212Pb-Radioimmunotherapy Induces G2 Cell-Cycle Arrest and Delays DNA Damage Repair in Tumor Xenografts in a Model for Disseminated Intraperitoneal Disease

In preclinical studies, targeted radioimmunotherapy using (212)pb-TCMC-trastuzumab as an in vivo generator of the high-energy a-particle emitting radionuclide Bi-212 is proving an efficacious modality for the treatment of disseminated peritoneal cancers. To elucidate mechanisms associated with this therapy, mice bearing human colon cancer LS-174T intraperitoneal xenografts were treated with (212)pb-TCMC-trastuzumab and compared with the nonspecific control (212)pb-TCMC-HuIgG, unlabeled trastuzumab, and HuIgG, as well as untreated controls. (212)pb-TCMC-trastuzumab treatment induced significantly more apoptosis and DNA double-strand breaks (DSB) at 24 hours. Rad51 protein expression was downregulated, indicating delayed DNA double-strand damage repair compared with (212)pb-TCMC-HuIgG, the nonspecific control. (212)pb-TCMC-trastuzumab treatment also caused G2-M arrest, depression of the S phase fraction, and depressed DNA synthesis that persisted beyond 120 hours. In contrast, the effects produced by (212)pb-TCMC-HuIgG seemed to rebound by 120 hours. In addition, (212)pb-TCMC-trastuzumab treatment delayed open chromatin structure and expression of p21 until 72 hours, suggesting a correlation between induction of p21 protein and modification in chromatin structure of p21 in response to (212)pb-TCMC-trastuzumab treatment. Taken together, increased DNA DSBs, impaired DNA damage repair, persistent G(2)-M arrest, and chromatin remodeling were associated with (212)pb-TCMC-trastuzumab treatment and may explain its increased cell killing efficacy in the LS-174T intraperitoneal xenograft model for disseminated intraperitoneal disease. Mol Cancer Ther; 11(3); 639-48. (C) 2012 AACR.