Emerging Technologies for Delivery of Biotherapeutics and Gene Therapy Across the Blood–Brain Barrier

Abstract

Antibody, immuno- and gene therapies developed for neurological indications face a delivery challenge posed by various anatomical and physiological barriers within the central nervous system (CNS); most notably, the blood–brain barrier (BBB). Emerging delivery technologies for biotherapeutics have focused on trans-cellular pathways across the BBB utilizing receptor-mediated transcytosis (RMT). ‘Traditionally’ targeted RMT receptors, transferrin receptor (TfR) and insulin receptor (IR), are ubiquitously expressed and pose numerous translational challenges during development, including species differences and safety risks. Recent advances in antibody engineering technologies and discoveries of RMT targets and BBB-crossing antibodies that are more BBB-selective have combined to create a new preclinical pipeline of BBB-crossing biotherapeutics with improved efficacy and safety. Novel BBB-selective RMT targets and carrier antibodies have exposed additional opportunities for re-targeting gene delivery vectors or nanocarriers for more efficient brain delivery. Emergence and refinement of core technologies of genetic engineering and editing as well as biomanufacturing of viral vectors and cell-derived products have de-risked the path to the development of systemic gene therapy approaches for the CNS. In particular, brain-tropic viral vectors and extracellular vesicles have recently expanded the repertoire of brain delivery strategies for biotherapeutics. Whereas protein biotherapeutics and bispecific antibodies enabled for BBB transcytosis are rapidly heading towards clinical trials, systemic gene therapy approaches for CNS will likely remain in research phase for the foreseeable future. The promise and limitations of these emerging cross-BBB delivery technologies are further discussed in this article.

The Therapeutic Potential of Nanobodies

“…Different mechanisms, including physical methods (disruption with osmotic shock, magnetic gradient and ultrasound) [118], use of cell-penetrating peptides [119] as well as receptor-mediated transcytosis (e.g., transferrin receptor, insulin receptor and lowdensity lipoprotein receptor-related protein) [120,121] have been explored for increasing BBB permeability. Among these strategies are the so-called 'Trojan horses' that are ligands or antibodies targeting receptors of the BBB resulting in the transfer of various molecules into the brain either by non-specific charge-mediated adsorptive endocytosis or by energy-dependant receptor-specific endocytosis/transcytosis [122]. One of the most interesting technologies is the transport pathway through the transferrin receptors on the surface of microvascular endothelial cells [123].…”

Differential expression of receptors mediating receptor-mediated transcytosis (RMT) in brain microvessels, brain parenchyma and peripheral tissues of the mouse and the human

“…This process is particularly important for brain delivery of essential macromolecules, including transferrin and insulin [3][4][5][6], across the blood-brain barrier (BBB) [1,2]. A ligand binding to a receptor on the luminal surface of brain endothelial cells (BEC) triggers ligand-receptor complex endocytosis, routing through various intracellular endosomal compartments where cargo is detached from the receptor and released on the abluminal side, while the receptor recycles 'back' to accept additional cargo molecules [7][8][9][10][11]. This pathway has been particularly well described for the transferrin receptor, which undergoes constitutive recycling and has been considered a 'prototypical' trigger of the RMT pathway [7][8][9][10][11][12][13].…”

“…Antibodies and peptide ligands against various endocytosing BBB receptors have been developed as potential molecular Trojan horses or shuttles to deliver therapeutic cargos across the BBB [2,8,9,[12][13][14][15][16]. Discovery strategies utilizing molecular ('omics') analyses of the BBB and screening of antibody libraries to select antibody species that can transmigrate the BBB [17][18][19][20][21][22] have yielded an experimental proof of concept for novel RMT-antibody pairs.…”

“…Discovery strategies utilizing molecular ('omics') analyses of the BBB and screening of antibody libraries to select antibody species that can transmigrate the BBB [17][18][19][20][21][22] have yielded an experimental proof of concept for novel RMT-antibody pairs. The literature survey of BBB targets shown to mediate transcytosis of ligands or antibodies in various model systems in vitro and in vivo, identi ed the following broad classes of RMT transporters: a) iron transporters: transferrin receptor (TFRC) [3][4][5][6]12,13]; b) insulin and insulin-like growth factors receptors: INSR, IGF1R, and IGF2R [8,16,23,24]; c) lipid transporters: lowdensity lipoprotein receptor (LDLR), LDLR-related protein 1 (LRP1), LRP8, and transmembrane protein 30A (TMEM30A/CDC50A) [8,9,[25][26][27][28][29][30][31][32]; d) solute carrier family transporters: glucose transporter-1 (GLUT-1)/SLC2A1 and SLC3A2/CD98hc [33,34]; and e) neuropeptide receptors: leptin receptor (LEPR) [35]. Structure/function relationships, ligand interactions and signaling, and specialized biology of these receptors is vastly different (summarized brie y in Supplementary Table 1), expanding the spectrum of potential RMT pathways across the BBB beyond the 'canonical' mechanism of TfR recycling.…”

“…For example, evaluation of bi-speci c antibodies where the TfR antibody was used as a BBB transport shuttle [36,37] revealed TfR antibody-driven fast systemic clearance, on-target complement-mediated toxicity on TfR-rich reticulocytes, as well as species differences in transport capacity in mice and non-human primates [7,36,38]. Ideally, RMT receptors targeted for the development of BBB delivery antibodies should demonstrate a high degree of BBB expression selectivity compared to peripheral organs, a high abundance in the brain endothelium, as well as low expression in the brain parenchyma [8,9]. This pro le would enable an extended systemic half-life and reduced toxicity, high e ciency of brain delivery and low target-mediated removal in the brain tissue.…”