Stimuli-Responsive Nanoparticles for siRNA Delivery.

Nanoparticles have been extensively employed to deliver many drugs, including siRNA, for the treatment of a variety of diseases, particularly cancer. Lately, there has been a great deal of effort to design nanoparticles with materials that are able to respond to intrinsic or extrinsic stimuli for "on demand" delivery of siRNA. These nanoparticles are able to trigger siRNA release upon different stimuli, such as a pH decrease, redox gradient, enzyme, light, magnetic field, temperature, ultrasound or electric current. Frequently, the stimuli cause the nanoparticles to undergo protonation, hydrolytic breakdown or phase transition for triggered release of siRNA, resulting in decreased side effects and better therapeutic outcome. While studies have demonstrated efficient in vitro and in vivo delivery, these "smart" nanoparticles have not yet reached the clinic. In this review, we address different classes of nanoparticles, such as polyplexes, lipoplexes, liposomes, polymeric micelles, polymeric, lipid and inorganic nanoparticles, that are able to respond to specific stimuli for siRNA triggered-release, emphasizing their application and discussing the latest advances.