Diphtheria toxin‐based anti‐human CD19 immunotoxin for targeting human CD19(+) tumors

CD19 is expressed on normal and neoplastic B cells and is a promising target for immunotherapy. However, there is still an unmet need to further develop novel therapeutic drugs for the treatment of the refractory/relapsing human CD19(+) tumors. We have developed a diphtheria toxin‐based anti‐human CD19 immunotoxin for targeting human CD19(+) tumors. We have constructed three isoforms of the CD19 immunotoxin: monovalent, bivalent, and foldback diabody. In vitro binding affinity and efficacy analysis demonstrated that the bivalent isoform had the highest binding affinity and in vitro efficacy. The in vivo efficacy of the CD19 immunotoxins was assessed using human CD19(+) JeKo‐1 tumor‐bearing NOD/SCID IL‐2 receptor γ(−/−) (NSG) mouse model. In these animals, CD19 immunotoxins significantly prolonged the median survival from 31 days in controls to 34, 36, and 40 days in animals receiving the monovalent isoform, foldback diabody isoform, and bivalent isoform, respectively. The bivalent CD19 immunotoxin is a promising therapeutic drug candidate for targeting relapsing/refractory human CD19(+) tumors.