Purpose PD1/PD-L1 signaling promotes tumor growth while inhibiting effector cell-mediated anti-tumor immune responses. Here we assessed the impact of single and dual blockade of PD1/PD-L1, alone or in combination with lenalidomide, on accessory and immune cell function as well as multiple myeloma (MM) cell growth in the BM milieu. Experimental Design Surface expression of PD1 on immune effector cells, and PD-L1 expression on CD138+MM cells and myeloid derived suppressor cells (MDSC) were determined on tumor cells from newly diagnosed (ND)-MM and relapsed/refractory (RR)-MM BM versus healthy donor (HD). We defined the impact of single and dual blockade of PD1/PD-L1, alone and with lenalidomide, on autologous anti-MM immune response and tumor cell growth. Results Both ND and RR patient MM cells have increased PD-L1 mRNA and surface expression compared to HD. There is also a significant increase in PD1 expression on effector cells in MM. Importantly, PD1/PD-L1-blockade abrogates BM-stroma cell (BMSC)-induced MM growth, and combined blockade of PD1/PD-L1 with lenalidomide further inhibits BMSC-induced tumor growth. These effects are associated with induction of intracellular expression of IFNγ and Granzyme-B in effector cells. Importantly, PD-L1 expression in MM is higher on MDSC than on antigen presenting cells, and PD1/PD-L1-blockade inhibits MDSC-mediated MM growth. Finally, lenalidomide with PD1/PD-L1-blockade inhibits MDSC-mediated immune suppression. Conclusion Our data therefore demonstrates that checkpoint signaling plays an important role in providing the tumor-promoting, immune-suppressive microenvironment in MM, and that PD1/PD-L1-blockade induces anti-MM immune response that can be enhanced by lenalidomide, providing the framework for clinical evaluation of combination therapy.

Direct targeting of RAS, which is frequently mutated, has proven to be challenging, and inhibition of individual downstream RAS mediators has resulted in limited clinical efficacy. We designed a chemical screen to identify compounds capable of potentiating mTOR inhibition in mutant RAS-positive leukemia, and identified a Wee1 inhibitor. Synergy was observed in both mutant NRAS- and mutant KRAS-positive acute myelogenous leukemia (AML) cell lines and primary patient samples. The observed synergy enhanced dephosphorylation of AKT, 4E-BP1 and S6K, and correlated with increased apoptosis. The specificity of Wee1 as the target of MK-1775 was validated by Wee1 knockdown (KD), as well as partial reversal of drug combination-induced apoptosis by a CDK1 inhibitor. Importantly, we also extended our findings to other mutant RAS-expressing malignancies, including mutant NRAS-positive melanoma, and mutant KRAS-positive colorectal cancer, pancreatic cancer, and lung cancer. We observed favorable responses with combined Wee1/mTOR inhibition in human cancer cell lines from multiple malignancies, and inhibition of tumor growth in in vivo models of mutant KRAS lung cancer and leukemia. The present study introduces for the first time Wee1 inhibition combined with mTOR inhibition as a novel therapeutic strategy to the selective treatment of mutant RAS-positive leukemia and other mutant RAS-expressing malignancies.

The tumor suppressor, p53, is a key regulator of apoptosis and functions upstream in the apoptotic cascade by both indirectly and directly regulating Bcl-2 family proteins. In cells expressing wild-type (wt) p53, the HDM2 protein binds to p53 and blocks its activity. Inhibition of HDM2:p53 interaction activates p53 and causes apoptosis or cell cycle arrest. Here, we investigated the ability of the novel HDM2 inhibitor, CGM097, to potently and selectively kill wt p53-expressing AML cells. The anti-leukemic effects of CGM097 were studied using cell-based proliferation assays (human AML cell lines, primary AML patient cells and normal bone marrow samples), apoptosis and cell cycle assays, ELISA, immunoblotting, and an AML patient-derived in vivo mouse model. CGM097 potently and selectively inhibited the proliferation of human AML cell lines and the majority of primary AML cells expressing wt p53, but not mutant p53, in a target-specific manner. Several patient samples that harbored mutant p53 were comparatively unresponsive to CGM097. Synergy was observed when CGM097 was combined with FLT3 inhibition against oncogenic FLT3-expressing cells cultured both in the absence as well as the presence of cytoprotective stromal-secreted cytokines, as well as when combined with MEK inhibition in cells with activated MAPK signaling. Finally, CGM097 was effective in reducing leukemia burden in vivo. These data suggest that CGM097 is a promising treatment for AML characterized as harboring wt p53 as a single agent, as well as in combination with other therapies targeting oncogene-activated pathways that drive AML.