Human kappa light chain targeted Pseudomonas exotoxin A--identifying human antibodies and Fab fragments with favorable characteristics for antibody-drug conjugate development.

Antibody-drug conjugates (ADC) represent promising agents for targeted cancer therapy. To allow rational selection of human antibodies with favorable characteristics for ADC development a screening tool was designed obviating the need of preparing individual covalently linked conjugates. Therefore, α-kappa-ETA' was designed as a fusion protein consisting of a human kappa light chain binding antibody fragment and a truncated version of Pseudomonas exotoxin A. α-kappa-ETA' specifically bound to human kappa light chains of human or human-mouse chimeric antibodies and Fab fragments. Antibody-redirected α-kappa-ETA' specifically inhibited proliferation of antigen-expressing cell lines at low toxin and antibody concentrations. Selected antibodies that efficiently delivered α-kappa-ETA' in the novel assay system were used to generate scFv-based covalently linked immunotoxins. These molecules efficiently triggered apoptosis of target cells, indicating that antibodies identified in our assay system can be converted to functional immunoconjugates. Finally, a panel of human epidermal growth factor receptor (EGFR) antibodies was screened--demonstrating favorable characteristics with antibody 2F8. These data suggest that antibodies with potential for Pseudomonas exotoxin A-based ADC development can be identified using the novel α-kappa-ETA' conjugate.