SELECTIVE ANTITUMOR EFFECT ON L10 HEPATO-CARCINOMA CELLS OF A POTENT IMMUNOCONJUGATE COMPOSED OF THE A-CHAIN OF ABRIN AND A MONOCLONAL-ANTIBODY TO A HEPATOMA-ASSOCIATED ANTIGEN

Abstract

The toxic A chain of abrin was isolated by affinity chromatography and was demonstrated to be a potent inhibitor of protein synthesis in a cell-free rabbit reticulocyte system with a complete inhibitory dose at 1 .times. 10-9 M. This A chain was coupled by a disulfide linkage to a purified monoclonal antibody directed against a tumor-associated antigen found on the line 10 hepatocarcinoma tumor in strain 2 guinea pigs. The immunoconjugate retained the functions of the individual components, i.e., antigen binding to the intact cell in vitro and inhibition of its protein synthesis. This conjugate was a selective antineoplastic agent with a cytocidal dose at 5 .times. 10-9 M toward antigen-bearing cells in vitro. Several antigen-negative cells were much less susceptible to its cytotoxic effect. The cytotoxicity of the conjugate appeared to be by antibody-mediated delivery of toxic A chain into the target cell. When cells were pretreated with excess free antibody followed by a brief exposure to conjugate, there was a reversal of the cytotoxicity to antigen-positive cells but not to the antigen-negative cells. The therapeutic efficacy of the conjugate was assayed by injecting a single dose s.c. or i.v. into syngeneic guinea pigs bearing established line 10 tumors. These in vivo studies showed that the conjugate was not toxic at a dosage of 60-1120 .mu.g/guinea pig, the conjugate decreased or abolished the growth of established solid tumors, the conjugate delayed or inhibited tumor metastases to lymph nodes and 20-40% of the animals in selective groups had a long-term complete regression.