Effective response of doxorubicin-sensitive and -resistant breast cancer cells to combinational siRNA therapy.

Chemotherapy is an effective approach to curb uncontrolled proliferation of malignant cells. However, most drugs rapidly lose their efficacy as a result of resistance development. We explored the potential of combinational siRNA silencing to prevent growth of drug-resistant breast cancer cells independent of chemotherapy. Resistance was induced in two breast cancer lines by chronic exposure to doxorubicin. Microarray analysis of apoptosis-related proteins showed Bcl2, survivin, NF B, and Mcl1 to be prominently up-regulated in drug-resistant cells. Human siRNA libraries against apoptosis-related proteins and kinases were screened using lipid-substituted polymers as non-viral carrier, and siRNAs were selected to diminish cell growth without affecting growth of skin fibroblasts. Surprisingly, the selected siRNAs led to similar responses in wild-type and drug-resistant cells, despite their phenotypic differences. Promising kinase siRNAs were co-delivered with anti-apoptotic Mcl-1 siRNA and Ribosomal Protein S6 Kinase (RPS6KA5) was found the most promising candidate for simultaneous silencing with Mcl-1. In both MDA435 wild type (WT) and MDA435 resistant (R) xenografts in nude mice, double silencing of Mcl-1/RPS6KA5 also led to improved inhibition of tumor growth in the absence of chemotherapy. We conclude that combinational silencing of well-selected targets could be a feasible therapeutic strategy in the absence of drug therapy and could provide a new avenue for therapy of drug-resistant breast cancers.