizure medications have been developed, but the proportion of people with drug-resistant epilepsy remains relatively unchanged. Developing strategies to address drug-resistant epilepsy is essential. Here, we define drug-resistant epilepsy and emphasize its relationship to the conceptualization of epilepsy as a symptom complex, delineate clinical risk factors, and characterize mechanisms based on current knowledge. We address the importance of ruling out pseudoresistance and consider the impact of nonadherence on determining whether an individual has drug-resistant epilepsy. We then review the principles of epilepsy drug therapy and briefly touch upon newly approved and experimental antiseizure medications.

P-glycoprotein: new insights into structure, physiological function, regulation and alterations in disease

The multidrug resistance phenomenon presents a major threat to the pharmaceutical industry. This resistance is a common occurrence in several diseases and is mediated by multidrug transporters that actively pump substances out of the cell and away from their target regions. The most well-known multidrug transporter is the P-glycoprotein transporter. The binding sites within P-glycoprotein can accommodate a variety of compounds with diverse structures. Hence, numerous drugs are P-glycoprotein substrates, with new ones being identified every day. For many years, the mechanisms of action of P-glycoprotein have been shrouded in mystery, and scientists have only recently been able to elucidate certain structural and functional aspects of this protein. Although P-glycoprotein is highly implicated in multidrug resistant diseases, this transporter also performs various physiological roles in the human body and is expressed in several tissues, including the brain, kidneys, liver, gastrointestinal tract, testis, and placenta. The expression levels of P-glycoprotein are regulated by different enzymes, inflammatory mediators and transcription factors; alterations in which can result in the generation of a disease phenotype. This review details the discovery, the recently proposed structure and the regulatory functions of P-glycoprotein, as well as the crucial role it plays in health and disease.

Major vault protein attenuates cardiomyocyte injury in doxorubicin-induced cardiomyopathy through activating AKT

Background

Doxorubicin (DOX) has limited chemotherapy application for malignancies due to cardiotoxicity. The pathogenesis of DOX-induced cardiomyopathy (DiCM) is yet to be elucidated. Increasing studies proved that activation of AKT prevented cardiomyocyte apoptosis and cardiac dysfunction in response to DOX insult. Our previous studies indicated that major vault protein (MVP) deficiency was accompanied by suppressed phosphorylation of AKT in metabolic diseases. This study aimed to investigate the role and underlying mechanism of MVP on cardiomyocyte apoptosis in DiCM.

Methods

Mice were intraperitoneally injected with DOX 5 mg/kg, once a week for 5 weeks, the total cumulative dose was 25 mg/kg. Cardiomyocyte-specific MVP overexpression was achieved using an adeno-associated virus system under the cTnT promoter after the fourth DOX injection. Cardiac function was examined by echocardiography followed by euthanasia. Tissue and serum were collected for morphology analysis and biochemical examination.

Results

Herein, we found that MVP expression was upregulated in DOX-treated murine hearts. Cardiac-specific MVP overexpression alleviated DOX-induced cardiac dysfunction, oxidative stress and fibrosis. Mechanistically, MVP overexpression activated AKT signaling and decreased cardiomyocyte apoptosis in DiCM.

Conclusions

Based on these findings, we supposed that MVP was a potential therapeutic agent against DiCM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-022-02517-9.

Emerging therapeutic targets for epilepsy: Preclinical insights

INTRODUCTION

Around 30% of patients with epilepsy suffer from drug-resistant seizures. Drug-resistant seizures may have significant consequences such as sudden death in epilepsy, injuries, memory disturbances, and childhood learning and developmental problems. Conventional and newer available antiepileptic drugs (AEDs) work via numerous mechanisms - mainly through inhibition of voltage-operated Na⁺ and/or Ca²⁺ channels, excitation of K⁺ channels, enhancement of GABA-mediated inhibition and/or blockade of glutamate-produced excitation. However, the discovery and development of novel brain targets may improve the future pharmacological management of epilepsy and hence is of pivotal importance.

AREAS COVERED

This article examines novel drug targets such as brain multidrug efflux transporters and inflammatory pathways; it progresses to discuss possible strategies for the management of drug-resistant seizures. Reduction of the consequences of blood brain barrier dysfunction and enhancement of anti-oxidative defense are discussed.

EXPERT OPINION

Novel drug targets comprise brain multidrug efflux transporters, TGF-β, Nrf2-ARE or m-TOR signaling and inflammatory pathways. Gene therapy and antagomirs seem the most promising targets. Epileptic foci may be significantly suppressed by viral-vector-mediated gene transfer, leading to an increased in situ concentration of inhibitory factors (for instance, galanin). Also, antagomirs offer a promising possibility of seizure inhibition by silencing micro-RNAs involved in epileptogenesis and possibly in seizure generation.

The role of efflux transporters and metabolizing enzymes in brain and peripheral organs to explain drug-resistant epilepsy

Drug‐resistant epilepsy has been explained by different mechanisms. The most accepted one involves overexpression of multidrug transporters proteins at the blood brain barrier and brain metabolizing enzymes. This hypothesis is one of the main pharmacokinetic reasons that lead to the lack of response of some antiseizure drug substrates of these transporters and enzymes due to their limited entrance into the brain and limited stay at the sites of actions. Although uncontrolled seizures can be the cause of the overexpression, some antiseizure medications themselves can cause such overexpression leading to treatment failure and thus refractoriness. However, it has to be taken into account that the inductive effect of some drugs such as carbamazepine or phenytoin not only impacts on the brain but also on the rest of the body with different intensity, influencing the amount of drug available for the central nervous system. Such induction is not only local drug concentration but also time dependent. In the case of valproic acid, the deficient disposition of ammonia due to a malfunction of the urea cycle, which would have its origin in an intrinsic deficiency of L‐carnitine levels in the patient or by its depletion caused by the action of this antiseizure drug, could lead to drug‐resistant epilepsy. Many efforts have been made to change this situation. In order to name some, the administration of once‐daily dosing of phenytoin or the coadministration of carnitine with valproic acid would be preferable to avoid iatrogenic refractoriness. Another could be the use of an adjuvant drug that down‐regulates the expression of transporters. In this case, the use of cannabidiol with antiseizure properties itself and able to diminish the overexpression of these transporters in the brain could be a novel therapy in order to allow penetration of other antiseizure medications into the brain.

Understanding mechanisms of drug resistance in epilepsy and strategies for overcoming it

INTRODUCTION

The present evidence indicates that approximately 70% of patients with epilepsy can be successfully treated with antiepileptic drugs (AEDs). A significant proportion of patients are not under sufficient control, and pharmacoresistant epilepsy is clearly associated with poor quality of life and increased morbidity and mortality. There is a great need for newer therapeutic options able to reduce the percentage of drug-resistant patients.

AREAS COVERED

A number of hypotheses trying to explain the development of pharmacoresistance have been put forward. These include: target hypothesis (altered AED targets), transporter (overexpression of brain efflux transporters), pharmacokinetic (overexpression of peripheral efflux transporters in the intestine or kidneys), intrinsic severity (initial high seizure frequency), neural network (aberrant networks), and gene variant hypothesis (genetic polymorphisms).

EXPERT OPINION

A continuous search for newer AEDs or among non-AEDs (blockers of efflux transporters, interleukin antagonists, cyclooxygenase inhibitors, mTOR inhibitors, angiotensin II receptor antagonists) may provide efficacious drugs for the management of drug-resistant epilepsy. Also, combinations of AEDs exerting synergy in preclinical and clinical studies (for instance, lamotrigine + valproate, levetiracetam + valproate, topiramate + carbamazepine) might be of importance in this respect. Preclinically antagonistic combinations must be avoided (lamotrigine + carbamazepine, lamotrigine + oxcarbazepine).

miR-485's anti-drug resistant epilepsy effects by regulating SV2A/PSD-95 and targeting ABCC1 and neuronal signaling-transduction proteins in hippocampus of rats

AIM

Drug-resistant epilepsy (DRE), most subsequently developing refractory epilepsy, causes a significant burden to the society. microRNAs have been demonstrated as key regulators and therapeutic targets in epilepsy. Accordingly, the aim of the present study was to test whether miR-485 could be a potential target for DRE.

METHODS AND RESULTS

An in vivo DRE model was developed in Sprague-Dawley rats by lithium chloride-pilocarpine and screened by antiepileptic drugs. We found that miR-485-5p in hippocampus was significant downregulated at early stage and recovered to normal level at late stage of DRE. Overexpression of miR-485-5p in dentate gyrus (DG) of hippocampus in DRE rats could significantly decrease the frequency of seizures and the numbers of epileptiform spikes of hippocampal DG neuron, and could specifically decrease SV2A expression without affecting PSD-95 expression in DG. Furthermore, miR-485-5p overexpression could significantly downregulate the expression of efflux transporter related to multidrug resistance (ABCC1) in hippocampus at late stage of DRE. Finally, a specific expression pattern of neuronal signaling-transduction proteins (LRP4, MDM4, p53, and TMBIM1) for DRE was observed, and miR-485-5p overexpression could modulate these proteins' expression levels toward normal in hippocampus both at early and late stage of DRE.

CONCLUSION

Collectively, these results suggest that miR-485 was a potential target for anti-DRE, and this effects might be partially via miR-485-5p/homeostatic-synaptic plasticity-molecule axis and/or targeting efflux transporter (ABCC1) and other neuronal signaling-transduction proteins (LRP4, MDM4, p53, and TMBIM1).

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

A novel approach of targeting refractory epilepsy: Need of an hour

The refractory epilepsy adds to the global burden of epilepsy as about 25 % of all patients with epilepsy present drug-resistant epilepsy. The P-glycoprotein (P-gp) plays a vital role in the mechanism of resistance in epilepsy. The AED levels in the brain are regulated by the P-gp transport. The upregulation of P-gp results in low concentration of AEDs inside the brain parenchyma and thus leads to resistance. There are three main conditions which lead to decrease transport of AEDs in refractory epilepsy. First being AEDs as substrate of P-gp; secondly, the elevated expression of P-gp in patients with drug resistant epilepsy as compared to drug-responsive patients; thirdly, the low brain AED concentration in refractory epilepsy in comparison to drug-responsive epilepsy. Therefore, determination of P-gp substrate should be a criterion for the selection of new AED for management of refractory epilepsy. This review highlights various tools which help in identification of P-gp substrates and also illustrates a concept of using various novel non-P-gp substrates which can cross the blood brain barrier and leads to enhanced accumulation inside the brain. Hence, these non P-gp substrates can be used as an add on treatment for the management of resistant epilepsy.

P-glycoprotein overactivity in epileptogenic developmental lesions measured in vivo using (R)-[11 C]verapamil PET

OBJECTIVE

Overexpression of the drug transporter P-glycoprotein (P-gp) is thought to be involved in drug-resistance in epilepsy by extrusion of antiepileptic drugs (AEDs). We used positron emission tomography (PET) and the P-gp substrate radiotracer (R)-[¹¹ C]verapamil (VPM) together with the third-generation P-gp inhibitor tariquidar (TQD) to evaluate P-gp function in individuals with drug-resistant epileptogenic developmental lesions.

METHODS

Twelve healthy controls (7 male, median age 45, range 35-55 years), and two patients with epileptogenic developmental lesions (2 male, aged 24 and 62 years) underwent VPM-PET scans before and 60 minutes after a 30-minute infusion of 2 and 3 mg/kg TQD. The influx rate constant, VPM-K₁ , was estimated from the first 10 minutes of dynamic data using a single-tissue compartment model with a VPM plasma input function. Statistical parametric mapping (SPM) analysis was used to compare individual patients with the healthy controls.

RESULTS

At baseline, SPM voxel-based analysis revealed significantly lower uptake of VPM corresponding to the area of the epileptogenic developmental lesion compared to 12 healthy controls (P < .048). This was accentuated following P-gp inhibition with TQD. After TQD, the uptake of VPM was significantly lower in the area of the epileptogenic developmental lesion compared to controls (P < .002).

SIGNIFICANCE

This study provides further evidence of P-gp overactivity in patients with drug-resistant epilepsy, irrespective of the type of lesion. Identifying P-gp overactivity as an underlying contributor to drug-resistance in individual patients will enable novel treatment strategies aimed at overcoming or reversing P-gp overactivity.

Focused Ultrasound Hyperthermia Augments Release of Glioma-derived Extracellular Vesicles with Differential Immunomodulatory Capacity

Background: Increasing evidence points to the critical role of extracellular vesicles (EVs) as molecular parcels that carry a diverse array of bioactive payloads for coordination of complex intracellular signaling. Focused ultrasound (FUS) hyperthermia is a technique for non-invasive, non-ionizing sublethal heating of cells in a near-instantaneous manner; while it has been shown to improve drug delivery and immunological recognition of tumors, its impact on EVs has not been explored to date. The goal of this study was to determine whether FUS impacts the release, proteomic profile, and immune-activating properties of tumor-derived EVs. Methods: Monolayered murine glioma cells were seeded within acoustically transparent cell culture chambers, and FUS hyperthermia was applied to achieve complete coverage of the chamber. Glioma-derived EVs (GEVs) were isolated for characterization by Nanoparticle Tracking Analysis, cryo-electron microscopy and mass spectrometry. An in vitro experimental setup was designed to further dissect the impact of GEVs on innate inflammation; immortalized murine dendritic cells (DCs) were pulsed with GEVs (either naïve or FUS hyperthermia-exposed) and assayed for production of IL-12p70, an important regulator of DC maturation and T helper cell polarization toward the interferon-γ-producing type 1 phenotype. Results: We confirmed that FUS hyperthermia significantly augments GEV release (by ~46%) as well as shifts the proteomic profile of these GEVs. Such shifts included enrichment of common EV-associated markers, downregulation of markers associated with cancer progression and resistance and modulation of inflammation-associated markers. When DCs were pulsed with GEVs, we noted that naïve GEVs suppressed IL-12p70 production by DCs in a GEV dose-dependent manner. In contrast, GEVs from cells exposed to FUS hyperthermia promoted a significant upregulation in IL-12p70 production by DCs, consistent with a pro-inflammatory stimulus. Conclusion: FUS hyperthermia triggers release of proteomically distinct GEVs that are capable of facilitating an important component of innate immune activation, lending both to a potential mechanism by which FUS interfaces with the tumor-immune landscape and to a role for GEV-associated biomarkers in monitoring response to FUS.

Why we urgently need improved epilepsy therapies for adult patients

PURPOSE

Up to a third of patients with epilepsy suffer from recurrent seizures despite therapeutic advances.

RESULTS

Current epilepsy treatments are limited by experiential data from treating different types of epilepsy. For example, we lack evidence-based approaches to efficacious multi-drug therapies or identifying potentially serious or disabling adverse events before medications are initiated. Despite advances in neuroscience and genetics, our understanding of epilepsy pathogenesis and mechanisms of treatment-resistance remains limited. For most patients with epilepsy, precision medicine for improved seizure control and reduced toxicity remains a future goal.

CONCLUSION

A third of epilepsy patients suffer from ongoing seizures and even more suffer from adverse effects of treatment. There is a critical need for more effective and safer therapies for epilepsy patients with frequent comorbitidies, including depression, anxiety, migraine, and cognitive impairments, as well as special populations (e.g., women, elderly). Advances from genomic sequencing techniques may identify new genes and regulatory elements that influence both the depth of the epilepsies' roots within brain circuitry as well as ASD resistance. Improved understanding of epilepsy mechanisms, identification of potential new therapeutic targets, and their assessment in randomized controlled trials are needed to reduce the burden of refractory epilepsy. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

Cytosolic phospholipase A2 is a key regulator of blood-brain barrier function in epilepsy

Blood-brain barrier dysfunction in epilepsy contributes to seizures and resistance to antiseizure drugs. Reports show that seizures increase brain glutamate levels, leading to barrier dysfunction. One component of barrier dysfunction is overexpression of the drug efflux transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). Based on our previous studies, we hypothesized that glutamate released during seizures activates cytosolic phospholipase A2 (cPLA2), resulting in P-gp and BCRP overexpression. We exposed isolated rat brain capillaries to glutamate ex vivo and used an in vivo-ex vivo approach of isolating brain capillaries from rats after status epilepticus (SE) and in chronic epileptic (CE) rats. Glutamate increased cPLA2, P-gp, and BCRP protein and activity levels in isolated brain capillaries. We confirmed the role of cPLA2 in the signaling pathway in brain capillaries from male and female mice lacking cPLA2. We also demonstrated, in vivo, that cPLA2 inhibition prevents overexpression of P-gp and BCRP at the blood-brain barrier in rats after status epilepticus and in CE rats. Our data support the hypothesis that glutamate signals cPLA2 activation, resulting in overexpression of blood-brain barrier P-gp and BCRP.-Hartz, A. M. S., Rempe, R. G., Soldner, E. L. B., Pekcec, A., Schlichtiger, J., Kryscio, R., Bauer, B. Cytosolic phospholipase A2 is a key regulator of blood-brain barrier function in epilepsy.

Exosomal miRNA: Small Molecules, Big Impact in Colorectal Cancer

Colorectal cancer (CRC) is one of the major causes of cancer-related deaths worldwide. Tumor microenvironment (TME) contains many cell types including stromal cells, immune cells, and endothelial cells. The TME modulation explains the heterogeneity of response to therapy observed in patients. In this context, exosomes are emerging as major contributors in cancer biology. Indeed, exosomes are implicated in tumor proliferation, angiogenesis, invasion, and premetastatic niche formation. They contain bioactive molecules such as proteins, lipids, and RNAs. More recently, many studies on exosomes have focused on miRNAs, small noncoding RNA molecules able to influence protein expression. In this review, we describe miRNAs transported by exosomes in the context of CRC and discuss their influence on TME and their potential as circulating biomarkers. This overview underlines emerging roles for exosomal miRNAs in cancer research for the near future.

ABC transporters in drug-resistant epilepsy: mechanisms of upregulation and therapeutic approaches

Drug-resistant epilepsy (DRE) affects approximately one third of epileptic patients. Among various theories that try to explain multidrug resistance, the transporter hypothesis is the most extensively studied. Accordingly, the overexpression of efflux transporters in the blood-brain barrier (BBB), mainly from the ATP binding cassette (ABC) superfamily, may be responsible for hampering the access of antiepileptic drugs into the brain. P-glycoprotein and other efflux transporters are known to be upregulated in endothelial cells, astrocytes and neurons of the neurovascular unit, a functional barrier critically involved in the brain penetration of drugs. Inflammation and oxidative stress involved in the pathophysiology of epilepsy together with uncontrolled recurrent seizures, drug-associated induction and genetic polymorphisms are among the possible causes of ABC transporters overexpression in DRE. The aforementioned pathological mechanisms will be herein discussed together with the multiple strategies to overcome the activity of efflux transporters in the BBB - from direct transporters inhibition to down-regulation of gene expression resorting to RNA interference (RNAi), or by targeting key modulators of inflammation and seizure-mediated signalling.

Drug-Resistant Epilepsy: Multiple Hypotheses, Few Answers

Epilepsy is a common neurological disorder that affects over 70 million people worldwide. Despite the recent introduction of new antiseizure drugs (ASDs), about one-third of patients with epilepsy have seizures refractory to pharmacotherapy. Early identification of patients who will become refractory to ASDs could help direct such patients to appropriate non-pharmacological treatment, but the complexity in the temporal patterns of epilepsy could make such identification difficult. The target hypothesis and transporter hypothesis are the most cited theories trying to explain refractory epilepsy, but neither theory alone fully explains the neurobiological basis of pharmacoresistance. This review summarizes evidence for and against several major theories, including the pharmacokinetic hypothesis, neural network hypothesis, intrinsic severity hypothesis, gene variant hypothesis, target hypothesis, and transporter hypothesis. The discussion is mainly focused on the transporter hypothesis, where clinical and experimental data are discussed on multidrug transporter overexpression, substrate profiles of ASDs, mechanism of transporter upregulation, polymorphisms of transporters, and the use of transporter inhibitors. Finally, future perspectives are presented for the improvement of current hypotheses and the development of treatment strategies as guided by the current understanding of refractory epilepsy.

Efflux proteins at the blood-brain barrier: review and bioinformatics analysis

1. Efflux proteins at the blood-brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics. 2. Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood-brain barrier. 3. We review efflux proteins at the blood-brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1). 4. This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET). 5. We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.

MVP-mediated exosomal sorting of miR-193a promotes colon cancer progression

Exosomes are emerging mediators of intercellular communication; whether the release of exosomes has an effect on the exosome donor cells in addition to the recipient cells has not been investigated to any extent. Here, we examine different exosomal miRNA expression profiles in primary mouse colon tumour, liver metastasis of colon cancer and naive colon tissues. In more advanced disease, higher levels of tumour suppressor miRNAs are encapsulated in the exosomes. miR-193a interacts with major vault protein (MVP). Knockout of MVP leads to miR-193a accumulation in the exosomal donor cells instead of exosomes, inhibiting tumour progression. Furthermore, miR-193a causes cell cycle G1 arrest and cell proliferation repression through targeting of Caprin1, which upregulates Ccnd2 and c-Myc. Human colon cancer patients with more advanced disease show higher levels of circulating exosomal miR-193a. In summary, our data demonstrate that MVP-mediated selective sorting of tumour suppressor miRNA into exosomes promotes tumour progression.

Exosomes are involved in the development of metastasis but how their composition is regulated is not well known. Here the authors propose that major vault protein-dependent loading of miR-193a into exosomes could be a general mechanism by which cancer cells get rid of oncosuppressor miRNAs.

P-Glycoprotein, not BCRP, Limits the Brain Uptake of [(18)F]Mefway in Rodent Brain

PURPOSE

The aim of this study was to determine whether the brain uptake of [(18)F]Mefway is influenced by the action of P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) in rodents.

PROCEDURES

[(18)F]Mefway was applied to rats pharmacologically inhibited with tariquidar (TQD) and to genetically disrupted mice.

RESULTS

Pretreatment of TQD results in 160% higher hippocampal uptake compared with control rats. In genetically disrupted mice, a maximal brain uptake value of 3.2 SUV in the triple knockout mice (tKO, Mdr1a/b((-/-))Bcrp1((-/-))) was comparable to that of the double knockout mice (dKO, Mdr1a/b((-/-))) and 2-fold those of the wild-type and Bcrp1((-/-)) knockout mice. The differences of binding values were statistically insignificant between control and experimental groups. The brain-to-plasma ratios for tKO mice were also two to five times higher than those for other groups.

CONCLUSIONS

[(18)F]Mefway is modulated by P-gp, and not by Bcrp in rodents.

The Inhibitor Ko143 Is Not Specific for ABCG2

Imaging ATP-binding cassette (ABC) transporter activity in vivo with positron emission tomography requires both a substrate and a transporter inhibitor. However, for ABCG2, there is no inhibitor proven to be specific to that transporter alone at the blood-brain barrier. Ko143 [[(3S,6S,12aS)-1,2,3,4,6,7,12,12a-octahydro-9-methoxy-6-(2-methylpropyl)-1,4-dioxopyrazino[1',2':1,6]pyrido[3,4- b]indole-3-propanoic acid 1,1-dimethylethyl ester], a nontoxic analog of fungal toxin fumitremorgin C, is a potent inhibitor of ABCG2, although its specificity in mouse and human systems is unclear. This study examined the selectivity of Ko143 using human embryonic kidney cell lines transfected with ABCG2, ABCB1, or ABCC1 in several in vitro assays. The stability of Ko143 in rat plasma was measured using high performance liquid chromatography. Our results show that, in addition to being a potent inhibitor of ABCG2, at higher concentrations (≥1 μM) Ko143 also has an effect on the transport activity of both ABCB1 and ABCC1. Furthermore, Ko143 was found to be unstable in rat plasma. These findings indicate that Ko143 lacks specificity for ABCG2 and this should be taken into consideration when using Ko143 for both in vitro and in vivo experiments.

Glutamate-Mediated Down-Regulation of the Multidrug-Resistance Protein BCRP/ABCG2 in Porcine and Human Brain Capillaries

Breast cancer resistance protein (BCRP) functions as a major molecular gatekeeper at the blood-brain barrier. Considering its impact on access to the brain by therapeutic drugs and harmful xenobiotics, it is of particular interest to elucidate the mechanisms of its regulation. Excessive glutamate concentrations have been reported during epileptic seizures or as a consequence of different brain insults including brain ischemia. Previously, we have demonstrated that glutamate can trigger an induction of the transporter P-glycoprotein. These findings raised the question whether other efflux transporters are affected in a comparable manner. Glutamate exposure proved to down-regulate BCRP transport function and expression in isolated porcine capillaries. The reduction was efficaciously prevented by coincubation with N-methyl-d-aspartate (NMDA) receptor antagonist MK-801. The involvement of the NMDA receptor in the down-regulation of BCRP was further confirmed by experiments showing an effect of NMDA exposure on brain capillary BCRP transport function and expression. Pharmacological targeting of cyclooxygenase-1 and -2 (COX-1 and -2) using the nonselective inhibitor indomethacin, COX-1 inhibitor SC-560, and COX-2 inhibitor celecoxib revealed a contribution of COX-2 activity to the NMDA receptor's downstream signaling events affecting BCRP. Translational studies were performed using human capillaries isolated from surgical specimens of epilepsy patients. The findings confirmed a glutamate-induced down-regulation of BCRP transport activity in human capillaries, which argued against major species differences. In conclusion, our data reveal a novel mechanism of BCRP down-regulation in porcine and human brain capillaries. Moreover, together with previous data sets for P-glycoprotein, the findings point to a contrasting impact of the signaling pathway on the regulation of BCRP and P-glycoprotein. The effect of glutamate and arachidonic acid signaling on BCRP function might have implications for brain drug delivery and for radiotracer brain access in epilepsy patients and patients with other brain insults.

Drug resistance in cortical and hippocampal slices from resected tissue of epilepsy patients: no significant impact of p-glycoprotein and multidrug resistance-associated proteins

Drug resistant patients undergoing epilepsy surgery have a good chance to become sensitive to anticonvulsant medication, suggesting that the resected brain tissue is responsible for drug resistance. Here, we address the question whether P-glycoprotein (Pgp) and multidrug resistance-associated proteins (MRPs) expressed in the resected tissue contribute to drug resistance in vitro. Effects of anti-epileptic drugs [carbamazepine (CBZ), sodium valproate, phenytoin] and two unspecific inhibitors of Pgp and MRPs [verapamil (VPM) and probenecid (PBN)] on seizure-like events (SLEs) induced in slices from 35 hippocampal and 35 temporal cortex specimens of altogether 51 patients (161 slices) were studied. Although in slice preparations the blood brain barrier is not functional, we found that SLEs predominantly persisted in the presence of anticonvulsant drugs (90%) and also in the presence of VPM and PBN (86%). Following subsequent co-administration of anti-epileptic drugs and drug transport inhibitors, SLEs continued in 63% of 143 slices. Drug sensitivity in slices was recognized either as transition to recurrent epileptiform transients (30%) or as suppression (7%), particularly by perfusion with CBZ in PBN containing solutions (43, 9%). Summarizing responses to co-administration from more than one slice per patient revealed that suppression of seizure-like activity in all slices was only observed in 7% of patients. Patients whose tissue was completely or partially sensitive (65%) presented with higher seizure frequencies than those with resistant tissue (35%). However, corresponding subgroups of patients do not differ with respect to expression rates of drug transporters. Our results imply that parenchymal MRPs and Pgp are not responsible for drug resistance in resected tissue.

Dynamic regulation of P-glycoprotein in human brain capillaries

Considering its role as a major blood-brain barrier gatekeeper, the dynamic regulation of the efflux transporter P-glycoprotein is of considerable functional relevance. In particular, disease-associated alterations in transport function might affect central nervous system drug efficacy. Thus, targeting regulatory signaling cascades might render a basis for novel therapeutic approaches. Using capillaries freshly prepared from patient tissue resected during epilepsy surgery, we demonstrate dynamic regulation of P-glycoprotein in human brain capillaries. Glutamate proved to up-regulate P-glycoprotein efflux transport in a significant manner via endothelial NMDA receptors. Both inhibition of cyclooxygenase-2 and antagonism at the glycine-binding site of the NMDA receptor prevented the glutamate-mediated induction of P-glycoprotein transport function in human capillaries. In conclusion, the data argue against species differences in the signaling factors increasing endothelial P-glycoprotein transport function in response to glutamate exposure. Targeting of cyclooxygenase-2 and of the NMDA receptor glycine-binding site was confirmed as an efficacious approach to control P-glycoprotein function. The findings might render a basis for translational development of add-on approaches to improve brain penetration and efficacy of drugs.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

A novel PET protocol for visualization of breast cancer resistance protein function at the blood-brain barrier

Breast cancer resistance protein (BCRP) is the most abundant multidrug efflux transporter at the human blood-brain barrier (BBB), restricting brain distribution of various drugs. In this study, we developed a positron emission tomography (PET) protocol to visualize Bcrp function at the murine BBB, based on the dual P-glycoprotein (P-gp)/Bcrp substrate radiotracer [(11)C]tariquidar in combination with the Bcrp inhibitor Ko143. To eliminate the contribution of P-gp efflux to [(11)C]tariquidar brain distribution, we studied mice in which P-gp was genetically knocked out (Mdr1a/b((-/-)) mice) or chemically knocked out by pretreatment with cold tariquidar. We found that [(11)C]tariquidar brain uptake increased dose dependently after administration of escalating doses of Ko143, both in Mdr1a/b((-/-)) mice and in tariquidar pretreated wild-type mice. After 15 mg/kg Ko143, the maximum increase in [(11)C]tariquidar brain uptake relative to baseline scans was 6.3-fold in Mdr1a/b((-/-)) mice with a half-maximum effect dose of 4.98 mg/kg and 3.6-fold in tariquidar (8 mg/kg) pretreated wild-type mice, suggesting that the presented protocol is sensitive to visualize a range of different functional Bcrp activities at the murine BBB. We expect that this protocol can be translated to the clinic, because tariquidar can be safely administered to humans at doses that completely inhibit cerebral P-gp.

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

Neuropathology of the blood-brain barrier and pharmaco-resistance in human epilepsy

Blood-brain barrier dysfunction is implicated in various neurological conditions. Modulating the blood-brain barrier may have therapeutic value. Progress is hindered by our limited understanding of the pathophysiology of the blood-brain barrier in humans, partly due to restricted availability of human tissue, and because human tissue can only provide limited data about temporal patterns of change. We addressed these important challenges by examining surgically resected brain tissue with various lengths of interval between intracranial depth electrode-related injury and resection, and post-mortem whole brain from patients with drug-sensitive or drug-resistant chronic epilepsy and controls. In this valuable set of resources, we found that: (i) there is a highly localized overexpression of P-glycoprotein in the epileptogenic hippocampus of patients with drug-resistant epilepsy; (ii) this overexpression appears specific to P-glycoprotein and does not affect other transporters; (iii) P-glycoprotein is expressed on the vascular endothelium and end-feet of vascular glia (forming a 'double cuff') in drug-resistant epileptic cases but not in post-mortem controls or surgical epilepsy tissue with electrode-related injuries; (iv) an acute insult from intracranial electrode recording causes localized inflammation, increased blood-brain barrier permeability and structural changes to vasculature detectable for up to at least 330 days and (v) chronic epilepsy is associated with inflammation, enhanced blood-brain barrier permeability and increased P-glycoprotein expression. The occurrence of seizures appears central to P-glycoprotein overexpression. Our findings have potential clinical impact because they directly improve our understanding of blood-brain barrier disruption and transporter expression in humans. In particular, our findings show that the expression of P-glycoprotein in humans is compatible with the inherent assumptions of one current hypothesis of multidrug resistance, and that the specific upregulation of P-glycoprotein expression is likely to be associated with ongoing chronic seizures. There may be a therapeutic window after initial acute injury for the prevention of P-glycoprotein overexpression, and thus this one potential component of drug resistance. Our findings add to the need for careful consideration of the benefit and risks of invasive electroencephalographic recording in surgical evaluation of drug-resistant epilepsy.

Lack of association between ABCB1 gene polymorphisms and pharmacoresistant epilepsy: an analysis in a western Chinese pediatric population

OBJECTIVE

The genetic polymorphisms of the ABCB1 (ATP-binding cassette B1) gene encoding P-glycoprotein have been proposed to be associated with pharmacoresistance phenotype in epilepsy patients. P-glycoprotein, a transmembrane transporter, works as an efflux pump by limiting antiepileptic drugs across the blood brain barrier, with correspondingly lowering drug concentrations in epileptogenic loci. In this study, we analyzed whether the three single nucleotide polymorphisms (C1236T, G2677T/A, and C3435T) in the ABCB1 gene were associated with pharmacoresistant epilepsy in a western Chinese pediatric population.

METHODS

A total of 350 children with epilepsy who had been prescribed antiepileptic drugs for at least 1year were included. Of this patient group 193 were drug responsive and 157 were drug resistant according to the presence of seizures. Genotypes of the three loci of ABCB1 gene were detected in 368 age- and sex-matched normal children and 350 epileptic children using the polymerase chain reaction (PCR)-restriction fragment length polymorphism technique. Normal population sample populace from the same ethnicity and territory was genotyped to check for population stratification. The allele, genotype, haplotype, and diplotype frequencies of ABCB1 polymorphisms were compared between drug-resistant and drug-responsive subjects.

RESULTS

No significant differences were observed in the frequencies of genotype, allele, haplotype, or diplotype of ABCB1 polymorphisms between patients with drug-resistant and drug-responsive epilepsy (p>0.05).

CONCLUSION

The above three polymorphisms in the ABCB1 gene were not found to be significantly associated with drug resistant epilepsy in a western Chinese pediatric population.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

Possible role of heme oxygenase-1 and prostaglandins in the pathogenesis of cerebral malaria: heme oxygenase-1 induction by prostaglandin D(2) and metabolite by a human astrocyte cell line

Astrocytes are the most abundant cells in the central nervous system that play roles in maintaining the blood-brain-barrier and in neural injury, including cerebral malaria, a severe complication of Plasmodium falciparum infection. Prostaglandin (PG) D(2) is abundantly produced in the brain and regulates the sleep response. Moreover, PGD(2) is a potential factor derived from P. falciparum within erythrocytes. Heme oxygenase-1 (HO-1) is catalyzing enzyme in heme breakdown process to release iron, carbon monoxide, and biliverdin/bilirubin, and may influence iron supply to the P. falciparum parasites. Here, we showed that treatment of a human astrocyte cell line, CCF-STTG1, with PGD(2) significantly increased the expression levels of HO-1 mRNA by RT-PCR. Western blot analysis showed that PGD(2) treatment increased the level of HO-1 protein, in a dose- and time-dependent manner. Thus, PGD(2) may be involved in the pathogenesis of cerebral malaria by inducing HO-1 expression in malaria patients.

Pharmacogenetics of ATP-binding cassette transporters and clinical implications

Drug resistance is a severe limitation of chemotherapy of various malignancies. In particular efflux transporters of the ATP-binding cassette family such as ABCB1 (P-glycoprotein), the ABCC (multidrug resistance-associated protein) family, and ABCG2 (breast cancer resistance protein) have been identified as major determinants of chemoresistance in tumor cells. Bioavailability depends not only on the activity of drug metabolizing enzymes but also to a major extent on the activity of drug transport across biomembranes. They are expressed in the apical membranes of many barrier tissues such as the intestine, liver, blood-brain barrier, kidney, placenta, testis, and in lymphocytes, thus contributing to plasma, liquor, but also intracellular drug disposition. Since expression and function exhibit a broad variability, it was hypothesized that hereditary variances in the genes of membrane transporters could explain at least in part interindividual differences of pharmacokinetics of a variety of anticancer drugs and many others contributing to the clinical outcome of certain leukemias and further malignancies.

Levetiracetam decreases the seizure activity and blood-brain barrier permeability in pentylenetetrazole-kindled rats with cortical dysplasia

This study investigates the effects of levetiracetam (LEV) on the functional and structural properties of blood-brain barrier (BBB) in pentylenetetrazole (PTZ)-kindled rats with cortical dysplasia (CD). Pregnant rats were exposed to 145 cGy of gamma-irradiation on embryonic day 17. In offsprings, kindling was induced by giving subconvulsive doses of PTZ three times per week for 45 days. While all kindled rats with CD died during epileptic seizures evoked by the administration of a convulsive dose of PTZ in 15 to 25 min, one week LEV (80 mg/kg) pretreatment decreased the mortality to 38% in the same setting. LEV caused a remarkable decrease (p<0.01) in extravasation of sodium fluorescein dye into the brain tissue of kindled animals with CD treated with convulsive dose of PTZ. Occludin immunoreactivity and expression remained essentially unchanged in all groups. Immunoreactivity for glial fibrillary acidic protein (GFAP) was observed to be slightly increased by acute convulsive challenge in kindled rats with CD while LEV pretreatment led to GFAP immunoreactivity comparable to that of controls. An increased c-fos immunoreactivity in kindled rats with CD exposed to convulsive PTZ challenge was also observed with LEV pretreatment. Tight junctions were ultrastructurally intact, whereas LEV decreased the increased pinocytotic activity in brain endothelium of kindled rats with CD treated with convulsive dose of PTZ. The present study showed that LEV decreased the increased BBB permeability considerably by diminishing vesicular transport in epileptic seizures induced by convulsive PTZ challenge in kindled animals with CD.

Blood-brain barrier damage and brain penetration of antiepileptic drugs: role of serum proteins and brain edema

PURPOSE

Increased blood-brain barrier (BBB) permeability is radiologically detectable in regions affected by drug-resistant epileptogenic lesions. Brain penetration of antiepileptic drugs (AEDs) may be affected by BBB damage. We studied the effects of BBB damage on brain distribution of hydrophilic [deoxy-glucose (DOG) and sucrose] and lipophilic (phenytoin and diazepam) molecules. We tested the hypothesis that lipophilic and hydrophilic drug distribution is differentially affected by BBB damage.

METHODS

In vivo BBB disruption (BBBD) was performed in rats by intracarotid injection of hyperosmotic mannitol. Drugs (H3-sucrose, 3H-deoxy-glucose, 14C-phenytoin, and C14-diazepam) or unlabeled phenytoin was measured and correlated to brain water content and protein extravasation. In vitro hippocampal slices were exposed to different osmolarities; drug penetration and water content were assessed by analytic and densitometric methods, respectively.

RESULTS

BBBD resulted in extravasation of serum protein and radiolabeled drugs, but was associated with no significant change in brain water. Large shifts in water content in brain slices in vitro caused a small effect on drug penetration. In both cases, total drug permeability increase was greater for lipophilic than hydrophilic compounds. BBBD reduced the amount of free phenytoin in the brain.

DISCUSSION

After BBBD, drug binding to protein is the main controller of total brain drug accumulation. Osmotic BBBD increased serum protein extravasation and reduced free phenytoin brain levels. These results underlie the importance of brain environment and BBB integrity in determining drug distribution to the brain. If confirmed in drug-resistant models, these mechanisms could contribute to drug brain distribution in refractory epilepsies.

Nanotechnology for delivery of drugs to the brain for epilepsy

Epilepsy results from aberrant electrical activity that can affect either a focal area or the entire brain. In treating epilepsy with drugs, the aim is to decrease seizure frequency and severity while minimizing toxicity to the brain and other tissues. Antiepileptic drugs (AEDs) are usually administered by oral and intravenous routes, but these drug treatments are not always effective. Drug access to the brain is severely limited by a number of biological factors, particularly the blood-brain barrier, which impedes the ability of AEDs to enter and remain in the brain. To improve the efficacy of AEDs, new drug delivery strategies are being developed; these methods fall into the three main categories: drug modification, blood-brain barrier modification, and direct drug delivery. Recently, all three methods have been improved through the use of drug-loaded nanoparticles.

Lack of association between ABCB1, ABCG2, and ABCC2 genetic polymorphisms and multidrug resistance in partial epilepsy

ATP-binding cassette (ABC) transporters participate in drug disposition and response in various conditions, and many polymorphisms in ABC transporter genes have been recognized in association with altered transporter functions of various drugs. Studies on epilepsy have focused on the C3435T polymorphism of the ABCB1 gene, but other ABC transporters are also thought to be involved in the transport of antiepileptic drugs. We have evaluated the functional polymorphisms of ABCB1, ABCG2, and ABCC2 genes with regard to epilepsy drug response in partial epilepsy, and have investigated the potential of combined effects of polymorphisms in more than one transporter gene. We studied 6 genetic polymorphisms in 3 transporter genes in 193 drug responders and 198 nonresponders. There was no significant difference between the two groups, and we did not observe any combined effects of ABCB1 and ABCG2 genetic polymorphisms. Our study suggests that genetic polymorphisms in ABC transporters may not be significant predictors of drug response in epilepsy patients.

Exploring the possible interaction between anti-epilepsy drugs and multidrug efflux pumps; in vitro observations

Approximately one-third of patients with epilepsy display an inherent resistance to pharmacological therapy, manifest as continuing seizures despite maximal tolerated doses of anti-epileptic drugs. One hypothesis for the underlying mechanism of anti-epileptic drug pharmacoresistance is lower drug entry to the epileptic neurones due to the activity of multidrug efflux pumps from the ATP Binding Cassette (ABC) superfamily at the blood-brain barrier. There has been a steady accumulation of animal and human data supporting this theory, particularly for ABC(B1) (P-glycoprotein). However, much of this evidence is indirect. In the present study, several anti-epileptic drugs (carbamazepine, valproic acid, phenytoin, lamotrigine and primidone) were examined for their ability to interact with three ABC transporters that have been implicated pharmacoresistance of anti-epileptic drugs - ABC(B1), ABC(C1) and ABC(G2). Interaction of anti-epileptic drugs with the transporters was assessed by determining whether they could reverse the ability of multidrug ABC transporters to confer a drug resistance phenotype on cancer cell lines. None of these compounds was able to affect the phenotype, suggesting an absence of any interaction with the multidrug transporters. This finding was further investigated by examination of transporter activity; namely the ability to reduce steady-state intracellular [(3)H]-radiolabelled drug accumulation. None of the anti-epileptic drugs affected labelled drug accumulation by any of the triumvirate of multidrug transporters examined, indicating that they are unlikely to be substrates. The lack of direct modulation by anti-epileptic drugs of ABC transporter function suggests that these proteins do not contribute significantly to resistance in epilepsy.

Alpha-methyl-l-tryptophan positron emission tomography in epilepsy with cortical developmental malformations

Preliminary studies suggest that alpha[(11)C]methyl-l-tryptophan positron emission tomography can detect the epileptic focus within malformations of cortical development. We determined the sensitivity and specificity of alpha-[(11)C]methyl-l-tryptophan positron emission tomography in identifying epileptic focus in children with intractable, neocortical epilepsy with and without malformations of cortical development. Seventy-three epileptic children were classified into lesional and nonlesional groups, and compared regarding focal increased alpha-[(11)C]methyl-l-tryptophan uptake. The sensitivity and specificity of focal increased alpha-[(11)C]methyl-l-tryptophan uptake, using intracranial electroencephalogram localization of seizure onset as the standard, were compared between lesional and nonlesional groups. The specificity of alpha-[(11)C]methyl-l-tryptophan positron emission tomography for detecting seizure onset lobe was equally high in lesional (97%) and nonlesional groups (100%), whereas sensitivity was higher in the lesional than the nonlesional group (47% versus 29%; P = 0.047). The incidence of alpha-[(11)C]methyl-l-tryptophan uptake abnormality was higher in the lesional than the nonlesional group (P < 0.01). Alpha-[(11)C]methyl-l-tryptophan positron emission tomography localized and visualized epileptogenic regions in 25% of patients with nonlocalizing magnetic resonance imaging. Although overall sensitivity of alpha-[(11)C]methyl-l-tryptophan positron emission tomography in identifying neocortical epileptic focus is modest, specificity is extremely high. When an alpha-[(11)C]methyl-l-tryptophan focus is detected, it likely represents the epileptogenic region to be resected.

ABC transporter (P-gp/ABCB1, MRP1/ABCC1, BCRP/ABCG2) expression in the developing human CNS

P-glycoprotein (P-gp/ABCB1), multidrug resistance associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) are plasma membrane efflux pumps that limit the intracellular uptake and retention of numerous lipophilic, amphipathic xeno- and endobiotics. Little is known about the neonatal and developmental expression of P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 in the human central nervous system (CNS), therefore post-mortem CNS tissue from infants born at 22 (0/7)-42 (0/7) weeks of gestation and adults was immunostained to determine their ontogeny and cellular localization. P-gp/ABCB1 immunostaining was observed in microvessel endothelial cells as early as 22 (0/7) weeks, increasing in prevalence and intensity with maturation, and later in gestation in large pyramidal neurons. MRP1/ABCC1 immunostaining was prominent early in the choroid plexus and ventricular ependyma, and noted later in large pyramidal neurons. BCRP/ABCG2 expression was limited to microvessel endothelial cells. P-gp/ABCB1, MRP1/ABCC1 and BCRP/ABCG2 in adult brain matched term newborn CNS but with more intense immunostaining. We conclude that P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 are expressed in a developmental, cell specific, fashion in the human CNS. The complementary pattern of P-gp/ABCB1 and BCRP/ABCG2 at the blood-brain with MRP1/ABCC1 at the blood-CSF barriers may limit CNS uptake and retention of drugs and toxins in neonates.