Radiolabeling of ortho/meta-[123I]iodo-Hoechst for proof of direct DNA strand breaks assisted by molecular modeling studies.

Abstract

The use of Auger-Meitner electron-emitting radionuclides in particular has been intensively studied in recent decades, as their extremely short range in the nanometer range enables highly localized damage to DNA and other intracellular structures. Radionuclides such as 123I and 125I are ideal for such applications because they release dense cascades of low-energy electrons during their electron capture decay. In this work, Hoechst derivatives were radiolabeled with 123I using isotopic exchange reaction or lactoperoxidase method in order to develop a tool for direct DNA damage dependent on the activity. Optimized radiolabeling yielded molar activities of 27.5 MBq/nmol and 84.1 MBq/nmol for isotopic exchange and lactoperoxidase method, respectively. Molecular modeling studies revealed a closer and more rigid coordination for the iodine of ortho-iodo-Hoechst (oIH) within the DNA in comparison with meta-iodo-Hoechst (mIH). DNA studies revealed 2.5 123I radionuclides per DNA which accounts for at least one double-strand break after one half-life of 123I. Cell studies with PC3 cells corroborated these findings with 27-34× higher impact of cell toxicity for oIH in comparison to mIH. The radiolabeled Hoechst derivatives are now available as robust tool for quantification of DNA damage, and the computational studies provide structural insights into the mechanism underlying the observed differences in DNA-damaging activity.

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