Chemically enhanced brain shuttle peptides.

The blood-brain barrier (BBB) constitutes a major impediment to central nervous system pharmacotherapy due to its highly selective permeability and enzymatic defense mechanisms. Brain shuttle peptides have emerged as a promising modality for overcoming this barrier, primarily via receptor-mediated transcytosis. To enhance their pharmacokinetic properties and transcytosis efficiency, several structural and chemical optimization strategies have been employed. Cyclization and retro-enantio modifications confer proteolytic resistance while preserving receptor affinity, as demonstrated by engineered peptides such as retro-D-THR, retro-D-T7, and BB4. Venom-derived scaffolds, including MiniAp-4 and MiniCTX3, further exemplify the potential of naturally occurring peptides for BBB penetration. Multivalent presentation, achieved through branched architectures or nanoparticle surface functionalization, significantly increases avidity and cellular uptake, thereby improving transcytosis. Dual-ligand systems, such as THR-TAT conjugates, have shown synergistic effects in glioma models, enhancing both BBB crossing and tumor targeting. While PEGylation is widely utilized in drug delivery to prolong circulation and reduce immunogenicity, its application in brain shuttle systems remains limited due to potential interference with receptor-mediated uptake. Collectively, these advances underscore the versatility of protease-resssistant brain shuttle peptides as targeted delivery vehicles for CNS therapeutics and provide a strong foundation for their translation into clinical applications.