Preclinical Characterization of AFM26, a Novel B Cell Maturation Antigen (BCMA)-Directed Tetravalent Bispecific Antibody for High Affinity Retargeting of NK Cells Against Myeloma

Natural killer (NK) cells are crucial effector cells of the innate immune system capable of rapidly recognizing and eliminating infected, stressed and malignant cells. NK cells are also the prime mediators of antibody-dependent cell-mediated cytotoxicity (ADCC), a potent mechanism of anti-viral immunity that has been applied to cancer therapy by targeting tumor-expressed surface antigens using monoclonal antibodies (mAbs). Classical ADCC is mediated by low affinity Fc-mediated engagement of NK cells via FcγRIIIA (CD16A) and is modulated by differences in target antigen expression levels. While high potency of therapeutic mAbs is achieved when target antigen is available at high density, potency and efficacy decrease substantially when copy numbers are low. Classical ADCC also needs to overcome the inhibitory effect of competing serum IgG and is negatively affected by a low affinity polymorphism of CD16A (158F) that is prevalent in approximately 8 of 10 individuals. Hence, classical Fc-mediated ADCC does not fully utilize the therapeutic potential of NK cell cytotoxicity.

B cell maturation antigen (BCMA) has emerged as a promising target for treatment of multiple myeloma (MM) due to its near universal expression on tumor cells and restricted expression in non-malignant tissues. Numerous therapeutic approaches are currently investigated clinically and pre-clinically and target BCMA, however, none of these are aimed at fully utilizing NK cell-mediated ADCC. Low copy numbers of BCMA (approx. 40-15,000) might limit the activity of classical mAbs against BCMA, especially in the presence of high serum concentrations of paraprotein. In addition, NK cells may be ideally suited to target minimal residual disease immediately before or after autologous stem cell transplantation. Consequently, by efficiently redirecting NK cell cytotoxicity to BCMA+ myeloma, AFM26 used alone or in combination with other approaches may provide a novel, optimized treatment strategy.

Here we describe development of AFM26, a BCMA and CD16A-directed tetravalent bispecific antibody that selectively engages CD16A+ effector cells, including NK cells, and is designed to overcome the limitations of classical ADCC. AFM26 is based on the recently launched Redirected, Optimized Cell Killing (ROCK) antibody platform and combines high affinity CD16A-directed effector cell engagement with IgG-like pharmacokinetics and manufacturability. We demonstrate that AFM26 interacts with NK cells with high avidity independently of CD16A polymorphism and in presence of competing IgG. NK cell-mediated lysis of BCMA+ target cell lines induced by AFM26 is largely independent of BCMA expression levels with high potency and efficacy observed at low copy numbers (<200), as confirmed by autologous lysis of primary MM cells in vitro. Despite more potent and efficacious in vitro lysis, release of inflammatory cytokines is comparable with classical antibody formats. We further demonstrate anti-tumor activity of AFM26 in combination with adoptive transfer of primary human NK cells in vivo using human IL-15-transgenic NOG mice.

AFM26 therefore is a promising agent currently in preclinical development to fully unlock NK cell cytotoxicity for BCMA-directed immunotherapy of MM.