A CD19-specific single-chain immunotoxin mediates potent apoptosis of B-lineage leukemic cells

CD19 is a B-lineage-specific transmembrane signaling protein participating in the control of proliferation and differentiation. It is present at high surface density on chronic B-lymphocytic leukemia (B-CLL) cells and cells of other B-cell malignancies, and is a prime target for therapy with antibody-derived agents. Many attempts have been made to target malignant cells via CD19, but to date none of these agents have received drug approval. Here we report the design of a monovalent immunotoxin consisting of a CD19-specific single-chain Fv antibody fragment fused to a derivative of Pseudomonas Exotoxin A. This fusion protein induced efficient antigen-restricted apoptosis of several human leukemia- and lymphoma-derived cell lines including Nalm-6, which it eliminated at an effective concentration (EC(50)) of 2.5 nM. The agent displayed synergistic toxicity when used in combination with valproic acid and cyclosporin A in cell-culture assays. It induced apoptosis of primary malignant cells in 12/12 samples from B-CLL patients, including patients responding poorly to fludarabine, and of cells from one pediatric acute lymphoblastic leukemia patient. In NOD/SCID mice transplanted with Nalm-6 cells, the toxin prevented engraftment and significantly prolonged survival of treated mice. Owing to its efficient antigen-restricted antileukemic activity, the agent deserves further development towards clinical testing.