Bioavailability and relative tissue distribution of [125I]‐recombinant human thrombin following intravenous or subcutaneous administration to non‐human primates

Summary.  Background: Recombinant human thrombin (rhThrombin) is being developed as a general adjunct to hemostasis. Endogenous thrombin is rapidly inactivated by complex formation with antithrombin III and other inhibitors. It follows that these inhibitors will also inactivate any rhThrombin that reaches the systemic circulation. Objectives: Study goals were to determine the pharmacokinetic characteristics of [125I]‐rhThrombin and [125I]‐rhThrombin complexed to endogenous inhibitors, and the tissue distribution of rhThrombin‐associated radioactivity in non‐human primates. Hematology, serum chemistry and coagulation status were also monitored. Methods: [125I]‐rhThrombin was administered intravenously (i.v.; 3.5 U kg−1) or subcutaneously (s.c.; 350 U kg−1) to male cynomolgus monkeys. Plasma was analyzed for rhThrombin‐associated radioactivity and non‐compartmental analysis was used to determine the corresponding pharmacokinetic parameters. A size exclusion–high pressure liquid chromatography (SE–HPLC) method was used to quantitate rhThrombin complexes, non‐complexed rhThrombin, and free [125I]. Whole‐body gamma scintigraphy was used to follow radioactivity localization up to 72 h postdose. Results: No adverse events were observed following [125I]‐rhThrombin administration. The pharmacokinetic profile of rhThrombin‐associated radioactivity following i.v. injection was multi‐exponential with an initial half‐life of approximately 10 min. Following both i.v. and s.c. dosing, the terminal half‐life was approximately 15 h. SE–HPLC analysis revealed that rhThrombin was rapidly complexed to antithrombin III and other inhibitors in the systemic circulation following i.v. administration. Thus, rhThrombin‐associated radioactivity in the blood was complexed and presumed inactive. [125I]‐rhThrombin inhibitor complexes accumulated and were eliminated in the liver following both routes of administration. Conclusions: These data suggest that rhThrombin rapidly binds to endogenous inhibitors following either i.v. or s.c. administration.