In Vivo Therapy of the BCL₁ Tumor: Effect of Immunotoxin Valency and Deglycosylation of the Ricin A Chain¹

The in vivo therapeutic effects of Fab′ and IgG anti-δ (anti-IgD)-ricin A chain immunotoxins were compared in mice bearing the surface IgD-positive BCL₁ leukemia. The immunotoxins were prepared with either native or deglycosylated ricin A chain. Immunotoxin therapy was assessed both by the number of cells bearing the BCL₁ immunoglobulin idiotype which remained in the spleen 24–48 h after injection with immunotoxin and by adoptive transfer of these spleen cells into normal mice. Immunotoxins prepared with either Fab′-anti-δ or IgG-anti-δ and native A chain induced a dose-dependent reduction in the number of idiotypepositive BCL₁ cells present in the spleens of the tumor-bearing mice. The maximal therapeutic response was achieved with 250 µg of immunotoxin (containing 80–100 µg A chain) per mouse for both immunotoxins, resulting in tumor reduction of approximately 90%. This represents an elimination of 3–4 × 10³ tumor cells. Reduction in the number of tumor cells was not observed with control reagents including antibody alone, antibody mixed with A chain, or an immunotoxin of irrelevant specificity. A Fab′ immunotoxin prepared with deglycosylated ricin A chain was approximately 5-fold more effective as an antitumor reagent than the same immunotoxin prepared with native A chain; thus, optimal therapy was achieved after injection of 50 µg of immunotoxin (containing 15–20 µg A chain). Since the immunotoxins prepared with deglycosylated A chain were only 2–3-fold more toxic to the mice than those prepared with native A chain, the former resulted in a 2–3-fold increase in the therapeutic index.