Abstract 649: Development and preclinical characterization of an Fc-optimized CD133 antibody for improved induction of NK cell reactivity against myeloid leukemia

Abstract

Abstract NK cells are cytotoxic lymphocytes that play a major role in anti-tumor immunity and largely contribute to the efficacy of allogenic stem cell transplantation (SCT) in leukemia. Another clinically important feature of NK cells is their ability to mediate antibody-dependent cellular cytotoxicity (ADCC) upon application of anti-tumor antibodies (mAb) such as Rituximab. Modifications of the human IgG1 Fc-part of anti-tumor antibodies may lead to markedly improved capability to recruit Fc-receptor bearing effector cells. Here we report on the development and preclinical characterization of an Fc-optimized mAb directed towards CD133, a molecule expressed on a wide variety of malignant cell types. Initially we evaluated binding of three different mouse anti-human CD133 mAbs (clones AC133, W6B3 and 293C3) to 20 AML and 6 CML samples to identify a clone with optimal binding characteristics. Binding of AC133 and W6B3 to the leukemic cells in 11/20 AML and 5/6 CML samples was comparable. In contrast, binding of 293C3 was observed in 18/20 AML cases and 5/6 CML cases. This indicates that 293C3 recognizes a particular epitope, which is expressed in a high proportion of AML cases. Therefore mAb 293C3 was selected for generation of chimeric mAbs with either a wildtype Fc part (293C3-WT) or a variant containing distinct modifications (S239D/I332E) to enhance its affinity to the Fc receptor CD16 (293C3-SDIE). The binding specificity of 293C3-WT and 293C3-SDIE was validated in analyses with CD133 transfectants and the respective mock controls. In cytotoxicity assays and analyses of NK cell degranulation, 293C3-WT clearly induced NK cell ADCC against primary leukemia cells. However, ADCC activity of the Fc-optimized mAb 293C3-SDIE markedly exceeded that of its counterpart containing the wild type Fc part. Considering the expression of CD133 on healthy hematopoietic progenitor cells, we presently evaluate potential toxicity of the antibodies towards these cells and other healthy tissue. We envisage that 293C3-SDIE might be an attractive immunotherapeutic reagent for the elimination of minimal residual disease in CD133 expressing leukemia, particularly in the context of allogenic SCT. Citation Format: Samuel Koerner, Julia Leibold, Ludger Grosse-Hovest, Hans-Jörg Bühring, Helmut R. Salih, Gundram Jung. Development and preclinical characterization of an Fc-optimized CD133 antibody for improved induction of NK cell reactivity against myeloid leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 649. doi:10.1158/1538-7445.AM2014-649