Blood-brain barrier transport of valproic acid

Design of experiment (DoE) of mucoadhesive valproic acid-loaded nanostructured lipid carriers (NLC) for potential nose-to-brain application

Epilepsy is a highly prevalent neurological disease and valproic acid (VPA) is used as a first-line chronic treatment. However, this drug has poor oral bioavailability, which requires the administration of high doses, resulting in adverse effects. Alternative routes of VPA administration have therefore been investigated, such as the nose-to-brain route, which allows the drug to be transported directly from the nasal cavity to the brain. Here, the use of nanostructured lipid carriers (NLC) to encapsulate drugs administered in the nasal cavity has proved advantageous. The aim of this work was to optimise a mucoadhesive formulation of VPA-loaded NLC for intranasal administration to improve the treatment of epilepsy. The Design of Experiment (DoE) was used to optimise the formulation, starting with component optimisation using Mixture Design (MD), followed by optimisation of the manufacturing process parameters using Central Composite Design (CCD). The optimised VPA-loaded NLC had a particle size of 76.1 ± 2.8 nm, a polydispersity index of 0.190 ± 0.027, a zeta potential of 28.1 ± 2.0 mV and an encapsulation efficiency of 85.4 ± 0.8%. The in vitro release study showed VPA release from the NLC of 50 % after 6 h and 100 % after 24 h. The in vitro biocompatibility experiments in various cell lines have shown that the optimised VPA-loaded NLC formulation is safe up to 75 µg/mL, in neuronal (SH-SY5Y), nasal (RPMI 2650) and hepatic (HepG2) cells. Finally, the interaction of the optimised VPA-loaded NLC formulation with nasal mucus was investigated and mucoadhesive properties were observed. The results of this study suggest that the use of intranasal VPA-loaded NLC may be a promising alternative to promote VPA targeting to the brain, thereby improving bioavailability and minimising adverse effects.

Effects of co-administration of lamotrigine on valproate transfer across the placenta and its brain entry in developing Genetic Absence Epilepsy Rats from Strasbourg (GAERS)

During development, embryos and foetuses may be exposed to maternally ingested antiseizure medications (ASM), valproate and lamotrigine, essential in some patients to control their epilepsy symptoms. Often, the two drugs are co-administered to reduce required doses of valproate, a known potential teratogen. This study used Genetic Absence Epilepsy Rat from Strasbourg to evaluate transfer of valproate and lamotrigine across late gestation placenta and their entry into cerebrospinal fluid (CSF) and brain of developing rats, in mono- and combination therapies. Animals at embryonic day (E) 19, postnatal day (P) 0, 4 and 21, and adults were administered valproate (30 mg/kg) or lamotrigine (6 mg/kg) with their respective [3H]-tracers, either alone or in combination. In chronic experiments, females consumed valproate-containing diet from 2 weeks prior to mating until offspring were used at E19 and P0. Drugs were injected 30 min before blood, CSF and brain samples were collected from terminally anaesthetised animals. Radioactivity in samples was measured. In acute monotherapy brain entry of valproate was higher in foetal than postnatal animals, correlating with its plasma protein binding. Brain entry of lamotrigine was not age-dependent. Combination therapy enhanced entry of lamotrigine into the adult brain but had no effects on brain and CSF entry of valproate. Following chronic valproate exposure, placental transfer of valproate decreased in combination therapy; however, foetal brain entry increased. Results suggest that during pregnancy, the use of combination therapy of valproate and lamotrigine may mitigate overall foetal exposure to valproate but potential risks to foetal brain development are less clear.

Valproic Acid Treatment after Traumatic Brain Injury in Mice Alleviates Neuronal Death and Inflammation in Association with Increased Plasma Lysophosphatidylcholines

The histone deacetylase inhibitor (HDACi) valproic acid (VPA) has neuroprotective and anti-inflammatory effects in experimental traumatic brain injury (TBI), which have been partially attributed to the epigenetic disinhibition of the transcription repressor RE1-Silencing Transcription Factor/Neuron-Restrictive Silencer Factor (REST/NRSF). Additionally, VPA changes post-traumatic brain injury (TBI) brain metabolism to create a neuroprotective environment. To address the interconnection of neuroprotection, metabolism, inflammation and REST/NRSF after TBI, we subjected C57BL/6N mice to experimental TBI and intraperitoneal VPA administration or vehicle solution at 15 min, 1, 2, and 3 days post-injury (dpi). At 7 dpi, TBI-induced an up-regulation of REST/NRSF gene expression and HDACi function of VPA on histone H3 acetylation were confirmed. Neurological deficits, brain lesion size, blood-brain barrier permeability, or astrogliosis were not affected, and REST/NRSF target genes were only marginally influenced by VPA. However, VPA attenuated structural damage in the hippocampus, microgliosis and expression of the pro-inflammatory marker genes. Analyses of plasma lipidomic and polar metabolomic patterns revealed that VPA treatment increased lysophosphatidylcholines (LPCs), which were inversely associated with interleukin 1 beta (Il1b) and tumor necrosis factor (Tnf) gene expression in the brain. The results show that VPA has mild neuroprotective and anti-inflammatory effects likely originating from favorable systemic metabolic changes resulting in increased plasma LPCs that are known to be actively taken up by the brain and function as carriers for neuroprotective polyunsaturated fatty acids.

Therapeutic potential of lithium chloride and valproic acid against neuronopathic types of mucopolysaccharidoses through induction of the autophagy process

Mucopolysaccharidoses (MPS) are a group of inherited disorders, caused by mutations in the genes coding for proteins involved (directly or indirectly) in glycosaminoglycan (GAG) degradation. A lack or drastically decreased residual activity of a GAG-degrading enzyme leads to the storage of these compounds, thus damaging proper functions of different cells, including neurons. The disease leads to serious psycho-motor dysfunctions and death before reaching the adulthood. Until now, induction of the autophagy process was considered as one of the therapeutic strategies for treatment of diseases caused by protein aggregation (Alzheimer's, Parkinson's, and Huntington's diseases). However, this strategy has only been recently suggested as a potential therapy for MPS. In this work, we show that the pharmacological stimulation of autophagy, by using valproic acid and lithium chloride, led to accelerated degradation of accumulated GAGs. Cytotoxicity tests indicated the safety of the use of the investigated compounds. We observed an increased number of lysosomes and enhanced degradation of heparan sulfate (one of GAGs). Induction of the autophagy process was confirmed by measuring abundance of the marker proteins, including LC3-II. Moreover, inhibition of this process resulted in abolition of the valproic acid- and LiCl-mediated reduction in GAG levels. This is the first report on the possibility of using valproic acid and lithium chloride for reducing levels of GAGs in neuronopathic forms of MPS.

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

A single dose of valproic acid improves neurologic recovery and decreases brain lesion size in swine subjected to an isolated traumatic brain injury

BACKGROUND

We lack specific treatments for traumatic brain injury (TBI), which remains the leading cause of trauma-related morbidity and mortality. Treatment with valproic acid (VPA) improves outcomes in models of severe TBI with concurrent hemorrhage. However, it is unknown if VPA will have similar benefits after isolated nonlethal TBI, which is the more common clinical scenario. The goal of this study was to evaluate the effect of VPA treatment in a preclinical isolated TBI swine model on neurologic outcomes and brain lesion size and to perform detailed pharmacokinetic analyses for a future clinical trial.

METHODS

Yorkshire swine (n = 10; 5/cohort) were subjected to TBI (8-mm controlled cortical impact). An hour later, we randomized them to receive VPA (150 mg/kg) or saline placebo (control). Neuroseverity scores were assessed daily (0 [normal] to 36 [comatose]), brain lesion size was measured on postinjury 3, and serial blood samples were collected for pharmacokinetic studies.

RESULTS

Physiologic parameters and laboratory values were similar in both groups. Valproic acid-treated animals demonstrated significantly better neuroseverity scores on postinjury 1 (control, 9.2 ± 4.4; VPA, 0 ± 0; p = 0.001). Valproic acid-treated animals had significantly smaller brain lesion sizes (mean volume in microliter: control, 3,130 ± 2,166; VPA, 764 ± 208; p = 0.02). Pharmacokinetic data confirmed adequate plasma and tissue levels of VPA.

CONCLUSION

In this clinically relevant model of isolated TBI, a single dose of VPA attenuates neurological impairment and decreases brain lesion size.

Life on the battlefield: Valproic acid for combat applications

The leading causes of death in military conflicts continue to be hemorrhagic shock (HS) and traumatic brain injury (TBI). Most of the mortality is a result of patients not surviving long enough to obtain surgical care. As a result, there is a significant unmet need for a therapy that stimulates a "prosurvival phenotype" that counteracts the cellular pathophysiology of HS and TBI to prolong survival. Valproic acid (VPA), a well-established antiepileptic therapy for more than 50 years, has shown potential as one such prosurvival therapy. This review details how VPA's role as a nonselective histone deacetylase inhibitor induces cellular changes that promote survival and decrease cellular pathways that lead to cell death. The review comprehensively covers more than two decades worth of studies ranging from preclinical (mice, swine) to recent human clinical trials of the use of VPA in HS and TBI. Furthermore, it details the different mechanisms in which VPA alters gene expression, induces cytoprotective changes, attenuates platelet dysfunction, provides neuroprotection, and enhances survival in HS and TBI. Valproic acid shows real promise as a therapy that can induce the prosurvival phenotype in those injured during military conflict.

Correction of Niemann-Pick type C1 trafficking and activity with the histone deacetylase inhibitor valproic acid

Niemann-Pick type C (NPC) disease is primarily caused by mutations in the NPC1 gene and is characterized by the accumulation of unesterified cholesterol and lipids in the late endosomal (LE) and lysosomal (Ly) compartments. The most prevalent disease-linked mutation is the I1061T variant of NPC1, which exhibits defective folding and trafficking from the endoplasmic reticulum to the LE/Ly compartments. We now show that the FDA-approved histone deacetylase inhibitor (HDACi) valproic acid (VPA) corrects the folding and trafficking defect associated with I1061T-NPC1 leading to restoration of cholesterol homeostasis, an effect that is largely driven by a reduction in HDAC7 expression. The VPA-mediated trafficking correction is in part associated with an increase in the acetylation of lysine residues in the cysteine-rich domain of NPC1. The HDACi-mediated correction is synergistically improved by combining it with the FDA-approved anti-malarial, chloroquine, a known lysosomotropic compound, which improved the stability of the LE/Ly-localized fraction of the I1061T variant. We posit that combining the activity of VPA, to modulate epigenetically the cellular acetylome, with chloroquine, to alter the lysosomal environment to favor stability of the trafficked I1061T variant protein can have a significant therapeutic benefit in patients carrying at least one copy of the I1061T variant of NPC1, the most common disease-associated mutation leading to NPC disease. Given its ability to cross the blood-brain barrier, we posit VPA provides a potential mechanism to improve the response to 2-hydroxypropyl-β-cyclodextrin, by restoring a functional NPC1 to the cholesterol managing compartment as an adjunct therapy.

Epigenetic Regulation of Multidrug Resistance Protein 1 and Breast Cancer Resistance Protein Transporters by Histone Deacetylase Inhibition

Multidrug resistance protein 1 (MDR1, ABCB1, P-glycoprotein) and breast cancer resistance protein (BCRP, ABCG2) are key efflux transporters that mediate the extrusion of drugs and toxicants in cancer cells and healthy tissues, including the liver, kidneys, and the brain. Altering the expression and activity of MDR1 and BCRP influences the disposition, pharmacodynamics, and toxicity of chemicals, including a number of commonly prescribed medications. Histone acetylation is an epigenetic modification that can regulate gene expression by changing the accessibility of the genome to transcriptional regulators and transcriptional machinery. Recently, studies have suggested that pharmacological inhibition of histone deacetylases (HDACs) modulates the expression and function of MDR1 and BCRP transporters as a result of enhanced histone acetylation. This review addresses the ability of HDAC inhibitors to modulate the expression and the function of MDR1 and BCRP transporters and explores the molecular mechanisms by which HDAC inhibition regulates these transporters. While the majority of studies have focused on histone regulation of MDR1 and BCRP in drug-resistant and drug-sensitive cancer cells, emerging data point to similar responses in nonmalignant cells and tissues. Elucidating epigenetic mechanisms regulating MDR1 and BCRP is important to expand our understanding of the basic biology of these two key transporters and subsequent consequences on chemoresistance as well as tissue exposure and responses to drugs and toxicants. SIGNIFICANCE STATEMENT: Histone deacetylase inhibitors alter the expression of key efflux transporters multidrug resistance protein 1 and breast cancer resistance protein in healthy and malignant cells.

Amino Acids in the Development of Prodrugs

Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.

Generation-6 hydroxyl PAMAM dendrimers improve CNS penetration from intravenous administration in a large animal brain injury model

Hypothermic circulatory arrest (HCA) provides neuroprotection during cardiac surgery but entails an ischemic period that can lead to excitotoxicity, neuroinflammation, and subsequent neurologic injury. Hydroxyl polyamidoamine (PAMAM) dendrimers target activated microglia and damaged neurons in the injured brain, and deliver therapeutics in small and large animal models. We investigated the effect of dendrimer size on brain uptake and explored the pharmacokinetics in a clinically-relevant canine model of HCA-induced brain injury. Generation 6 (G6, ~6.7nm) dendrimers showed extended blood circulation times and increased accumulation in the injured brain compared to generation 4 dendrimers (G4, ~4.3nm), which were undetectable in the brain by 48h after final administration. High levels of G6 dendrimers were found in cerebrospinal fluid (CSF) of injured animals with a CSF/serum ratio of ~20% at peak, a ratio higher than that of many neurologic pharmacotherapies already in clinical use. Brain penetration (measured by drug CSF/serum level) of G6 dendrimers correlated with the severity of neuroinflammation observed. G6 dendrimers also showed decreased renal clearance rate, slightly increased liver and spleen uptake compared to G4 dendrimers. These results, in a large animal model, may offer insights into the potential clinical translation of dendrimers.

Small molecule inhibitors of zinc-dependent histone deacetylases

Lysine acetylation is an ancient, evolutionarily conserved, reversible post-translational modification. A multitude of diverse cellular functions are regulated by this dynamic modification, including energy and metabolism, protein folding, transcription, and translation. Gene expression can be manipulated through changes in histone acetylation status, and this process is controlled by the function of 2 opposing enzymes: histone acetyl transferases and histone deacetylases (HDACs). The zinc-dependent HDACs are a family of hydrolases that remove acetyl groups from lysines, and their function can be modulated by the action of small molecule ligands. Inhibition through competitive binding of the catalytic domain of these enzymes has been achieved by a diverse array of small molecule chemotypes. Structural biology has aided the development of potent, and in some cases highly isoform-selective, inhibitors that have demonstrated utility in a number of neurological disease models. Continued development and characterization of highly optimized small molecule inhibitors of HDAC enzymes will help refine our understanding of their function and, optimistically, lead to novel therapeutic treatment alternatives for a host of neurological disorders.

Chronic central administration of valproic acid: Increased pro-survival phospho-proteins and growth cone associated proteins with no behavioral pathology

Valproic acid (VPA) is the most widely prescribed antiepileptic drug due to its ability to treat a broad spectrum of seizure types. However, potential complications of this drug include anticonvulsant polytherapy metabolism, organ toxicity and teratogenicity which limit its use in a variety of epilepsy patients. Direct delivery of VPA intracerebroventricularly (ICV) could circumvent the toxic effects normally seen with the oral route of administration. An additional potential benefit would be significantly reduced dosing while achieving high brain concentrations. Epileptogenic tissue from patients with intractable seizures has shown significant cell death which may be mitigated by maximizing cerebral VPA exposure. Here we show ICV administration of VPA localized to the periventricular zone increased pro-survival phospho-proteins (pAkt(Ser473), pAkt(Thr308), pGSK3β(Ser9), pErk1/2(Thr202/Tyr204)) and growth cone associated proteins (2G13p, GAP43) in a whole animal system. No significant changes in DCX, NeuN, synaptotagmin, and synaptophysin were detected. Assessment of possible behavioral alterations in rats receiving chronic central infusions of VPA was performed with the open field and elevated plus mazes. Neither paradigm revealed any detrimental effects of the drug infusion process.

Nanostructure-based drug delivery systems for brain targeting

CONTEXT

It is well-known fact that blood brain barrier (BBB) hinders the penetrance and access of many pharmacotherapeutic agents to central nervous system (CNS). Many diseases of the CNS remain undertreated and the inability to treat most CNS disorders is not due to the lack of effective CNS drug discovery, rather, it is due to the ineffective CNS delivery. Therefore, a number of nanostructured drug delivery carriers have been developed and explored over the past couple of years to transport the drugs to brain.

OBJECTIVE

The present review will give comprehensive details of extensive research being done in field of nanostructured carriers to transport the drugs through the BBB in a safe and effective manner.

METHODS

The method includes both the polymeric- and lipid-based nanocarriers with emphasis on their utility, methodology, advantages, and the drugs which have been worked on using a particular approach to provide a noninvasive method to improve the drug transport through BBB.

RESULTS

Polymeric- and lipid-based nanocarriers enter brain capillaries before reaching the surface of the brain microvascular endothelial cells without the disruption of BBB. These systems are further modified with specific ligands vectors and pegylation aiming to target and enhance their binding with surface receptors of the specific tissues inside brain and increase long circulatory time which favors interaction and penetration into brain endothelial cells.

CONCLUSION

This review would give an insight to the researchers working on neurodegenerative and non-neurodegenerative diseases of the CNS including brain tumor.

Brain delivery of valproic acid via intranasal administration of nanostructured lipid carriers: in vivo pharmacodynamic studies using rat electroshock model

The treatment of brain disorders is one of the greatest challenges in drug delivery because of a variety of main barriers in effective drug transport and maintaining therapeutic concentrations in the brain for a prolonged period. The objective of this study was delivery of valproic acid (VPA) to the brain by intranasal route. For this purpose, nanostructured lipid carriers (NLCs) were prepared by solvent diffusion method followed by ultrasonication and characterized for size, zeta potential, drug-loading percentage, and release. Six groups of rats each containing six animals received drug-loaded NLCs intraperitoneally (IP) or intranasally. Brain responses were then examined by using maximal electroshock (MES). The hind limb tonic extension:flexion inhibition ratio was measured at 15-, 30-, 60-, 90-, and 120-minute intervals. The drug concentration was also measured in plasma and brain at the most protective point using gas chromatography method. The particle size of NLCs was 154 ± 16 nm with drug-loading percentage of 47% ± 0.8% and drug release of 75% ± 1.9% after 21 days. In vivo results showed that there was a significant difference between protective effects of NLCs of VPA and control group 15, 30, 60, and 90 minutes after treatment via intranasal route (P < 0.05). Similar protective effect was observed in rats treated with NLCs of VPA in intranasal route and positive control in IP route (P > 0.05). Results of drug determination in brain and plasma showed that brain:plasma concentration ratio was much higher after intranasal administration of NLCs of VPA than the positive control group (IP route). In conclusion, intranasal administration of NLCs of VPA provided a better protection against MES seizure.

Physicochemical selectivity of the BBB microenvironment governing passive diffusion--matching with a porcine brain lipid extract artificial membrane permeability model

PURPOSE

To mimic the physicochemical selectivity of the blood-brain barrier (BBB) and to predict its passive permeability using a PAMPA model based on porcine brain lipid extract (PBLE 10%w/v in alkane).

METHODS

Three PAMPA (BD pre-coated and PBLE with 2 different lipid volumes) models were tested with 108 drugs. Abraham solvation descriptors were used to interpret the in vitro-in vivo correlation with 282 in situ brain perfusion measurements, spanning over 5 orders of magnitude. An in combo PAMPA model was developed from combining measured PAMPA permeability with one H-bond descriptor.

RESULTS

The in combo PAMPA predicted 93% of the variance of 197 largely efflux-inhibited in situ permeability training set. The model was cross-validated by the "leave-many-out" procedure, with q(2) = 0.92 ± 0.03. The PAMPA models indicated the presence of paramembrane water channels. Only the PBLE-based PAMPA-BBB model with sufficient lipid to fill all the internal pore space of the filter showed a wide dynamic range window, selectivity coefficient near 1, and was suitable for predicting BBB permeability.

CONCLUSION

BBB permeability can be predicted by in combo PAMPA. Its speed and substantially lower cost, compared to in vivo measurements, make it an attractive first-pass screening method for BBB passive permeability.

Histone deacetylase inhibitors de-repress tyrosine hydroxylase expression in the olfactory bulb and rostral migratory stream

Most olfactory bulb (OB) interneurons are derived from neural stem cells in the subventricular zone (SVZ) and migrate to the OB via the rostral migratory stream (RMS). Mature dopaminergic interneurons in the OB glomerular layer are readily identified by their synaptic activity-dependent expression of tyrosine hydroxylase (TH). Paradoxically, TH is not expressed in neural progenitors migrating in the RMS, even though ambient GABA and glutamate depolarize these progenitors. In forebrain slice cultures prepared from transgenic mice containing a GFP reporter gene under the control of the Th 9kb upstream regulatory region, treatment with histone deacetylase (HDAC) inhibitors (either sodium butyrate, Trichostatin A or Scriptaid) induced Th-GFP expression specifically in the RMS independently of depolarizing conditions in the culture media. Th-GFP expression in the glomerular layer was also increased in slices treated with Trichostatin A, but this increased expression was dependent on depolarizing concentrations of KCl in the culture media. Th-GFP expression was also induced in the RMS in vivo by intra-peritoneal injections with either sodium butyrate or valproic acid. Quantitative RT-PCR analysis of neurosphere cultures confirmed that HDAC inhibitors de-repressed Th expression in SVZ-derived neural progenitors. Together, these findings suggest that HDAC function is critical for regulating Th expression levels in both neural progenitors and mature OB dopaminergic neurons. However, the differential responses to the combinatorial exposure of HDAC inhibitors and depolarizing culture conditions indicate that Th expression in mature OB neurons and neural progenitors in the RMS are regulated by distinct HDAC-mediated mechanisms.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

[Drugs for status epilepticus treatment]

The pharmacokinetics and pharmacodynamics of major antiepileptic agents are presented. The onset of action and the factors leading to extraction across the blood brain barrier are described as well as the mechanism and extent of metabolism, and the main interactions with other drugs. For each class, the dosing scheme and practical issues related to administration are described, based on evidence when available in the literature.

Contribution of Carrier-Mediated Transport Systems to the Blood–Brain Barrier as a Supporting and Protecting Interface for the Brain; Importance for CNS Drug Discovery and Development

The blood-brain barrier (BBB) forms an interface between the circulating blood and the brain and possesses various carrier-mediated transport systems for small molecules to support and protect CNS function. For example, the blood-to-brain influx transport systems supply nutrients, such as glucose and amino acids. Consequently, xenobiotic drugs recognized by influx transporters are expected to have high permeability across the BBB. On the other hand, efflux transporters, including ATP-binding cassette transporters such as P-glycoprotein located at the luminal membrane of endothelial cells, function as clearance systems for metabolites and neurotoxic compounds produced in the brain. Drugs recognized by these transporters are expected to show low BBB permeability and low distribution to the brain. Despite recent progress, the transport mechanisms at the BBB have not been fully clarified yet, especially in humans. However, an understanding of the human BBB transport system is critical, because species differences mean that it can be difficult to extrapolate data obtained in experimental animals during drug development to humans. Recent progress in methodologies is allowing us to address this issue. Positron emission tomography can be used to evaluate the activity of human BBB transport systems in vivo. Proteomic studies may also provide important insights into human BBB function. Construction of a human BBB transporter atlas would be a most important advance from the viewpoint of CNS drug discovery and drug delivery to the brain.

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

Valproic acid is not a substrate for P-glycoprotein or multidrug resistance proteins 1 and 2 in a number of in vitro and in vivo transport assays

The antiepileptic drug valproic acid (VPA) is widely used in the treatment of epilepsy, bipolar disorders, and migraine. However, rather high doses are required for the clinical effects of VPA, which is due to its relatively inefficient delivery to the brain. The poor brain distribution of VPA is thought to reflect an asymmetric transport system at the blood-brain barrier (BBB). Based on recent data from in vitro experiments, multidrug resistance proteins (MRPs) have been proposed to be involved in the efflux transport of VPA at the BBB. In the present study, we used different experimental in vitro and in vivo strategies to evaluate whether VPA is a substrate for MRPs or the efflux transporter P-glycoprotein (Pgp). In contrast to known Pgp or MRP substrates, such as cyclosporin A or vinblastine, no directional transport of VPA was observed in cell monolayer efflux assays using the kidney cell lines Madin Darby canine kidney II and LLC-PK1, which had been transfected with either human or mouse cDNAs for the genes encoding Pgp, MRP1, or MRP2. Likewise, no indication for efflux transport of VPA was obtained in a rat microdialysis model, using inhibitors of either Pgp or MRPs. Furthermore, a significant role of MRP2 in brain efflux of VPA was excluded by using MRP2-deficient rats. Our data do not support the hypothesis that MRP1 or MRP2 is involved in the efflux of VPA from the brain. Thus, the molecular identity of the putative transporter(s) mediating the active efflux of VPA from the brain remains to be elucidated.

Interaction of valproic acid and carbapenem antibiotics with multidrug resistance-associated proteins in rat erythrocyte membranes

We recently reported that the decrease in plasma valproic acid (VPA) level by carbapenem antibiotics (CPs) may partly be due to the increased erythrocyte distribution of VPA. In order to clarify the mechanisms underlying altered VPA distribution in erythrocytes, we examined the role of multidrug resistance-associated proteins (Mrps). The uptake of 2,4-dinitrophenyl-S-glutathione (DNP-SG), a substrate of Mrps, by inside-out vesicles (IOVs) prepared from rat erythrocytes was an ATP-dependent, active process. DNP-SG uptake was mediated by high- and low-affinity transport systems, and was inhibited by various Mrp inhibitors such as probenecid and indomethacin. Glutathione stimulated only the high-affinity transport system. VPA inhibited the low-affinity transport of DNP-SG, while panipenem, a CP, inhibited both high- and low-affinity transport. ATP-dependent, Mrp-mediated transport of methotrexate, another Mrp substrate, in IOVs was also observed, and VPA and various CPs inhibited the transport. The uptake of [(3)H]VPA was examined, and found to be ATP-dependent. ATP-dependent uptake of [(3)H]VPA was inhibited by Mrp inhibitors and panipenem, while the inhibition was not observed in the absence of ATP. These results indicate that VPA and CPs interact with Mrp-mediated transport in erythrocyte membranes, and VPA itself is transported by Mrps, which is inhibited by panipenem. Thus, the increased erythrocyte distribution of VPA by CPs observed under in vivo conditions may partly be explained by their interaction with Mrps in erythrocyte membranes.

Factors affecting delivery of antiviral drugs to the brain

Although the CNS is in part protected from peripheral insults by the blood-brain barrier and the blood-cerebrospinal fluid barrier, a number of human viruses gain access to the brain, replicate within this organ, or sustain latent infection. The efficacy of antiviral drugs towards the cerebral viral load is often limited as both blood-brain interfaces impede their cerebral distribution. For polar compounds, the major factor restricting their entry lies in the tight junctions that occlude the paracellular pathway across these barriers. For compounds with more favourable lipid solubility properties, CNS penetration will be function of a number of physicochemical factors that include the degree of lipophilicity, size and ability to bind to protein or red blood cells, as well as other factors inherent to the vascular and choroidal systems, such as the local cerebral blood flow and the surface area available for exchange. In addition, influx and efflux transport systems, or metabolic processes active in both capillary endothelial cells and choroid plexus epithelial cells, can greatly change the bioavailability of a drug in one or several compartments of the CNS. The relative importance of these various factors with respect to the CNS delivery of the different classes of antiviral drugs is illustrated and discussed.

A mathematical model for prediction of drug molecule diffusion across the blood-brain barrier

BACKGROUND

Predicting the ability of drugs to enter the brain is a longstanding problem in neuropharmacology. The first step in creating a much-needed computational algorithm for predicting whether a drug will enter brain is to devise a rigorous mathematical model.

METHODS

Employing two experimental measures of blood-brain barrier (BBB) penetrability (brain/plasma ratio and the brain-uptake index) and 14 theoretically derived biophysical predictors, a mathematical model was developed to quantitatively correlate molecular structure with ability to traverse the BBB.

RESULTS

This mathematical model employs Stein's hydrogen bonding number and Randic's topological descriptors to correlate structure with ability to cross the BBB. The final model accurately predicts the ability of test molecules to cross the BBB.

CONCLUSIONS

A mathematical method to predict blood-brain barrier penetrability of drug molecules has been successfully devised. As a result of bioinformatics, chemoinformatics and other informatics-based technologies, the number of small molecules being developed as potential therapeutics is increasing exponentially. A biophysically rigorous method to predict BBB penetrability will be a much-needed tool for the evaluation of these molecules.

Small molecular drug transfer across the blood-brain barrier via carrier-mediated transport systems

Because of the physiological nature of the blood-brain barrier (BBB), transport of chemical compounds between blood and brain has been widely believed to occur by means of passive diffusion, depending upon the lipophilicity of the compounds. However, discrepancies exist between the lipophilicity and apparent BBB permeation properties in many cases, and these discrepancies can be ascribed to the existence of multiple mechanisms of drug transport through the BBB. Molecular identification and functional analysis of influx transport proteins (from blood to brain) and efflux transport proteins (from brain to blood) have progressed rapidly. Therefore, the BBB is now considered to be a dynamic interface that controls the influx and efflux of a wide variety of substances, including endogenous nutrients and exogenous compounds such as drugs, to maintain a favorable environment for the CNS. This review focuses on the role of transport systems in the uptake of xenobiotics, including organic anionic/cationic and neutral drugs, across the BBB into the brain, as well as on strategies to increase drug delivery into the brain by blocking efflux transport protein function, or to reduce CNS side effects by modulating BBB transport processes.

Blood-brain barrier efflux transport

Efflux transport at the blood-brain barrier (BBB) limits the brain tissue exposure to a variety of potential therapeutic agents, including compounds that are relatively lipophilic and would be predicted to permeate the endothelial lining of the brain microvasculature. Recent advances in molecular and cell biology have led to identification of several specific transport systems at the blood-brain interface. Refinement of classical pharmacokinetic experimentation has allowed assessment of the structural specificity of transporters, the impact of efflux transport on brain tissue exposure, and the potential for drug-drug interactions at the level of BBB efflux transport. The objective of this minireview is to summarize efflux transporter characteristics (location, specificity, and potential inhibition) for transport systems identified in the BBB. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on net brain tissue uptake of substrates also are presented. The potential impact of efflux transport on the pharmacodynamics of agents acting in the central nervous system are illustrated. Finally, general issues regarding the role of identifying efflux transport as part of the drug development process are discussed.

Relation of beta-casomorphin to apnea in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the most common cause of death in infants and its pathogenesis is complex and multifactorial. The aim of this review is to summarize recent novel findings regarding the possible association of beta-casomorphin (beta-CM) to apnea in SIDS, which has not been widely appreciated by pediatricians and scientists. beta-CM is an exogenous bioactive peptide derived from casein, a major protein in milk and milk products, which has opioid activity. Mechanistically, circulation of this peptide into the infant's immature central nervous system might inhibit the respiratory center in the brainstem leading to apnea and death. This paper will review the possible relationship between beta-CM and SIDS in the context of passage of beta-CM through the gastrointestinal tract and the blood-brain barrier (BBB), permeability of the BBB to peptides in infants, and characterization of the casomorphin system in the brain.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Manganese distribution across the blood-brain barrier. II. Manganese efflux from the brain does not appear to be carrier mediated

There is concern about manganese (Mn) neurotoxicity. Mn can enter the brain by carrier-mediated influx. There have been no previous reports of investigation of Mn efflux from the brain. We used an established method that determines the rate of efflux out of the brain across the blood-brain barrier (BBB) from the product of the brain distribution volume (Vbrain) and the apparent elimination rate constant (Kel). Vbrain is determined as 54Mn uptake into rat parietal brain slices versus time. Kel is determined from the percentage of 54Mn remaining in the brain at various times after its discrete injection into the parietal cortex, compared to a reference compound which is expected to very slowly diffuse out of the brain. The Mn ion, Mn citrate and Mn transferrin (Mn Tf) were studied. 14C-sucrose and 14C-dextran were used as reference compounds. The volume of distribution of the Mn species in brain slices was approximately 3-5 ml/g, indicating concentrative uptake. Mn, as the Mn ion or Mn citrate, was injected into the brain with sucrose or dextran to determine Kel. Based on the rapid exchange rate of Mn with ligands and on thermodynamic calculations, injection of Mn ion or Mn citrate into the brain would be expected to result in rapid formation of the same Mn species, predominantly the Mn ion, Mn citrates and Mn phosphate, in brain extracellular fluid. After injection into the brain Mn did not efflux from the brain more rapidly than sucrose or dextran, which diffuse across the BBB. Brain capillary diffusion of the Mn ion and Mn citrate would be expected to be slower than sucrose or dextran. The rate of Mn efflux from the brain is consistent with diffusion.

The impact of efflux transporters in the brain on the development of drugs for CNS disorders

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Assessment of tear concentrations on therapeutic drug monitoring. II. Pharmacokinetic analysis of valproic acid in guinea pig serum, cerebrospinal fluid, and tears

PURPOSE

To quantitatively describe the pharmacokinetics of valproic acid (VPA) in guinea pig serum (total [Cf+b] and free [Cf]), cerebrospinal fluid (CSF) [C]CSF and tears [C]T using a simple kinetic model, and to examine whether [Cf] and [C]CSF can be predicted by [C]T using the resulting pharmacokinetic parameters.

METHODS

[Cf+b], [Cf], [C]CSF and [C]T were determined after bolus i.v. injection of 10 or 20 mg/kg VPA using GC/ECNCI/MS.

RESULTS

[Cf+b] could be quantitatively described by a two compartment model with linear elimination kinetics. [Cf] was separately analyzed using multi-exponential equations. [C]CSF was analyzed using a simple kinetic model in which the CSF compartment is independently connected with the serum compartment by the apparent diffusion constants (KINCSF and KOUTCSF). [C]T was analyzed using the same simple kinetic model used for [C]CSF. The values of [C]CSF and [Cf] in the steady state can be represented by the following equations; [C]CSF = KINCSF/KOUTCSF x [Cf], [Cf] = KOUT/KINT x [C]T, and indicating that [Cf] and [C]CSF can be predicted by [C]T using the resulting pharmacokinetic parameters.

CONCLUSIONS

The measurement of [C]T which can be collected non-invasively and estimated the pharmacokinetic parameters for [Cf], [C]CSF, and [C]T might be a very useful method for TDM of VPA.

Probenecid-inhibitable efflux transport of valproic acid in the brain parenchymal cells of rabbits: a microdialysis study

Delivery of valproic acid (VPA) to the human brain is relatively inefficient as reflected by a low brain-to-unbound plasma concentration ratio (< or =0.5) at steady state. Previous pharmacokinetic studies suggested that the unfavorable brain-to-plasma gradient is maintained by coupled efflux transport processes at both the brain parenchymal cells and blood-brain barrier (BBB); one or both of the efflux transporters are inhibitable by probenecid. The present study in rabbits utilized microdialysis to measure drug concentration in the brain extracellular fluid (ECF) of the cerebral cortex during steady-state i.v. infusion with VPA alone or with VPA plus probenecid. Probenecid co-infusion elevated VPA concentration in the brain tissue surrounding the tip of the microdialysis probe to a greater extent than in the ECF (230% versus 47%). Brain intracellular compartment (ICC) concentration was estimated. In control rabbits, the ICC concentration was 2.8+/-0.28 times higher than the ECF concentration. Probenecid co-infusion elevated the ICC-to-ECF concentration ratio to 4.2+/-0.44, which confirms the existence of an efflux transport system in brain parenchymal cells. The ECF-to-unbound plasma concentration ratio was well below unity (0.029), indicating an uphill efflux transport of VPA across the BBB. Co-infusion of probenecid did not have a significant effect on VPA efflux at the BBB as evidenced by a minimal change in the ECF-to-unbound plasma concentration ratio. This study suggests the presence of distinctly different organic anion transporters for the efflux of VPA at the parenchymal cells and capillary endothelium in the brain.

Transporter-mediated permeation of drugs across the blood-brain barrier

Drug distribution into the brain is strictly regulated by the presence of the blood-brain barrier (BBB) that is formed by brain capillary endothelial cells. Since the endothelial cells are connected to each other by tight junctions and lack pores and/or fenestrations, compounds must cross the membranes of the cells to enter the brain from the bloodstream. Therefore, hydrophilic compounds cannot cross the barrier in the absence of specific mechanisms such as membrane transporters or endocytosis. So, for efficient supply of hydrophilic nutrients, the BBB is equipped with membrane transport systems and some of those transporter proteins have been shown to accept drug molecules and transport them into brain. In the present review, we describe mainly the transporters that are involved in drug transfer across the BBB and have been molecularly identified. The transport systems described include transporters for amino acids, monocarboxylic acids, organic cations, hexoses, nucleosides, and peptides. Most of these transporters function in the direction of influx from blood to brain; the presence of efflux transporters from brain to blood has also been demonstrated, including P-glycoprotein, MRPs, and other unknown transporters. These efflux transporters seem to be functional for detoxication and/or prevention of nonessential compounds from entering the brain. Various drugs are transported out of the brain via such efflux transporters, resulting in the decrease of CNS side effects for drugs that have pharmacological targets in peripheral tissues or in the reduction of efficacy in CNS because of the lower delivery by efflux transport. To identify the transporters functional at the BBB and to examine the possible involvement of them in drug transports by molecular and physiological approaches will provide a rational basis for controlling drug distribution to the brain.

Brain distribution of 6-mercaptopurine is regulated by the efflux transport system in the blood-brain barrier

6-mercaptopurine (6-MP) has been used clinically for 40 years to maintain remission in patients with acute lymphoblastic leukemia (ALL). However, central nervous system (CNS) relapses frequently occur in patients with ALL who continuously receive anticancer drugs, including 6-MP, during remission maintenance therapy. The cause of such CNS relapse is not well understood. One possible reason may involve the restricted distribution of 6-MP in the brain. This study, therefore, investigates the blood-brain barrier (BBB) transport which largely regulates 6-MP distribution in the brain using a quantitative microdialysis technique and centers on the efflux transport of 6-MP across the BBB. The brain tissue, cerebrospinal fluid (CSF), or hippocampal interstitial fluid (ISF) concentration of 6-MP was very low compared with the unbound plasma concentration, suggesting that 6-MP distribution in the brain is highly restricted. Kinetic analyses of this BBB transport showed that the efflux clearance from brain ISF to plasma across the BBB (CLout) is approximately 20-times greater than the influx clearance from plasma to brain (CLin). The CLout was significantly reduced by 1mM N-ethylmaleimide (NEM), a sulfhydryl-modifying agent, suggesting the participation of transport protein in the efflux of 6-MP across the BBB. In addition, efflux transport was inhibited by an intracerebral infusion of probenecid (1.5 mM), p-aminohippuric acid (PAH, 3.0 mM), benzoate (3.6 mM), or salicylate (3.7 mM) administered through a microdialysis probe, but neither choline (0.8 mM) nor tetraethylammonium (TEA, 0.7 mM) had any effect. These data suggest that the restricted 6-MP brain distribution may be ascribed to efficient efflux from the brain, possibly via both the organic anion transport system, shared with probenecid and PAH, and the monocarboxylic acid transport system, shared with benzoate and salicylate.

Molecular cloning and characterization of a new multispecific organic anion transporter from rat brain

A cDNA encoding the new member of the multispecific organic anion transporter family, OAT3, was isolated by the reverse transcription-polymerase chain reaction cloning method. Degenerate primers were designed based on the sequences conserved among OAT1, OAT2, and organic cation transporter 1 (OCT1), and reverse transcription-polymerase chain reaction was performed using rat brain poly(A)+ RNA. The 536-amino acid protein sequence encoded by OAT3 showed 49, 39, and 36% identity to those of OAT1, OAT2, and OCT1, respectively. Northern blot analysis revealed that rat OAT3 mRNA is expressed in the liver, brain, kidney, and eye. When expressed in Xenopus laevis oocytes, OAT3 mediated the uptake of organic anions, such as p-aminohippurate (Km = 65 microM), ochratoxin A (Km = 0.74 microM), and estrone sulfate (Km = 2.3 microM) and a cationic compound, cimetidine. OAT3-mediated uptake of [3H]estrone sulfate was sodium-independent. para-Aminohippuric acid, estrone sulfate or ochratoxin A did not show any trans-stimulatory effect on either influx or efflux of [3H]estrone sulfate via OAT3. Organic anions such as sulfobromophthalein, probenecid, indocyanine green, bumetanide, piroxicam, furosemide, azidodeoxythymidine, 4, 4'-diisothiocyanostilbene-3,3'-disulfonic acid, and benzylpenicillin inhibited OAT3-mediated estrone sulfate uptake, while ouabain and digoxin did not. Organic cations such as tetraethylammonium, guanidine, verapamil, and quinidine did not interact with OAT3. Acidic metabolites of neurotransmitters derived from dopamine, epinephrine, norepinephrine, and serotonin inhibited the uptake of estrone sulfate via OAT3. These results suggest an important role of OAT3 in the excretion/detoxification of endogenous and exogenous organic anions, especially from the brain.

Blood-brain barrier permeability to small and large molecules

The objective of this article is to provide the reader with an update of some of the BBB research highlights which have occurred in recent times, and to review the impact and contributions of immunogold electron microscopic studies on our understanding of the brain capillary endothelium. Glucose and monocarboxylic acids are two small molecules which this review will focus upon; and advances in immunogold characterization of the GLUT1 glucose transporter and the MCT1 and MCT2 monocarboxylic acid nutrient transporters will be discussed. Human serum albumin is chosen as a representative large molecule, and it has recently been shown that immunogold identification of this protein can serve as an indicator of compromised BBB function in a variety of pathophysiological conditions.

The blood-brain barrier efflux transporters as a detoxifying system for the brain

The role played by efflux transport systems across the blood-brain barrier (BBB) in the disposition of xenobiotics in the brain is described. Several drugs and organic anions are transported across the BBB via P-glycoprotein and other carrier-mediated efflux transport systems. Studies using in vitro cultured brain capillary endothelial cells, kinetic analysis, and mdr1a gene knock-out mice have shown that P-glycoprotein, located on the BBB, restricts the entry of vincristine and quinidine to the brain. Brain microdialysis studies have demonstrated that the brain interstitial fluid (ISF) concentrations of quinolone antibiotics are significantly lower than their corresponding unbound serum concentrations. A distributed model analysis supports the finding that efflux transport systems on the BBB restrict distribution of 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxyinosine (DDI), and quinolone antibiotics. A brain efflux index (BEI) method has been developed to provide direct evidence of an efflux transport system for carrying substrates from the cerebrum to the circulating blood across the BBB. The BEI method revealed the existence of carrier-mediated efflux organic anion transport systems for compounds such as p-aminohippuric acid, AZT, DDI, taurocholic acid, BQ-123, and estron sulfate. Moreover, cerebral neurotransmitters such as gamma-aminobutyric acid, L-glutamic acid, and L-aspartic acid are transported from brain to the circulating blood in the intact form via a carrier-mediated efflux transport system. The BBB not only restricts nonspecific permeation from the circulating blood to the brain, but also functions as an active efflux transport system for xenobiotics. Accordingly, the BBB plays a very important role by pumping xenobiotics and some endogenous compounds out of the brain, acting as a central nervous system (CNS)-specific detoxifying system supporting and maintaining normal cerebral function.

Effect of carnitine on valproic acid concentrations in serum, brain, and liver

This study investigates the influence of L-carnitine supplementation on valproic acid concentrations in rat serum, brain, and liver. Carnitine supplementation increased carnitine concentrations significantly in serum and liver but not in the brain. Free valproic acid concentrations in the brain were significantly increased by carnitine supplementation without any change of carnitine concentrations in the brain. The increase of serum-free valproic acid concentrations by carnitine supplementation apparently caused brain-free valproic acid concentrations to increase. This study suggests that L-carnitine supplementation to valproic acid therapy may potentiate valproic acid effects in the brain, even when the clinical dosage in humans is used.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.