89Zr-PET imaging of DNA double-strand breaks for the early monitoring of response following α- and β-particle radioimmunotherapy in a mouse model of pancreatic ductal adenocarcinoma

Rationale: The evaluation of early treatment response is critical for patient prognosis and treatment planning. When the current methods rely on invasive protocols that evaluate the expression of DNA damage markers on patient biopsy samples, we aim to evaluate a non-invasive PET imaging approach to monitor the early expression of the phosphorylated histone gamma H2AX in the context of pancreatic cancer targeted radionuclide therapy. Pancreatic ductal adenocarcinoma has a poor patient prognosis due to the absence of curative treatment for patients with advanced disease. There is therefore a critical need for the fast clinical translation of new therapeutic options. In line with these observations, our group has been focusing on the development of radiotheranostic agents based on a fully human monoclonal antibody (5B1) with exceptional affinity for CA19.9, an antigen overexpressed in PDAC. Two on-going clinical trials resulted from these efforts, one with Zr-89 (diagnosis) and one with Lu-177 (beta-particle therapy). More recently, we successfully developed and evaluated in PDAC mouse models a targeted a-therapy strategy with high clinical translation potential. We aim to expedite the clinical translation of the developed radioimmunotherapy approaches by investigating the early therapeutic response and effect of radiation therapy in a PDAC mouse model via PET imaging. Methods: Mice bearing BxPC3 tumor xenografts were treated with alpha- and beta-particle pretargeted radioimmunotherapy (PRIT), external beam radiotherapy (EBRT), or sham-treated (vehicle). The phosphorylated histone yH2AX produced as a response to DNA double strand breaks was quantified with the PET radiotracer, [Zr-89]Zr-DFO-anti-gamma H2AX-TAT. Results: PET imaging studies in BxPC3 PDAC mouse models demonstrated increased uptake of [Zr-89]Zr-DFO-anti-gamma H2AX-TAT (6.29 +/- 0.15 %IA/g) following beta-PRIT in BxPC3 PDAC xenografts as compared to the saline control group (4.58 +/- 0.76 %IA/g) and EBRT control group (5.93 +/- 0.76 %IA/g). Similarly, significantly higher uptake of [Zr-89]Zr-DFO-anti-gamma H2AX-TAT was observed in tumors of the 225 Ac-PRIT and EBRT (10 Gy) cohorts (7.37 +/- 1.23 and 6.80 +/- 1.24 %IA/g, respectively) compared to the negative control cohort (5.08 +/- 0.95 %IA/g). Ex vivo gamma H2AX immunohistochemistry and immunofluorescence analysis correlated with in vivo Zr-89-anti-gamma H2AX PET/CT imaging with increased gamma H2AX positive cell and gamma H2AX foci per cell in the treated cohorts. When a-PRIT resulted in prolonged overall survival of treated animals (107.5 days) as compared to beta-PRIT (73.0 days), no evidence of difference in [Zr-89]Zr-DFO-anti-gamma H2AX-TAT uptake at the tumor site was observed, highlighting that DNA damage is not the sole radiobiology paradigm and that off-targeted (bystander) effects should be considered. Conclusions: PET imaging studies with [Zr-89]Zr-DFO-anti-gamma H2AX-TAT following a- and beta-particle PRIT in a BxPC3 PDAC subcutaneous xenograft mouse model allowed the monitoring of tumor radiobiological response to treatment.