K16ApoE-mediated delivery across the blood-brain barrier: mechanisms, applications, and translational challenges

The blood-brain barrier (BBB) supporting the health of the central nervous system does this by limiting molecular exchange between circulation and brain tissue, although unfortunately this same restriction prevents therapeutically relevant macromolecules from entering the brain. Consequently, a number of protein-based therapies with success in peripheral tissues including enzymes, antibodies and biologics are unsuccessful in treating neurological disorders. The most recently described example is K16ApoE, a synthetic peptide that combines a polylysine-associated moiety with the receptor binding domain of apolipoprotein E (apoE), suggested to facilitate systemic delivery of large biomolecules across the BBB. Here, we review evidence that co-administration of K16ApoE with therapeutic proteins can mediate high concentrations of this protein in brain tissue, which can yield significant substrate clearance (pathological storage) and profound neurologic improvement and survival benefits in animal models of late-infantile neuronal ceroid lipofuscinosis (LINCL; CLN2 disease). In addition to applications in lysosomal storage disorders, the peptide has shown efficacy in promoting delivery of chemotherapeutic agents, antibodies and biologics related to brain tumors and neurodegenerative diseases. However, broad limitations persist including dose-dependent toxicity or nonspecific BBB permeability and immunogenicity as well high levels of uncertainty in referencing animal findings to humans. This review provides an integrated critical perspective on K16ApoE by connecting its proposed delivery mechanisms with therapeutic efficacy, toxicity, and translational barriers. In particular, we examine the unresolved balance between receptor-associated endothelial transport and transient BBB modulation and propose a mechanism-safety-translation framework to guide the rational development of K16ApoE and related peptide shuttles.