Anti-prostate immunotoxins: Cytotoxicity of E4 antibody–pseudomonas exotoxin constructs

E4 is a monoclonal antibody (MAb) that reacts with a surface antigen present on normal prostate and prostate cancers. Using this antibody, 2 immunotoxins were generated, one being a chemical conjugate with a mutant truncated form of Pseudomonas exotoxin A (PE), E4-PE35KDEL. The other is a recombinant single chain immunotoxin, E4(Fv)-PE38KDEL. The affinity of the conjugated immunotoxin was similar to the hybridoma-produced MAb E4, revealing that conjugation did not impair the binding ability. The affinity of the recombinant immunotoxin (10 nM) was 10-fold lower than that of the MAb, probably reflecting differences of bivalent (MAb) vs. monovalent (Fv) binding. Antigen positive prostate, breast and colon carcinoma cell lines showed cytotoxic response to the E4 immunotoxins while antigen negative cells were not affected. The IC50 value, representing a 50% inhibition of cellular protein synthesis, ranged from 0.3 to 20 ng/ml for E4-PE35KDEL and from 2 to 100 ng/ml for E4(Fv)-PE38KDEL. Therefore, the E4-derived immunotoxins may be useful for the treatment of prostate as well as breast and colon cancers.