Amplification of DNA damage by a γH2AX-targeted radiopharmaceutical

111In-DTPA-anti-γH2AX-Tat, which combines an anti-γH2AX antibody with a cell-penetrating peptide, Tat, and the Auger electron-emitting radioisotope, 111In, targets the DNA damage signalling protein, γH2AX, and has potential as a probe for imaging DNA damage in vivo. The goal of this study was to investigate whether 111In-DTPA-anti-γH2AX-Tat labelled to high specific activity (6 MBq/μg) can amplify treatment-related DNA damage for therapeutic gain.MethodsMDA-MB-468 and MDA-MB-231/H2N (231-H2N) breast cancer cells were incubated with 111In-DTPA-anti-γH2AX-Tat (3 MBq, 6 MBq/μg) or a control radioimmunoconjugate, 111In-DTPA-mIgG-Tat, and exposed to IR or bleomycin. DNA damage was studied by counting γH2AX foci and by neutral comet assay. Cytotoxicity was evaluated using clonogenic assays. 111In-DTPA-anti-γH2AX-Tat was administered intravenously to 231-H2N-xenograft-bearing Balb/c nu/nu mice in tumor growth inhibition studies.ResultsThe number of γH2AX foci was greater after exposure of cells to IR (10 Gy) plus 111In-DTPA-anti-γH2AX-Tat compared to IR alone (20.6 ± 2.5 versus 10.4 ± 2.3 foci/cell; P < .001).111In-DTPA-anti-γH2AX-Tat resulted in a reduced surviving fraction in cells co-treated with IR (4 Gy) versus IR alone (5.2% ± 0.9% versus 47.8% ± 2.8%; P < .001). Similarly, bleomycin (25–200 μg/mL) plus 111In-DTPA-anti-γH2AX-Tat resulted in a lower SF compared to bleomycin alone. The combination of a single exposure to IR (10 Gy) plus 111In-DTPA-anti-γH2AX-Tat significantly decreased the growth rate of 231-H2N xenografts in vivo compared to either 111In-DTPA-anti-γH2AX-Tat or IR alone (− 0.002 ± 0.004 versus 0.036 ± 0.011 and 0.031 ± 0.014 mm3/day, respectively, P < .001).Conclusion111In-DTPA-anti-γH2AX-Tat amplifies anticancer treatment-related DNA damage in vitro and has a potent anti-tumor effect when combined with IR in vivo.