Abstract B052: PHST001, a humanized anti-CD24 antibody, induces phagocytosis of ovarian tumor cells <i>in vitro</i> and inhibits their growth <i>in vivo</i>

Abstract Background: CD24 is a small and highly glycosylated tumor antigen with a restricted expression profile that acts as an important macrophage “don’t eat me” signal. CD24 is highly expressed by many human cancers, including ovarian, and it interacts with the macrophage receptor Siglec10 to protect cancer cells from being engulfed by tumor-associated macrophages. The purpose of this project is to assess efficacy of PHST001, a humanized anti-CD24 monoclonal antibody, against ovarian cancer (OC) in vitro and in vivo. Methods: In vitro phagocytosis: SKOV3 (PIK3CA mutated), OVCAR3 (TP53 mutated) and primary ovarian tumor cells were co-cultured with macrophages for 2 hours. Tumor target cells were engineered to express GFP. Phagocytosis was measured by flow cytometry as the number of CD11b+/GFP+ macrophages, quantified as a percentage of total CD11b+ macrophages. In vivo efficacy: NSG immunocompromised mice were engrafted SC with luciferase-expressing OVCAR3 cells or with a patient-derived xenograft (PDX) model of OC. Successfully engrafted mice were randomized between 4 treatment groups: isotype control, PHST001, cisplatin or carboplatin plus paclitaxel, or combination treatment. Tumor volume was assessed once a week by bioluminescence or caliper. Results: CD24 gene is highly amplified in ovarian tumors and CD24 copy number gain is more prevalent in higher grade tumors (∼30% in grades 2-4) compared with lower grade tumors (0% in grade 1). CD24 mRNA is highly expressed in OC samples and is upregulated greater than 500-fold compared to normal ovarian samples. PHST001 binds CD24 on the surface of OC cell lines and EpCam+ tumor cells from primary ovarian tumors with Kd ranging from 31nM to 117nM. PHST001 binds greater than 95% of the EpCam+ tumor cells derived from primary mucinous, neuroendocrine, and serous subtypes of ovarian carcinomas. PHST001 induces phagocytosis of OC cell lines by suppressive M2 macrophages in vitro with an average of 4-fold increase in phagocytosis of SKOV3 and OVCAR3 compared to the IgG4 isotype control. PHST001 promotes macrophage-mediated phagocytosis of primary human ovarian high grade serous carcinoma (average of 6-fold increase in phagocytosis compared to isotype control) and primary human ovarian tumor cells derived from ascites fluid (average of 7-fold increase in phagocytosis compared to IgG4 isotype control) in vitro. PHST001 dosed at 20mpk 3 times per week for a total of 18 doses significantly inhibited the growth of OVCAR3 and SKOV3 OC cell lines and significantly improved survival in vivo. PHST001 dosed at 20mpk 3 times per week for a total of 18 doses enhances chemotherapy efficacy, significantly inhibiting tumor growth and enhancing survival in the ovarian PDX model in vivo. Conclusions: PHST001, a humanized anti-CD24 monoclonal antibody, binds to CD24 on the surface of OC cells from multiple subtypes, induces phagocytosis of cancer cells by macrophages in vitro, and is significantly efficacious against OC models in vivo. A clinical study to investigate the safety and efficacy of PHST001 in cancer patients is ongoing. Citation Format: Suzana A. Kahn, Rachel Brewer, Grace Blacker, Joseane Sampaio, Giovanni Forcina, Alexandria Beans, Seth Ludwig, Jen Cao, John Burg, Amira Barkal, Raji Majeti, Irving Weissman, Roy Maute. PHST001, a humanized anti-CD24 antibody, induces phagocytosis of ovarian tumor cells in vitro and inhibits their growth in vivo [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl):Abstract nr B052.