Accepted poster session at ImmunoRad 2023: Targeted alpha therapy using Actinium-225 radiolabeled single domain antibodies induces antigen-specific immune responses and instills immunomodulation both systemically and at the tumor microenvironment

Background: Targeted radionuclide therapy (TRT) using targeting moieties labeled with α-particle emitting radionuclides is an intensely investigated treatment approach as the short range of α-particles allows effective treatment of local lesions and micro-metastases [1,2]. However, profound assessment of the immunomodulatory effect of α-TRT is lacking in literature. Methods: We studied immunological responses ensuing TRT with an anti-human CD20 single domain antibody radiolabeled with Actinium-225 in a human CD20 and ovalbumin expressing B16-melanoma model using flow cytometry of tumors, splenocyte restimulation and multiplex analysis of blood serum. Results: Tumor growth was delayed with α-TRT and increased blood levels of various cytokines such as interferon-γ, C-C motif chemokine ligand 5, granulocyte-macrophage colony-stimulating factor and monocyte chemoattractant protein-1. Peripheral anti-tumoral T-cell responses were detected upon α-TRT. At the tumor site, α-TRT modulated the “cold” tumor microenvironment (TME) to a more hospitable and “hot” habitat for anti-tumoral immune cells characterized by a decrease in pro-tumoral alternatively activated macrophages and an increase in anti-tumoral macrophages and dendritic cells. We also showed that α-TRT increased the percentage of PD-L1pos immune cells in the TME. To circumvent this immunosuppressive countermeasure we applied immune checkpoint blockade (ICB) of the PD-1|PD-L1 axis. Combination of α-TRT with PD-L1 blockade potentiated the therapeutic effect, however aggravated adverse events. A long-term toxicity study revealed severe kidney damage ensuing α-TRT. Conclusion: These data suggest that α-TRT alters the TME and induces systemic anti-tumoral immune responses, which explains why ICB enhances the therapeutic effect of α-TRT. However, further optimization is warranted to avoid adverse events.