Novel Cytokine-Mediated Mechanism of Action Identified By Quantitative Seroproteomics in Multiple Myeloma Patients Treated with Tagraxofusp, a Novel CD123-Directed Targeted Therapy

Introduction Plasmacytoid dendritic cells (pDCs) express CD123/IL-3Rα and promote tumor growth and immunosuppression in multiple myeloma (MM) (Chauhan et al, Cancer Cell 2009, 16:309-323; Ray et al, Leukemia, 2018, 32:843-846). Tagraxofusp is a novel targeted therapy directed against CD123, and is FDA-approved for the treatment of patients with blastic plasmacytoid dendritic cell neoplasm [BPDCN]). Tagraxofusp can also trigger anti-MM activity by reducing the viability of immunologically defective and tumor-promoting pDCs in MM. Furthermore, tagraxofusp synergistically enhances the anti-MM activity of anti-MM agents bortezomib and pomalidomide. Our preclinical findings led to a recently completed phase 1/2 clinical trial of tagraxofusp with pomalidomide/dexamethasone in relapsed/refractory MM patients (NCT02661022). Results demonstrated preliminary safety and efficacy, with 5 of 9 heavily pretreated patients achieving durable partial response (PR) (ASH 2019). Here, we report the early results of our translational correlative studies using bone marrow (BM), peripheral blood (PB), and serum from the study cohort.