Abstract 508: Combined MEK and PD-1 inhibition in acute myeloid leukemia: Effect on cell proliferation and phenotype
Authors: Moshofsky Kaycee B. , Cho Hyun J. , Kosaka Yoko , Newman Matthew T. , Tyner Jeffrey W. , Lind Evan F. 
Published: August 2019

Abstract
Although most patients with acute myeloid leukemia (AML) respond to standard of care chemotherapy, the majority of patients will relapse, and the 5-year overall survival rate persists at 25%. The high rates of relapse and poor survival have contributed to increased interest in the use of other therapies, including immunotherapies, in hematological malignancies in order to achieve more durable responses. In particular, immune checkpoint blockade, such as anti-PD-1, has elicited efficacious responses in Hodgkin’s lymphoma. In addition, trametinib, a small molecule MEK inhibitor, was shown to have activity in AML patients with a RAS-mutation in a phase 1/2 trial, but demonstrated the need for rational combinational therapies to produce longer lasting responses. Interestingly, in a murine model of colon carcinoma harboring a KRAS mutation, the combination of these two agents resulted in a more durable tumor response compared to either agent alone. We hypothesized that in AML, combined MEK and PD-1 inhibition would result in direct inhibition of blast proliferation while simultaneously promoting proliferation and activation of T cells with a mature/memory phenotype.
Bone marrow and peripheral blood samples isolated from patients with AML were cultured in our laboratory for 5 days with CD3 stimulation to induce T-cell activation. Cultures were treated with trametinib, anti-PD-1, or both. Additional samples were cultured with interferon-gamma (IFN-γ) to induce PD-L1 expression. Mouse spleen cells from a genetically engineered mouse model of AML (FLT3-ITD/TET2+/-) were cultured under the same conditions for 3 days. Samples were evaluated by flow cytometry to measure treatment effects on T-cell and blast proliferation and phenotype.
In the patient samples, trametinib consistently reduced PD-L1 expression in a dose-dependent manner on the myeloid cell population and was further able to attenuate PD-L1 expression induced by IFN-γ. A mean decrease of 36.6% was seen in PD-L1 expression after treatment with trametinib at 100 nm (p=0.0177). In most samples, trametinib decreased blast proliferation and increased blast cell death. Additionally, in a NRAS-mutant sample with high PD-L1 expression, the combination therapy best restored T cell proliferation and decreased the naive T cell population resulting in increased effector and memory T cells. We observed similar changes in our mouse model of AML.
In conclusion, IFN-γ increased PD-L1 expression on cancer cells suggesting a possible mechanism of immunosuppression inhibiting autologous immune responses, and the increased PD-L1 expression could be at least partially reversed by trametinib. Our study suggests that combined MEK and PD-1 inhibition may result in greater therapeutic efficacy over each agent alone and provide a rationale for administering this combination to patients with AML.
Citation Format: Kaycee B. Moshofsky, Hyun J. Cho, Yoko Kosaka, Matthew T. Newman, Jeffrey W. Tyner, Evan F. Lind. Combined MEK and PD-1 inhibition in acute myeloid leukemia: Effect on cell proliferation and phenotype [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 508.