Discovery of Non-Peptidyl Small-Molecule Human GCSF Receptor Agonists for the Potential Treatment of Neutropenia,

Abstract 3391

Granulocyte colony stimulating factor (GCSF) is the essential cytokine for the regulation of neutrophilic granulocytes. Binding of GCSF to its receptor (GCSFR) triggers receptor dimerization, leading to activation of JAK1 and JAK2, phosphorylation of GCSFR, STAT3, STAT5, and Ras/mitogen-activated protein kinase (MAPK), and results in proliferation and differentiation of granulocytic cells. Recombinant human GCSF (rhGCSF) is used successfully to alleviate chemotherapy-induced neutropenia, neutropenia associated with hematopoietic stem cell transplantation, and severe chronic neutropenia. A small molecule oral GCSFR agonist may offer a safer and more convenient alternative to the current injectable rhGCSF therapy. Whereas a previous effort to identify small-molecule mimetics of GCSF found SB-247464 that selectively activated the murine GCSFR, no small-molecule human GCSF mimetics have been developed. Recently, we have discovered a series of novel non-peptidyl small molecules that selectively activate human GCSFR (hGCSFR) function, and may provide a significant innovation in the treatment of neutropenia. In cells transiently transfected with an hGCSFR expression vector and a STAT3-responsive luciferase reporter, a lead compound, LG7455, activates luciferase expression with an efficacy of 50% relative to rhGCSF, and potency (EC₅₀) of 100 nM. LG7455 also activates luciferase expression in cells transfected with hGCSFR and a STAT5-responsive luciferase reporter (65%, 40 nM EC₅₀). The activity of LG7455 is dependent on the expression of hGCSFR, and LG7455 is not active in luciferase assays when human thrombopoietin receptor (hTPOR) or erythropoietin receptor (hEPOR) is expressed. In UT-7 cells made responsive to GCSF by stable transfection of hGCSFR (UTP-hGCSFR), LG7455 stimulated cell growth and increased the phosphorylation of STAT3 and STAT5. LG7455 did not increase growth of TPO- or EPO-responsive UT-7 cells. In CD34 positive human bone marrow hematopoietic cells (BM-HCs), LG7455, increased the percentage of cells positive for the granulocyte-specific marker CD15 (FUT4). The effect of LG7455 in BM-HCs was additive to the effect of rhGCSF. LG7455 is active in luciferase assays with expressed cynomolgus monkey GCSFR, but not mouse, guinea pig or rabbit GCSFR. Similar to what has been demonstrated for small-molecule human TPOR agonists such as eltrombopag, the activity of LG7455 is dependent on a specific residue in the hGCSFR transmembrane domain. When histidine 627 (His-627) in hGCSFR is changed to asparagine present at a similar location in the mouse GCSFR (Asp-602), unlike rhGCSF, LG7455 is no longer active. LG7455 is active, however, on mouse GCSFR with Asp-602 replaced by His. In radioligand-binding experiments using UTP-hGCSFR cells, LG7455 did not displace [¹²⁵I]rhGCSF, however binding of [¹²⁵I]rhGCSF was augmented in a concentration dependent manner consistent with allosteric receptor modulation. These data demonstrate that LG7455 is a novel small-molecule selective hGCSFR agonist that activates the receptor in a manner distinct from GCSF and similar to the mechanism of small-molecule hTPOR agonists. Further optimization of the LG7455 chemical series should provide orally-available molecules to treat neutropenia with improved safety and convenience compared to current injectable rhGCSF.