Novel CD123xCD3 Bispecific Igm Antibody, Igm-2537, Potently Induces T-Cell Mediated Cytotoxicity of Acute Myeloid Leukemia Cells In Vivo and in Vitro with Minimal Cytokine Release

Introduction: New therapeutics are needed for the effective treatment of acute myeloid leukemia (AML), as standard chemotherapeutics are poorly tolerated and ~50% of patients relapse primarily due to the incomplete elimination of leukemia stem cells (LSCs). CD123, the IL-3Rα chain, is highly expressed on leukemic blasts and LSCs, and is further increased in patients with poor prognostic factors. Bispecific T-cell engager (TCE) antibodies and CAR-T cells targeting CD123 have shown promising clinical efficacy in AML patients, but cytokine release syndrome limits their therapeutic window and remains a major safety concern. IGM-2537 is a novel pentameric IgM bispecific TCE antibody engineered with ten anti-CD123 binding sites, and an anti-CD3ε single chain Fv domain fused to the joining chain to engage T-cells. Here, we report that IGM-2537 not only potently induces T-cell mediated cytolytic killing of AML cells in vitro and in vivo but also demonstrates minimal cytokine release, and little to no effect on normal hematopoietic progenitor cells and human vascular endothelial cells (HUVECs) with in vitro and ex vivo studies. Additionally, an improved tolerability, excellent pharmacodynamic (PD) response and lower cytokine release was observed with a cynomolgus cross-reactive IgM TCE in monkeys.

Materials and Methods: Surface plasma resonance (SPR), cell membrane protein array, and alanine scanning were utilized to characterize human CD123 binding affinity, specificity and epitope mapping, respectively. T-cell dependent cellular cytotoxicity (TDCC) assays were employed to evaluate the potency of IGM-2537 target cytotoxicity, T cell activation and cytokine release. Ex vivo AML colony formation assays were used to assess the activity on AML blasts. Human MV-4-11 and MOLM-13 xenograft tumor models in humanized NSG^dKO mice were utilized to determine in vivo anti-tumor activity of IGM-2537. Healthy human PBMCs and bone marrow (BM) samples as well as HUVECs were used in ex vivo assays to address potential safety concerns of IGM-2537 mediated effects on hematopoietic progenitor cells and HUVECs, respectively. Lastly, to confirm that a CD123 directed IgM-based TCE molecule can potentially provide better safety profile in vivo, a cynomolgus-cross reactive IgM TCE comparator was assessed in monkeys for tolerability and PK/PD responses.

Results and Conclusions: IGM-2537 bound with high affinity and avidity to CD123. In vitro, IGM-2537 engaged both CD123 and CD3 to induce potent T-cell activation and T-cell mediated cytotoxicity of CD123⁺ AML cells, and autologous CD123⁺ basophils and plasmacytoid dendritic cells (pDCs). Though IGM-2537 demonstrated comparable maximal killing activity to a comparator IgG TCE, IGM-2537 demonstrated minimal cytokine release. In ex vivo patient-derived AML or normal BM colony formation assays, IGM-2537 eliminated AML colony forming cells at physiologically relevant effector/target (E/T) ratios but had little or no effect on normal hematopoietic progenitors. In addition, IGM-2537 demonstrated no cytotoxicity against HUVECs regardless of their CD123 expression at basal levels or at maximal levels upregulated by TNF-a and IFN-g, in contrast to a comparator IgG TCE. In vivo, IGM-2537 completely inhibited tumor growth in two AML xenograft tumor models, a subcutaneous MV-4-11 model, and a systemic MOLM-13 model in humanized NSG^dKO mice. To further evaluate the potency and safety of the CD123xCD3 IgM bispecific TCE format in vivo, a cynomolgus cross-reactive CD123xCD3 IgM bispecific TCE was evaluated for tolerability and PD responses in cynomolgus monkeys using single doses. All animals tolerated the cross-reactive bispecific IgM TCE well at doses up to 10 mg/kg (the maximal dose level evaluated), a dose 100-fold greater than published doses of a comparator IgG TCE. Complete depletion of CD123⁺ basophils and substantial reductions of pDCs were seen in blood and BM with minimal to no cytokine induction.

In summary, IGM-2537 demonstrated potent in vitro and in vivo T-cell mediated cytotoxicity of AML cell lines with minimal cytokine induction. The encouraging preclinical safety profiles were further supported by little/no effect on normal hematopoietic progenitor cells and HUVECs. These preclinical data support the clinical development of IGM-2537 for the treatment of AML and other hematological malignancies.