Immunotoxins are antibody-toxin fusion
proteins under development as
cancer therapeutics. In early clinical trials,
immunotoxins constructed with domains II and III of Pseudomonas
exotoxin (termed PE38), have produced a high rate of complete remissions in
Hairy Cell Leukemia and objective responses in other
malignancies.
Cholera exotoxin (also known as cholix toxin) has a very similar three-dimensional structure to Pseudomonas
exotoxin (PE) and when domains II and III of each are compared at the primary sequence level, they are 36% identical and 50% similar. Here we report on the construction and activity of an
immunotoxin made with domains II and III of
cholera exotoxin (here termed CET40). In cell viability assays, the CET40
immunotoxin was equipotent to tenfold less active compared to a PE-based
immunotoxin made with the same
single-chain Fv. A major limitation of toxin-based
immunotoxins is the development of
neutralizing antibodies to the toxin portion of the
immunotoxin. Because of structure and sequence similarities, we evaluated a CET40
immunotoxin for the presence of PE-related
epitopes. In western blots, three-of-three anti-PE antibody preparations failed to react with the CET40
immunotoxin. More importantly, in neutralization studies neither these
antibodies nor those from patients with neutralizing titers to PE38, neutralized the CET40-immunotoxin. We propose that the use of modular components such as antibody Fvs and toxin domains will allow a greater flexibility in how these agents are designed and deployed including the sequential administration of a second
immunotoxin after patients have developed
neutralizing antibodies to the first.