Design of a modular immunotoxin connected by polyionic adapter peptides.

Immunotoxins are genetically engineered fusion proteins of an antibody Fv fragment and a toxin from bacteria or plants, which function as anti-cancer therapeutics. Here, we describe a new generation of immunotoxins in which both proteins do not form a single fusion protein but are coupled specifically via cysteine-containing polyionic fusion peptides. The engineered Pseudomonas exotoxin PE38 was N-terminally fused to the peptide E(8)C. In combination with the disulfide-stabilized Fv fragment of the tumor-specific antibody B3, which was extended by the peptide R(8)CP, the fusion peptides ensured a specific and covalent coupling of the Fv fragment and the toxin. The resulting immunotoxin was as active and as specific as an immunotoxin consisting of a fusion protein of the same antibody fragment connected to the toxin.