Aptamers are a special class of nucleic acid molecules that are beginning to be 
investigated for clinical use. These small RNA/DNA molecules can form secondary 
and tertiary structures capable of specifically binding proteins or other 
cellular targets; they are essentially a chemical equivalent of antibodies. 
Aptamers have the advantage of being highly specific, relatively small in size, 
and non-immunogenic. Since the discovery of aptamers in the early 1990s, great 
efforts have been made to make them clinically relevant for diseases like 
cancer, HIV, and macular degeneration. In the last two decades, many aptamers 
have been clinically developed as inhibitors for targets such as vascular 
endothelial growth factor (VEGF) and thrombin. The first aptamer based 
therapeutic was FDA approved in 2004 for the treatment of age-related macular 
degeneration and several other aptamers are currently being evaluated in 
clinical trials. With advances in targeted-therapy, imaging, and nanotechnology, 
aptamers are readily considered as potential targeting ligands because of their 
chemical synthesis and ease of modification for conjugation. Preclinical studies 
using aptamer-siRNA chimeras and aptamer targeted nanoparticle therapeutics have 
been very successful in mouse models of cancer and HIV. In summary aptamers are 
in several stages of development, from pre-clinical studies to clinical trials 
and even as FDA approved therapeutics. In this review, we will discuss the 
current state of aptamers in clinical trials as well as some promising aptamers 
in pre-clinical development.