Influence of Structural Variations on Biological Activity of Anti-PSMA scFv and Immunotoxins Targeting Prostate Cancer

In this study, different variants of the anti-PSMA single-chain antibody fragment (scFv) D7 were cloned, varying in linker type, position of hexahistidine tags and VH-VL orientation. From these scFv, Pseudomonas exotoxin A-based immunotoxins were constructed and their biological activities against prostate cancer cells were compared. Binding of the constructs to PSMA-expressing prostate cancer cells was determined by flow cytometry. ADP-ribosyltransferase activity was analysed and cytotoxicity was measured with WST-1 assays. Different linker types did not influence the characteristics of the scFv or immunotoxins. The addition of an N-terminal hexahistidine-tag, however, resulted in decreased expression, binding and cytotoxicity. scFv in VH-VL orientation showed the highest expression and binding, whereas immunotoxins in VL-VH orientation exhibited the best binding and cytotoxicity. The present study showed how structure influences the characteristics of scFv and immunotoxins. It is therefore suggested that for each individual construct the optimal structure has to be determined separately.