Comparison of inhibitory effects of irreversible and reversible Btk inhibitors on platelet function

C-type lectin-like receptor 2: roles and drug target

C-type lectin-like receptor-2 (CLEC-2) is a member of the C-type lectin superfamily of cell surface receptors. The first confirmed endogenous and exogenous ligands of CLEC-2 are podoplanin and rhodocytin, respectively. CLEC-2 is expressed on the surface of platelets, which participates in platelet activation and aggregation by binding with its ligands. CLEC-2 and its ligands are involved in pathophysiological processes, such as atherosclerosis, cancer, inflammatory thrombus status, maintenance of vascular wall integrity, and cancer-related thrombosis. In the last 5 years, different anti- podoplanin antibody types have been developed for the treatment of cancers, such as glioblastoma and lung cancer. New tests and new diagnostics targeting CLEC-2 are also discussed. CLEC-2 mediates thrombosis in various pathological states, but CLEC-2-specific deletion does not affect normal hemostasis, which would provide a new therapeutic tool for many thromboembolic diseases. The CLEC-2-podoplanin interaction is a target for cancer treatment. CLEC-2 may be applied in clinical practice and play a therapeutic role.

Modelling arterial thrombus formation in vitro

PURPOSE OF REVIEW

Models of arterial thrombus formation represent a vital experimental tool to investigate platelet function and test novel antithrombotic drugs. This review highlights some of the recent advances in modelling thrombus formation in vitro and suggests potential future directions.

RECENT FINDINGS

Microfluidic devices and the availability of commercial chips in addition to enhanced accessibility of 3D printing has facilitated a rapid surge in the development of novel in-vitro thrombosis models. These include progression towards more sophisticated, 'vessel on a chip' models which incorporate vascular endothelial cells and smooth muscle cells. Other approaches include the addition of branches to the traditional single channel to yield an occlusive model; and developments in the adhesive coating of microfluidic chambers to better mimic the thrombogenic surface exposed following plaque rupture. Future developments in the drive to create more biologically relevant chambers could see a move towards the use of human placental vessels, perfused ex-vivo. However, further work is required to determine the feasibility and validity of this approach.

SUMMARY

Recent advances in thrombus formation models have significantly improved the pathophysiological relevance of in-vitro flow chambers to better reflect the in-vivo environment and provide a more translational platform to test novel antithrombotics.

Role of SHP2 (PTPN11) in glycoprotein VI-dependent thrombus formation: Improved platelet responsiveness by the allosteric drug SHP099 in Noonan syndrome patients

INTRODUCTION

The protein tyrosine phosphatase SHP2 (PTPN11) is a negative regulator of glycoprotein VI (GPVI)-induced platelet signal under certain conditions. Clinical trials with derivatives of the allosteric drug SHP099, inhibiting SHP2, are ongoing as potential therapy for solid cancers. Gain-of-function mutations of the PTPN11 gene are observed in part of the patients with the Noonan syndrome, associated with a mild bleeding disorder. Assessment of the effects of SHP2 inhibition in platelets from controls and Noonan syndrome patients.

MATERIALS AND METHODS

Washed human platelets were incubated with SHP099 and stimulated with collagen-related peptide (CRP) for stirred aggregation and flow cytometric measurements. Whole-blood microfluidics assays using a dosed collagen and tissue factor coating were performed to assess shear-dependent thrombus and fibrin formation. Effects on clot formation were evaluated by thromboelastometry.

RESULTS

Pharmacological inhibition of SHP2 did not alter GPVI-dependent platelet aggregation under stirring, but it enhanced integrin αIIbβ3 activation in response to CRP. Using whole-blood microfluidics, SHP099 increased the thrombus buildup on collagen surfaces. In the presence of tissue factor and coagulation, SHP099 increased thrombus size and reduced time to fibrin formation. Blood from PTPN11-mutated Noonan syndrome patients, with low platelet responsiveness, after ex vivo treatment with SHP099 showed a normalized platelet function. In thromboelastometry, SHP2 inhibition tended to increase tissue factor-induced blood clotting profiles with tranexamic acid, preventing fibrinolysis.

CONCLUSION

Pharmacological inhibition of SHP2 by the allosteric drug SHP099 enhances GPVI-induced platelet activation under shear conditions with a potential to improve platelet functions of Noonan syndrome patients.

Differential Regulation of GPVI-Induced Btk and Syk Activation by PKC, PKA and PP2A in Human Platelets

Bruton's tyrosine kinase (Btk) and spleen tyrosine kinase (Syk) are major signaling proteins in human platelets that are implicated in atherothrombosis and thrombo-inflammation, but the mechanisms controlling their activities are not well understood. Previously, we showed that Syk becomes phosphorylated at S297 in glycoprotein VI (GPVI)-stimulated human platelets, which limits Syk activation. Here, we tested the hypothesis that protein kinases C (PKC) and A (PKA) and protein phosphatase 2A (PP2A) jointly regulate GPVI-induced Btk activation in platelets. The GPVI agonist convulxin caused rapid, transient Btk phosphorylation at S180 (pS180↑), Y223 and Y551, while direct PKC activation strongly increased Btk pS180 and pY551. This increase in Btk pY551 was also Src family kinase (SFK)-dependent, but surprisingly Syk-independent, pointing to an alternative mechanism of Btk phosphorylation and activation. PKC inhibition abolished convulxin-stimulated Btk pS180 and Syk pS297, but markedly increased the tyrosine phosphorylation of Syk, Btk and effector phospholipase Cγ2 (PLCγ2). PKA activation increased convulxin-induced Btk activation at Y551 but strongly suppressed Btk pS180 and Syk pS297. PP2A inhibition by okadaic acid only increased Syk pS297. Both platelet aggregation and PLCγ2 phosphorylation with convulxin stimulation were Btk-dependent, as shown by the selective Btk inhibitor acalabrutinib. Together, these results revealed in GPVI-stimulated platelets a transient Syk, Btk and PLCγ2 phosphorylation at multiple sites, which are differentially regulated by PKC, PKA or PP2A. Our work thereby demonstrated the GPVI-Syk-Btk signalosome as a tightly controlled protein kinase network, in agreement with its role in atherothrombosis.

Zanubrutinib for the treatment of lymphoid malignancies: Current status and future directions

Zanubrutinib (BGB-3111, Brukinsa®, BeiGene) is a next-generation irreversible inhibitor of Bruton's tyrosine kinase (BTK), developed by BeiGene in 2012 for the treatment of B-cell malignancies. It was designed to minimize off-target inhibition of TEC- and EGFR-family kinases. Zanubrutinib is more selective than ibrutinib for BTK versus EGFR, FGR, FRK, HER2, HER4, ITK, JAK3, LCK, BLK and TEC. In addition, compared to ibrutinib, zanubrutinib has improved oral absorption and better target occupancy. Zanubrutinib demonstrated a lower incidence of off-target toxicities and reduced severity than ibrutinib. Moreover, zanubrutinib is similar to acalabrutinib, with less activity against TEC and ITK. The preliminary phase 1 results suggest that zanubrutinib has clinical activity and the drug is well tolerated in patients with B-cell lymphoid malignancies. Recent clinical trials have found it to demonstrate excellent efficacy and good tolerability in patients with chronic lymphocytic leukemia (CLL), Waldenstrom macroglobulinemia (WM) and mantle cell lymphoma (MCL). In recent phase 3 studies, zanubrutinib was compared with ibrutinib in patients with relapsed/refractory (R/R) MW and RR CLL. In both trials, zanubrutinib was found to demonstrate clinically meaningful advantages in safety and tolerability over ibrutinib; in particular, it was associated with a lower risk of atrial fibrillation/flutter and major bleeding events. In the recent SEQUOIA study, comparing zanubrutinib with bendamustine and rituximab (BR) in patients with previously untreated CLL, zanubrutinib significantly improved progression-free survival versus BR, with an acceptable safety profile consistent with previous studies. Zanubrutinib also demonstrated good activity and tolerability in patients with R/R MCL, marginal zone lymphoma and follicular lymphoma. Trials examining the efficacy and safety of the combination of zanubrutinib with obinutuzumab venetoclax and other drugs are ongoing. This review summarizes the clinical efficacy and safety of zanubrutinib in lymphoid malignancies.

Platelets and tyrosine kinase inhibitors: clinical features, mechanisms of action, and effects on physiology

Tyrosine kinase inhibitors (TKIs) have emerged as a promising class of target-directed, small molecule inhibitors used to treat hematologic malignancies, inflammatory diseases, and autoimmune disorders. Recently, TKIs have also gained interest as potential antiplatelet-directed therapeutics that could be leveraged to reduce pathologic thrombus formation and atherothrombotic complications, while minimally affecting platelet hemostatic function. This review provides a mechanistic overview and summarizes the known effects of tyrosine kinase inhibitors on platelet signaling and function, detailing prominent platelet signaling pathways downstream of the glycoprotein VI (GPVI) receptor, integrin αIIbβ3, and G protein-coupled receptors (GPCRs). This review focuses on mechanistic as well as clinically relevant and emerging TKIs targeting major families of tyrosine kinases including but not limited to Bruton's tyrosine kinase (BTK), spleen tyrosine kinase (Syk), Src family kinases (SFKs), Janus kinases (JAK), and signal transducers and activators of transcription (STAT) and evaluates their effects on platelet aggregation and adhesion, granule secretion, receptor expression and activation, and protein phosphorylation events. In summation, this review highlights current advances and knowledge on the effects of select TKIs on platelet biology and furthers insight on signaling pathways that may represent novel druggable targets coupled to specific platelet functional responses.

Antithrombotic Effects of Fostamatinib in Combination with Conventional Antiplatelet Drugs

New antithrombotic medications with less effect on haemostasis are needed for the long-term treatment of acute coronary syndromes (ACS). The platelet receptor glycoprotein VI (GPVI) is critical in atherothrombosis, mediating platelet activation at atherosclerotic plaque. The inhibition of spleen tyrosine kinase (Syk) has been shown to block GPVI-mediated platelet function. The aim of our study was to investigate if the Syk inhibitor fostamatinib could be repurposed as an antiplatelet drug, either alone or in combination with conventional antiplatelet therapy. The effect of the active metabolite of fostamatinib (R406) was assessed on platelet activation and function induced by atherosclerotic plaque and a range of agonists in the presence and absence of the commonly used antiplatelet agents aspirin and ticagrelor. The effects were determined ex vivo using blood from healthy volunteers and aspirin- and ticagrelor-treated patients with ACS. Fostamatinib was also assessed in murine models of thrombosis. R406 mildly inhibited platelet responses induced by atherosclerotic plaque homogenate, likely due to GPVI inhibition. The anti-GPVI effects of R406 were amplified by the commonly-used antiplatelet medications aspirin and ticagrelor; however, the effects of R406 were concentration-dependent and diminished in the presence of plasma proteins, which may explain why fostamatinib did not significantly inhibit thrombosis in murine models. For the first time, we demonstrate that the Syk inhibitor R406 provides mild inhibition of platelet responses induced by atherosclerotic plaque and that this is mildly amplified by aspirin and ticagrelor.