Sensitization of Tumor to ²¹²Pb Radioimmunotherapy by Gemcitabine Involves Initial Abrogation of G2 Arrest and Blocked DNA Damage Repair by Interference With Rad51

Purpose: To elucidate the mechanism of the therapeutic efficacy of targeted α-particle radiation therapy using ²¹²Pb-TCMC-trastuzumab together with gemcitabine for treatment of disseminated peritoneal cancers. Methods and Materials: Mice bearing human colon cancer LS-174T intraperitoneal xenografts were pretreated with gemcitabine, followed by ²¹²Pb-TCMC-trastuzumab and compared with controls. Results: Treatment with ²¹²Pb-TCMC-trastuzumab increased the apoptotic rate in the S-phase-arrested tumors induced by gemcitabine at earlier time points (6 to 24 hours). ²¹²Pb-TCMC-trastuzumab after gemcitabine pretreatment abrogated G2/M arrest at the same time points, which may be associated with the inhibition of Chk1 phosphorylation and, in turn, cell cycle perturbation, resulting in apoptosis. ²¹²Pb-TCMC-trastuzumab treatment after gemcitabine pretreatment caused depression of DNA synthesis, DNA double-strand breaks, accumulation of unrepaired DNA, and down-regulation of Rad51 protein, indicating that DNA damage repair was blocked. In addition, modification in the chromatin structure of p21 may be associated with transcriptionally repressed chromatin states, indicating that the open structure was delayed at earlier time points. Conclusion: These findings suggest that the cell-killing efficacy of ²¹²Pb-TCMC-trastuzumab after gemcitabine pretreatment may be associated with abrogation of the G2/M checkpoint, inhibition of DNA damage repair, and chromatin remodeling