Targeting Biological Barriers: Turning a Wall into a Therapeutic Springboard

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The past two decades have witnessed heavy investments in targeted delivery of biopharmaceuticals, including nucleic acid-based therapeutics, with a key challenge being the need to breach biological barriers to engage a target. For example, the blood-brain-barrier (BBB) is a considerable obstacle to delivery and accounts for failure of many experimental CNS therapies. Cerebral endothelial cells allow passive diffusion of low molecular weight hydrophobic compounds but are restrictive to hydrophilic macromolecular penetration. Likewise, the mucus-epithelial barrier remains a challenge to intestinal absorption of macromolecular drugs delivered orally. The pharmaceutical solution to overcoming these biological barriers has been the development of delivery technologies that can exploit endogenous transport pathways across barrier cells.¹ Nanoparticle-based drug carriers are one example, as they can be surface-functionalized with target-specific ligands. However, such non-viral delivery strategies often fail to exhibit effective penetration and concomitant therapeutic efficacy owing to non-defined (random) ligand decoration, poor transcellular transport, inadequate control of the cellular mechanisms required to ferry particles across barrier cells, and non-optimized barrier-specific design criteria. Furthermore, alternative barrier permeability technologies may compromise the integrity of biological barriers and their protective functions. Conversely, molecular and anatomical barrier alterations associated with disease or aging are often overlooked in targeted delivery designs. Due to these issues, there are continual calls for non-viral delivery solutions; it is clear new approaches are needed.

Aside from their protective roles, biological barriers maintain homeostasis and modulate immune surveillance. Thus, instead of trying to cross a barrier, perhaps the barrier cells themselves might serve as targets, specifically for genetic manipulation of barrier molecules involved in escalation of the disease condition or as “expression factories” for production of therapeutics.

Acute conditions (e.g., stroke, traumatic brain injury) and many chronic neuroinflammatory or neurodegenerative diseases involve cerebral blood capillary endothelial cells.² As an example, endothelial dysfunctions associated with Alzheimer’s disease (AD) can include defective degradation and clearance of amyloid-β and reduced barrier properties. In AD, expression of the receptor for advanced glycation end products (RAGE) increases by several fold in affected cerebral vessels where RAGE can bind different forms of amyloid-β.³ Likewise, low expression of mesenchyme homobox gene 2 in AD cerebral endothelium contributes to abnormal responses to angiogenic factors, leading to premature vessel regression and promoting proteosomal degradation of low-density lipoprotein receptor-related protein 1, which in turn lowers the capacity for amyloid-β clearance at the BBB level.⁴ Other neurological diseases (e.g., multiple sclerosis [MS], epilepsy, Parkinson’s disease, and amyotrophic lateral sclerosis) are also associated with brain capillary endothelial cell dysfunction or BBB disruption. For instance, immune cells have been shown to slip through a “leaky” BBB in MS.⁵ Another example is overexpression of caveolin-1, which increases during BBB breakdown following ischemic stroke. Indeed, enhanced endocytotic caveolae receptor-mediated transcytosis, which requires caveolin-1, initiates BBB dysfunction.⁶ Accordingly, specific nucleic acid therapeutic delivery to cerebral endothelial cells could offer opportunities to modulate multiple molecular targets and even protect and restore BBB function (e.g., vascular remodelling and “normalization”). Such combination genetic therapies may overcome therapeutic limitations associated with brain-specific biopharmaceuticals that need to cross the BBB and blood-cerebrospinal fluid barrier, and they might prove translatable to other endothelial barriers, such as the “blood-retina” barrier.

The gut microbiome and food antigen content, and their interaction with the intestinal immune system, is a determinant of an individual’s susceptibility for development of intestinal inflammation and food allergies. Inflammatory bowel disease (IBD) and food allergies are linked to dysregulated immunity to the gut microbiota or food antigens potentiated by antigen processing by intestinal epithelial receptors. The role of the neonatal Fc receptor (FcRn) in transport of antibacterial immunoglobulin G-immunocomplexes across the intestinal epithelial barrier into antigen-presenting cells in the underlying lamina propria⁷ and possible involvement in food antigen processing renders it a possible epithelial target to control immune induction in some disease states. Furthermore, intestinal epithelial cells have been shown to express the pro-inflammatory cytokine tumor necrosis factor (TNF-α), which is a key inflammatory molecule in IBD and a validated target for anti-TNF-α antibody intervention, which further supports the rationale for modulation of molecules within the biological barrier. Furthermore, intestinal enterocytes may serve as “expression factories” for production of many therapeutic proteins that exploit apical and basolateral secretory pathways.

There are, however, challenges to harnessing the biological barrier with non-viral systems, such as nanoparticles, since many engineered nanoparticles still exhibit poor endothelial and epithelial target engagement and penetration that calls for new approaches for improved and standardized nanoparticle engineering. Biomimetic designs with controllable size, shape, and precision cell-specific ligand patterning and avidity inspired by barrier-penetrating viruses⁸ may improve barrier cell-target engagement. Furthermore, multifunctional designs that incorporate both blood circulatory and target engagement properties are required for systemic applications. The further development of technologies that overcome current limitations could also, in the future, optimize barrier translocation properties of non-viral constructs.