RNA Aptamers are defined as RNA oligonucleotides that bind to a specific target with high affinity and specificity, similarly to how an antibody binds to an antigen. Isolation of aptamers from randomized pools of RNA by using a method called Systematic Evolution of Ligands by EXponential enrichment (SELEX) was first developed by Gold and Turek, and by Ellington and Szostak [1
]. These RNA molecules were termed as “aptamers,” with etymology stemming from the Greek word aptus
, which means “to fit” [1
]. To date, various aptamers have been successfully selected against different targets and have begun to show promise as diagnostic, prognostic and therapeutic tools in a wide-range of applications including the treatment for human diseases such as cancer, viral infection and macular degeneration [4
Although this review article focuses on the development and applications of RNA aptamers, it is important to note that aptamers can also be made of DNA and protein as well. However, there are numerous advantages to RNA aptamers as a pivotal tool of RNA nanotechnology when compared to DNA aptamers, protein aptamers, and antibodies [4
]. First, although RNA aptamers function similarly to antibodies, they are known to have low or no immunogenicity when compared to other macromolecules such as proteins. A second advantage to RNA aptamers is that when compared to their peptide and antibody counterparts, they are easier to synthesize in large quantities in a controlled manner, and achieve defined structure and stoichiometry. Furthermore, nucleic acids such as RNAs are generally considered to be more thermodynamically stable than peptides or antibodies. Importantly, RNA aptamers can be further chemically modified (e.g. 2’deoxy, 2’F, 2’NH3, 2’OMe), which has been found to greatly improve their stability in the blood stream and resistance to RNAase shearing. Moreover, RNA aptamers are single-stranded in nature, which allows for a unique tertiary structure and leads to tighter and more specific binding. The single-stranded composition of RNA aptamers also makes them smaller in size and easier to enter into cells than DNA aptamers of the same length in nucleotides. Importantly, this size-advantage and easy conjugation feature of RNA aptamers also aid their ability to carry additional ligands for specific targeting or therapeutic agents for intracellular drug delivery.