Myeloid cell-selective, oligonucleotide-based STAT3 inhibition combined with total marrow and lymphoid irradiation for immunotherapy of acute myeloid leukemia

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PROJECT SUMMARY There has been little or no long-term improvement in outcomes for patients with treatment-refractory acute myeloid leukemia (AML). The recently developed image-guided radiation strategy, total marrow and lymphoid irradiation (TMLI) delivers high radiation doses to major leukemia reservoirs while sparing normal tissues. Although TMLI prior to allogeneic hematopoietic cell transplantation (HCT) improved patients’ 2-year overall survival (OS) rate from <10% to 48%, relapses remained common. Such radiation resistance is a consequence of both intrinsic cancer cell properties and the extrinsic influence of the tumor microenvironment. It was previously demonstrated that radiation-induced cell death causes the release of danger signals recruiting Toll-like Receptor- 9 (TLR9)-positive myeloid cells, which jump-start tumor vascularization and regrowth. These cancer-promoting, rather than immunostimulatory, effects are mediated by TLR9-mediated secretion of cytokines such as IL-6, thereby leading to activation of Signal Transducer and Activator of Transcription 3 (STAT3). STAT3 is a multifaceted oncogene and a central immune checkpoint regulator operating in AML cells as well as in tumor- associated myeloid cells in patients. However, it remains an elusive target, with no FDA-approved direct small molecule STAT3 inhibitors. To overcome this challenge, we previously developed a strategy to deliver oligonucleotide STAT3 inhibitors, such as siRNA or decoy DNA, specifically into commonly TLR9-positive AML and normal myeloid cells. The nuclease-resistant, second-generation CpG-STAT3 decoy inhibitor (CSI-2) injected intravenously showed efficacy in targeting STAT3, suppressing leukemia cell survival and/or inducing immune responses against moderate burden of human and mouse AML in vivo. The hypothesis is that combining the immunostimulatory CSI-2 strategy with conditioning TMLI treatments will improve treatment efficacy against AML even at high burden (>50% leukemic blasts in the bone marrow) by providing time for the generation of adaptive T-cell driven immune responses. The preliminary results demonstrated that the TMLI regimen can improve uptake of CSI-2 by AML, thereby reducing leukemia-initiating potential, augmenting AML immunogenicity, and thereby inducing potent CD8+ T cell-mediated antileukemic immune responses. We propose to: 1. elucidate the molecular mechanisms of TMLI/CSI-2 effect on AML cell differentiation; 2. optimize TMLI to maximize the effect on leukemic bone marrow vascular structure, CSI-2 delivery, leukemogenic potential, and immunogenicity; 3. assess the efficacy and cellular mode-of-action of the TMLI/CSI-2 combination treatment compared to either treatment alone in human or mouse AML models in humanized or syngeneic mice, respectively. The overarching goal of this interdisciplinary proposal is to produce a clinically relevant, effective, and safe combinatorial radiation-immunotherapy for patients with relapsed/recurrent AML, representing the highest unmet need in cancer therapy.