Apolipoproteins in nanoparticle-mediated brain drug delivery: promise, mechanisms, and misconceptions

INTRODUCTION: Apolipoprotein E (apoE) plays a context-dependent role in nanomedicine for central nervous system drug delivery. It can facilitate cerebral endothelial recognition and increase brain exposure in nanoparticle systems while simultaneously promoting hepatic uptake, challenging its traditional interpretation as a brain-specific targeting ligand. AREAS COVERED: This review critically evaluates evidence from classical polysorbate-coated poly(butyl cyanoacrylate) (PBCA) and apoE-coupled albumin nanoparticles, apoE-derived peptide systems, solid lipid nanoparticles, and ionizable lipid nanoparticles (LNPs). Attention is given to apoE presentation and isoform biology, protein-corona dynamics, LDL receptor family interactions, hepatic clearance, blood-brain barrier (BBB) status, route-dependent disposition, and blood-cell interactions. Advances in BBB models, LNP engineering, and translational strategies are also considered. EXPERT OPINION: ApoE should be regarded as a tunable regulator of nanoparticle biodistribution rather than a universal BBB shuttle. Development should focus on therapeutically active drug exposure in the relevant brain compartment rather than apoE binding, endothelial uptake, or total brain-associated signal. This requires assessment of whole-blood interactions, hepatic partitioning, receptor dependence, compartment-resolved brain disposition, payload release, pharmacodynamic activity, disease-specific BBB biology, and potentially apoE genotype. ApoE-based systems should be advanced only when they demonstrate a reproducible therapeutic advantage over comparable unmodified carriers and alternative brain-delivery strategies.