Cloning of thrombopoietin and its therapeutic potential

Recently, four groups reported the cloning of thrombopoietin (TPO), also called c-Mpl ligand, from various species. In this study, we examined the in vitro and in vivo biological activity of TPO and its therapeutic efficacy in experimental animal models. Recombinant human TPO (rhTPO) supported the formation of only megakaryocyte (MK) colonies from rat marrow MK progenitor cells [colony-forming units-megakaryocyte (CFU-MK)] and predominantly acted on GpIIb/IIIa+ CFU-MK at the late stage of differentiation. MKs generated from rat GpIIb/IIIa+ CFU-MK after 3 days of liquid culture in the presence of rhTPO had mature characteristics. rhTPO stimulated an increase in the size of TPO-induced cultured rat MKs and in the number of elongated cytoplasmic processes, also called proplatelets, from these MKs in a dose-dependent manner. Administration of rhTPO to normal BALB/c mice daily for 5 days caused dose-dependent thrombocytosis. Treatment with rhTPO induced an increase in the size and number of marrow MKs and an expansion of the marrow CFU-MK pool. We further examined the effects of rhTPO on chemotherapy-induced thrombocytopenia in animal models. Following treatment with mitomycin C, mice received daily injections of various doses of rhTPO. Administration of rhTPO reduced the severity of thrombocytopenia and accelerated the recovery of platelets in a dose-dependent fashion: there was a significant reduction in the decrease in numbers of marrow MKs and CFU-MK with rhTPO treatment. Treatment with rhTPO also significantly improved neutropenia in mitomycin C-treated mice. Similar therapeutic efficacy was observed in cynomolgus monkeys with thrombocytopenia induced by nimustine. In addition, there was no significant change in several serum-chemistry parameters, in C-reactive protein, an acute phase protein, or in some variables involved in the blood-coagulation system. Furthermore, platelets from mice made thrombocytotic by repeated administration of rhTPO showed normal aggregation function. These results strongly suggest the clinical usefulness of rhTPO for the treatment of thrombocytopenia.