Aptamers : molecular binding agents with application in targeted cancer treatment

Aptamers are single stranded (ss) DNA or RNA oligonucleotides consisting of 25 to more than 200 nucleosides that can fold into three-dimensional structures and bind selectively to antigens on the surface of human cells, as well as small chemicals and proteins. Aptamers are selected from random sequence libraries and synthesized using a completely automated process called systematic evolution of ligands by exponential enrichment or SELEX (Fig. 1). They can also be chemically synthesized using solid-phase phosphoramidite chemistry. Alteration of their sugar residues from 2′-hydroxyl group of the ribose to 2′-fluoro or 2′-amino make aptamers resistant to cleavage by cellular enzymes. Poly(ethyleneglycol) (PEGylation) can be used to increase clearance times of the aptamer from blood serum.