Protein and Small-Molecule Leucopoiesis and Thrombopoiesis Stimulators.

Pluripotent stem cells of the bone marrow are stimulated by different cytokines to proliferation and differentiation into various types of blood cells. These cytokines are mostly glycoproteins. Erythropoietin stimulates stem cells to the formation of erythrocytes while colony-stimulating factors cause the formation of different types of white blood cells. Stem cell factors play an important role in the maintenance and survival of blood cells of all types. Thrombopoietin stimulates stem cells to proliferation and formation of blood platelets. Granulocyte colony-stimulating factor is probably the most important drug in use. It stimulates stem cells to the formation of neutrophile granulocytes. It is often used in recombinant forms such as filgrastim in the treatment of neutropenia in cancer chemotherapy or AIDS. Its pegylated conjugates such as pegfilgrastim are also available. Its activity can be supported by plerixafor, a small molecule - bicyclam derivative acting as an indirect agonist of stem cells factor. It acts as an antagonist of CXCR4 receptor activation of which brakes hematopoiesis. The treatment of conditions accompanied by thrombocytopenia such as idiopathic thrombocytopenic purpura is currently not performed by thrombopoietin but synthetic agonists of its receptor are preferred. Romiplostim is a peptibody. It consists of a protein part interacting with the thrombopoietin receptor which is, however, different from thrombopoietin, and of Fc fragment of immunoglobulin G1. In contrast, small molecule thrombopoietin receptor agonists represented by eltrombopag can be given orally unlike all of the above.