Antibody engineering of recombinant Fv immunotoxins for improved targeting of cancer: disulfide-stabilized Fv immunotoxins.

Recombinant immunotoxins are chimeric proteins in which a truncated toxin is fused to a recombinant antigen-binding domain such as a recombinant Fv or Fab. Recombinant immunotoxins target cell surface receptors and other antigens on tumor cells. The antigen-binding and -targeting domains in recombinant immunotoxins are usually single-chain Fvs (scFv), which are the antibody variable regions connected by a flexible peptide linker and fused directly to a bacterial toxin. However, Fabs have also been used. Recombinant immunotoxins have very good activity in vitro on cultured human tumor cell lines and have produced complete regressions and cures of established tumor xenografts in nude mouse models. Problems with the stability and binding of some scFv immunotoxins as well as scFvs not linked to toxin led to the development of a new type of recombinant Fv immunotoxin in which the targeting variable domains of the Fv are stabilized by an interchain disulfide bond located in structurally conserved framework positions of the VH and VL domains. These are termed disulfide-stabilized Fvs (dsFv) or dsFv immunotoxins. dsFvs and dsFv immunotoxins have several advantages over scFv immunotoxins. This review summarizes the design, construction, activities in vitro and in vivo, and biochemical characteristics of dsFv immunotoxins and compares them with scFv immunotoxins.