© 1996 Oxford University Press
research-article |
Immunogenicity of Tissue Plasminogen Activators in Rhesus Monkeys: Antibody Formation and Effects on Blood Level and Enzymatic Activity1
Toxicology Research Laboratories, Lilly Research Laboratories, a Division of Eli Lilly and Company P.O. Box 708, Greenfield, Indiana 46140
Received June 9, 1995; accepted October 23, 1995
The immunogenicity of a tissue-type plasminogen activator analog, mt-PA6, consisting of the second kringle and protease domains, was compared to that of the native-sequence protein (ntPA) in rhesus monkeys. Antibody responses were compared in groups of eight monkeys that were treated by iv injection twice, 1 month apart, using doses and regimens chosen to mimic therapy (0.5 mg/kg mt-PA6 bolus, 1.25 mg/kg nt-PA bolus + infusion). An additional group was treated with a 0.5 mg/kg nt-PA bolus. A single positive response was obtained in a monkey treated with 0.5 mg/kg nt-PA after the primary injection. Following the secondary injection, responses were obtained in 1/8, 3/8, and 6/8 monkeys treated with mt-PA6, nt-PA as a boles, or nt-PA as a bolus + infusion, respectively. Several monkeys were selected to determine whether circulating tPA antibody altered the pharmacokinetics of mt-PA6. Clearance was found to decrease without affecting peak blood levels as antibody concentrations increased from 0.02 to 100 µg/ml. In contrast, the peak blood level was reduced by 99% at an antibody concentration of 152 µg/ml in a monkey that had been exposed to mt-PA6 in adjuvant 14 months previously. Further, only the serum from this and three other hyperimmunized monkeys inhibited the enzymatic activity of tPA in vitro. It is concluded that mt-PA6 is not more immunogenic than nt-PA in rhesus, and that low levels of antibody are more likely to influence the pharmacokinetic properties of tPA than to inhibit its enzymatic activity. It is unlikely that mt-PA6 would present a serious immunogenic risk in humans.