The therapeutic potential of cell-internalizing aptamers

Aptamers that are evolved by the SELEX procedure (Systematic Evolution of Ligands by Exponential enrichment) can specifically recognize and tightly bind their cognate targets by means of well-defined secondary and three-dimensional structures. In comparison to antibodies, nucleic acid-based aptamers offer some exciting advantages, including the potential for chemical synthesis, convenient modification, chemical versatility, stability and lack of immunogenicity. During the past 20 years, aptamers have been developed for various applications such as diagnostics, drug development, target validation and therapeutics. Aptamers targeting cell surface proteins are being explored as promising delivery vehicles to target a distinct disease or tissue in a cell-type-specific manner. In this review, we summarize the recent developments in creatively using cell-internalizing aptamers as drug delivery escorts to deliver, enhance and modulate the activity of other therapeutic agents, including chemical drugs, toxins, small interfering RNAs and nanoparticle-encapsulated drugs. Specifically, several attractive aptamer-mediated cell-type specific siRNA delivery systems are highlighted, and their promise in clinical development is also discussed.