PARP Targeted Alpha-Particle Therapy Enhances Response to PD‑1 Immune-Checkpoint Blockade in a Syngeneic Mouse Model of Glioblastoma

We have previously demonstrated potent antitumor effects of PARP targeted alpha-therapy with astatine-211-MM4 ([211At]­MM4) in neuroblastoma preclinical models, although differential sensitivity suggests it is unlikely to be curative as a single-agent in all tumor types. Alpha-particle induced DNA damage can elicit an immune response that results in T-cell activation against tumor cells; however, tumor cells can evade immune surveillance through expression of programmed death ligand 1 (PD-L1). Therefore, we investigated the effects of α particle therapy in combination with immune-checkpoint blockade using astatine-211-MM4 and anti-programmed death receptor 1 (anti-PD-1) immunotherapy in a syngeneic mouse model of glioblastoma. We characterized the sensitivity of four human glioblastoma cell lines to [211At]­MM4 in vitro. To evaluate [211At]­MM4 treatment effects on hematological tissues, complete blood counts were performed after a single dose at 12, 24, or 36 MBq/kg. In vivo efficacy was evaluated in a syngeneic mouse model of glioblastoma using GL26 glioblastoma cells in CB57BL/6J mice treated with either 36 MBq/kg [211At]­MM4, anti-PD-1 antibody, or a combination of the two. Following a single dose of [211At]­MM4, lymphocytes are significantly decreased compared to control at both 72 h and 1 week following treatment followed by recovery of counts by 2 weeks. However, neutrophils showed an increase with all dose levels of [211At]­MM4 exhibiting higher levels than control. The average best tumor responses for combination, anti-PD-1, and [211At]­MM4 were 100%, 83.6%, and 58.2% decrease in tumor volume, respectively. Average progression free intervals for combination, anti-PD-1, [211At]­MM4, and control groups was 65, 36.4, 23.2, and 3 days, respectively. The percentages of disease-free mice at the end of the study for combination and anti-PD-1 were 100% and 60%, while [211At]­MM4 and control groups were both 0%. In summary, combination therapy was more effective than either single agent in all response categories analyzed, highlighting the potential for PARP targeted alpha-therapy to enhance PD-1 immune-checkpoint blockade.