Preclinical Characterization of KB003, a Novel Humaneered™ Monoclonal Anti-GM-CSF Antibody, Demonstrates That the GM-CSF Signaling Axis Is a Therapeutic Target in Chronic Myelomonocytic Leukemia (CMML)
Authors: Padron Eric , Painter Jeffrey S , Mailloux Adam W , McGraw Kathy L. , McDaniel Jessica M. , Bebbington Christopher , Baer Mark , Yarranton Geoffrey , Lancet Jeffrey E , Komrokji Rami S. , List Alan F. , Epling-Burnette P.K. 
Published: October 2019

Abstract
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<0.05). As shown in Figure 2d, hematopoetic colony formation assays confirmed the viability results. Using a one way ANOVA, there was a dose dependent decrease in the number of hematopoietic colonies, coupled with less organized and more diffuse appearance of colonies after KB003 treatment (Figure 2d).

Conclusion:
GM-CSF neutralization using KB003 suppresses CMML progenitor survival in vitro. These preclinical data suggest that KB003 and other GM-CSF signaling axis inhibitors merit further investigation in CMML. A more committed myeloid precursor expressing CD38 may represent the progenitor population with enhanced GM-CSF dependence in CMML. This is consistent with the results showing CD38 to be a marker of GM-CSF-dependent pSTAT5 sensitivity in JMML and may prove to be predictive biomarker for anti-GM-CSF therapy in CMML.

Disclosures:
No relevant conflicts of interest to declare.