Antibody therapy of cancer : Fc receptor-mediated mechanisms of action

Abstract

Cancer, a class of malignant diseases characterized by unregulated cell growth, is still a leading cause of death worldwide. The high specificity of antibodies combined with the ability to engage multiple mechanisms of action (MoA) and minimal side-effects makes them attractive agents for targeted treatment of cancer. Therapeutic monoclonal antibodies (mAbs) may induce anti-tumor effects after binding to tumor cells, by activation of an immune response via clustered Fc (fragment crystallizable)-regions of mAbs. Subsequent binding to Fc-gamma receptors (FcγRs) expressed on immune effector cells can lead to induction of antibody-dependent cellular cytotoxicity (ADCC) and/or antibody-dependent cellular phagocytosis (ADCP). The aim of this thesis was to gain insight into which, when and where FcγR-mediated mAb effector functions contribute to anti-tumor activity. The correlation between certain FcγR polymorphic variants, which impact interactions with IgG, and clinical responses provides evidence for the role of FcγR-mediated anti-tumor effects in clinical efficacy of mAbs. However, clinical efficacy is modulated by multiple factors, resulting in more or less pronounced effects by FcγR polymorphisms. By mapping interactions between human IgG isotypes, IgG1, IgG2, IgG3 and IgG4 and murine effector cells we demonstrated that human IgG1 is, as in human, also in mice the most potent human isotype. However, we also observed distinct differences compared to the activation of human effector cells. This detailed knowledge on the interactions obtained in this study, permits a better interpretation of human antibody efficacy studies in mouse xenograft models. Daratumumab (DARA), a human IgG1 CD38 mAb, was shown to induce ADCP with human macrophages of Burkitt’s lymphoma and multiple myeloma (MM) cell lines and on cells derived from MM patients. Using an ADCP-deficient variant of DARA, we showed that ADCP contributes to the in vivo MoA of DARA. In addition to ADCP, we also showed DARA induced FcγR-mediated programmed cell death (PCD) on MM cell lines. Inhibitory as well as activating FcγRs were shown to mediate DARA induced PCD in vivo. The role of ADCC in tumor inhibition was studied with a matched set of epidermal growth factor receptor (EGFR)-specific mAbs, each with a unique MoA, so that it was possible to distinguish between FcγR-mediated ADCC and Fab-mediated signaling inhibition. Comparing efficacy in several in vivo mouse xenograft models showed that ADCC mainly plays a role in preventing metastasis, as well as in inhibiting early stages of tumor development. Therefore, we studied the potential of EGFR targeting mAbs for peri-operative treatment of colorectal cancer (CRC). Zalutumumab, a human IgG1 EGFR mAb, induced ADCP on CRC cell lines, which was independent of EGFR downstream mutations. Peri-operative treatment of an experimentally induced liver metastasis model showed significant ADCP induction by zalutumumab in vivo. In conclusion, these detailed studies on FcγR-mediated anti-tumor mechanisms of therapeutic mAbs showed that multiple factors e.g. tumor microenvironment, tumor origin, timing of mAb treatment and the dose level of treatment impact the efficacy. The improved understanding of all anti-tumor mechanisms initiated by therapeutic mAbs provides a strong basis for the identification of effective combination therapies of mAbs and other agents.