Commentary [Research Highlights]

Stephen   D.   Skaper

Abstract

Drug delivery to the central nervous system remains a vexing problem because of the presence of the blood-brain barrier (BBB), whose endothelial cell tight junctions limit the paracellular flux of hydrophilic molecules. One solution may be vector-mediated delivery to the brain. This employs chimeric peptide technology, wherein the drug is conjugated to a molecular carrier of protein nature (e.g. cationized albumin or a transferrin receptor antibody). Transferrin receptor antibodies selectively target BBB endothelium due to the high levels of receptor expressed by these cells, thereby triggering receptor-mediated transport across the BBB via transcytosis. Chitosan, a naturally occurring, abundant, and biocompatible polysaccharide has found widespread pharmaceutical application. In particular, the cationic nature of chitosan facilitates its interaction with negative charges on the brain endothelium, Building on their previous work on brain-deliverable chitosan-polyethylene glycol nanoparticles functionalized with monoclonal anti-transferrin antibody, Karatas and colleagues have now designed caspase-3 inhibitor-loaded chitosan nanospheres conjugated with an anti-mouse transferrin receptor monoclonal antibody that selectively recognizes the transferrin receptor type 1 on the cerebral vasculature. Caspase-3 has been implicated in neuronal cell death in cerebral ischemia, as well as in the pathophysiology of other neurological disorders. When given intravenously, the caspase-3-loaded nanospheres were rapidly transported across the BBB and dose-dependently decreased infarct volume, neurological deficit, and ischemiainduced caspase-3 activity in mice subjected to focal cerebral ischemia/reperfusion. Because significant opening of the BBB starts only 6 h after ischemia/reperfusion, the rapid transfer of nanospheres to the brain was not likely caused by an increased permeability of the BBB. Similarly, nanospheres inhibited physiological caspase-3 activity during development in the neonatal mouse cerebellum on postnatal day 17, after closure of the BBB. These observations convincingly demonstrate that transferrin monoclonal antibody-conjugated chitosan nanospheres are an efficient delivery system for bringing neuroprotective, small peptide drugs into the brain. This opens intriguing new avenues for the treatment of central nervous system disorders in which the administration of therapeutic agents is handicapped by insufficient diffusion across the BBB. Further studies are eagerly awaited to see if this technology will be applicable to polypeptide factors, for example, brain-derived neurotrophic factor and ciliary neurotrophic factor.