Reconstitution of the blood-brain barrier in cell culture for studies of drug transport and metabolism

In vitro screening of nanomedicines through the blood brain barrier: A critical review

The blood-brain barrier accounts for the high attrition rate of the treatments of most brain disorders, which therefore remain one of the greatest health-care challenges of the twenty first century. Against this background of hindrance to brain delivery, nanomedicine takes advantage of the assembly at the nanoscale of available biomaterials to provide a delivery platform with potential to raising brain levels of either imaging or therapeutic agents. Nevertheless, to prevent later failure due to ineffective drug levels at the target site, researchers have been endeavoring to develop a battery of in vitro screening procedures that can predict earlier in the drug discovery process the ability of these cutting-edge drug delivery platforms to cross the blood-brain barrier for biomedical purposes. This review provides an in-depth analysis of the currently available in vitro blood-brain barrier models (both cell-based and non-cell-based) with the focus on their suitability for understanding the biological brain distribution of forthcoming nanomedicines. The relationship between experimental factors and underlying physiological assumptions that would ultimately lead to a more predictive capacity of their in vivo performance, and those methods already assayed for the evaluation of the brain distribution of nanomedicines are comprehensively discussed.

Brainpeps: the blood-brain barrier peptide database

Peptides are able to cross the blood-brain barrier (BBB) through various mechanisms, opening new diagnostic and therapeutic avenues. However, their BBB transport data are scattered in the literature over different disciplines, using different methodologies reporting different influx or efflux aspects. Therefore, a comprehensive BBB peptide database (Brainpeps) was constructed to collect the BBB data available in the literature. Brainpeps currently contains BBB transport information with positive as well as negative results. The database is a useful tool to prioritize peptide choices for evaluating different BBB responses or studying quantitative structure-property (BBB behaviour) relationships of peptides. Because a multitude of methods have been used to assess the BBB behaviour of compounds, we classified these methods and their responses. Moreover, the relationships between the different BBB transport methods have been clarified and visualized.

Lipophilicity and its relationship with passive drug permeation

In this review, we first summarize the structure and properties of biological membranes and the routes of passive drug transfer through physiological barriers. Lipophilicity is then introduced in terms of the intermolecular interactions it encodes. Finally, lipophilicity indices from isotropic solvent systems and from anisotropic membrane-like systems are discussed for their capacity to predict passive drug permeation across biological membranes such as the intestinal epithelium, the blood-brain barrier (BBB) or the skin. The broad evidence presented here shows that beyond the predictive power of lipophilicity parameters, the various intermolecular forces they encode allow a mechanistic interpretation of passive drug permeation.

Synthesis and Characterization of 9-[P-(N,N-dipropyl Sulfamide)] Benzoylamino-1,2,3,4-4H-acridine—A Potential Prodrug for the CNS Delivery of Tacrine

9-[P-(N,N-dipropylsulfamide)]benzoylamino-1,2,3,4-4H-acridine (PTHA) was synthesized as a prodrug to improve the curative effect of tacrine hydrochloride (THA), to prolong the activation time in brain, and to decrease the hepatic toxicity. The prodrug was prepared by attaching carboxyl group of probenecid to 9-amino group of THA. The structure of PTHA was confirmed by IR, HNMR, MS and UV. The physiochemical properties and stabilities of the prodrug under various conditions were investigated. Possibility of the prodrug passing through the blood-brain barrier (BBB) was evaluated in vitro by biopartition micellar chromatography (BMC). The concentrations of THA and PTHA in various tissues were determined by reversed-phase high-performance liquid chromatography after intravenous (i.v.) administration. The results showed PTHA was stable in various pHs and temperatures. It was indicated by partition coefficient in ethyl acetate/water that lipophilicity of the prodrug increased and it was predicted by BMC that the prodrug could improve the infiltration ability across BBB of THA. In-vivo experiment showed the concentrations of THA in brain and liver kept stable for about 4 h, which was beneficial for the treatment of Alzheimer's disease (AD). Compared with THA at the same time, AUC in liver decreases significantly, the overall targeting efficiency (TE) was enhanced from 10.97 to 16.11% and AUC(brain)/AUC(liver) increased from 1.52 to 2.53, which suggests the possibility to reduce the hepatic toxicity of THA by the way of the prodrug.

Functional role of adenosine receptor subtypes in the regulation of blood-brain barrier permeability: possible implications for the design of synthetic adenosine derivatives

The objective of this investigation was to determine the functional role of adenosine receptor subtypes in the regulation of blood-brain barrier (BBB) permeability. The presence of the equilibrative es and ei nucleoside transporters at the BBB was also determined. Studies were conducted in an experimental in vitro BBB model comprising bovine brain capillary endothelial cells (BCECs) and rat astrocytes (RAs). The presence of the receptors and transporters was investigated by a combination of RT-PCR and radioligand binding assays. Changes in paracellular permeability were investigated on basis of changes in trans-endothelial-electrical-resistance (TEER) and transport of paracellular markers. In BCECs the presence of A(2A) and A(3) receptors and the es nucleoside transporter was demonstrated. The A(1) receptor was absent, while the presence of the A(2B) receptor and the ei nucleoside transporter remained uncertain. In RAs the presence of all four receptor subtypes and the es and ei nucleoside transporters was demonstrated. Upon application of selective agonists no significant changes in TEER or the transport of the paracellular markers were observed. The functional role of adenosine receptor subtypes in regulating the paracellular permeability of the BBB is probably small. It is unlikely therefore that the BBB transport of synthetic adenosine analogues is modified by permeability changes. The es nucleoside transporter might play a role in the BBB transport of synthetic adenosine analogues.

Establishment and functional characterization of an in vitro model of the blood-brain barrier, comprising a co-culture of brain capillary endothelial cells and astrocytes

OBJECTIVE

The aim was to establish a flexible, abundantly available, reproducible and functionally characterized in vitro model of the blood-brain barrier (BBB).

METHODS

In a first step, bovine brain capillaries and newborn rat astrocytes were isolated. Subsequently, a co-culture of primary brain capillary endothelial cells (BCEC) on semi-permeable filter inserts, with astrocytes on the bottom of the filter was established. The cell material was characterized on the basis of specific cell-type properties and (functional expression of) specific BBB properties.

RESULTS

BCEC displayed: (1) characteristic endothelial cell morphology; (2) expression of endothelial cell markers (i.e., CD51, CD62P, CD71 and cadherin 5); (3) marginal F-actin localization; (4) tight junction formation between the cells; (5) expression of gamma-glutamyl-transpeptidase (gamma-GTP); (6) expression of P-glycoprotein (Pgp); (7) functional transendothelial transferrin transport and uptake; (8) restriction of paracellular transport; and (9) high transendothelial electrical resistance (TEER). Astrocytes displayed characteristic astrocyte morphology and expressed glial fibrillary acidic protein (GFAP). Co-culture with astrocytes increased TEER and decreased paracellular transport. In addition, expression of the glucocorticoid receptor (GR) was demonstrated in the endothelial cells of the BBB, while no expression of the mineralocorticoid receptor (MR) was found.

CONCLUSIONS

A high quality and mass-production in vitro BBB model was established in which experiments with physiological (e.g., regulation of BBB permeability), pharmacological (e.g., pharmacokinetics and pharmacodynamics) and pathophysiological (e.g., disease influence on BBB permeability) objectives can be reproducibly performed.

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.