Effective down-regulation of breast cancer resistance protein (BCRP) by siRNA delivery using lipid-substituted aliphatic polymers.

Breast Cancer Resistance Protein (BCRP, ABCG2) is an efflux protein whose aberrant activity has been linked to multidrug resistance in cancer. Although siRNA delivery to down-regulate BCRP expression is promising to sensitize tumor cells against drugs, therapeutic use of siRNA requires effective carriers that can deliver siRNA intracellularly with minimal toxicity on target cells. This study explored the feasibility of special class of cationic polymers, namely lipid-substituted low molecular weight (2kDa) polyethyleneimine (PEI), as a carrier for siRNA-mediated BCRP down-regulation. Structure-function studies methodically evaluated the effect of a range of lipophilic substitutions for siRNA delivery and BCRP down-regulation. Our results showed a significant increase in siRNA delivery as a function of lipid substitution for a range of lipids ranging from C8 to C18. The BCRP silencing was correlated to siRNA delivery efficiency of the polymers, and effectively lasted for ∼5days after a single treatment of siRNA. BCRP down-regulation sensitized the drug-resistant cells to cytotoxic effect of mitoxantrone by a ∼14-fold decrease in the IC(50) value, whose effect was evident even after 14days. This study demonstrated the possibility of functional siRNA delivery by lipid-modified low molecular weight PEI and highlighted the importance of the extent and nature of lipid substitution in effective siRNA delivery.