Despite their therapeutic potential across a wide range of central nervous system (CNS) disorders, nucleic acid-based therapeutics are limited by inefficient delivery to deep brain regions at clinically viable doses. Transferrin receptor 1 (TfR1) has emerged as an attractive target for receptor-mediated transcytosis across the blood-brain barrier (BBB), enabling systemic delivery of biologics such as lysosomal enzymes and monoclonal antibodies. In this study, we demonstrated the translational potential of recently described TfR1-targeting camelid-derived single-domain antibodies (VHHs) for CNS delivery of siRNAs. When conjugated 1:1 to different tool siRNAs, these VHHs promote rapid and robust intracellular uptake, resulting in potent RNAi activity at low nanomolar concentrations in neural cells. Systemic administration of VHH-siRNA conjugates in wild-type mice, hTfR1 transgenic-mice and non-human primates revealed a favourable pharmacokinetic profile characterized by rapid TfR-dependent distributional clearance and efficient functional uptake in deep brain structures. This translated into durable target knockdown of 50-80% at both mRNA and protein levels and with ED50 below 1 mg/kg siRNA. Collectively, these findings establish our TfR1 targeting VHHs as a fit-for-purpose platform for the systemic delivery of therapeutic oligonucleotides to deep brain structures at clinically relevant doses, opening new avenues for the treatment of diverse CNS disorders.
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