Drug Targeting Organ-Specific Strategies

34 2 Brain-Specific Drug Targeting Strategies
2.3.2.2 In Vitro Models
Freshly isolated capillaries from different species are a valuable model system of the BBB
[51], because they retain the native repertoire of receptors and enzymes. In particular, receptor
binding and carrier-mediated uptake or receptor-mediated endocytosis can be conveniently
studied. On which side binding or uptake takes place cannot be easily determined, as
both the luminal and abluminal side are exposed in such a preparation.
Numerous modifications of in vitro culture systems have been developed for the estimation
of BBB transfer [52]. Culture systems in use are either primary cultures of brain microvessel
endothelial cells (BMEC) or immortalized endothelial cell lines. BMEC may be
grown in co-culture with astrocytes or in astrocyte-conditioned medium. Astrocyte-derived
factors increase the tightness of the barrier as measured by transendothelial electrical resistance
(TEER) and by the permeability of hydrophilic markers such as sucrose.They also upregulate
the expression of BBB-enriched enzymes such as γ-glutamyl transpeptidase
(γ-GTP) and alkaline phosphatase.A setup of the in vitro technique in a transwell system for
transport studies is depicted in Figure 2.5.
Abluminal
lower chamber
C6 Astroglioma
Luminal upper chamber
BBMECs
6 well cluster plate
Microporous membrane
(0.4 µM pore size)
Millicell®
ERS
TEER
(ohms•cm 2 )
C6 Astroglioma
Figure 2.5. Setup for in vitro measurement of blood–brain barrier permeability with a co-culture of
bovine brain microvascular endothelial cells (BBMEC) and an astroglioma cell line, C6. The BBMEC
are grown on top of a filter insert. The C6 cells are either grown on the opposite side of the filter or on
the bottom of the wells. Transport across the BBMEC monolayer is measured by adding the test
substance to the upper chamber and sampling from the lower chamber. The tightness of the monolayer
is also characterized by the transendothelial electrical resistance (TEER). Courtesy of T. Abbruscato.
2.4 Drug Delivery Strategies
The approaches for developing strategies for the delivery of drugs to the brain are shown in
groups in the scheme in Figure 2.6. Pharmacological-based and physiological-based methods