Increased Nuclease Resistance
Nucleic acids are degraded rapidly once introduced in bodily fluids, RNA are more susceptible to degradation under normal laboratory conditions particularly due to RNase contamination. Special precautions must be taken to prevent RNA degradation. Nuclease resistant modifications can be introduced chemically in oligonucleotides that still retain its molecular structure and its shape based molecular interaction. These modifications are almost indispensable and have been used intensely in antisense applications. Also nucleic acids with mirror image chemistry have been developed that evade natural nucleases completely. Examples are converting the normal phosphodiester linkages to phosphorothioate or phosphorodithioate linkages, 2’O methyl, propyne bases etc.
Cellular delivery is an important functional aspect for therapeutic aptamers in addition to duplex stability and nuclease resistance. Polyethylene glycol, cholesterol, and various peptide structures have been used to attach to aptamers to facilitate cellular delivery or even targeting to the nucleus. Photo-cleavable linkers have been used that can be cleaved under controlled conditions.