[In vitro and in vivo antitumor activity of monoclonal anti-MM46 antibody-ricin A chain conjugate].

In an approach to antitumor agents with improved tumor specificity, the ricin toxic subunit A chain was covalently coupled with a monoclonal IgG2b antibody directed against MM antigen, a tumor-specific antigen on syngeneic mouse mammary tumor MM46 cells (anti-MM46 IgG), using N-succinimidyl-3-(2-pyridyldithio) propionate as cross-linking agent. The conjugate thus prepared (anti-MM46 conjugate) showed potent dose-dependent cytotoxicity against MM antigen-positive MM46 cells in vitro and inhibited the cell growth at concentrations above 1 micrograms/ml. The immunological specificity was verified by the observation that anti-MM46 conjugate did not show cytotoxicity against MM antigen-negative MM48 cells. In Winn-type tumor-neutralizing assay in which C3H/He mice were inoculated i.p. or s. c. with MM46 cells preincubated with a test material, anti-MM46 conjugate showed greater activity than did anti-MM46 IgG. When a group of five C3H/He mice inoculated i.p. with 5 X 10(4) MM46 cells were treated with an i.p. injection of 1 micrograms of anti-MM46 conjugate on days 1, 3, and 5, all five mice survived tumor free, although those treated with 1 micrograms of anti-MM46 IgG died before day 20 with a life span similar to those of mice treated with non-immune conjugate or phosphate-buffered saline (the control). Anti-MM46 conjugate also showed antitumor effects when injected i.v. to C3H/He mice bearing s. c. inoculated MM46 (inoculum, 4 X 10(6)) on day 1 at doses of 5 to 50 micrograms. Thus, the results demonstrated that in vivo efficacy of anti-MM46 conjugate over anti-MM46 IgG alone by therapeutic experiments as well as by tumor-neutralizing assay and suggested the potential use of monoclonal antibody-cytotoxic agent conjugates in cancer therapy.