Mannose-6-Phosphate-Tagged Liposomes Exhibit Increased Transcytosis Across Human Blood–Brain Barrier Model

Background/Objectives: The blood–brain barrier (BBB) presents a major challenge for delivering therapeutics to the central nervous system (CNS) due to its highly selective permeability. Human brain microvascular endothelial cells (hBMECs), the principal cellular component of the BBB, tightly regulate molecular transport and restrict the entry of many CNS-targeted therapies. Lipid-based nanoparticles have emerged as promising carriers for BBB transport because of their biocompatibility, tunable surface properties, and cargo encapsulation capabilities. One strategy to enhance nanoparticle transport involves surface functionalization with ligands that exploit endogenous transcytosis pathways. Mannose-6-phosphate (M6P), a glycan implicated in the brain entry of certain proteins and viruses, represents a potential targeting ligand for this purpose. Methods: In this study, we established a physiologically relevant in vitro BBB model using human-induced pluripotent stem cell-derived brain microvascular endothelial cells (hiPSC-BMECs) to evaluate M6P-functionalized liposomes for BBB transport. Fluorophore-labeled liposomes were used to monitor nanoparticle uptake and transcytosis. Results: M6P-functionalized liposomes exhibited significantly enhanced uptake in hiPSC-BMECs compared with non-functionalized control liposomes. Pharmacological inhibition studies supported the involvement of a clathrin-sensitive endocytic pathway. Transcytosis assays demonstrated enhanced BBB crossing of M6P-functionalized liposomes, with transport increasing according to ligand density and reaching approximately 55% of the transport observed for transferrin under the same experimental conditions. Following transcytosis, intact M6P-functionalized liposomes showed significantly higher uptake by downstream hiPSC-derived neurons and astrocytoma cells compared with control formulations. Conclusions: Together, these findings support M6P-functionalization as a promising strategy to enhance liposome uptake and transcytosis across a human-relevant in vitro BBB model. This work provides a proof-of-concept framework for the development and optimization of glycan-functionalized nanocarriers for CNS-directed delivery.