title: Comparative analyses of the small molecule thrombopoietin receptor agonist eltrombopag and thrombopoietin on in vitro platelet function.
abstract: OBJECTIVE
  The thrombopoietin receptor (TPOR) is a therapeutic target for treatment of thrombocytopenia because stimulation of this receptor results in enhanced megakaryocyte proliferation, differentiation, and ultimately platelet production. In addition to effects on megakaryocytes, TPOR stimulation also impacts platelet function. The present study examined platelet function following stimulation with the small molecule TPOR agonist eltrombopag.
  
  
  METHODS
  Platelets were obtained from healthy volunteers, and signal transduction pathway activation was examined in washed platelet preparations. Platelet aggregation was examined in both washed platelet preparations and platelet-rich plasma. Platelet alpha-granule release was determined via fluorescein-activated cell sorting measurement of CD62P.
  
  
  RESULTS
  In signal transduction studies of washed human platelets, eltrombopag induced the phosphorylation signal transducers and activators of transcription (STAT) proteins with no phosphorylation of Akt, whereas recombinant human TPO (rhTPO) induced the phosphorylation of Akt as well as STAT-1, -3, and -5. In studies conducted at subthreshold/submaximal concentrations of adenosine diphosphate (ADP) or collagen, eltrombopag pretreatment did not result in platelet aggregation. In contrast, rhTPO acted in synergy with submaximal concentrations of ADP or collagen to induce maximal aggregation under all conditions examined. Similarly, platelet activation as examined via surface expression of CD62P was not enhanced by eltrombopag pretreatment as compared to rhTPO.
  
  
  CONCLUSIONS
  These results demonstrate that the nonpeptidyl TPOR agonist eltrombopag stimulates platelet signal transduction with little or no effect on overall platelet function, in contrast to TPO, which significantly primes platelet activation. These data demonstrate that effects of TPOR ligands on platelet function can vary depending on the specific mechanism utilized to stimulate the TPOR.
authors.affiliation: Department of Oncology, GlaxoSmithKline, Collegeville, PA 19426, USA. Joseph.A.Erhardt@gsk.com
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Erhardt
authors.forename: Connie L
authors.initials: CL
authors.lastname: Erickson-Miller
authors.forename: Manuel
authors.initials: M
authors.lastname: Aivado
authors.forename: Melanie
authors.initials: M
authors.lastname: Abboud
authors.forename: Kodandaram
authors.initials: K
authors.lastname: Pillarisetti
authors.forename: John R
authors.initials: JR
authors.lastname: Toomey
mesh_terms: D001565:Benzoates / Q000494:pharmacology*; D001792:Blood Platelets / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D006834:Hydrazines / Q000494:pharmacology*; D008297:Male; D019007:P-Selectin / Q000096:biosynthesis; D010974:Platelet Aggregation / Q000187:drug effects*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D011720:Pyrazoles / Q000494:pharmacology*; D053628:Receptors, Thrombopoietin / Q000819:agonists* / Q000378:metabolism; D050791:STAT Transcription Factors / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D013921:Thrombocytopenia / Q000188:drug therapy / Q000378:metabolism; D013926:Thrombopoietin / Q000494:pharmacology*
chemical_list: D001565:Benzoates; D006834:Hydrazines; D019007:P-Selectin; D011720:Pyrazoles; D053628:Receptors, Thrombopoietin; D050791:STAT Transcription Factors; C082218:MPL protein, human; D013926:Thrombopoietin; D051057:Proto-Oncogene Proteins c-akt; C520809:eltrombopag