CAIX-targeted α therapy directed against hypoxic tumor cells in combination with immune checkpoint inhibitors in a syngeneic mouse tumor model

Tumor hypoxia is a major factor in therapy resistance. A potential strategy to treat hypoxic tumors is targeted α therapy (TAT), since α particles can cause complex DNA damage independent of oxygen levels. Here, we investigate the potential of TAT as monotherapy and in combination with immune checkpoint inhibitors (ICI) to treat hypoxic tumors. Methods: Monoclonal anti-CAIX antibody DOTA-MSC3 was labeled with indium-111 (111In) or actinium-225 (225Ac), and binding to CAIX-expressing hypoxic tumor cells was determined in vitro. Subsequently, the in vivo biodistribution and dosimetry of radiolabeled DOTA-MSC3 was assessed in B16F10-OVA tumor-bearing mice, and its spatial distribution in the tumor (autoradiography) was correlated to CAIX expression measured by immunofluorescence. Finally, tumor growth and survival were determined upon treatment with [225Ac]Ac-DOTA-MSC3 with and without ICI. Results: 111In- and 225Ac-labeled DOTA-MSC3 bound specifically to CAIX-expressing hypoxic tumor cells. In vivo, uptake of both radiopharmaceuticals in B16F10-OVA tumors was spatially correlated with CAIX-positive hypoxic tumor regions. [225Ac]Ac-DOTA-MSC3 significantly prolonged survival of mice compared with PBS control (p=0.0032). Furthermore, the combination of [225Ac]Ac-DOTA-MSC3 and ICI significantly delayed tumor growth and prolonged survival compared with PBS control (p=0.0022 and p=0.0019, respectively). Conclusion: Overall, these results demonstrate first proof-of-concept of the potential of CAIX-TAT to treat hypoxic tumors by targeting CAIX-positive hypoxic tumor regions. CAIX-TAT combined with ICI was most effective in inhibiting tumor growth and prolonging survival of tumor-bearing mice. Future studies are required to investigate the radiobiological and immunological effects of CAIX-TAT, to guide optimization of this treatment in combination with ICI.