Blood-brain barrier transport of therapeutics via receptor-mediation

Abstract

Drug delivery to the brain is hindered by the presence of the blood-brain barrier (BBB). Although the BBB restricts the passage of many substances, it is actually selectively permeable to nutrients necessary for healthy brain function. To accomplish the task of nutrient transport, the brain endothelium is endowed with a diverse collection of molecular transport systems. One such class of transport system, known as a receptor-mediated transcytosis (RMT), employs the vesicular trafficking machinery of the endothelium to transport substrates between blood and brain. If appropriately targeted, RMT systems can also be used to shuttle a wide range of therapeutics into the brain in a noninvasive manner. Over the last decade, there have been significant developments in the arena of RMT-based brain drug transport, and this review will focus on those approaches that have been validated in an in vivo setting.

Figure 1

Schematic of transport routes at the BBB. As a result of tight junctions closing down the paracellular space between adjacent endothelial cells, therapeutics must either diffuse through cell membranes or be transported by one of the mechanisms indicated in order to successfully reach brain tissue. A. Receptor-mediated transcytosis. B. Non-specific uptake either by cationization and absorptive-mediated transcytosis or by protein transduction domain. C. Carrier-mediated transport where nutrients enter the brain by traveling serially through transporters present in the apical and basolateral endothelial cell plasma membranes.

Figure 2

Schematic of strategies used for identifying novel BBB receptors and cognate targeting reagents. A diverse library of 10⁹-10¹² genes that encode antibody fragments or peptides can be displayed on the surface of phage, yeast, ribosomes or some other display platform. These libraries can then be screened for antibodies/peptides that bind to receptors on the apical side of the BBB. In the case of antibodies, these libraries often represent a significant fraction of the natural repertoire of the immune system. In contrast to traditional immunization strategies where a known antigen is used to raise antibodies in vivo, the library approach allows for in vitro selections even against antigens or receptors whose identities are not initially known. The figure depicts two such approaches. A. Transport-based selection. Selection using an in vitro model of the BBB grown on a permeable membrane. By simply applying the library to the blood side of the in vitro BBB, one can collect those particles that transport to the opposite chamber or brain side as a result of the antibody/peptide displayed on the particle surface. This process can be repeated until the target population is comprised almost exclusively of transporting particles, at which time the antibodies/peptides mediating the particle transcytosis process can be identified. B. Receptor-based selection. Alternatively, rather than using transport as the selection criterion, one can simply screen for antibodies or peptides that meet the requirement of binding to proteins located at the apical plasma membranes of the BBB endothelium. Subsequently, those binding antibodies or peptides can be screened for their capability to trigger endocytosis or transcytosis. Both approaches have been applied to BBB research as described in the main text.

Figure 3

Schematic of secondary targeting strategies for therapeutic delivery to the brain. The figure depicts an anti-TfR antibody-targeted PEGylated liposome as the delivery system, but similar techniques can be used for other BBB targeting vectors and therapeutic payloads. For each depicted technique, the anti-TfR MAb binds to the TfR, and allows crossing of the BBB by transcytosis. I. Transporter present on both BBB and target cell population. For this situation, the anti-TfR MAb can also bind to the TfR of the neurons and the astrocytes, yielding intracellular delivery of the payload to both brain cell types. II. Sequential targeting. In addition to the anti-TfR MAb, a second targeting antibody, such as the anti-HIR MAb, can be conjugated to the surface of the PEGylated liposome. Thus, once the anti-TfR MAb allows delivery across the mouse BBB, the secondary targeting reagent can bind to the HIR on the surface of xenografted human glioma cells allowing targeted delivery of payload. With this proof-of-concept example, the strategy was designed based on the species difference of the xenograft. However, one could instead envision using tumor specific antigens to provide this secondary targeting. III. Selective action. The anti-TfR MAb binds to the TfR of the neurons and the astrocytes. However, specificity can be attained since neurons may produce the cofactor necessary for the activity of an enzyme payload. Similarly, delivering an expression plasmid with a cell-type specific promoter would allow for selective action by driving expression of the gene payload only in the target cell population.