A Novel Thrombopoiesis-Stimulating Agent, AMG 531: Pharmacokinetics and Pharmacodynamics in FcRn Knock-Out and Wild Type Mice.
Authors: Sun Yu-Nien , Arends Rosalin , Smithson Anthony , Watson Ann , Nichol Janet L. 
Published: April 2020

AMG 531 is a novel platelet-stimulating peptibody that targets the thrombopoietin (TPO) receptor, resulting in an increased production of platelets. This molecule consists of an Mpl receptor binding domain of peptide and a carrier Fc domain. The purpose of this study was to evaluate pharmacokinetics (PK) and platelet response profiles (pharmacodynamics; PD) of AMG 531 in FcRn knock-out (KO) vs. wild type (WT) mice. Male C5BL/6J-B2m FcRn KO mice (n = 177) and male C57BL/6J WT (n = 162) received a single dose of 100 or 1000 μg/kg of AMG 531 intravenously (IV). One additional group of KO mice (n = 87) received a single IV dose of 300 μg/kg of AMG 531. At defined times, approximately 0.3 mL of blood per animal was collected by cardiac puncture from 3 mice for the measurement of serum AMG 531 concentration (PK) and platelet counts (PD). Serum AMG 531 concentrations were determined by a sandwich enzyme-linked immunosorbent assay (ELISA) with a lower limit of quantification (LLOQ) of 1.774 ng/mL. The composite mean values for PK and PD at each time point were analyzed by the non-compartmental method. Serum AMG 531 concentration-time profiles showed that AMG 531 was eliminated faster in KO mice compared to WT. After a single IV (100 – 1000 μg/kg) administration, AMG 531 exhibited non-linear pharmacokinetics in both KO and WT mice. A 24- and 16-fold increase in systemic clearance (CL) was observed in KO mice (128 and 318 mL/h/kg) relative to the WT (8.23 and 13.1 mL/h/kg) at the 100 and 1000 μg/kg dose levels, respectively. The estimated apparent half-life of AMG 531 was shorter in KO mice (1.3–2.2 hours) than in WT (6.5–12.5 hours). The estimated steady-state volumes of distribution were greater in KO mice (193–464 mL/kg) than in WT (104–158 mL/kg), suggesting that the absence of the binding of AMG 531 to the FcRn receptor in KO mice may enhance the extent of peripheral distribution. Pharmacodynamic response (increase of platelet counts after AMG 531 administration) was more pronounced in WT than in KO mice. The mean baseline platelet count at predose ranged from 1250 to 1390 x103/mm3 in KO mice and from 805 to 1280 x103/mm3 in WT. After single IV administration of AMG 531, a dose-dependent increase in the maximum platelet count (Pmax): initial minimum platelet count ratios of 1.8 to 2.1-fold for KO mice and 4.7 to 8.4-fold for WT were observed. When the dose increased from 100 to 1000 μg/kg, the Pmax values increased from approximately 2240 to 2550 x103/mm3 in KO mice and 4900 to 6722 x103/mm3 in WT. The Pmax values occurred at 96 to 120 hours (4–5 days) in KO mice and 92 to 264 hours (8–11 days) in WT. These results suggest that the FcRn receptor may act as a salvage receptor for AMG 531, maintaining serum concentrations in mice. The higher AMG 531 exposure in WT mice resulted in elevated platelet response relative to KO mice in a dose-dependent manner. This novel construct with a carrier Fc domain may sustain the exposure of the molecule and therefore provide added efficacy and increase the clinical benefit of AMG 531.