Lobachevsky, P. N. and Martin, R. F. DNA Breakage by Decay of Auger Electron Emitters: Experiments with ¹²³I-iodoHoechst 33258 and Plasmid DNA. Radiat. Res. 164, 766– 773 (2005).

The Auger electron-emitting isotope ¹²³I is of interest in the context of potential exploitation of Auger electron emitters in radioimmunotherapy. The efficiency of induction of cytotoxic lesions by decay of DNA-associated ¹²⁵I, the prototype Auger electron emitter, is well established, but its long half-life (60 days) is a limitation. However, the advantage of the much shorter half-life of ¹²³I (13.2 h) might be outweighed by its “weaker” Auger electron cascade with an average of 8–11 Auger electrons, compared to about 15–21 electrons for ¹²⁵I. Accordingly, the efficiency of DNA breakage for DNA-associated ¹²³I was investigated by incubation of ¹²³I-iodoHoechst 33258 with plasmid DNA. The efficiency of double-strand break induction by decay of ¹²³I was 0.62 compared to 0.82 per decay of ¹²⁵I in the same experimental system. In the presence of dimethylsulfoxide, the values were 0.54 and 0.65 for decay of ¹²³I and ¹²⁵I, respectively. The results also showed that at a very low ligand/plasmid molar ratio (<1), the majority of cleavage seemed to occur at a particular site on the plasmid molecule, indicating preferential binding of the ¹²³I-ligand to a unique site or a cluster of neighboring sites.