Effectiveness of siRNA Delivery via Arginine-Rich PEI-based Polyplex in Metastatic and Doxorubicin Resistant Breast Cancer Cells

Poor cellular uptake, rapid degradation in the presence of serum, and inefficient transfection are some of the major barriers in achieving therapeutic efficacy of naked small interfering RNAs (siRNAs). We investigated the efficacy of the polyplex formulated using our synthesized polymer, (P(SiDAAr)5PEG3)-- a polyethylene glycol (PEG)-modified L-arginine oligo (-alkylaminosiloxane) that is grafted with poly (ethyleneimine, PEI) for siRNA delivery. We hypothesized that the polyplex formulated using the polymer with a balanced composition of PEI for siRNA condensation and its protection, PEG for polyplex stability and to minimize the PEI-associated toxicity, and with arginine facilitating cellular uptake would overcome the above issues with siRNA delivery. We tested our hypothesis using anti-luciferase siRNA in luciferase-expressing metastatic breast cancer cells (MDA-MB-231-Luc-D3H2LN) and anti-ABCB1 siRNA against an efflux membrane protein, ABCB1, in doxorubicin (DOX) resistant breast cancer cells (MCF-7/Adr). The results demonstrated that the polyplex at an optimal nucleotide/polymer (N/P) ratio is stable in the presence of excess polyanions, has no cellular toxicity, and protects siRNA from RNase degradation. Transfection of MDA-MB-231-Luc-D3H2LN cells with anti-luciferase siRNA polyplex showed almost complete knockdown of luciferase expression. In MCF-7/Adr cells, transfection with anti-ABCB1 siRNA effectively down-regulated its target efflux protein, ABCB1, increased cellular uptake of DOX and enhanced its cytotoxic effect. However, the co-treatment did not completely overcome drug resistance, suggesting that further optimization is needed and/or mechanism(s) other than the efflux protein, ABCB1 may be involved in drug resistance. In conclusion, our polyplex is effective for siRNA delivery and can be explored for different therapeutic applications.