A Myristoylated Cell-Penetrating Peptide Bearing a Transferrin Receptor-Targeting Sequence for Neuro-Targeted siRNA Delivery

Many neurodegenerative disorders (NDDs) are characterized by aggregation of aberrant proteins and extensive oxidative stress in brain cells. As a treatment option for NDDs, RNA interference (RNAi) is a promising approach to suppress the activation of abnormal genes and negative regulators of antioxidant genes. Efficient neuro-targeted siRNA delivery requires a delicate optimization of nucleic acid carriers, quite distinct from putative pDNA carriers in regard to stable condensation and serum protection of siRNA, blood-brain barrier (BBB) bypass, effective siRNA delivery to brain cells, and functional release of bioactive siRNA at therapeutic levels. Here, we propose that a myristic acid conjugated, cell-penetrating peptide (transportan; TP), equipped with a transferrin receptor-targeting peptide (myr-TP-Tf), will lead to stable encapsulation of siRNA and targeted delivery of siRNA to brain cells overcoming the BBB. Myr-TP-Tf was successfully prepared by solid-phase peptide synthesis with high purity. Myr-TP-Tf-siRNA complexes formulated at a 20:1 (peptide-siRNA) molar ratio provided prolonged siRNA stability against serum and ribonuclease treatment. Fluorescence images clearly indicated that siRNA uptake was successfully achieved by myr-TP-Tf complexes in both a murine brain endothelioma and a human glioma cell line. The luciferase assay and the human placental alkaline phosphatase (hPAP) reporter assay results demonstrated the functional gene silencing effect of myr-TP-Tf-siRNA complexes in a human glioma cell line as well as in primary murine neurons/astrocytes, supportive of successful release of bioactive siRNA into the cytosol. Finally, the transcytosis assay revealed that favorable siRNA transport via receptor-mediated transcytosis was mediated by myr-TP-Tf complexes. In summary, these data suggest that myr-TP-Tf peptides possess promising properties as a vehicle for neuro-targeted siRNA delivery. We will further study this peptide in vitro and in vivo for transport mechanism kinetics and to validate its capability to deliver siRNA to the brain, respectively.