Increased sophistication of immunotoxins.

The article by Salvatore et al. (1) in this issue of Clinical Cancer Research describes the synthesis and biological activity of an improved, recombinant immunotoxin targeted to the CD22 B-cell antigen. This particularly well-performed study is one of a large series of significant innovations in the field by this and other groups of investigators in the last few years. To appreciate more fully the work in this issue and the recent developments in immunotoxin pharmacology, some historical perspective is useful. Immunotoxins consist of cell-selective ligands (usually monoclonal antibodies, antibody fragments, or cytokines) linked covalently to modified peptide toxins. The ligand binds cell surface receptors and triggers internalization. In defined intracellular vesicle compartments, the toxin moiety escapes to the cytosol, where it catalytically alters critical cell functions leading to cell death. Because immunotoxins kill by distinct mechanisms (e.g., inactivation of protein synthesis or signal transduction) than standard chemotherapy agents, which damage DNA or cell proliferation, it was envisioned that they would be active on chemoresistant malignancies. In addition, because immunotoxins possess a cell targeting function, the molecule should have distinct, nonoverlapping toxicities from chemotherapy drugs. Finally, immunotoxins may have additive or synergistic efficacy in combination with the standard chemotherapy agents. In the 1980s, clinical studies were conducted with antitumor monoclonal antibodies linked to protein synthesis inactivating toxins. The toxins (ricin and Pseudomonas exotoxin) had been either chemically or genetically altered to reduce normal tissue binding. Studies were done in patients with nonHodgkin’s lymphoma, chronic lymphocytic leukemia, cutaneous T-cell lymphoma, breast cancer, ovarian cancer, adult T-cell leukemia, colon cancer, peripheral T-cell lymphoma, and melanoma (2). However, few durable remissions were observed. Among the most active of these early immunotoxins was an anti-CD22 monoclonal antibody (RFB4) chemically linked to deglycosylated ricin A chain (3). In the 1990s, genetic engineering was used to fuse the catalytic and translocation domains of toxins (diphtheria toxin and Pseudomonas exotoxin) to cytokines and single chain Fvs, and these were administered to patients. These second generation immunotoxins were called fusion toxins or fusion proteins. In addition, several peptide toxins with absent normal tissue binding domains (gelonin, pokeweed antiviral protein, and saporin) were conjugated to monoclonal antibodies and tested in patients. Finally, a growth factor was coupled to a binding site mutant diphtheria toxin and infused directly into patients’ brain tumors. Improved remission rates were seen with DAB389IL2 2