Blood-brain interfaces: relevance to cerebral drug metabolism

Cerebrospinal fluid concentrations of ceftaroline and ceftazidime/avibactam in healthy volunteers: pharmacokinetics and probability of target attainment

OBJECTIVES

This study measured the penetration of ceftaroline and ceftazidime/avibactam into cerebrospinal fluid (CSF) to evaluate the potential of both drugs for treatment of central nervous system (CNS) infections.

METHODS

In this prospective, single-centre pharmacokinetic (PK) study, 24 healthy volunteers equally divided into two groups received four doses of either 600mg ceftaroline fosamil or 2000/500mg ceftazidime/avibactam as intravenous infusions over 2h at 8h intervals for 4 doses. Plasma samples were obtained on both study days and CSF was sampled once per subject at either 2h, 4h or 8h after the start of the last infusion via lumbar puncture. PK data were analysed using non-compartmental analysis, as well as using a population PK modelling approach. Monte Carlo simulations were performed to calculate probability of PK-pharmacodynamic (PK-PD) target attainment.

RESULTS

Two-compartment models described the plasma PK data for all three compounds. The ratios between the estimated distribution clearances into and out of the CSF were 0.021, 0.083 and 0.071 for ceftaroline, ceftazidime and avibactam, respectively, indicating limited CSF penetration. Ceftaroline and ceftazidime PK-PD targets were not attained in CSF for minimum inhibitory concentrations around commonly used susceptibility breakpoints, but exposure appears sufficient to treat several pathogens commonly causing CNS infections. Avibactam concentrations were well below reported threshold concentrations that are required for activity.

CONCLUSION

In healthy subjects, ceftaroline, ceftazidime and avibactam poorly distribute to CSF. Nonetheless, CSF exposure of both cephalosporins might be sufficient to cover certain, but not all, pathogens causative of CNS infections.

The barrier and interface mechanisms of the brain barrier, and brain drug delivery

Three different barriers are formed between the cerebrovascular and the brain parenchyma: the blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the cerebrospinal fluid-brain barrier (CBB). The BBB is the main regulator of blood and central nervous system (CNS) material exchange. The semipermeable nature of the BBB limits the passage of larger molecules and hydrophilic small molecules, Food and Drug Administration (FDA)-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Although the complexity of the BBB affects CNS drug delivery, understanding the composition and function of the BBB can provide a platform for the development of new methods for CNS drug delivery. This review summarizes the classification of the brain barrier, the composition and role of the basic structures of the BBB, and the transport, barrier, and destruction mechanisms of the BBB; discusses the advantages and disadvantages of different drug delivery methods and prospects for future drug delivery strategies.

In Vitro Models of the Blood–Cerebrospinal Fluid Barrier and Their Applications in the Development and Research of (Neuro)Pharmaceuticals

The pharmaceutical research sector has been facing the challenge of neurotherapeutics development and its inherited high-risk and high-failure-rate nature for decades. This hurdle is partly attributable to the presence of brain barriers, considered both as obstacles and opportunities for the entry of drug substances. The blood–cerebrospinal fluid (CSF) barrier (BCSFB), an under-studied brain barrier site compared to the blood–brain barrier (BBB), can be considered a potential therapeutic target to improve the delivery of CNS therapeutics and provide brain protection measures. Therefore, leveraging robust and authentic in vitro models of the BCSFB can diminish the time and effort spent on unproductive or redundant development activities by a preliminary assessment of the desired physiochemical behavior of an agent toward this barrier. To this end, the current review summarizes the efforts and progresses made to this research area with a notable focus on the attribution of these models and applied techniques to the pharmaceutical sector and the development of neuropharmacological therapeutics and diagnostics. A survey of available in vitro models, with their advantages and limitations and cell lines in hand will be provided, followed by highlighting the potential applications of such models in the (neuro)therapeutics discovery and development pipelines.

Xenobiotic transport and metabolism in the human brain

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

Cytochrome P450-mediated estrogen catabolism therapeutic avenues in epilepsy

Epilepsy is a neuropsychiatric disorder, which does not have any identifiable cause. However, experimental and clinical results have asserted that the sex hormone estrogen level and endocrine system function influence the seizure and epileptic episodes. There are available drugs to control epilepsy, which passes through the metabolism process. Cytochrome P-450 family 1 (CYP1A1) is a heme-containing mono-oxygenase that are induced several folds in most of the tissues and cells contributing to their differential expression, which regulates various metabolic processes upon administration of therapeutics. CYP1A1 gene family has been found to metabolize estrogen, a female sex hormone, which plays a central role in maintaining the health of brain altering the level of estrogen active neuropsychiatric disorder like epilepsy. Hence, in this article, we endeavor to provide an opinion of estrogen, its effects on epilepsy and catamenial epilepsy, their metabolism by CYP1A1 and new way forward to differential diagnosis and clinical management of epilepsy in future.

Intranasal administration of the chemotherapeutic perillyl alcohol results in selective delivery to the cerebrospinal fluid in rats

Perillyl alcohol (POH) has been extensively studied for the treatment of peripheral and primary brain tumors. The intranasal route of administration has been preferred for dosing POH in early-stage clinical trials associated with promising outcomes in primary brain cancer. However, it is unclear how intranasal POH targets brain tumors in these patients. Multiple studies indicate that intranasally applied large molecules may enter the brain and cerebrospinal fluid (CSF) through direct olfactory and trigeminal nerve-associated pathways originating in the nasal mucosa that bypass the blood–brain barrier. It is unknown whether POH, a small molecule subject to extensive nasal metabolism and systemic absorption, may also undergo direct transport to brain or CSF from the nasal mucosa. Here, we compared CSF and plasma concentrations of POH and its metabolite, perillic acid (PA), following intranasal or intravascular POH application. Samples were collected over 70 min and assayed by high-performance liquid chromatography. Intranasal administration resulted in tenfold higher CSF-to-plasma ratios for POH and tenfold higher CSF levels for PA compared to equal dose intravascular administration. Our preclinical results demonstrate POH undergoes direct transport from the nasal mucosa to the CSF, a finding with potential significance for its efficacy as an intranasal chemotherapeutic for brain cancer.

A blood-brain barrier overview on structure, function, impairment, and biomarkers of integrity

The blood–brain barrier is playing a critical role in controlling the influx and efflux of biological substances essential for the brain’s metabolic activity as well as neuronal function. Thus, the functional and structural integrity of the BBB is pivotal to maintain the homeostasis of the brain microenvironment. The different cells and structures contributing to developing this barrier are summarized along with the different functions that BBB plays at the brain–blood interface. We also explained the role of shear stress in maintaining BBB integrity. Furthermore, we elaborated on the clinical aspects that correlate between BBB disruption and different neurological and pathological conditions. Finally, we discussed several biomarkers that can help to assess the BBB permeability and integrity in-vitro or in-vivo and briefly explain their advantages and disadvantages.

Use of Intraventricular Medications in Critically Ill Patients: A Systematic Review

New evidence and increased use of intracranial devices have increased the frequency of intraventricular (IVT) medication administration in the neurologic intensive care unit. Significant benefits and risks are associated with administration of medications directly into the central nervous system. This review summarizes important literature, along with key information for clinicians regarding the administration, dosing, monitoring, and adverse effects related to IVT medication usage. Multiple medications have supporting literature for their use in critically ill patients including amphotericin B, aminoglycosides, colistimethate, daptomycin, quinupristin/dalfopristin, vancomycin, alteplase, and nicardipine. Sterile preparation and delivery, along with different types of devices that support medication administration, are also reviewed. One randomized, placebo-controlled trial of alteplase demonstrated decreased mortality but no change in good functional outcome. Other reports of IVT medication use are mainly limited to case reports and retrospective case series. There is a need for increased research on the topic; however, several practical barriers decrease the likelihood of a large, placebo-controlled, prospective study for most indications. Providers should consider implementing protocols to maximize safety of IVT medication delivery to ensure optimal patient outcomes.

Neurovascular Drug Biotransformation Machinery in Focal Human Epilepsies: Brain CYP3A4 Correlates with Seizure Frequency and Antiepileptic Drug Therapy

Pharmacoresistance is a major clinical challenge for approximately 30% of patients with epilepsy. Previous studies indicate nuclear receptors (NRs), drug efflux transporters, and cytochrome P450 enzymes (CYPs) control drug passage across the blood-brain barrier (BBB) in drug-resistant epilepsy. Here, we (1) evaluate BBB changes, neurovascular nuclear receptors, and drug transporters in lesional/epileptic (EPI) and non-lesional/non-epileptic (NON-EPI) regions of the same brain, (2) examine regional CYP expression and activity, and (3) investigate the association among CYP brain expression, seizure frequency, duration of epilepsy, and antiepileptic drug (AED) combination. We used surgically resected brain specimens from patients with medically intractable epilepsy (n = 22) where the epileptogenic loci were well-characterized by invasive and non-invasive methods; histology confirmed distinction of small NON-EPI regions from EPI tissues. NRs, transporters, CYPs, and tight-junction proteins were assessed by western blots/immunohistochemistry, and CYP metabolic activity was determined and compared. The relationship of CYP expression with seizure frequency, duration of epilepsy, and prescribed AEDs was evaluated. Decreased BBB tight-junction proteins accompanied IgG leakage in EPI regions and correlated with upregulated NR and efflux transporter levels. CYP expression and activity significantly increased in EPI compared to NON-EPI tissues. Change in EPI and NON-EPI CYP3A4 expression increased in patients taking AEDs that were CYP substrates, was downregulated when CYP- and non-CYP-substrate AEDs were given together, and correlated with seizure frequency. Our studies suggest focal neurovascular CYP-NR-transporter alterations, as demonstrated by the relationship of seizure frequency and AED combination to brain CYP3A4, might together impact biotransformation machinery of human pharmacoresistant epilepsy.

Microdialysis in awake macaque monkeys for central nervous system pharmacokinetics

BACKGROUND

The brain bioavailability of novel small molecules developed to address central nervous system disease is classically documented through ex vivo or in vivo analyses conducted in rodent models. Data acquired in rodent models are, however, not easily transferrable to human as the pharmacokinetic and pharmacodynamics profiles of the species are quite different.

METHODS

Using drugs selected for their differential transport across the blood-brain barrier, we here demonstrate the feasibility of brain microdialysis in normal vigil macaque monkey by measuring brain extracellular fluid bioavailability of carbamazepine, digoxin, oxycodone, and quinidine.

RESULTS

All drugs, but digoxin, were found in dialysate samples. Drugs that are substrate of P-glycoprotein show a difference of bioavailability or brain pharmacokinetic parameters between rodents and primates.

CONCLUSION

Data suggest that brain microdialysis in vigil macaque monkey, the species of choice for classic pharmacokinetic/pharmacodynamics studies could help predicting human brain bioavailability of a small molecule depending on the protein involved in the efflux transport from the brain.

Drug Delivery in an Orthotopic Tumor Stem Cell-Based Model of Human Glioblastoma

Grade IV gliomas, also known as glioblastoma multiforme (GBM), are incurable, lethal brain tumors, whose average life expectancy is around 15 months. There is a desperate need for a better understanding of the basic biology of these tumors, in order to devise novel, more specific and effective therapeutics. The handling of GBM represents a daunting challenge to clinicians, also considering the few therapeutic options available, none of which can significantly alter the inevitable lethal outcome of these tumors. Hence, the development of effective therapies would greatly benefit from the availability of in vivo GBM models that can reliably mimic the characteristics of malignant cells and the features of the human disease. Candidate new drugs have to be tested in these in vivo models by adopting settings concerning direct intra-brain delivery in order to define their overall therapeutic efficacy under clinical-like conditions. Here, we describe local intracranial delivery of drugs by osmotic mini-pumps.

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

Objective

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

Methods

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

Results

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

Significance

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

Hepatic and hippocampal cytochrome P450 enzyme overexpression during spontaneous recurrent seizures

OBJECTIVE

Available evidence points to a role of cytochrome P450 (Cyp) drug biotransformation enzymes in central nervous system diseases, including epilepsy. Deviations in drug pharmacokinetic profiles may impact therapeutic outcomes. Here, we ask whether spontaneous recurrent seizure (SRS) activity is sufficient to modulate the expression of major Cyp enzymes in the liver and brain.

METHODS

Unilateral intrahippocampal (IH) kainic acid (KA) injections were used to elicit nonconvulsive status epilepticus (SE), epileptogenesis, and SRS, as monitored by video-electroencephalography. Intraperitoneal (IP) KA injection was used to trigger generalized tonic-clonic SE. KA-injected mice and sham controls were sacrificed at 24-72 hours and 1 week post-SE (IH or IP KA), and during the chronic stage (SRS; 6 weeks post-IH KA). Liver and brain tissues were processed for histology, real-time quantitative polymerase chain reaction, Western blot, or microsomal enzymatic assay. Cyp2e1, Cyp3a13, glial fibrillary acidic protein (GFAP), IBA1, xenobiotic nuclear receptors nr1i2 (PXR), nr1i3 (CAR) and nr3c1 (glucocorticoid receptor [GR]) expression was examined. Serum samples were obtained to assay corticosterone levels, a GR activator.

RESULTS

A significant increase of Cyp3a13 and Cyp2e1 transcript level and protein expression was found in the liver and hippocampi during SRS, as compared to control mice. In the ipsilateral hippocampus, Cyp2e1 and Cyp3a protein upregulation during SRS positively correlated to GFAP expression. GFAP⁺ , and not IBA1⁺ , cells colocalized with Cyp2e1 or Cyp3a expression. In the liver, a trend increase in Cyp3a microsomal activity was found during SRS as compared to control mice. The transcript levels of the Cyp upstream regulators GR, xenobiotic nr1i2, and nr1i3 receptors were unchanged at SRS. Corticosterone levels, a GR ligand, were increased in the blood post-SE.

SIGNIFICANCE

SRS modifies Cyp expression in the liver and the hippocampus. Nuclear receptors or inflammatory pathways are candidate mechanisms of Cyp regulation during seizures.

Modification in CSF specific gravity in acutely decompensated cirrhosis and acute on chronic liver failure independent of encephalopathy, evidences for an early blood-CSF barrier dysfunction in cirrhosis

Although hepatic encephalopathy (HE) on the background of acute on chronic liver failure (ACLF) is associated with high mortality rates, it is unknown whether this is due to increased blood-brain barrier permeability. Specific gravity of cerebrospinal fluid measured by CT is able to estimate blood-cerebrospinal fluid-barrier permeability. This study aimed to assess cerebrospinal fluid specific gravity in acutely decompensated cirrhosis and to compare it in patients with or without ACLF and with or without hepatic encephalopathy. We identified all the patients admitted for acute decompensation of cirrhosis who underwent a brain CT-scan. Those patients could present acute decompensation with or without ACLF. The presence of hepatic encephalopathy was noted. They were compared to a group of stable cirrhotic patients and healthy controls. Quantitative brain CT analysis used the Brainview software that gives the weight, the volume and the specific gravity of each determined brain regions. Results are given as median and interquartile ranges and as relative variation compared to the control/baseline group. 36 patients presented an acute decompensation of cirrhosis. Among them, 25 presented with ACLF and 11 without ACLF; 20 presented with hepatic encephalopathy grade ≥ 2. They were compared to 31 stable cirrhosis patients and 61 healthy controls. Cirrhotic patients had increased cerebrospinal fluid specific gravity (CSF-SG) compared to healthy controls (+0.4 %, p < 0.0001). Cirrhotic patients with ACLF have decreased CSF-SG as compared to cirrhotic patients without ACLF (-0.2 %, p = 0.0030) that remained higher than in healthy controls. The presence of hepatic encephalopathy did not modify CSF-SG (-0.09 %, p = 0.1757). Specific gravity did not differ between different brain regions according to the presence or absence of either ACLF or HE. In patients with acute decompensation of cirrhosis, and those with ACLF, CSF specific gravity is modified compared to both stable cirrhotic patients and healthy controls. This pattern is observed even in the absence of hepatic encephalopathy suggesting that blood-CSF barrier impairment is manifest even in absence of overt hepatic encephalopathy.

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

Objective

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

Methods

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

Results

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

Significance

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

Optimal Management of Leptomeningeal Carcinomatosis in Breast Cancer Patients-A Systematic Review

The incidence of leptomeningeal carcinomatosis in breast cancer patients (LC-BC) is increasing. Despite significantly affecting patient quality of life (QoL) and overall survival (OS), little is known about its optimal management. A systematic review of treatment strategies for LC-BC was performed. EMBASE, Ovid Medline, Pubmed, and the Cochrane Central Register of Controlled Trials were searched from 1946 to 2015 for trials reporting on treatments for LC-BC. All treatment modalities and study types were considered. The outcome measures of interest included OS, time to neurologic progression (TTNP), QoL, and treatment toxicity. Of 718 unique citations, 173 studies met the prespecified eligibility criteria. Most were not specific to LC-BC patients. Of 4 identified randomized controlled trials (RCTs), 1 was specific to LC-BC patients and compared systemic therapy and involved-field radiotherapy with or without intrathecal (IT) methotrexate (35 patients), and the remaining 3 had compared different IT chemotherapy regimens (58 of 157 with LC-BC). Of the remaining studies, 19 were nonrandomized interventional studies (225 LC-BC patients), 148 were observational studies (3230 LC-BC patients), and 2 systematic reviews. Minimal prospective data were available on OS, TTNP, QoL, and toxicity. Owing to study heterogeneity, meta-analyses of the endpoint data could not be performed. Limited high-quality evidence exists regarding optimal treatment of LC-BC. The identified studies were heterogeneous and often methodologically poor. The only RCT that specifically assessed the role of IT chemotherapy showed no benefit, and, if anything, harm. Further prospective, tumor-specific trials with improved interstudy methodologic consistency and transparently reported data on OS, TTNP, QoL, and toxicity are urgently needed.

Pathophysiological implications of neurovascular P450 in brain disorders

Over the past decades, the significance of cytochrome P450 (CYP) enzymes has expanded beyond their role as peripheral drug metabolizers in the liver and gut. CYP enzymes are also functionally active at the neurovascular interface. CYP expression is modulated by disease states, impacting cellular functions, detoxification, and reactivity to toxic stimuli and brain drug biotransformation. Unveiling the physiological and molecular complexity of brain P450 enzymes will improve our understanding of the mechanisms underlying brain drug availability, pharmacological efficacy, and neurotoxic adverse effects from pharmacotherapy targeting brain disorders.

Prostaglandin transporter in the rat brain: its localization and induction by lipopolysaccharide

Prostaglandin E₂ (PGE₂) is produced in the brain during infectious/inflammatory diseases, and it mediates acute-phase responses including fever. In the recovery phase of such diseases, PGE₂ disappears from the brain through yet unidentified mechanisms. Rat prostaglandin transporter (PGT), which facilitates transmembrane transport of PGE₂, might be involved in the clearance of PGE₂ from the brain. Here, we examined the cellular localization of PGT mRNA and its protein in the brains of untreated rats and those injected intraperitoneally with a pyrogen lipopolysaccharide (LPS) or saline. PGT mRNA was weakly expressed in the arachnoid membrane of untreated rats and saline-injected ones, but was induced in blood vessels of the subarachnoidal space and choroid plexus and in arachnoid membrane at 5 h and 12 h after LPS injection. In the same type of cells, PGT-like immunoreactivity was found in the cytosol and cell membrane even under nonstimulated conditions, and its level was also elevated after LPS injection. PGT-positive cells in blood vessels were identified as endothelial cells. In most cases, PGT was not colocalized with cyclooxygenase-2, a marker of prostaglandin-producing cells. The PGE₂ level in the cerebrospinal fluid reached its peak at 3 h after LPS, and then dropped over 50% by 5 h, which time point coincides with the maximum PGT mRNA expression and enhanced level of PGT protein. These results suggest that PGT is involved in the clearance of PGE₂ from the brain during the recovery phase of LPS-induced acute-phase responses.

Expression of tight junction proteins and transporters for xenobiotic metabolism at the blood-CSF barrier during development in the nonhuman primate (P. hamadryas)

The choroid plexus (CP) is rich in barrier mechanisms including transporters and enzymes which can influence drug disposition between blood and brain. We have limited knowledge of their state in fetus. We have studied barrier mechanisms along with metabolism and transporters influencing xenobiotics, using RNAseq and protein analysis, in the CP during the second-half of gestation in a nonhuman primate (Papio hamadryas). There were no differences in the expression of the tight-junctions at the CP suggesting a well-formed fetal blood-CSF barrier during this period of gestation. Further, the fetal CP express many enzymes for phase I-III metabolisms as well as transporters suggesting that it can greatly influence drug disposition and has a significant machinery to deactivate reactive molecules with only minor gestational changes. In summary, the study suggests that from, at least, midgestation, the CP in the nonhuman primate is restrictive and express most known genes associated with barrier function and transport.

Effect of status epilepticus and antiepileptic drugs on CYP2E1 brain expression

P450 metabolic enzymes are expressed in the human and rodent brain. Recent data support their involvement in the pathophysiology of epilepsy. However, the determinants of metabolic enzyme expression in the epileptic brain are unclear. We tested the hypothesis that status epilepticus (SE) or exposure to phenytoin or phenobarbital affects brain expression of the metabolic enzyme CYP2E1. SE was induced in C57BL/6J mice by systemic kainic acid. Brain CYP2E1 expression was evaluated 18-24h after severe SE by immunohistochemistry. Co-localization with neuronal nuclei (NEUN), glial fibrillary acidic protein (GFAP) and CD31 was determined by confocal microscopy. The effect of phenytoin, carbamazepine and phenobarbital on CYP2E1 expression was evaluated in vivo or by using organotypic hippocampal cultures in vitro. CYP2E1 expression was investigated in brain resections from a cohort of drug-resistant epileptic brain resections and human endothelial cultures (EPI-EC). Immunohistochemistry showed an increase of CYP2E1 expression limited to hippocampal CA2/3 and hilar neurons after severe SE in mice. CYP2E1 expression was also observed at the astrocyte-vascular interface. Analysis of human brain specimens revealed CYP2E1 expression in neurons and vascular endothelial cells (EC). CYP2E1 was expressed in cultured human EC and over-expressed by EPI-EC. When analyzing the effect of drug exposure on CYP2E1 expression we found that, in vivo or in vitro, ethanol increased CYP2E1 levels in the brain and liver. Treatment with phenytoin induced localized CYP2E1 expression in the brain whereas no significant effects were exerted by carbamazepine or phenobarbital. Our data indicate that the effect of acute SE on brain CYP2E1 expression is localized and cell specific. Exposure to selected anti-epileptic drugs could play a role in determining CYP2E1 brain expression. Additional investigation is required to fully reproduce the culprits of P450 enzyme expression as observed in the human epileptic brain.

Clinical pharmacokinetics of antibacterials in cerebrospinal fluid

In the past 20 years, an increased discrepancy between new available antibacterials and the emergence of multidrug-resistant strains has been observed. This condition concerns physicians involved in the treatment of central nervous system (CNS) infections, for which clinical and microbiological success depends on the rapid achievement of bactericidal concentrations. In order to accomplish this aim, the choice of drugs is based on their disposition toward the cerebrospinal fluid (CSF), which is influenced by the physicochemical characteristics of antibacterials. A reduced distribution into CSF has been documented for beta-lactams, especially cephalosporins and carbapenems, on the basis of their hydrophilic nature. However, they represent a cornerstone of the majority of combined therapeutic schemes for their ability to achieve bactericidal concentrations, especially in the presence of inflamed meninges. The good tolerability of beta-lactams makes possible high daily dose intensities, which may be associated with increased probability of cure. Furthermore, the adoption of continuous infusion seems to be a fruitful option. Fluoroquinolones, namely moxifloxacin, and antituberculosis drugs, together with the agents such as linezolid, reach the highest CSF/plasma concentration ratio, which is greater than 0.8, and for most of these drugs it is near 1. For all drugs that are currently used for the treatment of CNS infections, the evaluation of pharmacokinetic/pharmacodynamic parameters, on the basis of dosing regimens and their time-dependent or concentration-dependent pattern of bacterial killing, remains an important aspect of clinical investigation and medical practice.

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

PURPOSE

Brain drug bioavailability is regulated by the blood-brain barrier (BBB). It was recently suggested that cytochrome P450 (CYP) enzymes could act in concert with multidrug transporter proteins to regulate drug penetration and distribution into the diseased brain. The possibility that phase II metabolic enzymes could be expressed in the epileptic brain has been not evaluated. Phase II enzymes are involved in the metabolism of common antiepileptic drugs (AEDs).

METHODS

Phase II enzyme UGT1A4 brain expression was evaluated in temporal lobe resections from patients with epilepsy. UGT1A4 expression was determined by western blot and immunocytochemistry in primary cultures of human drug-resistant brain endothelial human brain epileptic endothelial cells (EPI-EC)s and commercially available control cells human brain microvascular endothelial cells (HBMECs). Lack of DNA condensation measured by 4',6-diamidino-2-phenylindole (DAPI) was used as a surrogate marker of cell viability and was correlated to UGT1A4 expression high performance liquid chromatography ultraviolet detection (HPLC-UV) was used to quantify lamotrigine metabolism by EPI-EC and HBMEC. The appearance of the specific lamotrigine metabolite, 2-n glucuronide (MET-1), was also evaluated. Lamotrigine and MET-1 levels were measured in selected surgical brain and matched blood samples.

KEY FINDINGS

UGT1A4 expression was observed in BBB endothelial cells and neurons. Our quantification study revealed variable levels of UGT1A4 expression across the brain specimens analyzed. Neurons devoid of UGT1A4 expression displayed nuclear DAPI condensation, a sign of cellular distress. UGT1A4 overexpression in EPI-EC, as compared to HBMEC, was reflected by a proportional increase in lamotrigine metabolism. The lamotrigine metabolite, MET-1, was formed in vitro by EPI-EC and, to a lesser extent, by HBMEC. HPLC-UV measurements of brain and blood samples obtained from patients receiving lamotrigine prior to surgery revealed the presence of lamotrigine and its metabolites in the brain.

SIGNIFICANCE

These initial results suggest the presence of a phase II enzyme in the epileptic brain. Further studies are required to fully describe the pattern of brain UGT1A4 expression in relation to clinical variables and drug resistance.

Computational prediction of CNS drug exposure based on a novel in vivo dataset

PURPOSE

To develop a computational model for predicting CNS drug exposure using a novel in vivo dataset.

METHODS

The brain-to-plasma (B:P) ratio of 43 diverse compounds was assessed following intravenous administration to Swiss Outbred mice. B:P ratios were subjected to PLS modeling using calculated molecular descriptors. The obtained results were transferred to a qualitative setting in which compounds predicted to have a B:P ratio > 0.3 were sorted as high CNS exposure compounds and those below this value were sorted as low CNS exposure compounds. The model was challenged with an external test set consisting of 251 compounds for which semi-quantitative values of CNS exposure were available in the literature.

RESULTS

The dataset ranged more than 1700-fold in B:P ratio, with 16 and 27 compounds being sorted as low and high CNS exposure drugs, respectively. The model was a one principal component model based on five descriptors reflecting molecular shape, electronegativity, polarisability and charge transfer, and allowed 74% of the compounds in the training set and 76% of the test set to be predicted correctly.

CONCLUSION

A qualitative computational model has been developed which accurately classifies compounds as being high or low CNS exposure drugs based on rapidly calculated molecular descriptors.

Transport and metabolism at blood-brain interfaces and in neural cells: relevance to bilirubin-induced encephalopathy

Bilirubin, the end-product of heme catabolism, circulates in non-pathological plasma mostly as a protein-bound species. When bilirubin concentration builds up, the free fraction of the molecule increases. Unbound bilirubin then diffuses across blood-brain interfaces (BBIs) into the brain, where it accumulates and exerts neurotoxic effects. In this classical view of bilirubin neurotoxicity, BBIs act merely as structural barriers impeding the penetration of the pigment-bound carrier protein, and neural cells are considered as passive targets of its toxicity. Yet, the role of BBIs in the occurrence of bilirubin encephalopathy appears more complex than being simple barriers to the diffusion of bilirubin, and neural cells such as astrocytes and neurons can play an active role in controlling the balance between the neuroprotective and neurotoxic effects of bilirubin. This article reviews the emerging in vivo and in vitro data showing that transport and metabolic detoxification mechanisms at the blood-brain and blood-cerebrospinal fluid barriers may modulate bilirubin flux across both cellular interfaces, and that these protective functions can be affected in chronic unconjugated hyperbilirubinemia. Then the in vivo and in vitro arguments in favor of the physiological antioxidant function of intracerebral bilirubin are presented, as well as the potential role of transporters such as ABCC1 and metabolizing enzymes such as cytochromes P-450 in setting the cerebral cell- and structure-specific toxicity of bilirubin following hyperbilirubinemia. The relevance of these data to the pathophysiology of bilirubin-induced neurological diseases is discussed.

Modulation of Mrp1 (ABCc1) and Pgp (ABCb1) by bilirubin at the blood-CSF and blood-brain barriers in the Gunn rat

Accumulation of unconjugated bilirubin (UCB) in the brain causes bilirubin encephalopathy. Pgp (ABCb1) and Mrp1 (ABCc1), highly expressed in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) respectively, may modulate the accumulation of UCB in brain. We examined the effect of prolonged exposure to elevated concentrations of UCB on expression of the two transporters in homozygous, jaundiced (jj) Gunn rats compared to heterozygous, not jaundiced (Jj) littermates at different developmental stages (2, 9, 17 and 60 days after birth). BBB Pgp protein expression was low in both jj and Jj pups at 9 days (about 16-27% of adult values), despite the up-regulation in jj animals (2 and 1.3 fold higher than age matched Jj animals at P9 and P17-P60, respectively); Mrp1 protein expression was barely detectable. Conversely, at the BCSFB Mrp1 protein expression was rather high (60-70% of the adult values) in both jj and Jj at P2, but was markedly (50%) down-regulated in jj pups starting at P9, particularly in the 4(th) ventricle choroid plexuses: Pgp was almost undetectable. The Mrp1 protein down regulation was accompanied by a modest up-regulation of mRNA, suggesting a translational rather than a transcriptional inhibition. In vitro exposure of choroid plexus epithelial cells obtained from normal rats to UCB, also resulted in a down-regulation of Mrp1 protein. These data suggest that down-regulation of Mrp1 protein at the BSCFB, resulting from a direct effect of UCB on epithelial cells, may impact the Mrp1-mediated neuroprotective functions of the blood-cerebrospinal fluid barrier and actually potentiate UCB neurotoxicity.

Penetration of drugs through the blood-cerebrospinal fluid/blood-brain barrier for treatment of central nervous system infections

The entry of anti-infectives into the central nervous system (CNS) depends on the compartment studied, molecular size, electric charge, lipophilicity, plasma protein binding, affinity to active transport systems at the blood-brain/blood-cerebrospinal fluid (CSF) barrier, and host factors such as meningeal inflammation and CSF flow. Since concentrations in microdialysates and abscesses are not frequently available for humans, this review focuses on drug CSF concentrations. The ideal compound to treat CNS infections is of small molecular size, is moderately lipophilic, has a low level of plasma protein binding, has a volume of distribution of around 1 liter/kg, and is not a strong ligand of an efflux pump at the blood-brain or blood-CSF barrier. When several equally active compounds are available, a drug which comes close to these physicochemical and pharmacokinetic properties should be preferred. Several anti-infectives (e.g., isoniazid, pyrazinamide, linezolid, metronidazole, fluconazole, and some fluoroquinolones) reach a CSF-to-serum ratio of the areas under the curves close to 1.0 and, therefore, are extremely valuable for the treatment of CNS infections. In many cases, however, pharmacokinetics have to be balanced against in vitro activity. Direct injection of drugs, which do not readily penetrate into the CNS, into the ventricular or lumbar CSF is indicated when other effective therapeutic options are unavailable.

In vivo methods to study uptake of nanoparticles into the brain

Several in vivo techniques have been developed to study and measure the uptake of CNS compounds into the brain. With these techniques, various parameters can be determined after drug administration, including the blood-to-brain influx constant (K_in), the permeability-surface area (PS) product, and the brain uptake index (BUI). These techniques have been mostly used for drugs that are expected to enter the brain via transmembrane diffusion or by carrier-mediated transcytosis. Drugs that have limitations in entering the brain via such pathways have been encapsulated in nanoparticles (based on lipids or synthetic polymers) to enhance brain uptake. Nanoparticles are different from CNS compounds in size, composition and uptake mechanisms. This has led to different methods and approaches to study brain uptake in vivo. Here we discuss the techniques generally used to measure nanoparticle uptake in addition to the techniques used for CNS compounds. Techniques include visualization methods, behavioral tests, and quantitative methods.

Lack of a primary physicochemical determinant in the direct transport of drugs to the brain after nasal administration in rats: potential involvement of transporters in the pathway

The objectives of this study were to evaluate the relative contribution of the direct pathway in overall brain transport for 17 model drugs with different physicochemical properties after nasal administrations and to identify factors that govern the fraction of the dose transported to the brain via the direct pathway (F(a, direct)). When the model drugs were nasally administered to rats, 5 of the 17 model drugs were delivered to a significant extent to the brain via the direct pathway. Multiple linear regression analyses showed that the correlation between various physicochemical properties and F(a, direct) was not statistically significant, indicative of a lack of primary physicochemical determinants in the direct transport pathway. Transporters such as rOAT3 and rOCT2 were expressed at significant levels in rat olfactory epithelia, and uptakes of standard substrates were significantly decreased in HEK293 cells expressing rOAT3 and rOCT2 in the presence of the five model drugs that were delivered to appreciable extents to the brain via the direct pathway. Therefore, these observations indicate that carrier-mediated transport may play a role in the brain delivery of drugs from the nose via the direct transport pathway.

Brain endothelial cell-cell junctions: how to "open" the blood brain barrier

The blood-brain barrier (BBB) is a highly specialized structural and biochemical barrier that regulates the entry of blood-borne molecules into brain, and preserves ionic homeostasis within the brain microenvironment. BBB properties are primarily determined by junctional complexes between the cerebral endothelial cells. These complexes are comprised of tight and adherens junctions. Such restrictive angioarchitecture at the BBB reduces paracellular diffusion, while minimal vesicle transport activity in brain endothelial cells limits transcellular transport. Under normal conditions, this largely prevents the extravasation of large and small solutes (unless specific transporters are present) and prevents migration of any type of blood-borne cell. However, this is changed in many pathological conditions. There, BBB disruption (“opening”) can lead to increased paracellular permeability, allowing entry of leukocytes into brain tissue, but also contributing to edema formation. In parallel, there are changes in the endothelial pinocytotic vesicular system resulting in the uptake and transfer of fluid and macromolecules into brain parenchyma. This review highlights the route and possible factors involved in BBB disruption in a variety of neuropathological disorders (e.g. CNS inflammation, Alzheimer’s disease, Parkinson’s disease, epilepsy). It also summarizes proposed signal transduction pathways that may be involved in BBB “opening”.

The rat blood-brain barrier transcriptome

The blood-brain barrier (BBB) is the cellular interface between the circulating blood and neural environment, and is created by apposed endothelial cells and their intercellular tight junctions. Many aspects of how the BBB functions at the molecular level remain unresolved; therefore, we report for the first time a comprehensive gene expression profile of rat brain microvessels using serial analysis of gene expression (SAGE). We assembled a full and quantitative SAGE catalog containing 101,364 tags, of which 33% of the tags matched known genes, 51% matched expressed sequence tags (ESTs) in the Unigene database, and 16% of the tags were unassigned. The transcriptome catalog contains many new and novel transcripts among known BBB genes. A large compliment of junctional proteins and an extensive assortment of facilitated carrier and ATP-dependent transporters are included. To identify microvessel-enriched transcripts, we compared the microvessel SAGE catalog to cortex and hippocampus SAGE catalogs. This resulted in identification of 864 genes, including several known for their abundant expression at the BBB, such as the transferrin receptor (TrnR). Sorting enriched genes based on function revealed groups that encode transporters (11%), receptors (5%), proteins involved in vesicle trafficking (4%), structural proteins (10%), and components of signal transduction pathways (17%). This genomic repertoire emphasizes the unique cellular phenotype existing within the brain and further implicates the BBB as a mediator between the brain and periphery. These results may provide a useful resource and reference point from which to determine the effects of different physiological, developmental, and disease processes on BBB gene expression.

Enhanced Prospects for Drug Delivery and Brain Targeting by the Choroid Plexus–CSF Route

The choroid plexus (CP), i.e., the blood-cerebrospinal fluid barrier (BCSFB) interface, is an epithelial boundary exploitable for drug delivery to brain. Agents transported from blood to lateral ventricles are convected by CSF volume transmission (bulk flow) to many periventricular targets. These include the caudate, hippocampus, specialized circumventricular organs, hypothalamus, and the downstream pia-glia and arachnoid membranes. The CSF circulatory system normally provides micronutrients, neurotrophins, hormones, neuropeptides, and growth factors extensively to neuronal networks. Therefore, drugs directed to CSF can modulate a variety of endocrine, immunologic, and behavioral phenomema; and can help to restore brain interstitial and cellular homeostasis disrupted by disease and trauma. This review integrates information from animal models that demonstrates marked physiologic effects of substances introduced into the ventricular system. It also recapitulates how pharmacologic agents administered into the CSF system prevent disease or enhance the brain's ability to recover from chemical and physical insults. In regard to drug distribution in the CNS, the BCSFB interaction with the blood-brain barrier is discussed. With a view toward translational CSF pharmacotherapy, there are several promising innovations in progress: bone marrow cell infusions, CP encapsulation and transplants, neural stem cell augmentation, phage display of peptide ligands for CP epithelium, CSF gene transfer, regulation of leukocyte and cytokine trafficking at the BCSFB, and the purification of neurotoxic CSF in degenerative states. The progressively increasing pharmacological significance of the CP-CSF nexus is analyzed in light of treating AIDS, multiple sclerosis, stroke, hydrocephalus, and Alzheimer's disease.

Chlorpyrifos Alters Functional Integrity and Structure of an In Vitro BBB Model: Co-cultures of Bovine Endothelial Cells and Neonatal Rat Astrocytes

The blood-brain barrier (BBB) is a structural and functional interface between the circulatory system and the brain. Organophosphorous compounds such as chlorpyrifos (CPF) may cross the BBB and disrupt BBB integrity and function. To determine events that may contribute to CPF toxicity, we used an in vitro BBB model in which bovine microvascular endothelial cells (BMEC) and neonatal rat astrocytes were co-cultured. We hypothesized that CPF is metabolized by the BBB leading to an inhibition of esterase activity and a disruption of the BBB. The co-culturing of BMECs and astrocytes resulted in tight junction formation as determined by electron microscopy, electrical resistance and western blot analysis of two tight junction-associated proteins (ZO-1 and e-cadherin). We observed time dependent increases in ZO-1 and e-cadherin expression and electrical resistance during BBB formation, which were maximal after 9-13 days of co-culturing. The CPF concentration and production of its metabolites were monitored by HPLC following 24 h exposure to CPF on the luminal side of the BBB. We found that the BBB metabolized CPF, with the metabolite 2,3,6-trichloro-2-pyridinol being the major product. CPF and its metabolites were detected on the abluminal side of the BBB suggesting that CPF crossed this barrier. CPF was also detected intracellularly and on the membrane inserts. At tested concentrations (0.1-10 microM), CPF inhibited both carboxylesterase (CaE) and cholinesterase (ChE) activities in BMECs by 43-100%, while CPF-oxon totally inhibited CaE and ChE activity in concentrations as low as 0.1 microM. CPF also caused a concentration-dependent decrease in electrical resistance, with significant inhibition observed at 1 nM and complete loss at 1 microM. These data show that low concentrations of CPF and its metabolites are present within the BBB. CPF and its metabolites, especially CPF-oxon, contribute to the inhibition of CaE and ChE activity, as well as the alteration of BBB integrity and structure.

The study of codeine-gluthetimide pharmacokinetic interaction in rats

A high-performance liquid chromatographic (HPLC) assay with native fluorescence detection was developed for the simultaneous quantification of codeine and its two metabolites, morphine and morphine-3-glucuronide (M-3-G), in rat plasma. Solid-phase extraction was used to separate codeine and its metabolites from plasma constituents. Extraction efficiencies of codeine, morphine and M-3-G from rat plasma samples were 97, 92 and 93%, respectively. The chromatographic separation was performed using a reversed-phase C18 column and an elution gradient at ambient temperature. Using native fluorescence detection (excitation at 245 nm and emission at 345 nm), the detection limits of 50 ng/ml for morphine, 25 ng/ml for codeine and 20 ng/ml for M-3-G were obtained. The method had good precision, accuracy and linearity, and was applied to the study of glutethimide's influence on codeine metabolism in rat, following single doses of codeine-glutethimide association. The results confirmed the fact that glutethimide was responsible for a significant increase of morphine plasma levels and for their maintenance in time, concomitant with a significant decrease of M-3-G plasma levels, explained by the inhibition of morphine glucuronidation. In conclusion, glutethimide potentiates and prolongs the analgesic effect of codeine by a pharmacokinetic mechanism.

Targeted disruption of the PEPT2 gene markedly reduces dipeptide uptake in choroid plexus

The presence of multiple oligopeptide transporters in brain has generated considerable interest as to their physiological role in neuropeptide homeostasis, pharmacologic importance, and potential as a target for drug delivery through the blood-brain and blood-cerebrospinal fluid barriers. To understand further the purpose of specific peptide transporters in brain, we have generated PEPT2-deficient mice by targeted gene disruption. Homozygous PepT2 null mice lacked expression of PEPT2 mRNA and protein in choroid plexus and kidney, tissues in which PepT2 is normally expressed, whereas heterozygous mice displayed PepT2 expression levels that were intermediate between those of wild-type and homozygous null animals. Mutant PepT2 null mice were found to be viable, grew to normal size and weight, and were without obvious kidney or brain abnormalities. Notwithstanding the lack of apparent biological effects, the proton-stimulated uptake of 1.9 microm glycylsarcosine (a model, hydrolysis-resistant dipeptide) in isolated choroid plexus was essentially ablated (i.e. residual activity of 10.9 and 3.9% at 5 and 30 min, respectively). These novel findings provide strong evidence that, under the experimental conditions of this study, PEPT2 is the primary member of the peptide transporter family responsible for dipeptide uptake in choroid plexus tissue.

Brain drug delivery, drug metabolism, and multidrug resistance at the choroid plexus

The choroid plexuses (CPs) have the capability to modulate drug delivery to the cerebrospinal fluid (CSF) and to participate in the overall cerebral biodisposition of drugs. The specific morphological properties of the choroidal epithelium and the existence of a CSF pathway for drug distribution to different targets in the central nervous system suggest that the CP-CSF route is more significant than previously thought for brain drug delivery. In contrast to its role in CSF penetration of drugs, CP is also involved in brain protection in that it has the capacity to clear the CSF from numerous potentially harmful CSF-borne exogenous and endogenous organic compounds into the blood. Furthermore, CP harbors a large panel of drug-metabolizing enzymes as well as transport proteins of the multidrug resistance phenotype, which modulate the cerebral bioavailability of drugs and toxins. The use of an in vitro model of the choroidal epithelium suitable for drug transport studies has allowed the demonstration of the choroidal epithelium acting as an effective metabolic blood-CSF barrier toward some xenobiotics, and that a vectorial, blood-facing efflux of conjugated metabolites occurs at the choroidal epithelium. This efflux involves a specific transporter with characteristics similar to those of the multidrug resistance associated protein (MRP) family members. Indeed, at least one member, MRP1, is largely expressed at the CP epithelium, and localizes at the basolateral membrane. These metabolic and transport features of the choroidal epithelium point out the CP as a major detoxification site within the brain.

Relationship between permeability status of the blood-brain barrier and in vitro permeability coefficient of a drug

OBJECTIVE

The aim was to test the hypothesis that the assessment of basal and drug-induced changes in permeability of the blood-brain barrier (BBB) during in vitro drug transport assays is essential for an accurate estimation of the permeability coefficient of a drug.

METHODS

An in vitro BBB model was used, comprising of brain capillary endothelial cells (BCEC) and astrocytes co-cultured on semi-permeable filter inserts. Experiments were performed under control and challenged experimental circumstances, induced to simulate drug effects. The apparent BBB permeability coefficient for two markers for paracellular drug transport, sodium fluorescein (P(app,FLU), M(w) 376 Da) and FITC-labeled dextran (P(app,FD4), M(w) 4 kDa), was determined. Transendothelial electrical resistance (TEER) was used to quantify basal and (simulated) drug-induced changes in permeability of the in vitro BBB. The relationship between P(app) and TEER was determined. Drug effects were simulated by exposure to physiologically active endogenous and exogenous substances (i.e., histamine, deferroxamine mesylate, adrenaline, noradrenaline, bradykinin, vinblastine, sodium nitroprusside and lipopolysaccharide).

RESULTS

P(app,FLU) and P(app,FD4) in control experiments varied from 1.6 up to 17.6 (10(-6)cm/s) and 0.3 up to 7. 3 (10(-6)cm/s), respectively; while for individual filters P(app, FLU) was 4 times higher than P(app,FD4) (R(2)=0.97). As long as TEER remained above 131.Omega cm(2) for FLU or 122.Omega cm(2) for FD4 during the transport assay, P(app) remained independent from the basal permeability of the in vitro BBB. Below these TEER values, P(app) increased exponentially. This nonlinear relationship between basal BBB permeability and P(app) was described by a one-phase exponential decay model. From this model the BBB permeability status independent permeability coefficients for FLU and FD4 (P(FLU) and P(FD4)) were estimated to be 2.2+/-0.1 and 0.48+/-0.03 (10(-6)cm/s), respectively. In the experimentally challenged experiments, a reliable indication for P(FLU) and P(FD4) could be estimated only after the (simulated) drug-induced change in BBB permeability was taken into account.

CONCLUSIONS

The assessment of basal BBB permeability status during drug transport assays was essential for an accurate estimation of the in vitro permeability coefficient of a drug. To accurately extrapolate the in vitro permeability coefficient of a drug to the in vivo situation, it is essential that drug-induced changes in the in vitro BBB permeability during the drug transport assay are determined.

Human brain cytochrome P450 1B1: immunohistochemical localization in human temporal lobe and induction by dimethylbenz(a)anthracene in astrocytoma cell line (MOG-G-CCM)

CYP1B1, a new member of human cytochrome P450 family 1, is involved in the xenobiotic detoxification metabolism and possibly activation of numerous procarcinogens and promutagens. Localization of CYP1B1 in human temporal lobe and its induction in astrocytoma cell line (MOG-G-CCM) by 7,12-dimethylbenz(a)anthracene (DMBA) was investigated using antibodies against human CYP1B1. A single band of approximately 58 kDa size in both human temporal lobe and in MOG-G-CCM was detected by Western blot analysis. Treatment of MOG-G-CCM cells with DMBA resulted in approximately 2.8-fold induction of CYP1B1. CYP1B1 immunoreactivity was detected at the blood-brain interface areas of the temporal lobe as evidenced by co-localization with CD34 antigen. These results suggest that this enzyme may be important in brain xenobiotic metabolism acting as an enzymatic barrier.

Transport mechanisms for the antidepressant citalopram in brain microvessel endothelium

Blood-brain barrier transport of the selective serotonin reuptake inhibitor and antidepressant, citalopram, was studied using monolayers of bovine brain microvessel endothelial cells (BMECs). This study provides for the first time, evidence of a transport mechanism for a selective serotonin reuptake inhibitor (SSRI). Carrier-mediated transport, efflux mechanisms, as well as inhibition of metabolizing enzymes of citalopram were investigated. Citalopram transport was saturable and temperature-dependent suggesting that passage of the drug across BMECs was mediated by a carrier mechanism. Since the apical to basolateral and basolateral to apical permeability coefficients were similar and cyclosporin A, a P-glycoprotein inhibitor, does not modify the transport of citalopram, it appeared that no active efflux systems were involved in this transport. Citalopram is only available as a racemic drug and its pharmacological effect resides mainly in the S-(+)-enantiomer. However, the passage of citalopram enantiomers across BMEC monolayers was not stereoselective. Finally, inhibition of the metabolizing enzymes of citalopram and monoamine oxidases did not modify the permeation of citalopram across BMECs. Collectively, our results suggested that citalopram crosses the blood-brain barrier via a non-stereoselective, bidirectional and symmetrical carrier-mediated mechanism without influences of active efflux mechanisms or monoamine oxidases.

Cerebrospinal cuterebriasis in cats and its association with feline ischemic encephalopathy

Over the past 10 years, 10 cats with primary central nervous system infection by larvae of Cuterebra flies have been documented at Cornell University. Clinical abnormalities noted in all cats were progressive and most commonly consisted of depression (6/10), blindness (6/10), and behavior changes (2/10). Affected cats presented most commonly in July (2/10) and August (7/10); one cat was presented in September. The diverse histopathologic changes are unique to this aberrant migration and consist of a combination of five characteristic features: 1) parasitic track lesion (7/10), 2) superficial laminar cerebrocortical necrosis (10/10), 3) cerebral infarction (8/10), 4) subependymal rarefaction and astrogliosis with or without ependymal cell loss (7/10), and 5) subpial astrogliosis (7/10). Changes 2-5 occurred throughout the parenchyma unassociated with the track lesion or the parasite in the affected tissue. The larvae have been recovered most commonly in the region of the olfactory bulbs and peduncles, optic nerves, and cribriform plate, suggesting entry from the nasal cavity. Many of the changes noted are suggestive of a toxic factor elaborated by the parasite and borne within the cerebrospinal fluid, as well as vascular compromise as a component in those cats with brain infarction. Because of the prevalence of infarction associated with this syndrome and the lack of reported cases of such lesions in regions of the world devoid of the fly, we propose that aberrant cuterebral larval migration in the brain is the cause of feline ischemic encephalopathy.

Electronic spin resonance detection of superoxide and hydroxyl radicals during the reductive metabolism of drugs by rat brain preparations and isolated cerebral microvessels

A spin trapping technique was used to analyze by electron spin resonance (ESR) the formation of oxygen-derived free radicals during the cerebral reductive metabolism of xenobiotics able to undergo a single electron reduction, i.e. quinones, pyridinium compounds and nitroheterocyclics. Paraquat, menadione and nitrofurazone were used as model compounds of these three classes of molecules. ESR spectra indicative of superoxide and hydroxyl radical formation were obtained by incubation of brain homogenates directly within the ESR cavity at 37 degrees C for each of the three molecules tested. These signals were dependent on nucleotide cofactors, and increased in a time-dependent manner. The NADPH and NADH dependent free radical production was further characterized in brain microsomal and mitochondrial fractions, respectively. By using various combinations of reactive species inactivating enzymes (superoxide dismutase, catalase), a metal chelator (deferoxamine), and an hydroxyl trapping agent (dimethylsulfoxide), it was shown that (1) the primary radical generated was the superoxide anion; and (2) a significant production of the hydroxyl radical also occurred, that was secondary to the superoxide anion production. Consistent signals indicative of the production of both oxygen-derived free radicals were obtained when isolated cerebral microvessels which constitute the blood-brain barrier were incubated with the model molecules. This is of particular toxicological relevance, because this barrier represents a key element in the protection of the brain, and is in close contact with blood-born exogenous molecules.