Caveolae-Mediated Transport at the Injured Blood-Brain Barrier as an Underexplored Pathway for Central Nervous System Drug Delivery

Abstract

Drug delivery to the central nervous system (CNS) is generally hindered by the selectivity of the blood-brain barrier (BBB). However, there is strong evidence that the integrity of the BBB is compromised under certain pathological conditions, potentially providing a window to deliver drugs to injured brain regions. Recent studies suggest that caveolae-mediated transcytosis, a transport pathway suppressed in the healthy BBB, becomes elevated as an immediate response to ischemic stroke and at early stages of aging, where it may precede irreversible neurological damage. This article reviews early-stage caveolar transcytosis as a novel and promising drug delivery opportunity. We propose that albumin-binding and nanoparticle approaches have the potential to leverage this window of transcellular BBB disruption for trafficking therapeutic agents into the CNS.

Figure 1.. Transport pathways across the BBB.

Passive diffusion through the brain endothelium is restricted to small (<450 Daltons), lipophilic compounds including but not limited to caffeine, alcohol, and a limited subset of small molecule drugs that are not efflux transporter substrates. Transport of small and large molecules between cells (paracellular route) is effectively non-present due to a dense network of specialized tight junctions. Carrier-mediated transport of solutes involves binding of compounds to specific proteins expressed on the brain endothelium. Importantly, this pathway does not involve vesicles, and notable molecules that are transported through this route include glucose, amino acids, and vitamins. In terms of vesicular transport mechanisms, receptor-mediated transport involves specific receptor-ligand interactions that trigger clathrin-dependent endocytosis. After the vesicle is pinched off by dynamin, the clathrin coat is shed and the vesicle fuses with a sorting endosome. Depending on a number of factors, including the affinity of the ligand of the receptor, the vesicle will either be directed to a late endosome and lysosomal degradation pathway or be passed to a transcytotic vesicle. In adsorptive mediated transcytosis, a macromolecule needs to be positively charged, in which binding of the cation to the plasma membrane leads to endocytosis through caveolae. These uncoated vesicles are lipid rafts stabilized by caveolin-1. While many studies suggest that caveolae are directly passed across brain endothelial cells, it is not well understood if there are alternative routes involving degradation pathways.

Figure 2.. Overview of temporal changes in BBB integrity following an ischemic stroke.

The healthy BBB is characterized by low rates of transcytosis, an intact negatively charged glycocalyx, and a stringent paracellular barrier that collectively prevent non-specific transport of material into the brain parenchyma. At early stages of injury post-stroke (<6 hours), the dissolution of the glycocalyx coupled with activation of caveolae contribute to a robust transcytosis of plasma proteins across the brain endothelium without evidence of disruption to tight junctions. At late stages after injury (48-72 hours), breakdown of tight junctions allows paracellular transport of proteins in addition to caveolar transport. At this stage, the secondary injury cascade has developed to include neuroinflammation, excitotoxicity, and oxidative stress that all contribute to neuronal cell death.

Figure 3.. Approaches for leveraging caveolae-mediated transcytosis as a drug delivery route to the CNS.

One possibility for targeting caveolae is to conjugate therapeutic compounds to endogenous albumin, which is highly abundant in blood and accumulates in brain regions where BBB integrity is compromised. As a natural lipid carrier, albumin contains binding pockets that interact with fatty acid moieties, which can be chemically engineered to carry a variety of payloads. Liposomal nanocarriers have also been shown to traffic through caveolae following stroke and can deliver both hydrophobic and hydrophilic compounds. In each case, delivery would be expected only at sites of injury where caveolae-mediated transcytosis is selectively upregulated.