**5.2 Oligonucleotides/ PNAs/ MORF**

The natural ability of oligonucleotides and the oligonucleotide mimetic peptide nucleic acids (PNAs) and phosphorodiamidate morpholinos (MORF) to anneal with RNA and DNA makes them the appealing vehicles to bring radionuclides in close proximity to the RNA/DNA. Both 125I and 111In have been used to radiolabel oligonucleotides and have been applied successfully to target over-expression of certain genes involved with cancer (Cornelissen & Vallis, 2010).

Aptamers are synthetically based DNA or RNA oligonucleotides that are highly stable structures and are considered to have low immunogenicity. They are selected for their ability to bind to a target of interest (Perkins & Missailidis, 2007). Hicke et al. demonstrated that the aptamers cleared quickly from the blood, reaching maximum tumor uptake within 10 min, but then decreasing to approximately 2% by 3 h. However, the rapid clearance from the blood and tissues resulted in highly favorable tumor/blood and kidney ratios but there was considerable additional clearance of the 99mTc through the liver and intestines (Hicke et al., 2006). However, using aptamers as the radiopharmaceuticals needs further improvements.
