Yasui, L. S., Chen, K., Wang, K., Jones, T. P., Caldwell, J., Guse, D. and Kassis, A. I. Using Hoechst 33342 to Target Radioactivity to the Cell Nucleus. Radiat. Res. 167, 167–175 (2007).

We have explored the use of Hoechst 33342 (H33342) to carry radioactivity to the cell nucleus. H33342 enters cells and targets DNA at adenine-thymine-rich regions of the minor groove. Considerable membrane blebbing and ruffling occur in CHO cells within minutes after its addition to the culture medium in micromolar quantities. Blue vesicles are apparent in the cell cytoplasm, and by 30 min the nuclei are stained dark blue. Upon its binding to DNA, a visible emission shift of the dye can be observed with fluorescence microscopy. We have radioiodinated (¹²⁵I) H33342 and specifically irradiated nuclear DNA by incubating CHO cells with ¹²⁵I-H33342 at 37°C and accumulating ¹²⁵I decays at −90°C. At various times, the cells are thawed and assayed for survival (clonogenicity) and DSB (γ-H2AX) formation. ¹²⁵I-H33342 decay leads to a monoexponential decrease in cell survival with a D₀ of 122 ¹²⁵I decays per cell and a linear increase in DNA DSB induction (equivalent to 15 γ-H2AX foci/cell). Cell death is not modified by the radioprotective effects of H33342 because we use considerably lower concentrations than those that provide a slight protection against γ radiation. We conclude that cell killing by ¹²⁵I-H33342 and the induction of γ-H2AX foci are highly correlated.