123 ICC1: A PARP-Targeting, Auger Electron–Emitting Radiopharmaceutical for Radionuclide Therapy of Cancer

Poly(adenosine diphosphate ribose) polymerase (PARP) has emerged as an effective therapeutic strategy against cancer that targets the DNA damage repair enzyme. PARP-targeting compounds radiolabeled with an Auger electron–emitting radionuclide can be trapped close to damaged DNA in tumor tissue, where high ionizing potential and short range lead Auger electrons to kill cancer cells through the creation of complex DNA damage, with minimal damage to surrounding normal tissue. Here, we report on 123ICC1, an 123I-labeled PARP inhibitor for radioligand therapy of cancer. Methods: Copper-mediated 123I iododeboronation of a boronic pinacol ester precursor afforded 123ICC1. The level and specificity of cell uptake and the therapeutic efficacy of 123ICC1 were determined in human breast carcinoma, pancreatic adenocarcinoma, and glioblastoma cells. Tumor uptake and tumor growth inhibition of 123ICC1 were assessed in mice bearing human cancer xenografts (MDA-MB-231, PSN1, and U87MG). Results: In vitro and in vivo studies showed selective uptake of 123ICC1 in all models. Significantly reduced clonogenicity, a proxy for tumor growth inhibition by ionizing radiation in vivo, was observed in vitro after treatment with as little as 10 Bq 123ICC1. Biodistribution at 1 h after intravenous administration showed PSN1 tumor xenograft uptake of 0.9 ± 0.06 percentage injected dose per gram of tissue. Intravenous administration of a relatively low amount of 123ICC1 (3 MBq) was able to significantly inhibit PSN1 xenograft tumor growth but was less effective in xenografts that expressed less PARP. 123ICC1 did not cause significant toxicity to normal tissues. Conclusion: Taken together, these results show the potential of 123ICC1 as a radioligand therapy for PARP-expressing cancers.