A drug used to treat patients with immune thrombocytopenic purpura (ITP) who do not get better with other forms of treatment. In ITP, platelets (cells that cause blood clots to form) are destroyed by the immune system. AMG 531 is being studied as a way to treat low platelet counts caused by chemotherapy. It binds to the thrombopoietin receptor and causes the bone marrow to make more platelets. AMG 531 is also being studied in the treatment of myelodysplastic syndromes (a group of diseases in which the bone marrow does not make enough healthy blood cells). It is a type of thrombopoietin agonist. Also called Nplate and romiplostim.