The CD317-Targeted Immunotoxin HM1.24-ETA′ Efficiently Eliminates Malignant Plasma Cells in Vitro and in Vivo

Abstract Abstract 1838 Despite new treatment modalities, the clinical outcome of at least a subgroup of patients with multiple myeloma still needs improvement. Recently antibody-based targeted therapies with a toxic payload have documented impressing activity. HM1.24 (CD317), a surface molecule overexpressed on malignant plasma cells, is efficiently internalized and may represent a promising target for the development of myeloma-directed immunoconstructs. Here, the generation and characterization of a novel single-chain immunotoxin, HM1.24-ETA′, is described. HM1.24-ETA′ was generated by genetic fusion of a CD317-specific single-chain Fv antibody and a truncated variant of Pseudomonas aeruginosa exotoxin A (ETA′). The immunotoxin was expressed in E. coli and the protein was purified to homogeneity by affinity chromatography. HM1.24-ETA′ efficiently inhibited growth of myeloma cell lines (INA-6, RPMI8226, U266) analyzed in MTT assays. Half maximal growth inhibition was observed at low nanomolar concentrations. Target cell killing occurred via induction of apoptosis as indicated by annexin V / propidium iodide staining and analysis of PARP cleavage. The delivery of HM1.24-ETA′ to target cells is antigen-specific, because excess of unconjugated parental antibody completely blocked the cytotoxic effect. The proliferation of IL-6 dependent INA-6 was efficiently reduced by HM1.24-ETA′ even in co-culture experiments with bone marrow stromal cells that otherwise strongly support tumor cell growth. Importantly, HM1.24-ETA′ strongly triggered apoptosis (up to 80% annexin V-positive cells) of freshly isolated tumor cells from 5 of 5 myeloma patients. In a xenograft SCID mouse model, establishment of INA-6 plasma cell tumors was efficiently abrogated by treatment with HM1.24-ETA′ immunotoxin (p < 0.04). Thus, HM1.24-ETA′ immunotoxin in vitro and in the preclinical xenograft model in vivo demonstrates that the CD317 antigen may represent a promising target structure for immunotherapy of multiple myeloma using immunoconjugates with toxic payloads. Disclosures: No relevant conflicts of interest to declare.