Group Leader, Senior Scientist University Medical Center Schleswig-Holstein Kiel, Schleswig-Holstein, Germany
Introduction: The implementation of CD38-targeting antibodies daratumumab and isatuximab into combination regimens including a proteasome inhibitor (PI) significantly improved outcome and prolonged survival of patients with multiple myeloma (MM). Although many patients benefit, especially relapsed/refractory MM patients (RRMM) still have a very poor prognosis. In order to improve myeloma therapy, recently a phase II trial for RRMM patients was initiated combining the CD47 blocking antibody magrolimab with daratumumab or PIs (NCT04892446). Binding of CD47 highly expressed on MM cells to SIRPalpha on myeloid cells provides a strong ‘don’t eat me’ signal and diminishes phagocytosis of tumor cells and thus antibody-dependent cellular phagocytosis (ADCP) – a known mode of action of therapeutic CD38 antibodies. Thereby, blocking the CD47-SIRPalpha axis with a CD47 antibody may enhance phagocytosis and improve patients’ outcome.
Methods: To investigate whether blocking the CD47-SIRPa axis enhances ADCP of myeloma cells by CD38 antibodies with or without PI treatment, we generated a CD47-IgG2sigma antibody using the CD47 binding sequences of magrolimab and a silent Fc to prevent interactions with Fcg receptors (FcgR) on macrophages. Macrophages were generated from healthy donor monocytes by cultivation with M-CSF and were used at an effector-to-target cell ratio of 1:1 in real-time live cell imaging experiments. CD47-IgG2 was tested in combination with daratumumab and isatuximab, respectively, using various MM cell lines. Pre-treatment of MM cells with PIs was performed prior use in experiments. Expression levels of CD47 and CD38 were determined by quantitative flow cytometry.
Results: All tested myeloma cell lines express high levels of CD38 and CD47 and macrophages were SIRP-α+ and FcgR+. Importantly, ADCP of MM cells induced by monoclonal antibodies daratumumab and isatuximab could be enhanced by addition of the CD47 blocking antibody. However, improvement in phagocytosis strongly differs between myeloma cell lines not correlating with CD38 and CD47 levels. Thus, indicating that phagocytosis is influenced by additional factors. Of note, although high CD38 and CD47 expression was found on some MM cell lines, e.g., RPMI-8226, no ADCP could be induced at all. Pre-treating these resistant cells with PIs also did not induce significant phagocytosis. In responsive MM cell lines, ADCP mediated by CD38 antibodies in combination with the CD47 blocking antibody was slightly, but not significantly enhanced by pre-treatment with carfilzomib.
Conclusions: Our findings demonstrate that blocking the CD47-SIRPa axis with a CD47 blocking antibody can improve ADCP of myeloma cells by therapeutic antibodies targeting CD38 and that PI treatment may enhance phagocytosis. Interestingly, there might be also MM cells with particular phenotypes rendering them resistant to phagocytosis. Further investigating these factors may help to identify MM patients that benefit from CD47 blockade.
