Abstract 3467: Delivery of CpG and GM-CSF in a novel silica-based scaffold leads to differentiation of AML blasts and T cell-dependent immunity in syngeneic AML tumor models

Abstract Background: Acute Myeloid Leukemia (AML) has poor prognosis, however, differentiation therapy results in a high cure rate for a subset of patients with acute promyelocytic leukemia. Prior preclinical studies using a cryogel-based “antigen free” system demonstrated complete remission in AML models and a vaccine-like response. We set out to recapitulate these effects using a silica-based, biodegradable scaffold (mesoporous silica rod, MSRs) in order to capitalize on their ease of manufacturing and injectability. Building on this prior work, here we show that MSRs loaded with GM-CSF and CpG (ATT-01) can similarly induce differentiation of AML blasts in vitro, a significant vaccine-like T cell response, and durable remission in murine models of AML. Methods: Syngeneic mouse models [C1498, C1498-eGFP, and WEHI-3] were conducted by injecting tumor cells into the mouse tail vein. Tumors were treated with chemotherapy (ara-C, doxorubicin) on days 7-12 and then mice were treated with a subcutaneous injection of ATT-01 on 2 flanks on day 14. For takedown studies, flow cytometry was conducted on mouse organs on day 7 and 14 post-ATT-01 treatment. In a cellular depletion experiment, mice were treated with antibodies to decrease CD8+T-cells, CD4+T-cells and NK cells prior to running the C1498 therapeutic model. In vitro experiments were conducted by co-culturing MSRs with THP-1 cells and tumor cells, and APC functionality of THP1 cells (no PMA) were assayed using flow cytometric phenotyping. Results: Chemotherapy plus ATT-01 treatment resulted in complete remissions [CRs] in C1498 and WEHI-3 models, compared to chemotherapy or GM-CSF and CpG injection alone, which resulted in no CRs. The cellular depletion study demonstrated that CD8+ T cells were required for the therapeutic effect. The takedown study demonstrated that ATT-01 therapy resulted in increased CD8+ T cells in the bone marrow at day 7, compared to controls. In vitro, MSR and ATT-01 caused NETosis in THP-1 cells. In vitro studies demonstrated that Murine AML cell lines cultured with ATT-01 and MSR resulted in tumor cell death for a portion of cells. Surprisingly, a subset of murine AML cell lines expressed immune cell markers after culture with ATT-01 or MSR. In addition, THP-1 cells (human leukemic cell line, no PMA) cultured with MSR and ATT-01 differentiated into immune phenotypes. Conclusions: Treatment of murine AML with chemotherapy and ATT-01 resulted in durable remissions. We propose that generating peripheral inflammation in AML can lead to differentiation of AML blasts into APCs and presentation of AML antigens, leading to immune-mediated tumor control. Future experiments will further elucidate the differentiation effect of ATT-01 in vivo. Citation Format: Fernanda Langellotto, Jessica McDonough, Benjamin Seiler, Brena Barra, Edward Doherty, Robert Pierce. Delivery of CpG and GM-CSF in a novel silica-based scaffold leads to differentiation of AML blasts and T cell-dependent immunity in syngeneic AML tumor models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3467.