Platelet Aggregation Analysis of AMG 531 in a Randomized, Placebo-Controlled Study in Healthy Japanese Subjects.

AMG 531 is a novel thrombopoiesis-stimulating peptibody currently being investigated for the treatment of chronic immune thrombocytopenic purpura. This study was a double-blind, phase 1 evaluation of safety (including a platelet aggregation analysis), pharmacodynamics, and pharmacokinetics of AMG 531 in healthy Japanese men. Thirty subjects were randomly assigned 4:1 (AMG 531:placebo) to each of 3 sequential dose cohorts (0.3, 1, and 2 μg/kg) of 10 subjects each. A single dose of AMG 531 or placebo was administered by subcutaneous injection, and subjects were evaluated for 6 weeks. Concentrations of 0.2 and 2 μg/mL collagen and 0.5 and 2 μmol/L adenosine phosphate were used as reagents in the platelet aggregation analysis conducted on day -1, day 2, day 15, and at the end of the study. No enhancement or reduction in platelet aggregation was detected in samples collected from subjects treated with AMG 531 compared with either pretreatment or placebo samples. Overall, adverse events were similar between the AMG 531 and placebo groups. Treatment-related adverse events (headache, “feeling hot,” migraine without aura, and/or malaise) were reported for 5 of 24 subjects treated with AMG 531. Four of 8 subjects who received the 1 μg/kg dose and 7 of 8 subjects who received the 2 μg/kg dose had platelet count increases ≥ 1.5-fold over baseline, and 3 of 8 subjects who received the 2 μg/kg dose had platelet count increases ≥ 2-fold over baseline, a pharmacodynamic effect similar to that seen in previous studies of non-Japanese subjects. Serum concentrations of AMG 531 were below the lower limit of quantification in all but 2 subjects who received the 2 μg/kg dose. In summary, platelets in subjects treated with AMG 531 functioned normally compared with pretreatment and placebo samples. AMG 531 was generally well tolerated and was effective at raising platelet counts after a single subcutaneous administration.