Antitumor activity of L6-ricin immunotoxin against the H2981-T3 lung adenocarcinoma cell line in vitro and in vivo.
The monoclonal antibody L6 recognizes a determinant that is expressed on lung, breast, colon, and ovarian carcinomas and is present only at trace levels in normal tissues. L6 was covalently linked to intact ricin by a thioether bond to produce an immunotoxin (IT). Gel analysis revealed that this IT was heterogeneous, but mostly one monoclonal antibody molecule linked to one ricin molecule. The L6-ricin IT selectively bound and was selectively toxic to L6-positive H2981-T3 adenocarcinoma cells in protein synthesis inhibition assays in which lactose was added to block the native ricin binding site. Clonogenic studies showed that 1 microgram/ml L6-ricin could inhibit about 99.99% of H2981-T3 growth in a limiting dilution assay, even in the presence of a 20-fold excess of human bone marrow cells. Treatment of bone marrow cells with the same dose of L6-ricin resulted in the growth of ample numbers of bone marrow progenitor cells (colony-forming units-mixed, colony-forming units granulocyte/macrophage, and blast-forming units erythroid) after 14 days. We also evaluated the antitumor effect of L6-ricin administered intratumorally with lactose against established H2981-T3 tumors in a nude mouse model. Thirty % of the tumor-bearing animals responded completely to single-dose treatment, while 60% gave partial responses. The in vivo effects were not absolutely specific, since irrelevant anti-CD5 IT also induced tumor regression in this model (10% responded completely, while 30% gave partial responses). However, irrelevant IT gave higher systemic toxicity (50% mortality) than L6-ricin (23% mortality). The nonspecific activity of IT was possibly due to Fc binding, which was demonstrated in vitro, or due to ricin B-chain binding. Ricin alone was too toxic for sustained tumor protection. Unconjugated L6 had no antitumor effect. The data suggest that L6-ricin may be useful for in vitro purging of autologous bone marrow from patients with solid tumors and marrow involvement and for in vivo regional therapy of L6-positive carcinomas.
Department of Therapeutic Radiology, University of Minnesota Hospitals and Clinics, Minneapolis 54455.
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King
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Lasky
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D000230:Adenocarcinoma / Q000378:metabolism / Q000628:therapy*; D000911:Antibodies, Monoclonal / Q000378:metabolism / Q000627:therapeutic use; D000943:Antigens, Differentiation / Q000276:immunology; D018956:CD5 Antigens; D006801:Humans; D007167:Immunotherapy; D000922:Immunotoxins / Q000378:metabolism / Q000627:therapeutic use*; D008175:Lung Neoplasms / Q000378:metabolism / Q000628:therapy*; D012276:Ricin / Q000276:immunology / Q000378:metabolism / Q000627:therapeutic use*; D014407:Tumor Cells, Cultured
D000911:Antibodies, Monoclonal; D000943:Antigens, Differentiation; D018956:CD5 Antigens; D000922:Immunotoxins; D012276:Ricin