RT Journal Article SR Electronic T1 The combination of 225Ac-anti-CD33 radioimmunoconjugate and CD47 blockade enhances survival in a preclinical model of acute myeloid leukemia JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 2570 OP 2570 VO 63 IS supplement 2 A1 Jennings, Caroline A1 Li, Jason A1 McCloskey, Megan A1 Pachhal, Sagarika A1 Greer, Emily A1 Chin, Amanda A1 Geoghegan, Eileen A1 Liang, Qing A1 Hwang, Jesse A1 Roy, Monideepa A1 Chen, Mary A1 Kotanides, Helen A1 Ludwig, Dale A1 Beckford, Denis YR 2022 UL http://jnm.snmjournals.org/content/63/supplement_2/2570.abstract AB 2570 Introduction: CD47, a macrophage checkpoint that suppresses phagocytosis, is upregulated in several cancers including acute myeloid leukemia (AML). The CD47 “don’t eat me” signal is a mechanism which enables AML cells to evade immunosurveillance. By contrast, calreticulin (CRT) is a pro-phagocytic “eat me” signal that enhances the innate immune response to kill tumor cells. Since CD33 is overexpressed in AML cells, we hypothesized that radiation-induced endoplasmic reticulum stress resulting from 225Ac-radiolabeled anti-CD33 (225Ac-anti-CD33) treatment will upregulate CRT and potentially synergize with a CD47 blocking agent such as magrolimab to trigger the innate immune response and improve antitumor efficacy. Here, we investigate the potential benefit of combining CD47 blockade with 225Ac-anti-CD33 (Actimab-A) in a preclinical model of human AML. Methods: Anti-CD33 (lintuzumab) monoclonal antibody was radiolabeled with 225Ac via p-SCN-Bn-DOTA. The biological activity and cytotoxicity of 225Ac-anti-CD33 was evaluated using CD33 positive tumor cell lines. 225Ac-anti-CD33 mediated CRT upregulation was evaluated in vitro in a panel of heme cell lines using flow cytometry and in vivo in mice bearing U937 xenograft tumors using Positron Emission Tomography (PET). The ability of the 225Ac-anti-CD33 and anti-CD47 antibody (magrolimab) combination to enhance macrophage phagocytosis in vitro was assessed by flow cytometry. The efficacy resulting from the combination of 225Ac-anti-CD33 and anti-CD47 antibodies was evaluated in the human MV-4-11 disseminated AML model in vivo. Results: 225Ac-anti-CD33 binds CD33 antigen similar to the native antibody and demonstrates specific cytotoxicity to receptor positive cells. 225Ac-anti-CD33 treatment led to the upregulation of CRT in vitro. Furthermore, treatment with 225Ac-anti-CD33 induced CRT upregulation in mice bearing U937 tumors as demonstrated by PET. Phagocytosis of CD33 positive cells was significantly enhanced by the 225Ac-anti-CD33 and anti-CD47 combination compared to each agent alone. The 225Ac-anti-CD33 and anti-CD47 combination therapy demonstrated a reproducible increase in survival and reduced toxicity in a human AML preclinical model. Conclusions: This study suggests that the combination of CD47 (“don’t eat me” signal) blockade and CRT (“eat me” signal) upregulation by 225Ac-anti-CD33 can synergize to increase innate immune response and significantly improve survival in a preclinical model of AML. Our findings warrant further investigation of the potential for this combination to enhance responses in hematological cancers.