Pharmacokinetics of Tumor-reactive Immunotoxins in Tumor-bearing Mice: Effect of Antibody Valency and Deglycosylation of the Ricin A Chain on Clearance and Tumor Localization¹

The blood clearance and tissue distribution of immunotoxins composed of either intact or Fab′ fragments of tumor-reactive, monoclonal rat anti-murine immunoglobulin D (anti-δ) or nonreactive, normal rat immunoglobulin G and ricin A chain (native or chemically deglycosylated) were determined in BCL₁ tumor-bearing mice. In the presence of accessible target cells, neither the valency of the antibody nor deglycosylation of the A chain affect the initial rate of clearance (α phase) of immunotoxins from the blood; however, both factors affect the levels of tumor-specific and nonspecific localization of the immunotoxins. Thus, the use of immunotoxins with deglycosylated A chain greatly reduced the levels of nonspecific uptake by the liver and concomitantly increased tumorspecific localization. Immunotoxins prepared with Fab′ fragments of anti-δ antibody and deglycosylated A chain were twice as effective at localizing to splenic tumor than immunotoxins prepared with intact immunoglobulin G and deglycosylated A chain. Approximately 90% of tumor-specific localization of anti-δ immunotoxins occurred within 1 h after injection and less than 5% of the immunotoxin remained in the circulation 4 h after injection. In addition, the antigen-binding capacity of the remaining circulating immunotoxins decreased in a linear manner over the first 10 h to approximately 20% of the initial binding activity. Thus, by 10 h after injection, only 2–3% of injected immunotoxin remained in the circulation and this material expressed little antigen reactivity. Analysis of the in vivo stability of circulating ¹²⁵I-labeled immunotoxins in tumor-bearing mice demonstrated that Fab′ immunotoxins are more stable than IgG immunotoxins prepared with N-succinimidyl-3-(2-pyridylthio)propionate. In BCL₁-bearing mice, significant splitting of the anti-δ immunotoxins did not occur until tumor localization was virtually complete.