Abstract 609: Anti-HER3 radioimmunotherapy enhances the anti-tumor effects of CD47 blockade in solid tumors
Authors: Beckford-Vera Denis , Li Jason , Jennings Caroline , McCloskey Megan , Chin Amanda , Liang Qing , Hwang Jesse , Roy Monideepa , Chen Mary , Kotanides Helen 
Published: June 2022

Abstract
Background: Cancer immunotherapy strategies targeting blockade of the CD47-SIRPα immunosuppressive signal have made significant progress in recent years. However, monotherapies have not shown meaningful clinical responses in solid tumors. Therefore, therapeutic combinations are being explored to improve patient outcomes. CD47 is a macrophage checkpoint inhibitor that acts as a “don’t eat me” signal on cancer cells to evade innate immune detection and destruction. Targeted radiation to cancer cells will upregulate calreticulin (CRT), a pro-phagocytic “eat me” signal. We therefore hypothesize that we can enhance the efficacy of anti-CD47 antibodies by combining them with appropriate targeted antibody radioconjugates (ARC). In this experiment we chose to study an anti-HER3 radioconjugate, as HER3 is overexpressed in a variety of cancers including breast, ovarian, lung, gastric and prostate and is associated with poor clinical prognosis. Additionally, upregulation of HER3 is implicated in the acquired resistance against HER1 or HER2 targeted therapies. Here, we demonstrate enhanced therapeutic efficacy of a novel Actinium-225 (225Ac) armed HER3 specific targeting ARC (225Ac-HER3-ARC) and a CD47 blocking antibody (anti-CD47) combination in preclinical solid tumor models.
Methods: The anti-HER3 antibody (AT-02) was radiolabeled with 225Ac. 225Ac-HER3-ARC biological activity was evaluated using human recombinant HER3 and receptor positive tumor cell lines. 225Ac-HER3-ARC mediated CRT upregulation and cytotoxicity was evaluated using flow cytometry and MTS assay, respectively. The benefits of the 225Ac-HER3-ARC and anti-CD47 combination to enhance macrophage phagocytosis was evaluated by flow cytometry. We further evaluated the therapeutic benefits of the 225Ac-HER3-ARC and CD47 combination in human HER3+ tumor xenograft mouse model.
Results: The 225Ac-HER3-ARC retains similar binding properties to native antibody and demonstrates specific cytotoxicity on tumor cells. CRT was upregulated by 225Ac-HER3-ARC in HER3+ cells. Furthermore, the combination of 225Ac-HER3-ARC and anti-CD47 enhances in vitro macrophage mediated tumor cell phagocytosis compared to each agent alone. Importantly, the in vivo 225Ac-HER3-ARC and CD47 antibody combination shows enhanced antitumor effect with reduced toxicity and improved survival benefit in a human preclinical solid tumor model compared to anti-CD47 agent alone.
Conclusions: We demonstrate enhanced efficacy of the 225Ac-HER3-ARC and CD47 blocking antibody combination in vitro and in a preclinical solid tumor animal model. This approach is an encouraging strategy to potentially improve antitumor responses in patients with HER3+ tumors. Consequently, the findings obtained in this study along with the need to develop better therapies for patients with HER3+ tumors support the further preclinical development of HER3-ARC.
Citation Format: Denis Beckford-Vera, Jason Li, Caroline Jennings, Megan McCloskey, Amanda Chin, Qing Liang, Jesse Hwang, Monideepa Roy, Mary Chen, Helen Kotanides. Anti-HER3 radioimmunotherapy enhances the anti-tumor effects of CD47 blockade in solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 609.