Live-Cell Imaging of Colloidal Mesoporous Silica Nanoparticles for Drug Delivery: Drug Loading, Pore Sealing and Controlled Release

Abstract

Colloidal mesoporous silica (CMS) nanoparticles are promising candidates for drug delivery. However, after the drug is loaded into the pores, sealing of the pores against premature drug release and controlled intracellular drug release are challenging tasks. We studied the mechanisms of nanoparticle uptake into living cells, trafficking and intracellular delivery of drugs by highly-sensitive fluorescence microscopy and real-time single-particle tracking. As an approach to seal the mesopores, we encapsulated CMS nanoparticles of 50 nm diameter with a supported lipid bilayer (SLB). We demonstrated the delivery of the microtubule-depolymerising drug colchicine from SLB-coated CMS nanoparticles into liver cancer cells. The microtubule network of the cells was destroyed within 2 h of incubation. With this method, colchicine is released directly into the cell showing characteristic effects at concentrations below the amount necessary for application in solution. In another approach, we linked a fluorescently labelled model drug (cystein-ATTO633) via a redox-sensitive disulfide bridge inside the pores of CMS. For reduction of the disulfide bonds and release of the drug, the CMS nanoparticles need to reach the cytoplasmic reducing milieu after internalization. We showed that nanoparticles were endocytosed by HuH7 cells, but endosomal escape seems to be a bottleneck for drug release. Using a photosensitizer to induce endosomal escape, successful release of the drug was achieved. Additionally, we found that grafting of ATTO633 at high concentration in the pores of silica nanoparticles results in self-quenching of the ATTO633 fluorescence. Release of dye from the pores promotes a de-quenching effect with an excellent readout signal. As the EGF-receptor is overexpressed on a variety of tumors, CMS particles were equipped with EGF as ligand for specific targeting. Internalization and cellular motion of the particles was studied by single-particle tracking.