title: The Syk-kinase inhibitor R406 impairs platelet activation and monocyte tissue factor expression triggered by heparin-PF4 complex directed antibodies.
abstract: BACKGROUND
  Heparin-induced thrombocytopenia (HIT) is a rare but severe complication of heparin therapy in which immunoglobulin G (IgG) antibodies against the platelet factor 4-heparin complex activate platelets through the FcγRIIA receptor. Clustering of FcγRIIA initiates signaling cascades involving tyrosine kinases including the spleen tyrosine kinase (Syk). Moreover, besides the critical role of platelets, the expression of tissue factor (TF) by human monocytes triggered by HIT antibodies has been shown to contribute to the hypercoagulability and the thrombotic complications in HIT patients.
  
  
  OBJECTIVE
  We investigated the effect of R406, a small molecule inhibitor of Syk developed as a potential treatment of autoimmune diseases, allergic disorders and B-cell related hematological malignancies, on FcγRIIA-mediated platelet activation. To further assess the potential activity of Syk inhibitors in HIT treatment, the effect of R406 was also evaluated on HIT antibodies-induced expression of TF and procoagulant activity of monocytic cells.
  
  
  RESULTS
  We show that R406 is a potent inhibitor of platelet signaling and functions initiated by FcγRIIA cross-linking by specific antibodies or by sera from HIT patients. Syk inhibition efficiently prevents FcγRIIA-induced LAT phosphorylation and activation of phosphoinositide 3-kinase, Akt, phospholipase Cγ2 and p38 MAP-kinase. As a consequence, FcγRIIA-induced platelet aggregation, granule secretion and microparticles production are strongly inhibited by R406. Moreover, the Syk inhibitor efficiently impairs the expression of TF and the procoagulant activity of human monocytes triggered by HIT antibodies.
  
  
  CONCLUSIONS
  Syk inhibitors may be of therapeutic interest in the treatment of HIT by reducing HIT antibodies-mediated platelet activation and monocyte procoagulant activity.
authors.affiliation: Inserm, U1048 and Université Toulouse 3, I2MC, 31432 Toulouse Cedex 04, France.
authors.forename: T
authors.initials: T
authors.lastname: Lhermusier
authors.forename: J
authors.initials: J
authors.lastname: van Rottem
authors.forename: C
authors.initials: C
authors.lastname: Garcia
authors.forename: J-M
authors.initials: JM
authors.lastname: Xuereb
authors.forename: A
authors.initials: A
authors.lastname: Ragab
authors.forename: V
authors.initials: V
authors.lastname: Martin
authors.forename: M-P
authors.initials: MP
authors.lastname: Gratacap
authors.forename: P
authors.initials: P
authors.lastname: Sié
authors.forename: B
authors.initials: B
authors.lastname: Payrastre
mesh_terms: D001483:Base Sequence; D017931:DNA Primers; D006493:Heparin / Q000276:immunology*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000037:antagonists & inhibitors*; D009000:Monocytes / Q000378:metabolism*; D010078:Oxazines / Q000494:pharmacology*; D015539:Platelet Activation / Q000187:drug effects*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D011725:Pyridines / Q000494:pharmacology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000072377:Syk Kinase; D013925:Thromboplastin / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D047908:Intracellular Signaling Peptides and Proteins; C510788:N4-(2,2-dimethyl-3-oxo-4H-pyrid(1,4)oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine; D010078:Oxazines; D047428:Protein Kinase Inhibitors; D011725:Pyridines; D006493:Heparin; D013925:Thromboplastin; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase