We have compared the effectiveness in causing DNA strand breaks of${}^{111}{\rm In}$ bound to DNA or free in aqueous solution with that of γ rays. Supercoiled DNA from pBR322 plasmid labeled with [3 H]thymidine was purified and mixed with${}^{111}{\rm InCl}{}_{3}$ in the absence or presence of diethylenetriaminepentaacetic dianhydride (DTPA), a metal chelator which prevents the binding of indium to DNA. The reaction mixtures were stored at 4°C to accumulate radiation dose from the decay of${}^{111}{\rm In}$. The DNA was then resolved by gel electrophoresis into supercoiled, nicked circular and linear forms, representing undamaged DNA, single-strand breaks (SSBs) and double-strand breaks (DSBs), respectively. The D0 values of pBR322 DNA exposed to γ radiation from an external137 Cs source and the decay of${}^{111}{\rm In}$ dispersed in solution (+DTPA) are 3.1 ± 0.1 and 2.8 ± 0.1 Gy, respectively. In terms of accumulated${}^{111}{\rm In}$ disintegrations cm-3 of plasmid DNA solution, the D0 value is$15.3\ (\pm \ 0.7)\times 10^{10}$ disintegrations in the absence of DTPA and$38.2\ (\pm \ 1.1)\times 10^{10}$ disintegrations in its presence. Since only 14.6 ± 5% of the${}^{111}{\rm In}$ was bound to DNA in the absence of DTPA, an effective D0 for bound${}^{111}{\rm In}$ of$3.4\ (\pm \ 1.1)\times 10^{10}$ disintegrations is obtained. The 11-fold (range 9- to 17-fold) increased effectiveness of this Auger electron emitter when in proximity to DNA appears to be due mainly to the higher yield of SSBs.

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