Preclinical Evaluation of Stx-0712, a CCR2 Cytotoxicity Targeting Chimera (CCR2-CyTAC) for the Treatment of Chronic Myelomonocytic Leukemia

Introduction: Chronic myelomonocytic leukemia (CMML) is a myeloid stem cell neoplasm with few effective therapies and poor prognosis. This disease is characterized by an expansion of malignant monocytes which express high and homogeneous levels of the homing chemokine receptor CCR2. We have developed a CCR2 targeting Cytotoxicity Targeting Chimera (CCR2-CyTAC, STX-0712), which potently binds CCR2 on cells, and when combined with the CyTAC platform antibody, eliminates target cells via enhanced effector mediated killing. We present here the characterization of CCR2 expression in CMML patient samples, ex vivo depletion of CMML cells via treatment with STX-0712, and the dose dependent depletion of MO1 monocytes (classical monocytes) in non-human primates, supporting CMML as a promising indication for STX-0712. Methods: IRB approval was obtained from Mayo Clinic for patient sample use. For CCR2 characterization and depletion experiments, we used bone marrow biopsy specimens and peripheral blood mononuclear cells (PBMCs) obtained from CMML patients and controls from healthy donors. CCR2 expression and monocyte depletion were evaluated via flow cytometry, as previously described. In vivo pharmacokinetic (PK) and pharmacodynamic (PD) studies were conducted in cynomolgus monkeys. Results: First, we evaluated CCR2 expression in peripheral blood and bone marrow in CMML samples by flow cytometry. In 6/6 (100%) peripheral blood samples CCR2 was highly expressed on >98% of the monocytes. Also, in 3/3 (100%) bone marrow samples, CCR2 was expressed on a subset of CD34+ progenitors, while low/no expression was observed in healthy controls. This progenitor subpopulation had characteristics of the inflammatory granulocyte-macrophage progenitor (GMP)-like cells. Next, STX-0712 was added to freshly isolated PBMCs from CMML donors and CCR2+ monocyte depletion, mediated by endogenous Fc gamma receptor 3a positive (FcγRIIIa) effector cells, was assessed. In 5/5 (100%) CMML patient samples, the drug selectively induced depletion of CD14+, CCR2+ cells with an average potency of 3nM and 66-91% target cell killing vs no drug control. Finally, we evaluated the effects of STX-0712 in cynomolgus monkeys to evaluate PK and PD effects on CCR2 MO1 monocytes in vivo. STX-0712 demonstrated PK supportive of dosing in man. In 9/9 (100%) animals, STX-0712 effectively depleted >95% of CCR2+ monocytes which returned after drug levels dropped below ~10ng/mL. All doses were well tolerated and the findings of comprehensive immunophenotyping were to be as expected, predictable and mechanism directed. Conclusion: These results demonstrate high and homogenous CCR2 expression on malignant monocytes, ex vivo human target engagement and pharmacodynamics, and in vivo depletion of CCR2 MO1 monocytes in a potent and dose dependent manner. Based on this data we plan a First in Human, phase 1 clinical trial for patients with relapsed/refractory CMML.