Pharmacokinetics and Pharamcodynamics of Recombinant Human Thrombopoietin Following Single Injection in Rats

Background: Thrombopoietin (TPO) is the primary regulator of platelet production. Recombinant human thrombopoietin (rHuTPO) and its derivative have been reported as potent drugs in treatment of chemotherapy induced thrombocytopenia and idiopathic thrombocytopenic purpura. Due to severe immunogenic adverse affects clinical applications of rHuTPO have been limited. However, new TPO receptor agonists that lack this adverse effect are developed. RHuTPO exhibits nonlinear kinetics and complex dynamics both mediated by binding to TPO receptor. Studies of rHuTPO pharmacokinetics (PK) and pharmacodynamics (PD) will facilitate future use of new TPO receptor agonists.

Purpose: To analyze PK and PD of rHuTPO following a single IV bolus administration in rats.

Methods: Adult male Wistar rats of body weight 300–600 g received a single dose of rHuTPO (ProSpec Ltd) (2.4, 4.8 and 9.6 μg/kg) and a placebo via an intravenous injection in tail. Blood samples (100–200μL) were collected at various hours during 3 days for PK analysis and every day for consecutive 14 days for PD assessment. RHuTPO plasma concentrations were measured using ELISA (R&D Systems Inc). Platelet count was determined using hematology analyzer (BC-2800 VET, Mindray). Noncompartmental parameters such as area under concentration curve (AUC), clearance (CL), volume at steady-state (Vss), and terminal half-life (t_1/2) were calculated by WinNonlin 5.2 (Pharsight). Percent increase over the baseline for platelet maximum (%MPC) and area under the platelet response curve (%AUC_PLT) were applied as PD markers. Student t-test was used to compare the means between control and treatment groups (SAS 9.1, SAS Institute).

Results: CL decreased with increasing dose (0.185, 0.131, and 0.047 L/h/kg), similarly to Vss (3.28, 1.10, and 0.51 L). The t_1/2 values were also dose dependent (13.2, 6.1, and 8.2 h). The %MPC values were 43, 77, and 45% for corresponding doses and the peaks occurred on days 5, 6, and 6 following rHuTPO administration. The %AUC_PLT values were relatively insensitive to increasing doses (20, 23, and 13%). Both mean peak platelet counts and mean AUC_PLT were significantly different (p < 0.05) from the control values with an exception of AUC_PLT for dose 2.4 μg/kg.

Conclusions: RHuTPO exhibits nonlinear pharmacokinetics in rat. CL and Vss dependence on dose suggests receptor mediated drug disposition as a major reason of nonlinear PK. IV bolus doses 2.4, 4.8 and 9.6 μg/kg of rHuTPO moderately increase platelet count in rats that peaks on day 5–6 after drug administration. The observed lack of the PD marker increase with increased doses implicates that the applied doses are in a flat region of the dose-response curve. Compared to similar data reported for humans, rHuTPO is less efficacious in rats than in humans.