Although siRNA is a promising technology for cancer gene therapy, effective cytoplasmic delivery has remained a significant challenge. In this paper, a potent siRNA transfer system with active targeting moieties toward cancer cells and a high loading capacity is introduced to inhibit drug resistance. Mesoporous silica nanoparticles are of great potential for developing targeted gene delivery. Amino-modified MSNs (NH2-MSNs) were synthesized using a modified sol-gel method and characterized by FTIR, BET, TEM, SEM, X-ray diffraction, DLS, and H-1-NMR. MDR1-siRNA was loaded within NH2-MSNs, and the resulting negative surface was capped by functionalized chitosan as a protective layer. Targeting moieties such as TAT and folate were anchored to chitosan via PEG-spacers. The loading capacity of siRNA and the protective effect of chitosan for siRNA were determined by gel retardation assay. MTT assay, flow cytometry, real-time PCR, and western blot were performed to study the cytotoxicity, cellular uptake assay, targeting evaluation, and MDR1 knockdown efficiency. The synthesized NH2-MSNs had a particle size of approximate to 100 nm and pore size of approximate to 5 nm. siRNA was loaded into NH2-MSNs with a high loading capacity of 20% w/w. Chitosan coating on the surface of siRNA-NH2-MSNs significantly improved the siRNA protection against enzyme activity compared to naked siRNA-NH2-MSNs. MSNs and modified MSNs did not exhibit significant cytotoxicity at therapeutic concentrations in the EPG85.257-RDB and HeLa-RDB lines. The folate-conjugated nanoparticles showed a cellular uptake of around two times higher in folate receptor-rich HeLa-RDB than EPG85.257-RDB cells. The chitosan-coated siRNA-NH2-MSNs produced decreased MDR1 transcript and protein levels in HeLa-RDB by 0.20 and 0.48-fold, respectively. The results demonstrated that functionalized chitosan-coated siRNA-MSNs could be a promising carrier for targeted cancer therapy. Folate-targeted nanoparticles were specifically harvested by folate receptor-rich HeLa-RDB and produced a chemosensitized phenotype of the multidrug-resistant cancer cells
