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.

Fig. 1

A schematic depiction of the design and mechanisms of blood–brain barrier (BBB) transmigration of three emerging brain delivery technologies: BBB-crossing antibodies (a), brain-tropic adenoviral vectors (b) and engineered extracellular vesicles (c, d). a BBB-crossing antibodies are raised against a select number of BBB receptors that undergo receptor-mediated transcytosis (RMT) (Table 1). Therapeutic antibodies (TH-Ab)  or other therapeutic cargoes, bio-engineered to incorporate BBB-crossing function, internalize via clathrin-coated vesicles and are trafficked through the early endosome pathway, including multivesicular bodies (MVB). They are released on the abluminal side of the BBB, as free antibodies or, in some cases, via exosomes, where therapeutic cargoes engage central targets (such as misfolded proteins or receptors on parenchymal cells) or replace missing molecules (for example growth factors or enzymes). The carrier receptor is recycled back to the luminal membranes via recycling endosomes to accept new cargoes from the circulatory compartment. b Brain-tropic and non-brain-tropic AAVs, AAV9 and AAV2 respectively, recognize discrete receptors on brain endothelial cells that initiate internalization. Thereafter, distinct intracellular trafficking routes are utilized by AAV9 and AAV2, with the former undergoing active transcytosis across the BBB and the later peri-nuclear/nuclear localization and transduction of the endothelial cells. Following transcytosis across the BBB, AAV9 is taken up by and subsequently transduces parenchymal cells. AAV9 is also capable of spreading within the brain by anterograde, retrograde and trans-synaptic neuronal transport. c Exosomes can be used as brain delivery vehicles in either their natural or in engineered forms. They can be loaded with various gene-based (siRNA, miRNA, DNA) and protein-based (antibodies, peptides) biotherapeutics. Specific targeting ligands are genetically engineered into the exosomal membranes in producing cells to achieve BBB crossing and/or drug delivery to target cells. d Natural exosomes may internalize into brain endothelial cells by fusion with the plasma membrane and release their cargoes into endothelial cells. Exosomes expressing RMT ligands likely undergo clathrin-dependent endocytosis, trafficking and abluminal release. Transcytosing exosomes could target parenchymal cells or release their cargo into the brain extracellular space. AAV adeno-associated virus, MVB multivesicular body; TH Ab therapeutic antibody