Bispecific Antibodies for Cancer Immunotherapy Current Perspectives

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Müller & Kontermann
antigen [CEA]) were selected.f'-' Although the principle and the targets on effector and tumor cells have remained fundamentally the same over the time, the format of the bispecific antibody has varied considerably, influenced by the technology progress and guided by the clinical requirements. Thus, the first generation of bispecific antibodies was created by hybridization of two antibody-producing cells, or by chemical cross-linking of two different monoclonal antibodies or fragments thereof. The development of recombinant antibody technology then allowed the design of the second generation, which consists of several smaller formats where the antibody is essentially reduced to the domains involved directly in the antigen binding. The main problems of the first generation, consisting of low efficacy and severe adverse effects such as cytokine storm syndrome, could be ascribed to immunogenieity and Fc-mediated effects, whieh were addressed and improved by the next generation.'^-*'
Current Perspectives
Dafne Müller and Roland E. Kontermann
Institut für Zellbiologie und Immunologie, Universität Stuttgart, Stuttgart, Germany
Contents
Abstract 1. Recombinant Bispecific Antibody Formats 2. Trifunctionai Antibodies for Cancer immunotherapy 3. Bispecific T-Ceii Enganger for Concer immunotherapy 4. Other Formats for T-Celi fîetargeting 5. Dual Targeting Strategies 6. Bispecific Antibodies for Pre-Targeting Radiotherapy 7. Targeting of Tumor Necrosis Factor for improved Radiotherapy 8. Haif-Life Extension Strategies 9. Conciusions 89 90 91 93 93 94 94 95 95 96
the most potent effector cells of the immune system, which cannot be activated by the effector mechanism of conventional therapeutic monoclonal antibodies. In this context, CD3 has become widely accepted as the appropriate trigger molecule. Nevertheless, retargeting of other effector cells such as natural killer cells, macrophages, and neutrophils have also been on trial, where the Fc receptors FcyRIII (CD16), FcyRI (CD64). and FcaR (CD89) were chosen accordingly as trigger molecules. On the side ofthe tumor cell, established tumor-associated antigens of hématologie malignancies (e.g. CD19, CD20, CD30) and solid tumors (e.g. epidermal growth factor receptor [EGFR], human epidermal growth factor receptor 2 [HER2/ERBB2/neu], epithelial cell adhesion molecule [EPCAM], cardnoembryonic
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Bispecific Antibodies for Cancer Immunotherapy
assemble into different immunoglobulins, including nonfunctional, monospecific, but also bispecific antibodies. These bispeciilc antibodies resemble more or less normal IgGs, depending on the heavy chain subclass. For example, fusion of two hybridomas, one mouse and the other rat, as utilized by trifunctional triomabs (see section 2), results in hybrid bispeeific IgGs possessing one mouse y2a heavy chain and one rat 72b heavy chainj^^l Alternatively, bispecific antibodies can be produced from purified antibodies by chemical conjugation of whole immunoglobulins or their antigen-binding (Fab') fragments (figure 1). A plethora of different recombinant bispecific antibody formats have been developed during the last two decades.''-' These formats include IgG-Hke molecules utilizing the Fc region for dimerization. but also small recombinant formats such as tandem single-chain variable fragment (scFv) molecules (taFv), diabodies (Db), single-chain Db (scDb), and various derivatives thereof^^' (figure 1). Fc-mediated dimerization is often used to produce bispecific tetravalent molecules, i.e. molecules possessing two binding sites for each antigen. This can be achieved, for example, by fusing a single-chain Fv fragment to the C-terminus of an antibody heavy chain or substituting the Fab arm with a bispecific single-chain antibody
The application of bispecific antibodies in cancer immunotherapy is based mainly on the concept of retargeting effector cells to tumor cells, but new strategies have emerged recently, for example, the recruitment of effector molecules.t'l Effector cell retargeting is achieved by simultaneous binding of the bispecific molecule to a tumor antigen on the target cell and a trigger molecule on the effector cell, leading to effective effector cell activation and ultimately to tumor cell destruction. Thus, the redirection of an immune response to the tumor site, bypassing Fc-mediated or major histocompatibility complex (MHC)-restricted activation of effector cells, is achieved. Among the effector cells, cytotoxic T cells have been mostly employed for this approach, considering that they constitute