Conjugates of elliptinium acetate with mouse monoclonal anti-α-fetoprotein antibodies or fab fragments: In vitro cytotoxic effects upon human hepatoma cell lines

Elliptinium acetate (EA) is a new anti-cancer compound displaying cytostatic activity against various malignancies including hepatoma. Using 3 hepatoma cell lines, we compared the in vitro activity of doxorubicin (reference drug), of EA and of conjugates made up with this latter drug and monoclonal antibodies (MAbs). The linkage was performed by a direct oxidation method. Specific immunoconjugates were prepared with an anti-alphafetoprotein (AFP) MAb (AF01) or its Fab fragment (Fab AF01). Non-specific conjugates were obtained with an anti-thyroglobulin MAb (TG01) or its Fab (Fab TG01). Direct membrane injury (51Cr-release), DNA and protein synthesis as well as AFP release were investigated for all compounds. Free EA displayed only weak activity on DNA and protein synthesis, at 10-fold higher molar concentration than doxorubicin. Conjugation of EA with whole AF01 allowed significant potentiation of protein synthesis inhibition without affecting the 3 other tests. In contrast, Fab AF01 × EA conjugates displayed a marked effect in the 4 tests; in particular, this conjugate was at least 100 times more efficient than any other compound when tested in the 51Cr-release test. Neither Fab AF01 nor free EA alone or in combination exhibited such an effect. Fab TG01 × EA conjugate was not directly cytotoxic but potentiated inhibition of DNA and protein synthesis between 2- and 10-fold. The mechanism of the direct cytotoxic effect of anti-AFP Fab × EA conjugate, which has never been described in any other immunodrug model, was investigated.