Alpha- and beta-emitter radiopharmaceutical therapy combined with dual immune checkpoint inhibitors yields distinct control of primary and metastatic tumors

Background: The therapeutic challenge of advanced and metastatic solid tumors may be addressed by immune checkpoint inhibitors (ICIs) for some patients, but most exhibit primary or acquired resistance. Combination with radiopharmaceutical therapy (RPT) provides promise in overcoming ICI resistance by delivering radiation to all lesions. Selecting radionuclides of differing physical properties, such as linear energy transfer (LET) and path length, are an optimizable feature of RPT. For instance, alpha-emitters such as 225 Ac deliver high LET radiation over short ranges suitable for targeting microscopic disease, whereas beta-emitters such as 90 Y release lower LET radiation over longer distances suitable for macroscopic primary tumors. The optimal radionuclide type and dose to control coexisting macro- and micro-disease in advanced tumors remain undefined. To define rational strategies for metastatic disease, we evaluate differential therapeutic efficacy of alpha- and beta-emitting RPTs when combined with dual ICIs in controlling concurrent macroscopic and microscopic tumors. Methods: Dosimetry was estimated using the Monte Carlo-based RAPID platform informed by serial PET/CT or SPECT/CT imaging and/or ex vivo biodistribution. Female C57BL/6 mice (6-8 weeks) were implanted subcutaneously with 2 × 10 6 MOC2 cells in the flank on day -5 resulting in a macroscopic (~70 mm 3 ) tumor at the time of RPT. To model micro-metastatic disease, we injected 1 × 10 6 MOC2 cells intravenously on the morning prior to RPT. Mice received either 90 Y-NM600 (2, 12, or 20 Gy) or 225 Ac-NM600 (0.5, 2, or 6 Gy) on day 1, or were assigned to a no-radiation control group. Dual ICIs (anti-CTLA-4 + anti-PD-L1; 100 µg each) were administered intraperitoneally on days −1, 2, and 5 relative to RPT. Lymphopenia was monitored weekly using an HM5 analyzer up to expected isotope decay. Mice were euthanized when macroscopic tumors exceeded 20 mm in any dimension or on independent health-monitoring criteria for moribundity. Results: We identified an optimal dose of either 225 Ac- or 90 Y-NM600 that inhibited primary tumor growth and increased survival in a mixed tumor model. However, proportions of mice that were euthanized at the 20 mm primary tumor cut-off versus those that were euthanized prior to that endpoint because of moribundity differed between 225 Ac- and 90 Y-NM600. 50% of 225 Ac-NM600 mice were euthanized for reaching the 20 mm primary tumor endpoint, while no mice in the 90 Y-NM600 group were euthanized for primary tumor size and were instead euthanized for moribundity. Conclusions: Further testing is required to identify the cause of differing moribundity amongst the 225 Ac-NM600 versus 90 Y-NM600 treatment groups; however, these findings suggest differing effects on disease progression in a mixed tumor model when treated with radionuclides of differing properties.