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AbstractEffective drug delivery to the central nervous system (CNS) remains a challenge due to the blood–brain barrier (BBB). Macromolecules such as proteins and peptides are unable to cross BBB and have poor therapeutic efficacy due to little or no drug distribution. A promising alternative is the conjugation of a drug to a shuttle molecule that can reach the CNS via receptor-mediated transcytosis (RMT). Several receptors have been described for RMT, such as low-density lipoprotein receptor-related protein 1 (LRP1). We used phage display technology combined with an in vitro BBB model to identify LRP1 ligands. A single domain antibody (dAb) library was used to enrich for species that selectively bind to immobilised LRP1 ligand. We obtained a novel nanobody, dAb D11, that selectively binds to LRP1 receptor and mediates in vitro internalisation of phage particles in brain endothelial cells, with a dissociation constant Kd of 183.1 ± 85.8 nM. The high permeability of D11 was demonstrated by an in vivo biodistribution assay in mice. We discovered D11, the first LRP1 binding dAb with BBB permeability. Our findings will contribute to the development of RMT-based drugs for the treatment of CNS diseases. [ABSTRACT FROM AUTHOR]