In vitro activity: Eltrombopag demonstrates a half maximal effective concentration (EC50) of 0.27 μM in murine BAF3 cells transfected with the luciferase reporter gene under direction of the STAT-activated IRF-1 promoter and human TpoR (BAF3/IRF-1/hTpoR). Eltrombopag activates the receptor by association with metal ions (i.e., Zn2+) and specific amino acids within the transmembrane and juxtamembrane domains of the TpoR. Eltrombopag (30 μM) results in activation of STAT5 in N2C-Tpo cells, as detected with an antiphospho-STAT5 antibody on Western blots. Eltrombopag stimulates proliferation after a 2-day incubation with an EC50 of 0.03 μM in a BrdU assay conducted in BAF3/hTpoR cells. Eltrombopag also induces differentiation of hematopoietic stem cells into committed megakaryocyte progenitor cells. Eltrombopag increases the differentiation of bone marrow CD34+ cells into CD41+ megakaryocytes in a dose-dependent manner with an EC50 of 0.1 μM. Eltrombopag inhibits N2C-Tpo cell and HEL92.1.7 cell proliferating with IC50 of 20.7 μg/mL and 2.3 μg/mL. Eltrombopag (20 μg/mL) leads to a decreased cell division rate, a block in G(1) phase of cell cycle, and increased differentiation in human and murine leukemia cells.

Cell Assay: Eltrombopag (5 μg/mL) shows clear signs of differentiation, significant changes in the organization of the nuclear contents, and an increase in the cytoplasm/nucleus ratio in HL60 cells. Eltrombopag (5 μg/mL) causes an increase in CD11b, which is consistent with a premacrophage state in U937 cells, and also causes an increase in CD11b in URE cells. Eltrombopag leads to a reduction in free intracellular iron in leukemic cells in a dose-dependent manner in HL60 cells.