ALBERT

Description: The transcription factor CCAAT/enhancer binding protein β (C/EBPβ, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1β. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBPβ. One factor was shown to be tumor necrosis factor- (TNF-), which increased C/EBPβ expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBPβ localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBPβ correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBPβ to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBPβ. These observations suggest that diffuse distribution of C/EBPβ in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor.

Description: Key Points GM-CSF–dependent STAT5 hypersensitivity is detected in 90% of CMML samples and is enhanced by signaling mutations. Treatment with a GM-CSF–neutralizing antibody and JAK2 inhibitors reveals therapeutic potential.

Description: Abstract 2897 EphA3 is a receptor tyrosine kinase expressed at high level on the surface of some solid tumors and certain leukemias. KB004 is a high-affinity non-fucosylated, recombinant antibody developed by KaloBios derived from a mouse monoclonal anti-EphA3 antibody using Antibody Humaneering™ Technology. The antibody binds EphA3 from human and cynomolgus macaque with equivalent affinity (610 pM for human and 570 pM for cynomolgus as determined by surface plasmon resonance (spr) analysis) making the latter a relevant toxicology species. KB004 has a direct pro-apoptotic effect on EphA3-positive cells and a induces potent antibody-dependent cellular cytotoxicity (ADCC) activity mediated by CD16-expressing effector cells. The non-fucosylated antibody has enhanced affinity for both human and cynomolgus CD16a. The affinity of KB004 for cynomolgus CD16a (7.2 nM, determined by spr analysis) was comparable to the affinity for the high-affinity variant of CD16a from humans (6.6 nM), indicating that KB004 has similar potential for ADCC in cynomolgus macaques. We first demonstrated cell-surface expression of EphA3 by flow cytometry in approximately 50% of primary human leukemia cells. Here we extend the analysis to multiple myeloma (MM) in which 3/5 patient samples show EphA3 expression. EphA3 expression was confirmed in a panel of chronic myeloid leukemia (CML) samples [n=14] and in acute myeloid leukemia (AML) [n=40] in which EphA3+ cells were identified in each French American British (FAB) classification sub-type except M0, for which only two samples were studied. CD123-positive leukemia stem cells (CD34+ CD38− CD123+) also displayed surface expression of EphA3. KB004 induced apoptosis in EphA3+ primary cells from each of the diseases analyzed, including imatinib-sensitive and resistant CML, with no activity in EphA3-negative specimens, including normal CD34+ bone marrow cells. KB004 inhibited myeloid leukemia colony formation (CFU-L) from primary AML samples in methylcellulose colony assays without affecting normal hematopoietic colony recovery and also showed dose-dependent inhibition of long-term culture-initiating cells (LTC-IC) from AML stem cells. EphA3 expression was not detected on primary human CD34+ bone marrow cells from healthy individuals. These data support the development of an anti-EphA3 antibody for the treatment of hematologic malignancies. The selective activity against leukemic stem cells supports a novel strategy for therapeutic targeting of leukemia-initiating cells. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 1713 Background: Chronic Myelomonocytic Leukemia (CMML) and Juvenile Myelomonocytic leukemia (JMML) are classified as MDS/MPN in the WHO classification system. Despite sharing clinical and histological features, CMML is characterized by a heterogeneous collection of molecular lesions while JMML is defined by well-established molecular aberrations clustered along the RAS pathway leading directly to GM-CSF hypersensitivity; a pathognomonic characteristic of JMML. Here we test whether a molecular signature for GM-CSF hypersensitivity in JMML, determined by the pSTAT5 activation assay, is also present in CMML and whether this signature clusters within a specific CMML subgroup. Methods: Cryopreserved bone marrow aspirates from 24 patients with newly diagnosed or relapsed CMML were obtained from the Moffitt Cancer Center Tissue Repository. Cells were thawed and rested in Stem Span H3000 with 10% FBS for 2 hours and then either starved for one hour in serum-free media, serum free group (n=12), or rested in Stem Span for an additional hour, serum group (n=12), prior to stimulation with G-CSF, IL-3, or GM-CSF for 15 minutes and then fixed and permeabilized with formaldehyde and methanol, as previously described. Samples were stained with an anti-pSTAT5(Y-694) antibody and analyzed by flow cytometry (Kotecha, Cancer Cell. 2009). Cells stained with isotype-control antibody were used to establish the threshold for basal STAT5 phosphorylation. Because STAT5 was constitutively phosphorylated in serum, and to a lesser extent in serum-free conditions, inducible cytokine activation was defined as the percentage of pSTAT5 positive cells above untreated samples in both CMML and healthy controls. A retrospective chart review was performed to obtain clinical variables including age, sex, WHO classification, Dusseldorf scoring system, MD Anderson scoring system, WBC, peripheral monocyte count, blast percentage, anemia, platelet count, splenomegaly, and metaphase cytogenetics. Results: The percentage of pSTAT5 responsive cells after G-CSF stimulation with doses up to 10 ng/ml was similar in cases and normal BM controls (p=0.14), whereas, a statistically significant increase in the percentage of inducible pSTAT5 positive cells was observed with GM-CSF 0.1 ng/ml (p=0.04), GM-CSF 1 ng/ml (p=0.02), and GM-CSF 10 ng/ml (p=0.01) in CMML BM cells compared to healthy donor BM cells, as shown in Figure 1. Using one standard deviation below the mean as a cut point, only 5 patients failed to show GM-CSF hypersensitivity in the serum (n=3) and serum-free groups (n=2), respectively. IL-3 and GM-CSF play similar roles in hematopoietic growth through the activation of JAK2/STAT5 and share a common beta-chain required for signaling. Signaling mediated by GM-CSF and IL3 converge to activate RAS and other downstream intermediates that regulate DNA synthesis, cell-cycle progression and suppression of apoptosis. The concentration of IL3 required to induce STAT5 phosphorylation was 10-fold greater than GM-CSF in CMML cells, but the percentage of cells responsive to IL3 was greater in CMML cases compared to controls at 10 ng/ml (p=0.02). Analysis of the percentage of GM-CSF hypersensitive cells and clinical parameters revealed no associations with age at onset, WHO classification, Dusseldorf scoring system, MD Anderson scoring system, blast percentage, anemia, platelet count, splenomegaly, or karyotype. The percentage of pSTAT5 positive cells with GM-CSF 0.1 ng/ml positively correlated with the total leukocyte (p=0.03) and total monocyte (p=0.02) count indicating that the JAK2/STAT5 signaling response is indicative of disease burden. Conclusions: Based on the threshold for cytokine stimulation and percentage of cells that display pSTAT5 induction, CMML appears to preferentially utilize GM-CSF for survival and/or expansion. Although RAS mutations were not assessed, CMML cells were preferentially sensitive to GM-CSF in newly diagnosed cases independent of cytogenetic abnormalities suggesting that JMML and CMML share biological features of GM-CSF hypersensitivity. Disclosures: Padron: KaloBios Pharmaceuticals, Inc.: Research Funding. Bebbington:KaloBios Pharmaceuticals, Inc.: Employment, Equity Ownership. Baer:KaloBios Pharmaceuticals, Inc.: Employment, Equity Ownership.

Description: Abstract 3793 Introduction: Chronic Myelomonocytic Leukemia (CMML) is a genetically diverse hematologic malignancy characterized by cytopenias with or without leukocytosis, marrow dysplasia, monocytosis, splenomegaly, and a propensity to transform to Acute Myeloid Leukemia (AML). It is among the most aggressive chronic myeloid malignancies with a three year overall survival approximating twenty percent. The current treatment armamentarium is generally ineffective and mostly derived from studies that focused on Myelodysplastic Syndromes (MDS). As has been well defined in Juvenile Myelomonocytic Leukemia (JMML), we previously reported that primary CMML bone marrow mononuclear cells exhibit GM-CSF-dependent pSTAT5 hypersensitivity. To determine whether this signaling pathway is a viable therapeutic target, we present the preclinical characterization of KB003 activity on primary CMML bone marrow mononuclear cells. Methods: KB003 was engineered and provided by KaloBios Pharmaceuticals. The ability of KB003 to neutralize GM-CSF was tested in two ways using doses that ranged from .01 to 10,000 ng/ml of KB003. First, 10 ng/ml of GM-CSF was used to induce IL-8 production by the U937 cell line as measured by ELISA. Second, the GM-CSF-dependent cell line, MO7e, was cultured in the absence and in the presence of 0.1, 1, and 10 ng/ml of GM-CSF with or without KB003. Apoptosis and viability was assessed by annexin and 4′,6-diamidino-2-phenylindole (DAPI) staining by flow cytometry. To determine anti-GM-CSF sensitivity in CMML, primary cryoperserved bone marrow samples from 10 patients were reconstituted in prewarmed StemSpan H3000 with 10% FBS and microcultured in a 96 well plate with increasing doses of GM-CSF and KB003. After a 48-hour incubation period, cells were stained with myeloid-specific antibodies (CD34, CD38, CD14, CD33) and measured by multi-color flow cytometry. DAPI was used to measure cell viability. MethoCult hematopoietic progenitor assays were used to quantify the expansion and differentiation of CMML mononuclear cells in the presence of GM-CSF and escalating concentrations of KB003 in those samples with sufficient cell number for analysis (n=7). Results: KB003 effectively neutralized GM-CSF-induced IL-8 secretion in U937 cells with an IC50 of 48.2 ng/ml, as shown in Figure 1a. A dose of 0.1, 1, and 10 ng/ml of GM-CSF was used to protect MO7e cells from apoptosis and a survival benefit was achieved at each dose tested (Fig 1b). In this assay, KB003 neutralized 0.1 ng/ml of GM-CSF, but was unable to neutralize GM-CSF at higher doses (10 ng/ml). Using CMML samples, doses ranging from 10 to 100 μg/ml of KB003 were tested in apoptosis and in colony formation assays. Using the gating strategy shown in Figure 2a, the percentage of mature (CD33+/CD14+) and immature (CD33+/CD14-) myeloid subpopulations within the viable gate after KB003 treatment was decreased (Figure 2b). As expected, KB003 had no effect on CD3+ T-cells in the mixed culture populations. In addition to myeloid subpopulations identified by CD14, the CD33+/CD38+ cells were also more sensitive than CD33+/CD38- or CD38+/CD34+ inhibition by KB003 (Figure 2c, p

Description: The transcription factor CCAAT/enhancer binding protein β (C/EBPβ, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1β. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBPβ. One factor was shown to be tumor necrosis factor- (TNF-), which increased C/EBPβ expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBPβ localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBPβ correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBPβ to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBPβ. These observations suggest that diffuse distribution of C/EBPβ in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor.