The CD33xCD123xCD70 Multispecific CD3-Engaging DARPin MP0533 Induces Selective T Cell-Mediated Killing of AML Leukemic Stem Cells

The prognosis of patients with acute myeloid leukemia (AML) is limited, especially for elderly or unfit patients not eligible for hematopoietic stem cell (HSC) transplantation. The disease is driven by leukemic stem cells (LSCs), which are characterized by clonal heterogeneity and resistance to conventional therapy. These cells are therefore believed to be a major cause of progression and relapse. We designed MP0533, a multispecific CD3-engaging DARPin (designed ankyrin repeat protein) that can simultaneously bind to three antigens on AML cells (CD33, CD123, and CD70), aiming to enable avidity-driven T cell-mediated killing of AML cells co-expressing at least two of the antigens. In vitro, MP0533 induced selective T cell-mediated killing of AML cell lines, as well as patient-derived AML blasts and LSCs, expressing two or more target antigens, while sparing healthy HSCs, blood, and endothelial cells. The higher selectivity also resulted in markedly lower levels of cytokine release in normal human blood compared to single antigen-targeting T-cell engagers. In xenograft AML mouse models, MP0533 induced tumor-localized T-cell activation and cytokine release, leading to complete eradication of the tumors while having no systemic adverse effects. These studies show that the multispecific-targeting strategy used with MP0533 holds promise for improved selectivity towards LSCs and efficacy against clonal heterogeneity, potentially bringing a new therapeutic option to this group of patients with high unmet need. MP0533 is currently being evaluated in a dose-escalation phase 1 study in patients with relapsed or refractory AML (NCT05673057).

Abstract 525: Novel DARPin multi-specific T-cell engager with an improved therapeutic window to overcome dose limiting toxicities in AML therapies

Purpose: The medical need due to high mortality in acute myeloid leukemia (AML) remains high, and the treatment of relapsed or refractory AML continues to be therapeutically challenging. MYLOTARG, the only approved anti-CD33 antibody drug conjugate (ADC), has provided proof-of-concept for targeted immunotherapies in AML. Currently, a plethora of ADCs and T-cell engager (TCE) therapies have entered clinical development in AML, but those therapies are often accompanied by dose limiting toxicities, preventing dose escalation to desired anti-tumor efficacy. The biggest challenges seem to be limited target specificity and hyperstimulation of the immune system leading to e.g. myelotoxicities and cytokine release syndrome, respectively. Therefore, more selective therapies are needed to allow for robust anti-tumor activity with a more acceptable safety profile.

Experimental design: To address the selectivity challenge, we have generated multi-specific T-cell engaging DARPin® molecules, targeting two different tumor associated antigens (TAAs) with optimized affinity for their targets. In order to find the right target combination, the optimal affinity to increase tumor specificity via avidity, as well as the best molecular architecture, we took advantage of our unique modular DARPin® platform and screened 1000s of combinations of multi-specific DARPin® molecules, binding simultaneously to multiple TAAs in conjunction with our CD3-binding DARPin® molecule.

Results: We constructed multi-specific TCEs targeting two different AML antigens with optimized affinity leading to a substantial avidity gain when both targets are co-expressed on tumor cells. The avidity gain resulted in strongly enhanced in vitro potency as shown by activation of both CD8+ and CD4+ T cells and subsequent killing of AML tumor cells, with bioactivities in the range of established TCE benchmark formats (e.g. BiTE® and DART®). In contrast, in an ex vivo whole blood assay the multi-specific DARPin® constructs induced profoundly less cytokine release as compared to benchmark molecules indicating an improved therapeutic window. Finally, we also demonstrated tumor regression in PMBC humanized mouse models bearing MOLM-13 tumors, using both half-life extended (HLE) and non-HLE lead constructs.

In conclusion, we have generated TCEs based on multi-specific DARPin® constructs with high potency, selectivity and ultimately with the potential for an improved therapeutic window for the treatment of AML.

Citation Format: Nina Reschke, Thamar Looser, Jennifer Krieg, Matteo Bianchi, Patricia Schildknecht, Nicole Bassler, Yvonne Gruebler, Sebastian Grimm, Laura Jeanbart, Tanja Hospodarsch, Alexandra Neculcea, Daniel Steiner, Bernd Schlereth, Christian Reichen. Novel DARPin multi-specific T-cell engager with an improved therapeutic window to overcome dose limiting toxicities in AML therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 525.