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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.

A number of amphiphilic cyclic peptides—[FR]4, [WR]5, and [WK]5—containing hydrophobic and positively-charged amino acids were synthesized by Fmoc/tBu solid-phase peptide methods and evaluated for their efficiency in intracellular delivery of siRNA to triple-negative breast cancer cell lines, MDA-MB-231 and MDA-MB-468, in the presence and absence of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Among the peptides, [WR]5, which contains alternate tryptophan (W) and arginine (R) residues, was found to be the most efficient in the delivery of siRNA by improving the delivery by more than 3-fold when compared to other synthesized cyclic peptides that were not efficient. The data also showed that co-formulation of [WR]5 with lipid DOPE significantly enhanced the efficiency of siRNA delivery by up to ~2-fold compared to peptide alone. Based on the data indicating the efficiency of [WR]5 in siRNA delivery, peptides containing arginine residues on the ring and tryptophan residues on the side chain, [R6K]W6 and [R5K]W5, were also evaluated, and demonstrated improved delivery of siRNA. The presence of DOPE again enhanced the siRNA delivery in most cases. [WR]5, [R5K]W5, and [R6K]W6 did not show any significant toxicity in MDA-MB-231, MDA-MB-468, and AU565 WT cells at N/P ratios of 20:1 or less, in the presence and absence of DOPE. Silencing of kinesin spindle protein (KSP) and Janus kinase 2 (JAK2) was evaluated in MDA-MB-231 cells in the presence of the peptides. The addition of DOPE significantly enhanced the silencing efficiency for all selected peptides. In conclusion, peptides containing tryptophan and arginine residues were found to enhance siRNA delivery and to generate silencing of targeted proteins in the presence of DOPE.