Polymeric delivery of siRNA for dual silencing of Mcl-1 and P-glycoprotein and apoptosis induction in drug-resistant breast cancer cells.

Enhanced survival mechanisms of malignant cells in combination with elevated levels of drug transporters can sustain an undesirable resistance against drug therapy. Short interfering RNA (siRNA) delivery against targets involved in aberrant mechanisms is a promising approach and we hypothesize that simultaneous silencing of multiple targets could prove more advantageous than common approach to silence individual targets. To explore this approach, we targeted anti-apoptotic proteins myeloid cell leukemia 1 (Mcl-1) and survivin along with the efflux pump P-glycoprotein (P-gp) in drug-resistant breast cancer cells. Polymeric siRNA delivery was employed for this purpose by using small polyethylenimine (PEI) substituted with lipids. While silencing Mcl-1 caused ∼90% cell death in wild-type cells, this effect was less significant in P-gp over-expressing cells. An additive effect for Mcl-1 and P-gp silencing was evident in the latter cells, where simultaneous silencing of these targets created a significantly higher effect compared with silencing each individual target. Prolonged exposure of wild-type cells to doxorubicin (DOX) resulted in upregulation of P-gp, breast cancer resistance protein, survivin and Mcl-1. Dual silencing of P-gp and Mcl-1 again resulted in an additive effect in resistance-induced cells, which displayed an increased dependency on Mcl-1 for survival. Cytotoxic effect of DOX was also enhanced in resistance-induced cells after silencing Mcl-1. We conclude that polymer-mediated siRNA delivery can silence multiple targets simultaneously and reverse drug resistance.