Abstract 2860: A CD135 immunocytokine with target cell-restricted IL-15 activity for treatment of AML

Abstract After diagnosis, a primary goal of treatment in acute myeloid leukemia (AML) is to achieve a morphological complete remission (CR) by induction chemotherapy. However, about half of the patients in CR show remaining minimal residual disease (MRD), which constitutes the basis for relapse. With the aim to eradicate MRD and thus improve long-term survival in AML, we recently conducted a phase I trial evaluating safety/tolerability and preliminary efficacy of an Fc-optimized antibody targeting CD135/FLT3 (FLYSYN, NCT02789254) to induce NK cell anti-leukemia reactivity. This trial enrolling AML patients in CR with detectable MRD revealed that FLYSYN is safe and well tolerated, with about 45% of patients achieving a molecular response after treatment with the target dose (Heitmann et al., Blood 2020). Besides by reinforcing the ADCC-inducing capability of antibodies, NK cell immunity can be further increased by cytokines like IL-15, and multiple efforts presently aim to exploit the latter cytokine for cancer treatment. However, clinical efficacy of IL-15 so far is limited as systemic application causes substantial side effects due to unspecific immune activation (Conlon et al, JCO 2015). To overcome this limitation, we aimed to develop an immunocytokine consisting of our Fc-optimized CD135 mAb fused to IL-15. However, the activity of the cytokine moiety within classical ICs does not depend on antigen binding, and thus application of clinically effective doses is still prevented by toxicity due to unspecific immune activation. To overcome this problem, we took advantage of the unique mechanism of action of IL-15, which stimulates IL-15Rβ/γ on cytotoxic lymphocytes as membrane-bound complex with IL-15Rα on monocytes and DCs (trans-presentation) and generated an IL-15 mutant with abolished IL-15Rα binding. The latter allows to substitute physiological trans-presentation by binding of the construct to the target antigen, thereby limiting side effects. Functional analysis using primary AML cells as targets revealed that our modified immunocytokine (MIC135) induced target-restricted NK cell anti-leukemia reactivity in a profoundly greater extent than the Fc-optimized FLYSYN antibody. Notably, in stark contrast to FLYSYN, MIC135 induced prominent NK cell proliferation, and target cell killing upon treatment with MIC135 was likewise clearly superior. Analyses regarding off-target toxicity confirmed the target-antigen restricted efficacy of MIC135 compared to anti-CD135 immunocytokine with wildtype IL-15 (IC135). Furthermore, our MIC135 did not induce unwanted effects against healthy FLT3 expressing cells. Taken together, MIC135 induces NK cell reactivity against leukemia cells in a highly target cell-restricted manner and displays higher efficacy than Fc-optimized antibodies, thus constituting a promising treatment option for AML. Citation Format: Martina S. Lutz, Bastian J. Schmied, Fabian Riegg, Latifa Zekri, Jonas S. Heitmann, Melanie Maerklin, Martin Pfluegler, Gundram Jung, Helmut R. Salih. A CD135 immunocytokine with target cell-restricted IL-15 activity for treatment of AML [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 2860.