Antitumor Effect of Actinomycin D Entrapped in Liposomes Bearing Subunits of Tumor-specific Monoclonal Immunoglobulin M Antibody¹

Sonicated liposomes containing actinomycin D in the membranes were chemically coated with the subunits of monoclonal immunoglobulin M (IgM) antibody against a mouse mammary tumor-associated antigen (MM antigen) and examined for their in vitro and in vivo antitumor effects against MM46 (MM⁺) and MM48 (MM^-) tumors of C3H/He mouse origin. The antibody-bearing, actinomycin D-containing liposomes (chemoimmunoliposomes) were selectively bound to MM⁺ tumor cells and showed much more in vitro cytotoxicity against the tumor cells than that shown by free actinomycin D. The in vivo antitumor effect of the chemoimmunoliposomes was tested on the mammary tumor cells (5 × 10⁴ to 5 × 10⁶) transplanted i.p. into syngeneic mice. A single i.p. injection of the chemoimmunoliposomes containing 0.3, 0.5, or 1 µg of actinomycin D into MM46 tumor-bearing mice resulted in the cure of some mice and a prolonged survival time in the rest of the mice as compared to results in controls. In this test, free actinomycin D, anti-MM IgM antibody, and bovine serum albumin-coated liposomes containing actinomycin D were marginally effective or ineffective. To examine a systemic antitumor effect of chemoimmunoliposomes, mice were inoculated with MM46 tumor cells and then treated with a single i.v. injection of liposomes 4 days later. If the mice were pretreated with an i.v. injection of unmodified multilamellar liposomes, an injection of the chemoimmunoliposomes containing 1 µg of actinomycin D resulted in a significant inhibition of tumor growth. Both free actinomycin D and bovine serum albumin-coated liposomes containing actinomycin D were ineffective against the s.c. tumor. These results indicate that an antitumor drug entrapped in the membranes of small sonicated liposomes bearing antitumor monoclonal antibodies can be delivered to antigenic tumor cells and exert more efficient antitumor activity than does the free drug.