Monoclonal anti-MM46 antibody:ricin A chain conjugate: in vitro and in vivo antitumor activity.

In an approach to antitumor agents with improved tumor specificity, the ricin toxic subunit A chain was covalently cou pled with a monoclonal lgG2b antibody directed against MM antigen, a tumor-specific antigen on syngeneic mouse mam mary tumor MM46 cells (anti-MM46 IgG), using N-succinimidyl 3-(2-pyridyldithio)propionate as cross-linking agent. The con jugate thus prepared (anti-MM46 conjugate) showed potent dose-dependent cytotoxicity against MM antigen-positive MM46 cells in vitro and inhibited the cell growth at concentra tions above 1 jug/ml. The immunological specificity was verified by the observation that anti-MM46 conjugate did not show cytotoxicity against MM antigen-negative MM48 cells. Neither nonimmune conjugate similarly prepared from mouse nonim mune IgG nor unconjugated anti-MM46 IgG alone exhibited cytotoxicity against MM46 cells. Anti-MM46 IgG still retained considerable in vitro complement-dependent cytotoxicity against MM46 cells after conjugation with ricin A chain. 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. A markedly enhanced efficacy of antiMM46 conjugate was also observed in therapeutic experi ments. When a group of five C3H/He mice inoculated i.p. with 5 x 104 MM46 cells were treated with an i.p. injection of 1 fig of anti-MM46 conjugate on Days 1, 3, and 5, all five mice survived tumor-free, although those treated with 1 ,ug of antiMM46 IgG died before Day 20 with a life span similar to those of mice treated with nonimmune conjugate or phosphatebuffered saline (control). Thus, the in vivo efficacy of antiMM46 conjugate over anti-MM46 IgG alone was demonstrated by therapeutic experiments as well as by tumor-neutralizing assays. Although anti-MM46 conjugate showed no antitumor effect when injected i.p. to C3H/He mice bearing s.c.-inocu lated MM46 tumor (inoculum, 4 x 105) on Days 1, 3, 5, and 7 at a dose of 10 /ng, it inhibited tumor growth when injected intraregionally to tumor-bearing mice, suggesting that the con jugate is effective also to solid-type MM46 tumor if a sufficient amount of anti-MM46 conjugate reaches the tumor site.