Influence of immune checkpoint inhibitor target and timing on combination treatment with antibody-based β-emitting targeted radionuclide therapy across tumor immunogenicity profiles.

INTRODUCTION: Targeted radionuclide therapy (TRT) and immune checkpoint inhibition (ICI) could be a highly effective combination treatment due to potential synergistic effects. In this study, we investigated two clinically relevant antibody-based radiopharmaceuticals labeled with lutetium-177: [177Lu]Lu-DOTA-hG250 targeting carbonic anhydrase IX (CAIX) and rosopatamab tetraxetan ([177Lu]Lu-DOTA-rosopatamab) targeting prostate-specific membrane antigen (PSMA). We aimed to assess whether ICI target and treatment timing influence the efficacy of TRT+ICI combinations across three syngeneic murine tumor models with distinct immunogenicity profiles. Understanding these factors is critical for optimizing combination strategies. METHODS: C57BL6 hPSMA transgenic mice bearing subcutaneous MC38-hPSMA or B16F10-hPSMA tumors were injected with [177Lu]Lu-DOTA-rosopatamab. Balb/c mice bearing subcutaneous renca-hCAIX tumors were injected with [177Lu]Lu-DOTA-hG250. Tumor growth and overall survival were evaluated for different combination treatments of hPSMA-TRT or hCAIX-TRT with aPD-1, aCTLA-4, or dual-ICI (n = 10 per group). To evaluate the impact of timing on therapeutic outcomes, different concurrent and sequential treatment schedules were compared. RESULTS: In the MC38-hPSMA model, a subtherapeutic [177Lu]Lu-DOTA-rosopatamab dose combined with aPD-1 resulted in robust anti-tumor activity. aCTLA-4 monotherapy was already curative, precluding detection of a combination effect under the conditions tested. In contrast, in the B16F10-hPSMA model, only minimal benefit was seen when combining the same radiopharmaceutical with dual-ICI. The renca-hCAIX model demonstrated differential outcomes, responding most strongly to a subtherapeutic dose of [177Lu]Lu-DOTA-hG250 combined with dual-ICI, while combining with aPD-1 provided moderate benefit and with aCTLA-4 had no therapeutic benefit. Lastly, exploring treatment timing in the MC38-hPSMA and renca-hCAIX models revealed only limited impact on therapeutic outcome under the assessed conditions. DISCUSSION: These findings indicate that 177Lu-labeled antibodies can synergize with ICI at subtherapeutic activity levels in certain settings, supporting their potential to enhance efficacy while potentially reducing toxicity. Combination outcomes varied across tumor models and ICI targets and were likely influenced by both biological and experimental context. The impact of treatment timing appeared limited within the experimental conditions evaluated. These findings should be further explored in preclinical models incorporating mechanistic analyses, while the impact of treatment timing warrants additional investigation in clinically relevant settings. These insights highlight key factors for optimizing TRT+ICI strategies in future clinical studies.