Cyclic SLS Peptide Fusion Enables Efficient Brain Delivery of Monoclonal Antibodies across the Blood−Brain Barrier

Abstract The blood−brain barrier (BBB) significantly limits the delivery of therapeutic antibodies to the central nervous system, thereby reducing their clinical effectiveness against neurological disorders. Receptor-mediated transcytosis (RMT) has been explored as a promising strategy to improve antibody transport across the BBB. However, many existing approaches require complex molecular engineering and may affect systemic pharmacokinetics (PK). In this study, we developed a compact, genetically encodable BBB shuttle platform using a cyclic SLS peptide, hereafter referred to as SLS, that can be fused to monoclonal antibodies. Using trastuzumab (TZB) as a model antibody, we conducted structure−function optimization by assessing peptide cyclization, linker configuration, and peptide valency. This evaluation identified a representative construct, SLS-F-TZB, in which one SLS moiety is fused to the C-terminus of each TZB heavy chain via a (G4S)3 linker. SLS-F-TZB was produced with high purity comparable to that of unmodified TZB and preserved HER2-dependent cytotoxic activity. SLS-F-TZB showed increased cell-associated antibody signals in human brain endothelial cells and improved transport across both 2D and 3D in vitro BBB models. Upon intravenous administration in mice, SLS-F-TZB achieved higher brain concentrations than unmodified TZB while maintaining a similar systemic PK profile. Brain PK analysis revealed a biphasic distribution profile with enhanced brain exposure over time. Brain capillary isolation analysis showed that capillary-associated levels of SLS-F-TZB were comparable to those of unmodified TZB, suggesting that the increased whole-brain levels were not primarily attributable to vascular retention. Importantly, SLS fusion also enhanced the brain delivery of aducanumab, suggesting that this strategy may be applicable to antibodies beyond TZB. Collectively, these results indicate that fusion of a small SLS moiety enhances BBB transport and brain exposure of monoclonal antibodies without markedly affecting systemic PK or antigen-dependent antibody function. This genetically encodable platform offers a simple, versatile, and scalable approach for enhancing the delivery of therapeutic antibodies to the central nervous system.