Daratumumab- 225 Actinium conjugate demonstrates greatly enhanced antitumor activity against experimental multiple myeloma tumors.

Daratumumab is an anti-CD38 directed monoclonal antibody approved for the treatment of multiple myeloma (MM) and functions primarily via Fc-mediated effector mechanisms such as complement-dependent cytotoxicity (CDC), antibody-dependent cell cytotoxicity (ADCC), antibody-dependent cellular phagocytosis, and T-cell activation. However, not all patients respond to daratumumab therapy and management of MM remains challenging. Radioimmunotherapy with alpha particle-emitting radionuclides represents a promising approach to significantly enhance the potency of therapeutic antibodies in cancer treatment. Here we report the results of mechanistic and feasibility studies using daratumumab radiolabeled with an alpha-emitter ²²⁵ Actinium for therapy of MM. CD38-positivelymphoma Daudi cell line and MM cell lines KMS-28BM and KMS-28PE were treated in vitro with ²²⁵ Ac-daratumumab. ²²⁵ Ac-daratumumab Fc-functional properties were assessed with C1q binding and ADCC assays. The pharmacokinetics and tumor uptake of ¹¹¹ In-daratumumab in Daudi tumor-bearing severe combined immunodeficiency (SCID) mice were measured with microSPECT/CT. The therapeutic effects of ²²⁵ Ac-daratumumab on Daudi and KSM28BM tumors in mice and treatment side effects were evaluated for 50 days posttreatment. The safety of ²²⁵ Ac-labeled antimurine CD38 mAb in immunocompetent mice was also evaluated. ²²⁵ Ac-daratumumab efficiently and specifically killed CD38-positive tumor cells in vitro , while its complement binding and ADCC functions remained unaltered. MicroSPECT/CT imaging demonstrated fast clearance of the radiolabeled daratumumab from the circulation and tissues, but prolonged retention in the tumor up to 10 days. Therapy and safety experiments with ²²⁵ Ac-daratumumab showed a significant increase in the antitumor potency in comparison to naked antibody without any significant side effects. Our results highlight the potential of targeting alpha-emitters to tumors as a therapeutic approach and suggest that ²²⁵ Ac-daratumumab may be a promising therapeutic strategy for the treatment of hematologic malignancies.