Co-delivery of PD-L1- and EGFR-targeting siRNAs by synthetic PEG-KL4 peptide to the lungs as potential strategy against non-small cell lung cancer.

Background: Small interfering RNA (siRNA) holds great promise for treating various lung diseases, but the lack of safe and efficient pulmonary siRNA delivery systems has hindered its advance into the clinics. The epidermal growth factor receptor (EGFR) which promotes cell proliferation, and the programmed cell death ligand 1 (PD-L1) which plays a crucial role in suppressing cytotoxic T cells activity, are two important targets for treating non-small cell lung cancer (NSCLC). Here, we explored the potential of PEG-KL4, a synthetic peptide, to deliver siRNA to various NSCLC cells and to lung tissues in mice.

Methods: PEG-KL4 was used to transfect siRNAs targeted at both EGFR and PD-L1 into NSCLC cells. Immunoblotting was used to evaluate the siRNA silencing effects in HCC827 and NCI-H1975 NSCLC cells. CD8+ T cell-mediated NSCLC cell killing was employed to demonstrate the functional effects of PD-L1 siRNA knock-down. Fluorescent siRNAs were used to visualise siRNA uptake in cells as well as to enable biodistribution studies in BALB/c mice.

Results: Our results showed that PEG-KL4 was efficient in mediating siRNA knock-down of EGFR and PD-L1 in various NSCLC cells. Importantly, the PEG-KL4 peptide enabled significantly better siRNA delivery than the commercial Lipofectamine 2000 reagent. We hypothesised that PEG-KL4 peptide enabled siRNA to either escape from or bypass endosomal degradation as indicated by confocal fluorescence imaging. Notably, combined knock-down of EGFR and PD-L1 in NCI-H1975 cells resulted in better effector T cell-mediated cancer cell killing than knock-down of PD-L1 alone. Moreover, biodistribution of PEG-KL4/siRNA complexes following intravenous administration revealed poor lung delivery with the fluorescent siRNA accumulating in the liver. In contrast, intratracheal delivery of PEG-KL4/siRNA complexes resulted in the fluorescent siRNA to be detected in the lung with retarded renal excretion.

Conclusion: In conclusion, we demonstrated that the co-delivery of siRNAs targeting EGFR and PD-L1 using PEG-KL4 is feasible and represents a promising future strategy to treat NSCLC, whereby pulmonary siRNA delivery is favourable to intravenous administration.