Non-Competitive Binding of Isatuximab and Daratumumab to CD38: Implications for Targeted Therapy in Multiple Myeloma.

Background/Objectives: CD38-targeting monoclonal antibodies isatuximab and daratumumab have revolutionized multiple myeloma (MM) treatment, but a deeper understanding of their distinct mechanisms is crucial for therapeutic optimization. Methods: We used flow cytometry to assess isatuximab and daratumumab binding competition in MM cell lines and patient-derived bone marrow cells. The dynamics of CD38 expression were evaluated at different time points before and after antibody-mediated removal. The effects of IMiDs (pomalidomide, lenalidomide) on CD38 expression and isatuximab-induced apoptosis, either alone or in combination with IMiDs, were also examined. Moreover, MM cell migration was assessed through CXCR4-mediated assays, and cell adhesion was evaluated via CD49d-dependent assays. Results: Isatuximab and daratumumab did not compete for CD38 binding, confirming distinct epitope recognition. Following depletion with either antibody, CD38 expression on the MM cell surface began to recover within 2 h, suggesting a dynamic regulation of CD38 availability. While daratumumab lacked direct apoptosis, isatuximab induced significant direct cell death. Pomalidomide enhanced isatuximab-induced apoptosis by increasing CD38 expression, whereas lenalidomide had no significant effect. Additionally, both antibodies effectively inhibited MM cell migration and significantly reduced cell adhesion. Conclusions: Their non-competitive binding and shared impact on cell dynamics suggest opportunities for optimizing treatment strategies through combinatorial or sequential approaches in MM therapy.