Toxin-Based Therapeutic Approaches

Shapira, Assaf; Benhar, Itai

First generation immunotoxins were prepared by chemically conjugating antibodies to natural-intact toxin units or to toxins with attenuated cell binding capability. However, these constructs were heterogeneous and unspecific because of the multiplicity of potential sites available for chemical conjugation and as the presence of the cell binding domain of the toxin led to intoxication of "normal" cells, respectively. Immunotoxins of the second generation were also based on chemical conjugation between the targeting moiety and the toxin. Nevertheless, cumulative knowledge on the structure and function of the toxins enabled the removal of their native non-specific cell binding domain, generating much more target-specific immunotoxins when conjugated to monoclonal antibodies. Although more specific, and thus better tolerated by animals, immunotoxins from the second generation were still chemically heterogeneous and their large size hindered them from penetrating solid tumors. In order to avoid heterogeneity, improve tumor penetration and reduce production complexity and costs, recombinant DNA techniques were applied in the production of third generation immunotoxins. In these constructs, which are mostly produced in the bacterium Escherichia coli, the cell binding domain of the toxin is genetically replaced with a ligand or with the Fv portion of an antibody in which its light and heavy chain variable fragments are either genetically linked (scFv) or held together by a disulfide bond (dsFv) (Figure 2 ).