[ 123 I]CC1, a radiopharmaceutical for Targeted radionuclide therapy (TRT), exploits PARP binding and trapping to amplify DNA damage and cytotoxicity in human cancer cell lines.

We investigated whether the antitumor effects of [ 123 I]CC1, a PARP-binding radiopharmaceutical emitting very short-range ionising Auger electrons with potential use in Targeted Radionuclide Therapy (TRT), are driven by PARP trapping. Trapping will prolong its residence time on DNA and triggers more DNA damage. We tested this in PSN1 and U87 human cancer cell lines exposed to [ 123 I]CC1, by assessing PARP levels in the chromatin fraction via western blot and quantifying 123 I counts in nuclear and cytoplasmic fractions using a gamma counter. Downstream effects were evaluated using gammaH2AX immunofluorescence as a marker of DNA damage and DNA fiber assays to measure replication speed. Furthermore, we modulated PARP association with DNA by using methyl methanesulfonate (MMS) to increase and the PARG inhibitor JA2131 to decrease trapped PARP, using western blot and gamma counter to analyze how [ 123 I]CC1 tracks PARP movements from chromatin to nuclear soluble fraction. Finally, gammaH2AX immunofluorescence was used to assess whether combinatory treatments with MMS or PARGi affect the ability of [ 123 I]CC1 in producing DNA damage. [ 123 I]CC1 elevated trapped PARP levels on DNA in PSN1 and U87 cells, followed by a rapid increase in gammaH2AX foci. Interestingly, DNA fiber assays showed that replication was markedly hindered in PSN1 at 30 min and 24 h, indicating substantial DNA damage. Pretreatment with 0.01% MMS before [ 123 I]CC1 addition further elevated PARP levels compared to [ 123 I]CC1 alone, resulting in greater chromatin 123 I accumulation. In contrast, PARG inhibitor treatment reduced PARP residence time on DNA, and in combination with [ 123 I]CC1 decreased DNA 123 I delivery. Finally, assessment of gammaH2AX at 30 min and 24 hours showed that MMS pretreatment enhanced DNA damage induced by [ 123 I]CC1 greater damage than MMS alone, demonstrating an additive effect, whereas PARGI pretreatment had no significant effect.The results indicate that the cytotoxic effect of [ 123 I]CC1 is mediated by PARP trapping, which underlies its potent antitumoral activity previously observed by our group. Furthermore, modulation of the PARylation cycle alters delivery of radioactivity to DNA and the extent of DNA damage, suggesting a strategy to enhance the therapeutic benefit of [ 123 I]CC1 and its potential for clinical translation.