Spleen tyrosine kinase (SYK) is an important regulatory molecule of signal transduction pathways involved in the pathogenesis of autoimmune diseases such as immune thrombocytopenia (ITP), and the SYK-signaling pathway has emerged as a potential target for the treatment of numerous diseases. The aim of this narrative review is to summarize the biological properties of SYK and its involvement in disease pathways, provide an update on SYK inhibitors in the treatment of ITP, and consider other potential applications. Fostamatinib, the only licensed SYK inhibitor to date, produces clinical response in ITP patients, including those who are refractory to other treatments. It appears to reduce the risk of thrombotic events and may therefore be a drug to consider for patients with an increased thrombotic risk. Encouraging results have also been obtained in the treatment of warm autoimmune hemolytic anemia. Several other SYK inhibitors have entered clinical trials for a range of indications, reflecting the ability of these drugs to affect multiple signaling pathways. SYK inhibitors have the potential to target several aspects of COVID-19 pathogenesis including thrombosis, without affecting normal hemostasis, and data from the first study of fostamatinib in COVID-19 are encouraging. It is hoped that ongoing trials in autoimmune indications other than ITP, as well as in hematological malignancies and other disorders, confirm the promise of SYK inhibitors. Immune thrombocytopenia (ITP) is an autoimmune disease that usually happens when your immune system mistakenly attacks and destroys platelets, which are cells that help blood to clot. Individuals with ITP can experience easy or excessive bruising and bleeding. Scientists have identified that an enzyme called spleen tyrosine kinase (SYK) is involved in numerous biological processes that are associated with the immune system response, inflammation, and some types of cancer in humans. Therefore, it has become a target for new drugs which inhibit the action of SYK. In this review article, the authors provide a summary of the biological properties and actions of SYK and its involvement in various diseases, discuss information about drugs that have been developed as SYK inhibitors for the treatment of ITP, and consider other potential uses for drugs that inhibit SYK. Although several drugs are being developed, the only SYK inhibitor that is currently available for the treatment of ITP is a drug called fostamatinib. In patients with ITP, including those who no longer respond to other treatments, fostamatinib has been shown to improve platelet counts and reduce bleeding events. Researchers are also currently investigating the use of drugs that inhibit SYK, including fostamatinib, for the potential treatment of other diseases associated with inflammation (e.g. rheumatoid arthritis, COVID-19), autoimmunity (e.g. warm autoimmune hemolytic anemia), and blood cancers (e.g. lymphoma, chronic lymphocytic leukemia, and acute myeloid leukemia).

Ibrutinib and acalabrutinib are irreversible inhibitors of Bruton9s tyrosine kinase (Btk) used in the treatment of B cell malignancies. They bind irreversibly to cysteine 481 of Btk, blocking autophosphorylation on tyrosine (Y) 223 and phosphorylation of downstream substrates including phospholipase-ɣ2 (PLCɣ2). In the present study, we demonstrate that concentrations of ibrutinib and acalabrutinib that block Btk kinase activity as shown by loss of phosphorylation at Y223 and PLCɣ2 delay but do not block aggregation to a maximally-effective concentration of CRP or collagen. In contrast, 10- 20 fold higher concentrations of ibrutinib or acalabrutinib block platelet aggregation to GPVI agonists. Ex vivo studies on patients treated with ibrutinib, but not acalabrutinib, show a reduction of platelet aggregation to CRP indicating that the clinical dose of ibrutinib but not acalabrutinib is supramaximal for Btk blockade. Unexpectedly, low concentrations of ibrutinib inhibit aggregation to CRP in patients deficient in Btk. The increased bleeding seen with ibrutinib over acalabrutinib is due to off-target actions of ibrutinib that occur because of unfavourable pharmacodynamics.

Inhibitors of the tyrosine kinase Btk have been proposed as novel antiplatelet agents. In this study we show that low concentrations of the Btk inhibitor ibrutinib block CLEC-2-mediated activation and tyrosine phosphorylation including Syk and PLCγ2 in human platelets. Activation is also blocked in patients with X-linked agammaglobulinemia (XLA) caused by a deficiency or absence of Btk. In contrast, the response to GPVI is delayed in the presence of low concentrations of ibrutinib or in patients with XLA, and tyrosine phosphorylation of Syk is preserved. A similar set of results is seen with the second-generation inhibitor, acalabrutinib. The differential effect of Btk inhibition in CLEC-2 relative to GPVI signalling is explained by the positive feedback role involving Btk itself, as well as ADP and thromboxane A2 mediated activation of P2Y12 and TP receptors, respectively. This feedback role is not seen in mouse platelets and, consistent with this, CLEC-2-mediated activation is blocked by high but not by low concentrations of ibrutinib. Nevertheless, thrombosis was absent in 8 out of 13 mice treated with ibrutinib. These results show that Btk inhibitors selectively block activation of human platelets by CLEC-2 relative to GPVI suggesting that they can be used at 'low dose' in patients to target CLEC-2 in thrombo-inflammatory disease.

Platelet factor 4 (PF4) is an abundant chemokine that is released from platelet α-granules on activation. PF4 is central to the pathophysiology of vaccine-induced immune thrombocytopenia and thrombosis (VITT) in which antibodies to PF4 form immune complexes with PF4, which activate platelets and neutrophils through Fc receptors. In this study, we show that PF4 binds and activates the thrombopoietin receptor, cellular myeloproliferative leukemia protein (c-Mpl), on platelets. This leads to the activation of Janus kinase 2 (JAK2) and phosphorylation of signal transducer and activator of transcription (STAT) 3 and STAT5, leading to platelet aggregation. Inhibition of the c-Mpl–JAK2 pathway inhibits platelet aggregation to PF4, VITT sera, and the combination of PF4 and IgG isolated from VITT patient plasma. The results support a model in which PF4-based immune complexes activate platelets through binding of the Fc domain to FcγRIIA and PF4 to c-Mpl.

Antiplatelet medications comprise the cornerstone of treatment for diseases that involve arterial thrombosis, including acute coronary syndromes (ACS), stroke and peripheral arterial disease. However, antiplatelet medications may cause bleeding and, furthermore, thrombotic events may still recur despite treatment. The interaction of collagen with GPVI receptors on the surface of platelets has been identified as one of the major players in the pathophysiology of arterial thrombosis that occurs following atherosclerotic plaque rupture. Promisingly, GPVI deficiency in humans appears to have a minimal impact on bleeding. These findings together suggest that targeting platelet GPVI may provide a novel treatment strategy that provides additional antithrombotic efficacy with minimal disruption of normal hemostasis compared to conventional antiplatelet medications. CLEC-2 is gaining interest as a therapeutic target for a variety of thrombo-inflammatory disorders including deep vein thrombosis (DVT) with treatment also predicted to cause minimal disruption to hemostasis. GPVI and CLEC-2 signal through Src, Syk and Tec family tyrosine kinases, providing additional strategies for inhibiting both receptors. In this review, we summarize the evidence regarding GPVI and CLEC-2 and strategies for inhibiting these receptors to inhibit platelet recruitment and activation in thrombotic diseases.

(2020). A rationale for blocking thromboinflammation in COVID-19 with Btk inhibitors. Platelets: Vol. 31, No. 5, pp. 685-690.