A recombinant protein with megakaryopoiesis stimulating activity. Romiplostim mimics endogenous thrombopoietin (TPO), directly binding to and activating the platelet thrombopoietin receptor (TpoR, Mpl, or CD110 antigen), a cytokine receptor belonging to the hematopoietin receptor superfamily. Activation of TpoR stimulates the proliferation and differentiation of megakaryocytes, resulting in an increase in the production of blood platelets. or 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. Romiplostim 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. Romiplostim 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 AMG 531 and Nplate.