YM477, a Novel Orally-Active Thrombopoietin Receptor Agonist.

Thrombopoietin (TPO) is the principal physiologic regulator of platelet production. The search for an orally-active nonpeptidyl small molecule TPO receptor agonist has resulted in the discovery of YM477. YM477 acted specifically on the TPO receptor and stimulated megakaryocytopoiesis throughout the development and maturation of megakaryocytes just as TPO does. YM477, however, was shown to have high species specificity, effective in only humans and chimpanzees. Recently, nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice were characterized as an efficient engraftment model for human hematopoietic stem cells, as this model results in the production of human platelets. In this way, we examined the in vivo platelet-increasing effect of YM477 in human platelet-producing NOD/SCID mice in which human hematopoietic stem cells were transplanted. In this study, we used commercially available cryopreserved human fetal liver CD34⁺ cells as a source of human hematopoietic stem cells. The cells were transplanted into sublethally irradiated (240 cGy) NOD/SCID mice. Human platelets started to appear in peripheral blood of these mice 4 weeks after transplantation. The production of human platelets continued up to one year post-transplant. Various doses of YM477 (0, 0.3, 1, and 3 mg/kg/day) were orally administered for 14 days to NOD/SCID mice that had been confirmed to produce human platelets stably. Oral administration of YM477 dose-dependently increased the number of human platelets produced by these mice, with significance at 1 mg/kg/day and above. The increase in the human platelet count reached about 2.7-fold at 1 mg/kg/day and about 3.0-fold at 3 mg/kg/day on day 14. Withdrawal of YM477 administration caused the human platelet count to return to the pretreatment level. The number of murine platelets did not change during the study period. Next, to evaluate the function of human platelets produced in peripheral blood of these mice, the expression of activation-dependent marker CD62P (P-selectin) on human platelets stimulated with thrombin receptor agonist peptide (TRAP) were examined. CD62P expression on human platelets was induced by the stimulation of blood from transplanted mice with TRAP, suggesting that human platelets produced in NOD/SCID mice were functional. Furthermore, the maximum response of CD62P expression on human platelets induced by TRAP was evaluated before and after administration of YM477 at 3 mg/kg/day for 14 days. CD62P expression was not changed by administration of YM477, which was similar to the results obtained with a vehicle group. These results suggest that YM477 is an orally active TPO receptor agonist useful for treating patients with thrombocytopenia.