Antibody radioconjugates in combination with CD47 Immunotherapy Improves Antitumor Efficacy in preclinical models of solid tumors

Introduction: Resistance to therapy and low response rates continue to be the ultimate challenges for achieving cures in patients with cancer, despite the progress made with single agent targeted cancer therapies in the clinic. To overcome resistance, particularly in settings of tumor heterogeneity, the combined administration of agents with non-overlapping mechanisms of action is one strategy that can significantly improve patient outcome. This treatment approach can increase the probability of antitumor responses by making cancer cells incapable of developing resistance to two independent inhibitory effects. We have recently shown that radiation directed to cells with antibody radioconjugates (ARCs) can upregulate calreticulin (CRT), a pro-phagocytic “eat me” signal in cancer cells as part of the cytotoxic mechanism of targeted radiation. The immunogenic feature of ARCs may synergize with the CD47 “don’t eat me” signal overexpressed on cancer cells to evade innate immune response. Hence, we hypothesize that the immunogenic and cytotoxic properties of ARCs will upregulate CRT signal, thereby synergizing with CD47 blocking therapies such as magrolimab to enhance antitumor activity. Here we explore the benefits of an ARC and magrolimab combination therapy in preclinical human solid tumor models.

Methods: Anti-HER2 (Trastuzumab) monoclonal antibody was radiolabeled with ²²⁵Ac. The biological activity of the ²²⁵Ac-anti-HER2 was evaluated using human recombinant protein and receptor positive tumor cell lines. ²²⁵Ac-anti-HER2 mediated CRT upregulation and cytotoxicity was evaluated in a panel of solid tumor cell lines using flow cytometry and MTS assay, respectively. The ability of the ²²⁵Ac-anti-HER2 and anti-CD47 antibody (magrolimab) combination to enhance macrophage phagocytosis in vitro was monitored by flow cytometry. The efficacy of a combinatorial treatment with ²²⁵Ac-anti-HER2 and anti-CD47 antibodies was assessed in preclinical models of human ovarian (SKOV3) and breast (BT474) cancers.

Results: ²²⁵Ac-anti-HER2 has similar target binding properties to that of the native antibody and demonstrates specific cytotoxicity to receptor positive cells. ²²⁵Ac-anti-HER2 treatment led to the upregulation of CRT. Likewise, phagocytosis was significantly enhanced by ²²⁵Ac-anti-HER2 and CD47 combination compared to single agent alone. In vivo efficacy studies of the ²²⁵Ac-anti-HER2 and anti-CD47 antibody combination showed significant enhanced antitumor activity with reduced toxicity and improved survival benefit in human solid tumor xenograft models.

Conclusions: Our findings suggest a mechanism by which ARC-mediated CRT upregulation potentiates a pro-phagocytic innate immune response when combined with CD47-SIRPα axis inhibition by blocking agents such as magrolimab, thereby enhancing antitumor activity. This approach could be a promising therapeutic strategy to increase antitumor immunity thereby improving cancer patient outcomes.