Abstract A14: Ex vivo immune activation with the macrophage-targeting immunotherapy, anti-Clever-1 antibody bexmarilimab, in acute myeloid leukemia and myelodysplastic syndrome

Abstract The potential of immunotherapies in acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) remains still under investigation. Clever-1 (also known as Stabilin-1) is a multifunctional scavenger and adhesion receptor expressed by monocytes and immunosuppressive macrophages. Bexmarilimab, a Clever-1 targeting antibody, demonstrates many immunomodulatory effects1 along with promising anti-tumor activity against solid tumors in patients with multiple lines of previous treatment2. In AML, high STAB1 (mRNA) levels associate with poor survival and resistance to therapy3. The aim of this study was to profile Clever-1 expression in AML and MDS and test in preclinical models the growth inhibitory and immunomodulatory potential of bexmarilimab, as a single agent and in combination with azaciditine and venetoclax. AML cell lines (n=11) and frozen mononuclear cells, extracted from AML (n=42) and very high risk MDS (n=4) patient bone marrow (BM) aspirates provided by the Finnish Hematology Registry and Biobank, were used. Samples were treated for 48h with bexmarilimab alone, or in combination with azacytidine and/or venetoclax. Flow cytometry was used to detect different myeloid and TBNK cell populations along with Clever-1, HLA-DR, PD-(L)1 and additional T cell activation markers. Our results confirmed Clever-1 protein expression in AML cell lines and bexmarilimab treatment induced metabolic and growth inhibition of KG1 blasts (Clever-1high). In patients with AML, FAB M4/M5 subtypes exhibited highest Clever-1 levels, along with FAB M2 patients with consequent rapid relapse. Clever-1 expression correlated negatively with monocyte MHC class II molecule, HLA-DR, expression, and BM T-cell frequency, in line with the immunosuppressed state associated with high Clever-1. Ex vivo treatment of the primary AML BM cells with bexmarilimab resulted in a notable, 5-10x fold increase in monocyte HLA-DR in samples with low basal HLA-DR and high Clever-1. The combination of azacitidine with bexmarilimab augmented HLA-DR induction by 20-110% (mean 44%). FAB M1/M2 AML showed increase in activation markers, such as Ki67, CXCR3 and Granzyme B after ex vivo bexmarilimab treatment in CD8+ T-cells. Furthermore, bexmarilimab reduced PD-1 expression in NK- and CD8+CXCR3+ T-cell populations of FAB M0-M2 AML and MDS-EB2. This project is the first comprehensive investigation of Clever-1 expression in AML and MDS patient BM blasts and monocytes. Ex vivo treatment with bexmarilimab, alone or in combination with azacytidine or venetoclax, indicate enhanced antigen presentation capability and immunological activation. These results validate the therapeutic potential of bexmarilimab in myeloid malignancies. The safety, tolerability and preliminary efficacy of bexmarilimab is now further investigated in combination with venetoclax and/or azacytidine in a phase I/II clinical trial BEXMAB (NCT05428969).1Viitala et al., Clin Cancer Res 2019;25:3289-303; 2 Bono et al., Annals of Oncology; 2021. p 32, 5: S1283-S346; 3 Lin et al., Mol Ther Nucleic Acids 2019;18:476-84 Citation Format: Arno Ylitalo, Sofia Aakko, Heikki Kuusanmäki, Mari Björkman, Juho Jalkanen, Marie-Louise Fjällskog, Caroline Heckman, Maija Hollmén, Mika Kontro. Ex vivo immune activation with the macrophage-targeting immunotherapy, anti-Clever-1 antibody bexmarilimab, in acute myeloid leukemia and myelodysplastic syndrome [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A14.