332 PASTAN ET AL toxins kill many types of animal cells. Because of their potency, they have been coupled to antibodies and growth factors in order to kill target cells with specific surface properties. These agents are now being evaluated for treat ญ ment of cancer, autoimmune diseases, and chronic infectious diseases (1-4). DNA sequences encoding Pseudomonas exotoxin, diphtheria toxin, ricin, and several other toxins have been cloned and expressed in Escherichia coli, where these toxins can be produced in large amounts because they do not kill bacterial cells. Initially, toxins produced in bacteria were chemically coupled to antibodies to make classical immunotoxins. A significant advance has been the preparation of recombinant chimeric toxins by fusing cell-targeting genes to modified toxin genes (5, 6). The approaches used to design, express, and purify chimeric toxins, their properties, and their mechanisms of action are major areas of focus of this review.

Directly targeting cytotoxic agents to cancer cells is an old idea, but it has recently become a popular approach to therapy because modem biological techniques have identified molecules on the surfaces of cancer cells that can be used as targets for this type of therapy. These target proteins include oncoproteins that cause the cells to grow abnormally, differentiation antigens that are present on cancers derived from organs with dispensable functions, and other antigens commonly found on cancer cells, which may represent fetal antigens inappropriately expressed in these cancers. Because bacterial and plant toxins are very active cell-killing agents, one approach to targeted therapy is to attach one of these toxins to an antibody or a growth factor that preferentially binds to a cancer cell. Many toxins have been used for this purpose, but three of these, Pseudomonas exotoxin A (PE), ricin, and diph ญ theria toxin (DT) have been most intensively employed. Initially, toxins purified from natural sources and chemically attached to antibodies to make immunotoxins had to be chemically modified to reduce binding to toxin receptors in normal cells and allow preferential binding to the targets on the cancer cells. Several recent reviews describe the construction and activity of immunotoxins produced by the chemical linkage of toxins to antibodies (7-10). Recently, genes encoding these three toxins have been cloned (11-13), and expressed in E. coli, and novel molecules with more desirable biological properties produced (14-17). In this review, we discuss how these toxins act, how they can be genetically modified to produce more active and specific molecules, the results of animal experiments, and prospects for the future.