Tumor immunity following radiopharmaceutical + checkpoint blockade varies with treatment sequence, radioisotope, and tumor immunogenicity

Radiopharmaceutical therapy (RPT) enhances tumor response to immune checkpoint inhibitors (ICI) in preclinical models, but the effects of different radioisotopes have not been compared. To evaluate mechanisms of response to RPT+ICI, we used NM600, an alkylphosphocholine selectively taken up by most tumors. Effects of 90Y-, 177Lu-, and 225Ac-NM600 + ICIs were compared in syngeneic murine models, B78 melanoma (poorly immunogenic) and MC38 colorectal cancer (immunogenic). 90Y-/177Lu-/or 225Ac-NM600 delivering 2 Gy mean tumor dose promoted tumor regression and improved survival when combined with ICIs in syngeneic mice bearing B78 or MC38 tumors. Regardless of the administered isotope, this combination was optimized with early ICI administration (days -3/0/3) relative to day 1 RPT. 90Y-NM600+ICI produced the greatest anti-tumor response for MC38, whereas high linear energy transfer (LET) alpha particle radiation from 225Ac-NM600+ICI was most effective against poorly immunogenic B78 tumors. Flow cytometry and single cell RNA and T cell receptor (TCR) sequencing illuminated distinct mechanisms of 90Y- or 177Lu-NM600 in promoting expansion of existing adaptive immunity and of 225Ac-NM600 in promoting immune priming when combined with ICI. Antitumor immune response can be achieved with appropriate application of a- or ß- emitting RPT in combination with ICIs in diverse murine tumor models. Overall design: B78 melanoma tumor-bearing mice received either 2 Gy 90Y-, 177Lu-, 225AcNM600 or EBRT on day 1 + ICI (anti-CTLA4 and anti-PD-L1) on days -3/0/3 or ICI alone. Tumors were harvested on day 15, dissociated, and immune cells were isolated by Ficoll density gradient centrifugation for single cell RNA sequencing analysis. All samples had viability > 90%, and were resuspended at a concentration of 1.2 *106 cells/mL in RPMI-1640 for single-cell RNA sequencing.