Pharmacokinetics of tumor-reactive immunotoxins in tumor-bearing mice: effect of antibody valency and deglycosylation of the ricin A chain on clearance and tumor localization.

The blood clearance and tissue distribution of immunotoxins composed of either intact or Fab' fragments of tumor-reactive, monoclonal rat antimurine immunoglobulin D (anti-delta) 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 (alpha 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 tumor-specific localization. Immunotoxins prepared with Fab' fragments of anti-delta 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-delta 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-delta immunotoxins did not occur until tumor localization was virtually complete.