Bexmarilimab is a new humanized monoclonal antibody against common lymphatic endothelial and vascular endothelial receptor-1 (CLEVER-1), and is in clinical trials for macrophage-guided cancer immunotherapy. In addition to cancer, CLEVER-1 is also associated with fibrosis. To facilitate prospective human PET studies, we preclinically evaluated ⁸⁹Zr-labeled bexmarilimab in rabbits.

Methods: Bexmarilimab was conjugated with desferrioxamine (DFO) and radiolabeled with ⁸⁹Zr. Retained immunoreactivity was confirmed by flow cytometry. Distribution kinetics of intravenously administered ⁸⁹Zr-DFO-bexmarilimab (0.1 mg/kg) for up to 7 days in a rabbit model of renal fibrosis mediated by unilateral ureteric obstruction (UUO). The in-vivo stability of ⁸⁹Zr-DFO-bexmarilimab was evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in combination with autoradiography. Additionally, we estimated the human radiation dose from data obtained in healthy rabbits.

Results: ⁸⁹Zr-DFO-bexmarilimab cleared rapidly from the blood circulation and distributed to the liver and spleen. At 24 hours post-injection, PET/CT, ex-vivo gamma counting and autoradiography demonstrated that there was significantly higher ⁸⁹Zr-DFO-bexmarilimab uptake in UUO-operated fibrotic renal cortex, characterized by abundant CLEVER-1-positive cells, than in contralateral or healthy kidneys. The estimated effective dose for a 70-kg human was 0.70 mSv/MBq.

Conclusion: The characteristics of ⁸⁹Zr-DFO-bexmarilimab support future human PET studies to, for example, stratify patients for bexmarilimab treatment, evaluate the efficacy of treatment, or monitor disease progression.