Experimental methods and transport models for drug delivery across the blood-brain barrier

Abstract

The blood-brain barrier (BBB) is a dynamic barrier essential for maintaining the micro-environment of the brain. Although the special anatomical features of the BBB determine its protective role for the central nervous system (CNS) from blood-born neurotoxins, however, the BBB extremely limits the therapeutic efficacy of drugs into the CNS, which greatly hinders the treatment of major brain diseases. This review summarized the unique structures of the BBB, described a variety of in vivo and in vitro experimental methods for determining the transport properties of the BBB, e.g., the permeability of the BBB to water, ions, and solutes including nutrients, therapeutic agents and drug carriers, and presented newly developed mathematical models which quantitatively correlate the anatomical structures of the BBB with its barrier functions. Finally, on the basis of the experimental observations and the quantitative models, several strategies for drug delivery through the BBB were proposed.

Figure 1

Schematic of the cross-sectional view of: (a) a peripheral microvessel (the microvessel in non-brain organs), and (b) the blood-brain barrier (BBB) or cerebral microvessel (the microvessel in the brain). In addition to other structures as in a peripheral microvessel, the BBB is wrapped by astrocyte foot processes (AP). BM, Basement membrane (or basal lamina); E, endothelial cell; EN, nucleus of endothelial cell; P, pericytes; G, surface glycocalyx layer; TJ, tight junction. Modified from [40].

Figure 2

Schematic of junctional complex in the paracellular pathway of the BBB. Modified from [27, 28].

Figure 3

Transport pathways across the brain endothelial cell. Modified from [33].

Figure 4

Schematic for the in vivo permeability measurement of rat cerebral microvessels. The fluorescence solution was injected into the brain via a carotid artery with a syringe pump. The fluorescence images were captured by a CCD camera, which was connected to an inverted microscope. The image analysis software was then used to measure the fluorescence intensity for the region of interest in each image.

Figure 5

Quantitative fluorescence imaging method for the measurement of solute permeability in a rat pial microvessel. The images were collected during the in vivo experiments and the fluorescence intensity was analyzed off-line. When the fluorescence labeled test solute was injected into the carotid artery, the pial microvessel lumen filled with fluorescent solute (red frame in b), producing ΔI₀. With continued perfusion, the measured fluorescence intensity increased indicating further transport of the solute out of the microvessel and into the surrounding tissue. The initial solute flux into the tissue was measured from the slope (dI/dt)₀ (a). The solute permeability P was calculated by P = 1/ΔI₀ (dI/dt)₀ r/2. Here r is the microvessel radius. Redrawn from [50]. The scale bar in (b) is 50 μm.

Figure 6

Model geometry for the paracellular pathway of the BBB (Not in scale). The thickness of the endothelial surface glycocalyx layer is L_f. The inter-endothelial cleft has a length of L and a width of 2B. The length of the tight junction strand in the inter-endothelial cleft is L_jun. The width of the small continuous slit in the junction strand is 2B_s. The distance between the junction strand and luminal front of the cleft is L₁. The width of the basement membrane is 2L_b and the length of the astrocyte foot processes is 2W_a. The cleft between astrocyte foot processes has a length of L_a and a width of 2B_a. The surface glycocalyx layer and the endothelial cells are defined as the Endothelium only while the BBB is defined to include the endothelium, the basement membrane and the astrocytes. Redrawn from [40].

Figure 7

Model predictions for hydraulic conductivity L_p (a) as a function of B_s, the half width of the small slit in the junction strand under two cases: when considering transport across the endothelium only (Endothelium only, green line), and when considering transport across the entire BBB (BBB). In the BBB case, three different fiber densities were considered for the basement membrane: the same as the fiber density in the surface glycocalyx layer (K_b=3.16cm², the dash-dot-dash line), ten times lower (K_b=31.6cm², the dashed line) and higher (K_b=0.316cm², the solid line); (b) as a function of the surface glycocalyx layer thickness L_f. Redrawn from [40].