A novel nonpeptidyl human c-Mpl activator stimulates human megakaryopoiesis and thrombopoiesis

NIP-004 is a novel synthetic compound developed to display human thrombopoietin (TPO) receptor (c-MpI) agonist activity. NIP-004 displays species specificity, stimulating proliferation or differentiation of human c-MpI-expressing cells such as UT-7/TPO and human CD34(+) cells but not murine c-MpI-expressing cells or cynomolgus monkey cells. To test the mechanism of its action, we constructed mutant forms of c-MpI; murine c-MpI(L490H) displayed a response to NIP-004, whereas human C-MpI(H499L) lost this response, indicating that histidine in the transmembrane domain of c-MpI is essential for its activity. Because histidine is not present in the c-MpI transmembrane domain of rats, hamsters, rhesus macaques, and cynomolgus monkeys, we examined the in vivo efficacy of NIP-004 using mice that received xenotransplants. In immunodeficient nonobese diabetic (NOD)/Shi-scid, IL-2R gamma(null) (NOG) mice receiving transplants of umbilical cord blood-derived CD34(+) cells, NIP-004 increased human megakaryoblasts, mature megakaryocytes, and circulating human platelets 6-fold, the latter being morphologically and functionally indistinguishable from normal human platelets. These observations indicate that NIP-004 is a novel human c-MpI activator and induces human thrombopoiesis.