PET imaging of DNA damage

Objectives The efficacy of most anti-cancer treatments, including ionizing radiation (IR) and many cytotoxic drugs, depends on an ability to cause DNA double strand breaks (DSBs). Very early during DNA damage signalling , the histone isoform H2AX is phosphorylated on Serine-139 to form γH2AX. With the aim of PET imaging of DNA damage, we synthesised a ⁸⁹Zr-labelled anti-γH2AX antibody, modified with the cell-penetrating peptide, TAT, which includes a nuclear localisation sequence.

Methods ⁸⁹Zr-anti-γH2AX-TAT was synthesized using EDC/NHS chemistry for TAT-peptide (GRKKRRQRRRPPQGYG) linkage. Desferroxamine conjugation allowed labelling with ⁸⁹Zr. Uptake and retention of ⁸⁹Zr-anti-γH2AX-TAT was evaluated in the breast adenocarcinoma cell line MDA-MB-468 in vitro, or grown as xenografts in athymic mice. Irradiated by external beam (3-10 Gy) was used to induce DSBs and expression of γH2AX. Since ⁸⁹Zr emits either a positron or low-energy electrons as well as a 909 keV gamma rays, detailed radiobiological measurements were included to ensure the radiolabelled compound itself does not cause any additional DSBs.

Results Uptake of ⁸⁹Zr-anti-γH2AX-TAT by cells was similar to results previously obtained with ¹¹¹In-anti-γH2AX-TAT. Retention of the ⁸⁹Zr-anti-γH2AX-TAT was 8-fold longer in irradiated cells expressing γH2AX, compared to non-irradiated cells, or to non-specific IgG control. PET imaging of mice showed higher uptake of ⁸⁹Zr-anti-γH2AX-TAT in irradiated xenografts, compared to non-irradiated or non-specific controls (12.1±1.6 vs. 5.2±1.9 and 5.1±0.8 %ID/g, respectively; P<0.0001). Monte Carlo modelling showed that the absorbed dose per decay of ⁸⁹Zr was up to 24-fold lower compared to ¹¹¹In. In vitro and ex vivo analysis showed that exposure of naïve or irradiated cells or xenografts to ⁸⁹Zr-anti-γH2AX-TAT did not significantly change the number of observable DNA double strand breaks, γH2AX foci or clonogenic survival.

Conclusions ⁸⁹Zr-anti-γH2AX-TAT allows PET imaging of DNA DSBs in a tumor xenograft mouse model.

Research Support Funding was supplied by CRUK, MRC and the Generalitat de Catalunya