[Preparation and anti-tumor activity of monoclonal antibody against ascitic mammary tumor MM46 cells].

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 a 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 microgram/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 compared that of 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 microgram of anti-MM46 conjugate on days 1, 3, and 5, all mice survived tumor free, although those treated with 1 microgram of anti-MM46 IgG died before day 20 with a life span similar to those of mice treated with nonimmune conjugate or phosphate-buffered saline (control). Anti-MM46 conjugate also showed antitumor effects when injected i.v. to C3H/He mice bearing s.c. inoculated MM46 (inoculum, 2 X 10(6] on Day 1 at a dose of 5 to 50 micrograms. Thus, the in vivo efficacy of anti-MM46 conjugate over anti-MM46 IgG alone was demonstrated by therapeutic experiments as well as by tumor-neutralizing assays.