Eltrombopag (formerly SB-497115; SB 497115; SB497115; trade name PROMACTA), a member of the biarylhydrazone class, is a novel nonpeptide agonist of the thrombopoietin receptor (TpoR) approved for use in the treatment of chronic hepatitis C-associated thrombocytopenia and chronic immune (idiopathic) thrombocytopenia (ITP). Eltrombopag demonstrates an 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. 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.