The CD19 Antibody MOR208 Efficiently Triggers Natural Killer Cell-Mediated Cytotoxicity Against Acute Lymphoblastic Leukemia Cells From Pediatric and Adult Patients

Abstract Abstract 1502 CD19 represents a promising target antigen for therapeutic antibodies in the treatment of B-lineage acute lymphoblastic leukemia (ALL), because it is expressed very early during B-cell development and thus is highly displayed by the majority of both precursor and mature B-ALL cells. Several CD19-targeting molecules are in different stages of preclinical and clinical development. However, no conventional CD19 antibodies have been approved to date for clinical application. This may be due to the limited effector functions triggered by the wild type Fc-domain of first generation CD19 antibodies. These limitations may be overcome by next generation antibodies with enhanced potency such as MOR208 (formerly XmAb5574), an Fc-engineered humanized CD19 antibody that has shown high activity in preclinical models of multiple B cell neoplasms and is currently evaluated in a phase I clinical trial in chronic lymphocytic leukemia. Here, MOR208 and its non-engineered IgG1 analogue were evaluated for their potential to trigger antibody-dependent cell-mediated cytotoxicity (ADCC) of freshly isolated ALL cells from both pediatric and adult patients. To quantify Fc-mediated effector function, MOR208 and its native IgG1 analogue were evaluated for their ability to induce lysis of primary ALL cells in standard 51Cr release assays using different effector cell populations, or human serum as a source of complement. MOR208 induced potent ADCC in the presence of natural killer (NK) cells, whereas no cytotoxicity was observed for either CD19 antibody when myeloid effector cells were used. Neither antibody triggered complement-dependent cytotoxicity. MOR208 induced NK-cell mediated lysis of a panel of freshly isolated ALL samples obtained from seven adult and eight pediatric ALL patients. MOR208 triggered lysis at picomolar concentrations (EC50 = 26 pM) and was more effective than the native CD19 IgG1 analogue. MOR208 activated NK cells more potently as indicated by upregulation of CD69. In addition, MOR208 required lower effector-to-target cell ratios and antibody concentrations, and achieved higher maximum extents of lysis (30% and 15% target cells lysis by MOR208 and the native CD19 IgG1 analogue, respectively). The improved ADCC potential of NK cells was observed irrespective of the FcγRIIIA allotype at amino acid position 158. Moreover, MOR208 induced ADCC with patient-derived NK cells and mediated lysis of autologous ALL cells despite expression of inhibitory MHC class I molecules. MOR208 also displayed high cytotoxicity with allogeneic donor-derived NK cells isolated from a patient previously transplanted with allogeneic hematopoietic progenitor cells. These experiments demonstrate that MOR208 exhibits enhanced cytotoxicity compared to a native non-engineered anti-CD19 antibody when employing patient-derived tumor cells. CD19 antibody therapy with MOR208 may represent a promising approach for the treatment of pediatric or adult B-ALL, by overcoming the limitations of conventional CD19 antibodies. Application of MOR208 may be especially promising in the eradication of minimal residual disease cells in a post-transplantation context where high numbers of allogeneic NK effector cells are available. Disclosures: Zhukovsky: Xencor, Inc. / Affimed Therapeutics AG: Employment.