Bruton tyrosine kinase is essential for botrocetin/VWF-induced signaling and GPIb-dependent thrombus formation in vivo

Hemin regulates platelet clearance in hemolytic disease by binding to GPIbα

Hemolysis is associated with thrombosis and vascular dysfunction, which are the pathological components of many diseases. Hemolytic products, including hemoglobin and hemin, activate platelets (PLT). Despite its activation, the effect of hemolysis on platelet clearance remains unclear, It is critical to maintain a normal platelet count and ensure that circulating platelets are functionally viable. In this study, we used hemin, a degradation product of hemoglobin, as a potent agonist to treat platelets and simulate changes in vivo in mice. Hemin treatment induced activation and morphological changes in platelets, including an increase in intracellular Ca2+ levels, phosphatidylserine (PS) exposure, and cytoskeletal rearrangement. Fewer hemin-treated platelets were cleared by macrophages in the liver after transfusion than untreated platelets. Hemin bound to glycoprotein Ibα (GPIbα), the surface receptor in hemin-induced platelet activation and aggregation. Furthermore, hemin decreased GPIbα desialylation, as evidenced by reduced Ricinus communis agglutinin I (RCA- I) binding, which likely extended the lifetime of such platelets in vivo. These data provided new insight into the mechanisms of GPIbα-mediated platelet activation and clearance in hemolytic disease.

Activation of Platelet mTORC2/Akt Pathway by Anti-β2GP1 Antibody Promotes Thrombosis in Antiphospholipid Syndrome

BACKGROUND

Anti-β2GP1 (β2-glycoprotein 1) antibodies are the primary pathogenic antibody to promote thrombosis in antiphospholipid syndrome (APS), yet the underlying mechanism remains obscure. We aimed to explore the intracellular pathway that mediated platelet activation.

METHODS

Platelets were isolated from patients with APS and subjected to RNA sequencing. Platelet aggregation, the release of platelet granules, platelet spreading, and clot retraction were detected to evaluate platelet activation. We purified anti-β2GP1 antibodies from patients with APS and the total IgG from healthy donors to stimulate platelets with/without FcγRIIA (Fcγ receptor IIA) blocking antibody or Akt (protein kinase B) inhibitor. Platelet-specific Sin1 (stress-activated protein kinase-interacting protein) deficiency mice were established. The thrombus model of inferior vena cava flow restriction, ferric chloride-induced carotid injury model, and laser-induced vessel wall injury in cremaster arterioles model were constructed after administration of anti-β2GP1 antibodies.

RESULTS

Combined RNA sequencing and bioinformatics analysis suggested that APS platelets exhibited increased levels of mRNA associated with platelet activation, which was in line with the hyperactivation of APS platelets in response to stimuli. Platelet activation in APS platelets was accompanied by upregulation of the mTORC2 (mammalian target of the rapamycin complex 2)/Akt pathway and increased levels of SIN1 phosphorylation at threonine 86. Anti-β2GP1 antibody derived from patients with APS enhanced platelet activation and upregulated the mTORC2/Akt pathway. Moreover, the Akt inhibitor weakened the potentiating effect of the anti-β2GP1 antibody on platelet activation. Notably, Sin1 deficiency suppresses anti-β2GP1 antibody-enhanced platelet activation in vitro and thrombosis in all 3 models.

CONCLUSIONS

This study elucidated the novel mechanism involving the mTORC2/Akt pathway, which mediates the promotion of platelet activation and induction of thrombosis by the anti-β2GP1 antibody. The findings suggest that SIN1 may be a promising therapeutic target for the treatment of APS.

CD36 mediates SARS-CoV-2-envelope-protein-induced platelet activation and thrombosis

Aberrant coagulation and thrombosis are associated with severe COVID-19 post-SARS-CoV-2 infection, yet the underlying mechanism remains obscure. Here we show that serum levels of SARS-CoV-2 envelope (E) protein are associated with coagulation disorders of COVID-19 patients, and intravenous administration of the E protein is able to potentiate thrombosis in mice. Through protein pull-down and mass spectrometry, we find that CD36, a transmembrane glycoprotein, directly binds with E protein and mediates hyperactivation of human and mouse platelets through the p38 MAPK-NF-κB signaling pathway. Conversely, the pharmacological blockade of CD36 or p38 notably attenuates human platelet activation induced by the E protein. Similarly, the genetic deficiency of CD36, as well as the pharmacological inhibition of p38 in mice, significantly diminishes E protein-induced platelet activation and thrombotic events. Together, our study reveals a critical role for the CD36-p38 axis in E protein-induced platelet hyperactivity, which could serve as an actionable target for developing therapies against aberrant thrombotic events related to the severity and mortality of COVID-19.

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.

Research progress on the interaction between oxidative stress and platelets: Another avenue for cancer?

Oxidative stress (OS) is a chemical imbalance between an oxidant and an antioxidant, causing damage to redox signaling and control or causing molecular damage. Unbalanced oxidative metabolism can produce excessive reactive oxygen species (ROS). These excess ROS can cause drastic changes in platelet metabolism and further affect platelet function. It will also lead to an increase in platelet procoagulant phenotype and cell apoptosis, which will increase the risk of thrombosis. The creation of ROS and subsequent platelet activation, adhesion, and recruitment are then further encouraged in an auto-amplifying loop by ROS produced from platelets. Meanwhile, cancer cells produce a higher concentration of ROS due to their fast metabolism and high proliferation rate. However, excessive ROS can result in damage to and modification of cellular macromolecules. The formation of cancer and its progression is strongly associated with oxidative stress and the resulting oxidative damage. In addition, platelets are an important part of the tumor microenvironment, and there is a significant cross-communication between platelets and cancer cells. Cancer cells alter the activation status of platelets, their RNA spectrum, proteome, and other properties. The "cloaking" of cancer cells by platelets providing physical protection，avoiding destruction from shear stress and the attack of immune cells, promoting tumor cell invasion.We explored the vicious circle interaction between ROS, platelets, and cancer in this review, and we believe that ROS can play a stimulative role in tumor growth and metastasis through platelets.

Role of GPR56 in Platelet Activation and Arterial Thrombosis

The adhesion G protein-coupled receptor GPR56 mediates cell-cell and cell-extracellular matrix interactions. To examine the function of GPR56 in platelet activation and arterial thrombosis, we generated GPR56-knockout mice and evaluated GPR56 expression in human and mouse platelets. The results revealed that the levels of the GPR56 N-terminal fragment were significantly higher on the first day after myocardial infarction than on the seventh day in the plasma of patients with ST-segment-elevation myocardial infarction. Next, we investigated the effects of GPR56 on platelet function in vitro and in vivo. We observed that collagen-induced aggregation and adenosine triphosphate release were reduced in Gpr56 ^-/- platelets. Furthermore, P-selectin expression on the Gpr56 ^-/- platelet surface was also reduced, and the spreading area on immobilized collagen was decreased in Gpr56 ^-/- platelets. Furthermore, collagen-induced platelet activation in human platelets was inhibited by an anti-GPR56 antibody. Gpr56 ^-/- mice showed an extended time to the first occlusion in models with cremaster arteriole laser injury and FeCl₃-induced carotid artery injury. GPR56 activated the G protein 13 signaling pathway following collagen stimulation, which promoted platelet adhesion and thrombus formation at the site of vascular injury. Thus, our study confirmed that GPR56 regulated the formation of arterial thrombosis. Inhibition of the initial response of GPR56 to collagen could significantly inhibit platelet activation and thrombus formation. Our results provide new insights for research into antiplatelet drugs.

Dual roles of fucoidan-GPIbα interaction in thrombosis and hemostasis: implications for drug development targeting GPIbα

BACKGROUND

Platelet GPIbα-von Willebrand factor (VWF) interaction initiates platelet adhesion, activation, and thrombus growth, especially under high shear conditions. Therefore, the GPIb-VWF axis has been suggested as a promising target against arterial thrombosis. The polysaccharide fucoidan has been reported to have opposing prothrombotic and antithrombotic effects; however, its binding mechanism with platelets has not been adequately studied.

OBJECTIVE

The objective of this study was to explore the mechanism of fucoidan and its hydrolyzed products in thrombosis and hemostasis.

METHODS

Natural fucoidan was hydrolyzed by using hydrochloric acid and was characterized by using size-exclusion chromatography, UV-visible spectroscopy, and fluorometry techniques. The effects of natural and hydrolyzed fucoidan on platelet aggregation were examined by using platelets from wild-type, VWF and fibrinogen-deficient, GPIbα-deficient, and IL4Rα/GPIbα-transgenic and αIIb-deficient mice and from human beings. Platelet activation markers (P-selectin expression, PAC-1, and fibrinogen binding) and platelet-VWF A1 interaction were measured by using flow cytometry. GPIbα-VWF A1 interaction was evaluated by using enzyme-linked immunosorbent assay. GPIb-IX-induced signal transduction was detected by using western blot. Heparinized whole blood from healthy donors was used to test thrombus formation and growth in a perfusion chamber.

RESULTS

We found that GPIbα is critical for fucoidan-induced platelet activation. Fucoidan interacted with the extracellular domain of GPIbα and blocked its interaction with VWF but itself could lead to GPIbα-mediated signal transduction and, subsequently, αIIbβ3 activation and platelet aggregation. Conversely, low-molecular weight fucoidan inhibited GPIb-VWF-mediated platelet aggregation, spreading, and thrombus growth at high shear.

CONCLUSION

Fucoidan-GPIbα interaction may have unique therapeutic potential against bleeding disorders in its high-molecular weight state and protection against arterial thrombosis by blocking GPIb-VWF interaction after fucoidan is hydrolyzed.

Platelet caspase-1 and Bruton tyrosine kinase activation in patients with COVID-19 is associated with disease severity and reversed in vitro by ibrutinib

Background

Severity of coronavirus disease 2019 (COVID-19) is often associated with thrombotic complications and cytokine storm leading to intensive are unit (ICU) admission. Platelets are known to be responsible for abnormal hemostasis parameters (thrombocytopenia, raised D-dimers, and prolonged prothrombin time) in other viral infections through the activation of the nucleotide-binding domain leucine repeat rich containing protein 3 inflammasome induced by signaling pathways driven by Bruton tyrosine kinase (BTK) and leading to caspase-1 activation.

Objectives

We hypothesized that caspase-1 activation and the phosphorylation of BTK could be associated with the severity of the disease and that ibrutinib, a BTK inhibitor, could inhibit platelet activation.

Methods and Results

We studied caspase-1 activation by flow cytometry and the phosphorylation of BTK by Western blot in a cohort of 51 Afro-Carribean patients with COVID-19 disease (19 not treated in ICU and 32 treated in ICU). Patients with a platelet count of 286.7 × 109/L (69-642 × 109/L) were treated by steroids and heparin preventive anticoagulation. Caspase-1 and BTK activation were associated with the severity of the disease and with the procoagulant state of the patients. Furthermore, we showed in vitro that the plasma of ICU patients with COVID-19 was able to increase CD62P expression and caspase-1 activity of healthy platelets and that ibrutinib could prevent it.

Conclusions

Our results show that caspase-1 and BTK activation are related to disease severity and suggest the therapeutic hope raised by ibrutinib in the treatment of COVID-19 by reducing the procoagulant state of the patients.

BTK inhibitors in the treatment of hematological malignancies and inflammatory diseases: mechanisms and clinical studies

Bruton's tyrosine kinase (BTK) is an essential component of multiple signaling pathways that regulate B cell and myeloid cell proliferation, survival, and functions, making it a promising therapeutic target for various B cell malignancies and inflammatory diseases. Five small molecule inhibitors have shown remarkable efficacy and have been approved to treat different types of hematological cancers, including ibrutinib, acalabrutinib, zanubrutinib, tirabrutinib, and orelabrutinib. The first-in-class agent, ibrutinib, has created a new era of chemotherapy-free treatment of B cell malignancies. Ibrutinib is so popular and became the fourth top-selling cancer drug worldwide in 2021. To reduce the off-target effects and overcome the acquired resistance of ibrutinib, significant efforts have been made in developing highly selective second- and third-generation BTK inhibitors and various combination approaches. Over the past few years, BTK inhibitors have also been repurposed for the treatment of inflammatory diseases. Promising data have been obtained from preclinical and early-phase clinical studies. In this review, we summarized current progress in applying BTK inhibitors in the treatment of hematological malignancies and inflammatory disorders, highlighting available results from clinical studies.

Risk of bleeding associated with BTK inhibitor monotherapy: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND

The risk of bleeding associated with Bruton's tyrosine kinase inhibitor (BTKi) monotherapy remains to be understood. This systematic review aims to evaluate BTKi monotherapy related bleeding risk.

RESEARCH DESIGN AND METHODS

PubMed, Embase, and CENTRAL were searched up to December 5, 2021. We included randomized controlled trials (RCTs) comparing BTKi monotherapy with control drugs or placebo, or comparing different BTKi monotherapies. The risk ratios (RR) with 95% confidence intervals (95% CI) were calculated.

RESULTS

10 studies with 3139 patients were included. Ibrutinib (vs. control drugs or placebo) significantly increased the risk of overall bleeding and major bleeding (RR=2.22, 95% CI 1.80-2.75, P<0.00001; RR=1.80, 95% CI 1.02-3.18, P=0.04, respectively). Acalabrutinib (vs. control drugs) had a significantly increased overall bleeding risk (RR=3.45, 95% CI 2.39-4.99, p<0.00001). A significant difference was found in overall bleeding between ibrutinib and acalabrutinib (RR=1.35, 95% CI 1.11-1.64, P=0.002). Compared to zanubrutinib, ibrutinib tended to increase the risk of major bleeding (RR=1.55, 95% CI 0.57-4.18, P=0.39).

CONCLUSIONS

Ibrutinib and acalabrutinib (vs. control drugs or placebo) have a higher risk of bleeding and overall bleeding, respectively. Limited evidence suggests that ibrutinib (vs. acalabrutinib) significantly increases overall bleeding risk, but the differences are not observed in other comparisons.

Regulation of the Tec family of non-receptor tyrosine kinases in cardiovascular disease

Tyrosine phosphorylation by protein tyrosine kinases (PTKs) is a type of post-translational modification. Tec kinases, which are a subfamily of non-receptor PTKs, were originally discovered in the hematopoietic system and include five members: Tec, Btk, Itk/Emt/Tsk, Etk/Bmx, and Txk/Rlk. With the progression of modern research, certain members of the Tec family of kinases have been found to be expressed outside the hematopoietic system and are involved in the development and progression of a variety of diseases. The role of Tec family kinases in cardiovascular disease is receiving increasing attention. Tec kinases are involved in the occurrence and progression of ischemic heart disease, atherosclerosis, cardiac dysfunction associated with sepsis, atrial fibrillation, myocardial hypertrophy, coronary atherosclerotic heart disease, and myocardial infarction and post-myocardial. However, no reviews have comprehensively clarified the role of Tec kinases in the cardiovascular system. Therefore, this review summarizes research on the role of Tec kinases in cardiovascular disease, providing new insights into the prevention and treatment of cardiovascular disease.

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

CRACking the Molecular Regulatory Mechanism of SOCE during Platelet Activation in Thrombo-Occlusive Diseases

Thrombo-occlusive diseases such as myocardial infarction, ischemic stroke and deep vein thrombosis with subsequent pulmonary embolism still represent a major health burden worldwide. Besides the cells of the vasculature or other hematopoietic cells, platelets are primarily responsible for the development and progression of an occluding thrombus. The activation and function of platelets crucially depend on free cytosolic calcium (Ca2+) as second messenger, which modulates platelet secretion, aggregation and thrombus formation. Ca2+ is elevated upon platelet activation by release of Ca2+ from intracellular stores thus triggering of the subsequent store-operated Ca2+ entry (SOCE), which is facilitated by Ca2+ release-activated channels (CRACs). In general, CRACs are assembled by the pore-forming unit Orai in the plasma membrane and the Ca2+-sensing stromal interaction molecule (STIM) in the endoplasmic reticulum after the depletion of internal Ca2+ stores. In the last few years, there is a growing body of the literature demonstrating the importance of STIM and Orai-mediated mechanism in thrombo-occlusive disorders. Thus, this review provides an overview of the recent understanding of STIM and Orai signaling in platelet function and its implication in the development and progression of ischemic thrombo-occlusive disorders. Moreover, potential pharmacological implications of STIM and Orai signaling in platelets are anticipated and discussed in the end.

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

All irreversible Bruton tyrosine kinase (Btk) inhibitors including ibrutinib and acalabrutinib induce platelet dysfunction and increased bleeding risk. New reversible Btk inhibitors were developed, like MK-1026. The mechanism underlying increased bleeding tendency with Btk inhibitors remains unclear. We investigated the effects of ibrutinib, acalabrutinib and MK-1026 on platelet function in healthy volunteers, patients and Btk-deficient mice, together with off-target effects on tyrosine kinase phosphorylation. All inhibitors suppressed GPVI- and CLEC-2-mediated platelet aggregation, activation and secretion in a dose-dependent manner. Only ibrutinib inhibited thrombus formation on vWF-co-coated surfaces, while on collagen this was not affected. In blood from Btk-deficient mice, collagen-induced thrombus formation under flow was reduced, but preincubation with either inhibitor was without additional effects. MK-1026 showed less off-target effects upon GPVI-induced TK phosphorylation as compared to ibrutinib and acalabrutinib. In ibrutinib-treated patients, GPVI-stimulated platelet activation, and adhesion on vWF-co-coated surfaces were inhibited, while CLEC-2 stimulation induced variable responses. The dual inhibition of GPVI and CLEC-2 signalling by Btk inhibitors might account for the increased bleeding tendency, with ibrutinib causing more high-grade bleedings due to additional inhibition of platelet-vWF interaction. As MK-1026 showed less off-target effects and only affected activation of isolated platelets, it might be promising for future treatment.

How I treat and prevent venous thrombotic complications in patients with lymphoma

Venous thromboembolism (VTE) is a common complication occurring in 5-10% of patients with lymphoma. As the complexity of lymphoma management has increased with novel therapies, so too has the treatment of VTE. Therapeutic options for the treatment of cancer-associated VTE have expanded from only warfarin and low-molecular-weight heparins (LMWHs) to include the direct oral anticoagulants (DOACs) apixaban, edoxaban and rivaroxaban. There have been no head-to-head trials comparing different DOACs in this setting and randomized trials comparing a DOAC with LMWH dalteparin differ in trial design and results. Drug-drug interactions, drug-specific side effects and patient selection are important considerations when prescribing anticoagulant therapy. In all patients, the relative risks of thrombosis and bleeding, the availability of the anticoagulant, and the life expectancy of the patient are vital elements in selecting the most appropriate anticoagulant (which can vary over time) for the individual patient. We describe the intricacies and challenges of treating thrombotic complications in patients with lymphoma with an emphasis on evidence and guideline-based care.

Keeping a balance in chronic lymphocytic leukemia (CLL) patients taking ibrutinib: ibrutinib-associated adverse events and their management based on drug interactions

INTRODUCTION

Ibrutinib is a highly effective drug for patients with chronic lymphocytic leukemia (CLL), and is well tolerated even by older patients and those unfit to receive conventional immuno-chemotherapy.

AREAS COVERED

The occurrence of adverse events was revealed as a major cause of ibrutinib failure in the real-world. Ibrutinib-induced lymphocytosis carries the risk of an untimely interruption of therapy because it may be misinterpreted as disease progression. In addition, drug interactions can worsen ibrutinib-associated toxicities by increasing the plasma concentration of ibrutinib. In this review, we present a case of major hemorrhage and atrial fibrillation (AF) during ibrutinib use and summarize the adverse events associated with ibrutinib. Furthermore, the practical management of ibrutinib-associated toxicities was covered with reference to a drug interaction mechanism.

EXPERT OPINION

Clinicians should examine the prescribed drugs prior to ibrutinib initiation and carefully monitor toxicities while taking ibrutinib. A reduced dose of ibrutinib with the concurrent use of CYP3A inhibitors such as antifungal agents could be an attractive strategy to reduce toxicities and may confer financial benefits. Reducing unexpected toxicities is as significant as achieving treatment response in the era of life-long therapy with ibrutinib in patients with CLL.

Targeting Bruton's Tyrosine Kinase in CLL

Targeting the B-cell receptor signaling pathway through BTK inhibition proved to be effective for the treatment of chronic lymphocytic leukemia (CLL) and other B-cell lymphomas. Covalent BTK inhibitors (BTKis) led to an unprecedented improvement in outcome in CLL, in particular for high-risk subgroups with TP53 aberration and unmutated immunoglobulin heavy-chain variable-region gene (IGHV). Ibrutinib and acalabrutinib are approved by the US Food and Drug Administration for the treatment of CLL and other B-cell lymphomas, and zanubrutinib, for patients with mantle cell lymphoma. Distinct target selectivity of individual BTKis confer differences in target-mediated as well as off-target adverse effects. Disease progression on covalent BTKis, driven by histologic transformation or selective expansion of BTK and PLCG2 mutated CLL clones, remains a major challenge in the field. Fixed duration combination regimens and reversible BTKis with non-covalent binding chemistry hold promise for the prevention and treatment of BTKi-resistant disease.

Vascular Impact of Cancer Therapies: The Case of BTK (Bruton Tyrosine Kinase) Inhibitors

Novel targeted cancer therapies have revolutionized oncology therapies, but these treatments can have cardiovascular complications, which include heterogeneous cardiac, metabolic, and vascular sequelae. Vascular side effects have emerged as important considerations in both cancer patients undergoing active treatment and cancer survivors. Here, we provide an overview of vascular effects of cancer therapies, focusing on small-molecule kinase inhibitors and specifically inhibitors of BTK (Bruton tyrosine kinase), which have revolutionized treatment and prognosis for B-cell malignancies. Cardiovascular side effects of BTK inhibitors include atrial fibrillation, increased risk of bleeding, and hypertension, with the former 2 especially providing a treatment challenge for the clinician. Cardiovascular complications of small-molecule kinase inhibitors can occur through either on-target (targeting intended target kinase) or off-target kinase inhibition. We will review these concepts and focus on the case of BTK inhibitors, highlight the emerging data suggesting an off-target effect that may provide insights into development of arrhythmias, specifically atrial fibrillation. We believe that cardiac and vascular sequelae of novel targeted cancer therapies can provide insights into human cardiovascular biology.

Bruton Tyrosine Kinase Inhibitors in Chronic Lymphocytic Leukemia: Beyond Ibrutinib

Bruton tyrosine kinase inhibitors have indisputably transformed the treatment landscape of chronic lymphocytic leukemia, but require continuous therapy to maintain response. This places emphasis on their unique toxicity profile and potential loss of efficacy owing to resistance. Data from single-arm clinical studies are suggestive of comparable efficacy and favorable toxicity profiles of next-generation Bruton tyrosine kinase inhibitors. This is supported by the ASPEN study in Waldenstrom's macroglobulinemia, which convincingly demonstrated that zanubrutinib has a better toxicity profile than ibrutinib. Novel, reversible Bruton tyrosine kinase inhibitors are showing the potential to improve long-term efficacy by overcoming common mechanisms of resistance.

A Review of the Bruton Tyrosine Kinase Inhibitors in B-Cell Malignancies

The B-cell receptor signaling pathway plays an integral role in the proliferation and survival of malignant B cells. Targeting the B-cell receptor pathway via the inhibition of Bruton tyrosine kinase (BTK) has evolved the treatment of a variety of B-cell malignancies, including chronic lymphocytic leukemia, mantle cell lymphoma, marginal zone lymphoma, and Waldenström macroglobulinemia. Currently, there are three BTK inhibitors approved by the U.S. Food and Drug Administration: ibrutinib, acalabrutinib, and zanubrutinib. This article reviews the pharmacology, clinical efficacy, safety, dosing, drug-drug interactions, and implications for advanced practitioners of BTK inhibitors in the treatment of B-cell malignancies.

Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT): Targeting Pathomechanisms with Bruton Tyrosine Kinase Inhibitors

A series of cases with rare thromboembolic incidents including cerebral sinus vein thrombosis (some of them fatal) and concomitant thrombocytopenia occurring shortly after vaccination with the coronavirus disease 2019 (COVID-19) vaccine AZD1222 (Vaxzevria) have caused significant concern and led to its temporary suspension in many countries. Immediate laboratory efforts in four of these patients have identified a tentative pathomechanism underlying this syndrome termed initially vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) and renamed recently vaccine-induced immune thrombotic thrombocytopenia (VITT). It encompasses the presence of platelet-activating antibodies to platelet factor-4/heparin complexes, possibly emulated by polyanionic constituents of AZD1222, and thus resembles heparin-induced thrombocytopenia (HIT). Because these immune complexes bind and activate platelets via Fcγ receptor IIA (FcγRIIA), high-dose intravenous immunoglobulin G has been suggested for treatment of VITT in addition to non-heparin anticoagulants. Here we propose inhibitors of Bruton tyrosine kinase (Btk) approved for B cell malignancies (e.g., ibrutinib) as another therapeutic option in VITT, as they are expected to pleiotropically target multiple pathways downstream of FcγRIIA-mediated Btk activation, for example, as demonstrated for the effective inhibition of platelet aggregation, dense granule secretion, P-selectin expression and platelet-neutrophil aggregate formation stimulated by FcγRIIA cross-linking. Moreover, C-type lectin-like receptor CLEC-2- and GPIb-mediated platelet activation, the interactions and activation of monocytes and the release of neutrophil extracellular traps, as encountered in HIT, could be attenuated by Btk inhibitors. As a paradigm for emergency repurposing of approved drugs in COVID-19, off-label use of Btk inhibitors in a low-dose range not affecting haemostatic functions could thus be considered a sufficiently safe option to treat VITT.

Platelet transfusion for cancer secondary thrombocytopenia: Platelet and cancer cell interaction

Chemoradiotherapy and autoimmune disorder often lead to secondary thrombocytopenia in cancer patients, and thus, platelet transfusion is needed to stop or prevent bleeding. However, the effect of platelet transfusion remains controversial for the lack of agreement on transfusion strategies. Before being transfused, platelets are stored in blood banks, and their activation is usually stimulated. Increasing evidence shows activated platelets may promote metastasis and the proliferation of cancer cells, while cancer cells also induce platelet activation. Such a vicious cycle of interaction between activated platelets and cancer cells is harmful for the prognosis of cancer patients, which results in an increased tumor recurrence rate and decreased five-year survival rate. Therefore, it is important to explore platelet transfusion strategies, summarize mechanisms of interaction between platelets and tumor cells, and carefully evaluate the pros and cons of platelet transfusion for better treatment and prognosis for patients with cancer with secondary thrombocytopenia.

Bleeding by Bruton Tyrosine Kinase-Inhibitors: Dependency on Drug Type and Disease

Bruton tyrosine kinase (Btk) is expressed in B-lymphocytes, myeloid cells and platelets, and Btk-inhibitors (BTKi) are used to treat patients with B-cell malignancies, developed against autoimmune diseases, have been proposed as novel antithrombotic drugs, and been tested in patients with severe COVID-19. However, mild bleeding is frequent in patients with B-cell malignancies treated with the irreversible BTKi ibrutinib and the recently approved 2nd generation BTKi acalabrutinib, zanubrutinib and tirabrutinib, and also in volunteers receiving in a phase-1 study the novel irreversible BTKi BI-705564. In contrast, no bleeding has been reported in clinical trials of other BTKi. These include the brain-penetrant irreversible tolebrutinib and evobrutinib (against multiple sclerosis), the irreversible branebrutinib, the reversible BMS-986142 and fenebrutinib (targeting rheumatoid arthritis and lupus erythematodes), and the reversible covalent rilzabrutinib (against pemphigus and immune thrombocytopenia). Remibrutinib, a novel highly selective covalent BTKi, is currently in clinical studies of autoimmune dermatological disorders. This review describes twelve BTKi approved or in clinical trials. By focusing on their pharmacological properties, targeted disease, bleeding side effects and actions on platelets it attempts to clarify the mechanisms underlying bleeding. Specific platelet function tests in blood might help to estimate the probability of bleeding of newly developed BTKi.

Management of Ibrutinib Toxicities: a Practical Guide

PURPOSE OF REVIEW

Ibrutinib is a first-in-class, highly potent Bruton tyrosine kinase inhibitor which has become standard of care for patients with chronic lymphocytic leukaemia and other lymphoproliferative disorders. It requires indefinite administration which places emphasis on toxicity and long-term tolerance.

RECENT FINDINGS

Extensive use of ibrutinib in studies and clinical practice has better defined its full toxicity profile which has made its use more challenging than initially foreseen. In particular, dysrhythmias, bleeding, infections and constitutional symptoms have been reported and can result in dose reduction or discontinuation of ibrutinib. Herein, we review the common as well as rare but important toxicities and discuss approach and management on a practical level. We also highlight that patients should be regularly monitored for adverse events and proactively treated to minimise side effects and avoid disruption.

Effects of novel Btk and Syk inhibitors on platelet functions alone and in combination in vitro and in vivo

BACKGROUND

Inhibitors of tyrosine kinases downstream of the B-cell receptor, such as Bruton's tyrosine kinase (Btk) or Spleen tyrosine kinase (Syk), used alone or in combination are new therapeutic options in the treatment of B-cell malignancies. A challenge in the development of second-generation Btk inhibitors is to limit their side effects such as the increased bleeding risk. Considering the pivotal role of Syk in immunoreceptor tyrosine-based activation motif mediated platelet signaling, the impact of inhibiting this kinase on platelet functions is also worth analyzing.

OBJECTIVES

We investigated the effect of a novel Btk inhibitor, tirabrutinib, and a Syk inhibitor, entospletinib, alone and in combination on platelet signaling and functions in vitro and ex vivo.

METHODS

Platelet aggregation, secretion, and signaling responses as well as thrombus growth under flow were analyzed in the presence of the inhibitors alone or in combination in vitro, at clinically relevant doses, and ex vivo in patients treated with these inhibitors in the context of a phase I trial.

RESULTS

Although tirabrutinib alone had modest effects on platelet activation in vitro and ex vivo, entospletinib alone efficiently inhibited washed platelet aggregation in response to collagen. However, entospletinib weakly affected platelet activation in platelet-rich plasma, in whole blood and ex vivo. Importantly, the combination of tirabrutinib and entospletinib induced a significant decrease in platelet response to collagen in vitro and ex vivo correlating with mild bleedings reported in some of the treated patients.

CONCLUSION

These new results should contribute to improve the safety of these targeted therapies.

Platelet function and bleeding in chronic lymphocytic leukemia and mantle cell lymphoma patients on ibrutinib

BACKGROUND

Therapy with irreversible Bruton's tyrosine kinase inhibitor ibrutinib in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) is associated with bleeding.

OBJECTIVES

To propose the predictive markers of such bleeding, as well as mechanisms responsible for decreased bleeding at later therapy stages.

PATIENTS/METHODS

We investigate platelet functional activity in 50 CLL and 16 MCL patients on ibrutinib using flow cytometry and light transmission aggregometry.

RESULTS

Prior to treatment, both patient groups had decreased platelet counts; impaired aggregation with adenosine diphosphate (ADP); and decreased binding of CD62P, PAC1, and annexin V upon stimulation. Bleeding in patients treated with ibrutinib was observed in 28 (56%) CLL patients, who had decreased aggregation with ADP and platelet count before therapy. Their platelet count on therapy did not change, platelet aggregation with ADP steadily improved, and aggregation with collagen first decreased and then increased in anticorrellation with bleeding. Bleeding in MCL was observed in 10 (62%) patients, who had decreased dense granule release before therapy. ADP and ristocetin induced platelet aggregation in ibrutinib-treated MCL patients increased on therapy, while collagen-induced aggregation evolved similarly to CLL patients.

CONCLUSIONS

Our results suggest that ibrutinib-dependent bleeding in CLL patients involves three mechanisms: decreased platelet count (the most important discriminator between bleeding and non-bleeding patients), impaired platelet response to ADP caused by CLL, and inhibition by ibrutinib. Initially, ibrutinib shifts the balance to bleeding, but then it is restored because of the improved response to ADP.

Oral Bruton tyrosine kinase inhibitors block activation of the platelet Fc receptor CD32a (FcγRIIA): a new option in HIT?

Activation of the platelet Fc-receptor CD32a (FcγRIIA) is an early and crucial step in the pathogenesis of heparin-induced thrombocytopenia type II (HIT) that has not been therapeutically targeted. Downstream FcγRIIA Bruton tyrosine kinase (BTK) is activated; however, its role in Fc receptor-induced platelet activation is unknown. We explored the potential to prevent FcγRIIA-induced platelet activation by BTK inhibitors (BTKi's) approved (ibrutinib, acalabrutinib) or in clinical trials (zanubrutinib [BGB-3111] and tirabrutinib [ONO/GS-4059]) for B-cell malignancies, or in trials for autoimmune diseases (evobrutinib, fenebrutinib [GDC-0853]). We found that all BTKi's blocked platelet activation in blood after FcγRIIA stimulation by antibody-mediated cross-linking (inducing platelet aggregation and secretion) or anti-CD9 antibody (inducing platelet aggregation only). The concentrations that inhibit 50% (IC50) of FcγRIIA cross-linking-induced platelet aggregation were for the irreversible BTKi's ibrutinib 0.08 µM, zanubrutinib 0.11 µM, acalabrutinib 0.38 µM, tirabrutinib 0.42 µM, evobrutinib 1.13 µM, and for the reversible BTKi fenebrutinib 0.011 µM. IC50 values for ibrutinib and acalabrutinib were four- to fivefold lower than the drug plasma concentrations in patients treated for B-cell malignancies. The BTKi's also suppressed adenosine triphosphate secretion, P-selectin expression, and platelet-neutrophil complex formation after FcγRIIA cross-linking. Moreover, platelet aggregation in donor blood stimulated by sera from HIT patients was blocked by BTKi's. A single oral intake of ibrutinib (280 mg) was sufficient for a rapid and sustained suppression of platelet FcγRIIA activation. Platelet aggregation by adenosine 5'-diphosphate, arachidonic acid, or thrombin receptor-activating peptide was not inhibited. Thus, irreversible and reversible BTKi's potently inhibit platelet activation by FcγRIIA in blood. This new rationale deserves testing in patients with HIT.

Changes in primary and secondary hemostasis in patients with CLL treated with venetoclax and ibrutinib

Bleeding is a common adverse event following ibrutinib monotherapy. However, it remains unclear how hemostasis is affected by venetoclax in combination with ibrutinib. Here we investigated hemostasis in patients with chronic lymphocytic leukemia (CLL) at baseline, during ibrutinib monotherapy, and during venetoclax and ibrutinib combination therapy or venetoclax monotherapy. Primary hemostasis, assessed by Multiplate using adenosine diphosphate (ADP), arachidonic acid (AA), and thrombin receptor agonist peptide (TRAP-6), was impaired in all CLL patients at baseline, remained unchanged upon ibrutinib monotherapy, and improved significantly following venetoclax added to ibrutinib or as monotherapy. Secondary hemostasis assessed by thromboelastography (TEG) was normal and unchanged throughout treatment. The frequency of clinical bleeding events was the highest during ibrutinib monotherapy, in line with the demonstrated improved primary hemostasis upon addition of venetoclax, thus pointing toward a treatment option for CLL patients with increased bleeding risk.

Safety considerations with targeted therapy drugs for B-cell non-Hodgkin lymphoma

INTRODUCTION

B-cell non-Hodgkin lymphomas (B-NHLs) are the most frequent hematologic malignant cancers. Molecular targeted therapy is an important aspect of B-NHL treatment alongside cytotoxic chemotherapy, radiotherapy, and immunotherapy.

AREAS COVERED

Molecular targeted therapies have changed the landscape of treatment strategies for B-NHLs since the approval of rituximab, an anti-CD20 monoclonal antibody, by the US Food and Drug Administration in 1997. Currently, several targeted therapies have been approved or are in the later-phase of clinical trials including naked antibodies, antibody-drug conjugates, and small molecules, such as Bruton's tyrosine kinase (BTK) inhibitors, phosphatidylinositol 3-kinase (PI3 K) inhibitors, enhancer of zeste homolog 2 (EZH2) inhibitors, and B-cell lymphoma 2 (Bcl-2) inhibitors. These drugs have various toxicities because of their unique mechanisms of action. In this review, the available toxicity data of the targeted therapies for B-NHLs have been summarized.

EXPERT OPINION

Recent clinical developments of targeted therapies for B-NHLs have provided several useful effective therapeutic options for patients. However, there are unique toxicities that need to be resolved. It is necessary to find out the toxicity mechanism; optimal treatment strategy for these toxicities; and novel targeted therapies that might potentially overcome the toxicities of previously approved targeted therapies.

Current Role and Emerging Evidence for Bruton Tyrosine Kinase Inhibitors in the Treatment of Mantle Cell Lymphoma

The Bruton tyrosine kinase inhibitors (BTKi), acalabrutinib, ibrutinib, and zanubrutinib, are all approved in the United States for the treatment of relapsed mantle cell lymphoma (MCL). BTKi as a class have become the preferred therapy for most of the patients with relapsed MCL, and ongoing clinical trials are evaluating whether combining BTKi with other targeted agents may deepen response and further improve outcomes. Emerging evidence supports the efficacy of BTKi-containing combinations as frontline treatment, and clinical studies to define the role of this class of drugs for newly diagnosed patients with MCL are in progress.

Ibrutinib for the treatment of chronic lymphocytic leukemia

INTRODUCTION

Chemoimmunotherapy has improved outcomes in chronic lymphocytic leukemia, yet it is not curative, with very high relapse rates, and is associated with a significant risk of toxicities. Moreover, patients with higher-risk genetic abnormalities continue to experience poorer outcomes and lower survival. Recently, novel targeted therapies have been developed to increase efficacy and reduce toxicity. Areas covered: Ibrutinib is an oral irreversible inhibitor of Bruton's tyrosine kinase, a mediator of B-cell receptor signaling, which plays a vital role in various B-cell neoplasms. The drug has been approved for the treatment of several hematological malignancies, including chronic lymphocytic leukemia/small lymphocytic lymphoma, where large trials have shown outcomes never seen before even in high-risk patients. The safety profile appeared furthermore favorable, even in elderly and unfit patients. Expert opinion: Therapy with ibrutinib rarely provides MRD-negative complete remission; an indefinite maintenance is therefore needed, with the risk of developing adverse events (AE) or resistance resulting in treatment interruption or discontinuation. Novel, extremely promising, combination strategies, based on the association of ibrutinib with chemoimmunotherapy, anti-CD20 monoclonal antibody or other targeted agents, are currently being investigated, with the goal of achieving greater depth of remission, especially MRD-negativity, and removing the need for indefinite treatment.

How I manage ibrutinib intolerance and complications in patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) therapy has changed dramatically with the introduction of several targeted therapeutics. Ibrutinib was the first approved for use in 2014 and now is used for initial and salvage therapy of CLL patients. With its widespread use in clinical practice, ibrutinib's common and uncommon adverse events reported less frequently in earlier clinical trials have been experienced more frequently in real-world practice. In particular, atrial fibrillation, bleeding, infections, and arthralgias have been reported. The management of ibrutinib's adverse events often cannot be generalized but must be individualized to the patient and their long-term risk of additional complications. When ibrutinib was initially developed, there were limited therapeutic alternatives for CLL, which often resulted in treating through the adverse events. At the present time, there are several effective alternative agents available, so transition to an alternative CLL directed therapy may be considered. Given the continued expansion of ibrutinib across many therapeutic areas, investigation of the pathogenesis of adverse events with this agent and also clinical trials examining therapeutic approaches for complications arising during therapy are needed. Herein, we provide strategies we use in real-world CLL clinical practice to address common adverse events associated with ibrutinib.

Differences and similarities in the effects of ibrutinib and acalabrutinib on platelet functions

While efficient at treating B-cell malignancies, Bruton tyrosine kinase (BTK) inhibitors are consistently reported to increase the risk of bleeding. Analyzing platelet aggregation response to collagen in platelet-rich plasma allowed us to identify two groups in the healthy population characterized by low or high sensitivity to ibrutinib in vitro. Inhibition of drug efflux pumps induced a shift from ibrutinib low-sensitive platelets to high-sensitive ones. At a clinically relevant dose, acalabrutinib, a second-generation BTK inhibitor, did not affect maximal collagen-induced platelet aggregation in the ibrutinib low-sensitive group but did inhibit aggregation in a small fraction of the ibrutinib high-sensitive group. Consistently, acalabrutinib delayed aggregation, particularly in the ibrutinib high-sensitive group. In chronic lymphocytic leukemia patients, acalabrutinib inhibited maximal platelet aggregation only in the ibrutinib high-sensitive group. Acalabrutinib inhibited collagen-induced tyrosine-753 phosphorylation of phospholipase Cγ2 in both groups, but, in contrast to ibrutinib, did not affect Src-family kinases. Acalabrutinib affected thrombus growth under flow only in the ibrutinib high-sensitive group and potentiated the effect of cyclooxygenase and P2Y₁₂ receptor blockers in both groups. Since the better profile of acalabrutinib was observed mainly in the ibrutinib low-sensitive group, replacement therapy in patients may not systematically reduce the risk of bleeding.

Sin1 (Stress-Activated Protein Kinase-Interacting Protein) Regulates Ischemia-Induced Microthrombosis Through Integrin αIIbβ3-Mediated Outside-In Signaling and Hypoxia Responses in Platelets

Objective—

Microthrombosis as a serious consequence of myocardial infarction, impairs the microvascular environment and increases the occurrences of heart failure, arrhythmia, and death. Sin1 (stress-activated protein kinase-interacting protein) as an essential component of mTORC2 (mammalian target of rapamycin complex 2) is required for cell proliferation and metabolism in response to nutrients, stress, and reactive oxygen species and activates Akt and PKC (protein kinase C). However, the activation and function of Sin1/mTORC2 in ischemia-induced microthrombosis remain poorly understood.

Approach and Results—

The phosphorylation of the mTORC2 target Akt at S473 (serine 473) was significantly elevated in platelets from the distal end of left anterior descending obstructions from patients who underwent off-pump coronary artery bypass grafting compared with platelets from healthy subjects. Consistent with this finding, phosphorylation of T86 in Sin1 was also dramatically increased. Importantly, the augmented levels of phosphorylated Sin1 and Akt in platelets from 61 preoperative patients with ST-segment—elevation myocardial infarction correlated well with the no-reflow phenomena observed after revascularization. Platelet-specific Sin1 deficiency mice and Sin1 T86 phosphorylation deficiency mice were established to explore the underlying mechanisms in platelet activation. Mechanistically, Sin1 T86 phosphorylation amplifies mTORC2-mediated downstream signals; it is also required for αIIbβ3-mediated outside-in signaling and plays a role in generating hypoxia/reactive oxygen species through NAD + /Sirt3 (sirtuin 3)/SOD2 (superoxide dismutase 2) pathway. Importantly, Sin1 deletion in platelets protected mice from ischemia-induced microvascular embolization and subsequent heart dysfunction in a mouse model of myocardial infarction.

Conclusions—

Together, the results of our study reveal a novel role for Sin1 in platelet activation. Thus, Sin1 may be a valuable therapeutic target for interventions for ischemia-induced myocardial infarction deterioration.

Ibrutinib-related bleeding: pathogenesis, clinical implications and management

: Ibrutinib is the first drug of a new family of Bruton's tyrosine kinases (Btk)-inhibiting agents, which have proved to be useful for the treatment of several B-cell lymphoid malignancies. This drug is associated to an increased bleeding risk from initial clinical trials especially in association with warfarin. Although Btk plays an important role in platelet signalling, increased bleeding tendency in patients on ibrutinib is more complex than Btk inhibition alone and is because of several antiplatelet mechanisms, namely inhibition of Btk and Tec kinases, which play a key role in platelet activation downstream of the collagen GPVI and Glycoprotein Ib. This risk is increased by concomitant antiplatelet and anticoagulant therapy; both dual antiplatelet therapy and vitamin K antagonists are contraindicated in these patients. Potential ibrutinib users often have age-associated cardiovascular risk factors or conditions and the drug itself may trigger atrial fibrillation requiring antithrombotic therapy. Aspirin and direct oral anticoagulants can be regarded as the antithrombotic therapies of choice if required. Heparin and fondaparinux have also been used in clinical trials. Therefore, the need and duration of antithrombotic therapy must be carefully evaluated and treatment individualized according to clinical circumstances. Ibrutinib withdrawal and platelet transfusion are key for the management of major bleeding not involving the central nervous system.

Akt-mediated platelet apoptosis and its therapeutic implications in immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by low platelet count which can cause fatal hemorrhage. ITP patients with antiplatelet glycoprotein (GP) Ib-IX autoantibodies appear refractory to conventional treatments, and the mechanism remains elusive. Here we show that the platelets undergo apoptosis in ITP patients with anti-GPIbα autoantibodies. Consistent with these findings, the anti-GPIbα monoclonal antibodies AN51 and SZ2 induce platelet apoptosis in vitro. We demonstrate that anti-GPIbα antibody binding activates Akt, which elicits platelet apoptosis through activation of phosphodiesterase (PDE3A) and PDE3A-mediated PKA inhibition. Genetic ablation or chemical inhibition of Akt or blocking of Akt signaling abolishes anti-GPIbα antibody-induced platelet apoptosis. We further demonstrate that the antibody-bound platelets are removed in vivo through an apoptosis-dependent manner. Phosphatidylserine (PS) exposure on apoptotic platelets results in phagocytosis of platelets by macrophages in the liver. Notably, inhibition or genetic ablation of Akt or Akt-regulated apoptotic signaling or blockage of PS exposure protects the platelets from clearance. Therefore, our findings reveal pathogenic mechanisms of ITP with anti-GPIbα autoantibodies and, more importantly, suggest therapeutic strategies for thrombocytopenia caused by autoantibodies or other pathogenic factors.

Bone Marrow Defects and Platelet Function: A Focus on MDS and CLL

The bloodstream typically contains >500 billion anucleate circulating platelets, derived from megakaryocytes in the bone marrow. This review will focus on two interesting aspects of bone marrow dysfunction and how this impacts on the quality of circulating platelets. In this regard, although megakaryocytes are from the myeloid lineage leading to granulocytes (including neutrophils), erythrocytes, and megakaryocytes/platelets, recent evidence has shown that defects in the lymphoid lineage leading to B cells, T cells, and natural killer (NK) cells also result in abnormal circulating platelets. Current evidence is limited regarding whether this latter phenomenon might potentially arise from (a) some form of as-yet-undetected defect common to both lineages; (b) adverse interactions occurring between cells of different lineages within the bone marrow environment; and/or (c) unknown disease-related factor(s) affecting circulating platelet receptor expression/function after their release from megakaryocytes. Understanding the mechanisms underlying how both myeloid and lymphoid lineage bone marrow defects lead to dysfunction of circulating platelets is significant because of the potential diagnostic and predictive value of peripheral platelet analysis for bone marrow disease progression, the additional potential effects of new anti-cancer drugs on platelet function, and the critical role platelets play in regulation of bleeding risk, inflammation, and innate immunity.

Oral Bruton tyrosine kinase inhibitors selectively block atherosclerotic plaque-triggered thrombus formation in humans

Interaction of von Willebrand factor (VWF) with platelet glycoprotein Ib (GPIb) and interaction of collagen with GPVI are essential for thrombus formation on ruptured or eroded atherosclerotic plaques (atherothrombosis). GPIb and GPVI signal through Bruton tyrosine kinase (Btk), which can be blocked irreversibly by oral application of ibrutinib, an established therapy for chronic lymphocytic leukemia (CLL) with long-term safety. We found that ibrutinib and the novel Btk inhibitors acalabrutinib and ONO/GS-4059 block GPVI-dependent static platelet aggregation in blood exposed to human plaque homogenate and collagen but not to ADP or arachidonic acid. Moreover, Btk inhibitors prevented platelet thrombus formation on human atherosclerotic plaque homogenate and plaque tissue sections from arterially flowing blood, whereas integrin α₂β₁ and VWF-dependent platelet adhesion to collagen, which is important for physiologic hemostasis, was not affected. This plaque-selective platelet inhibition was also observed in CLL patients taking 450 mg of ibrutinib and in volunteers after much lower and intermittent dosing of the drug. We conclude that Btk inhibitors, by targeting GPIb and GPVI signal transduction, suppress platelet thrombus accretion from flowing blood on atherosclerotic plaque but spare hemostatic platelet function. Btk inhibitors hold promise as the first culprit lesion-focused oral antiplatelet drugs and are effective at low doses.

BTKCys481Ser drives ibrutinib resistance via ERK1/2 and protects BTKwild-type MYD88-mutated cells by a paracrine mechanism

Acquired ibrutinib resistance due to BTK^Cys481 mutations occurs in B-cell malignancies, including those with MYD88 mutations. BTK^Cys481 mutations are usually subclonal, and their relevance to clinical progression remains unclear. Moreover, the signaling pathways that promote ibrutinib resistance remain to be clarified. We therefore engineered BTK^Cys481Ser and BTK^WT expressing MYD88-mutated Waldenström macroglobulinemia (WM) and activated B-cell (ABC) diffuse large B-cell lymphoma (DLBCL) cells and observed reactivation of BTK-PLCγ2-ERK1/2 signaling in the presence of ibrutinib in only the former. Use of ERK1/2 inhibitors triggered apoptosis in BTK^Cys481Ser-expressing cells and showed synergistic cytotoxicity with ibrutinib. ERK1/2 reactivation in ibrutinib-treated BTK^Cys481Ser cells was accompanied by release of many prosurvival and inflammatory cytokines, including interleukin-6 (IL-6) and IL-10 that were also blocked by ERK1/2 inhibition. To clarify if cytokine release by ibrutinib-treated BTK^Cys481Ser cells could protect BTK^WT MYD88-mutated malignant cells, we used a Transwell coculture system and showed that nontransduced BTK^WT MYD88-mutated WM or ABC DLBCL cells were rescued from ibrutinib-induced killing when cocultured with BTK^Cys481Ser but not their BTK^WT-expressing counterparts. Use of IL-6 and/or IL-10 blocking antibodies abolished the protective effect conferred on nontransduced BTK^WT by coculture with BTK^Cys481Ser expressing WM or ABC DLBCL cell counterparts. Rebound of IL-6 and IL-10 serum levels also accompanied disease progression in WM patients with acquired BTK^Cys481 mutations. Our findings show that the BTK^Cys481Ser mutation drives ibrutinib resistance in MYD88-mutated WM and ABC DLBCL cells through reactivation of ERK1/2 and can confer a protective effect on BTK^WT cells through a paracrine mechanism.

Bruton tyrosine kinase inhibitor ONO/GS-4059: from bench to bedside

The Bruton tyrosine kinase (BTK) inhibitor, ibrutinib, has been approved for the treatment of chronic lymphocytic leukemia, mantle cell lymphoma, and Waldenstroms macroglobulinemia. Acquired resistance to ibrutinib due to BTK C481S mutation has been reported. Mutations in PLC?2 can also mediate resistance to ibrutinib. Untoward effects due to off-target effects are also disadvantages of ibrutinib. More selective and potent BTK inhibitors (ACP-196, ONO/GS-4059, BGB-3111, CC-292) are being investigated. This review summarized the preclinical research and clinical data of ONO/GS-4059.

Severe platelet dysfunction in NHL patients receiving ibrutinib is absent in patients receiving acalabrutinib

The Bruton tyrosine kinase (Btk) inhibitor ibrutinib induces platelet dysfunction and causes increased risk of bleeding. Off-target inhibition of Tec is believed to contribute to platelet dysfunction and other side effects of ibrutinib. The second-generation Btk inhibitor acalabrutinib was developed with improved specificity for Btk over Tec. We investigated platelet function in patients with non-Hodgkin lymphoma (NHL) receiving ibrutinib or acalabrutinib by aggregometry and by measuring thrombus formation on collagen under arterial shear. Both patient groups had similarly dysfunctional aggregation responses to collagen and collagen-related peptide, and comparison with mechanistic experiments in which platelets from healthy donors were treated with the Btk inhibitors suggested that both drugs inhibit platelet Btk and Tec at physiological concentrations. Only ibrutinib caused dysfunctional thrombus formation, whereas size and morphology of thrombi following acalabrutinib treatment were of normal size and morphology. We found that ibrutinib but not acalabrutinib inhibited Src family kinases, which have a critical role in platelet adhesion to collagen that is likely to underpin unstable thrombus formation observed in ibrutinib patients. We found that platelet function was enhanced by increasing levels of von Willebrand factor (VWF) and factor VIII (FVIII) ex vivo by addition of intermediate purity FVIII (Haemate P) to blood from patients, resulting in consistently larger thrombi. We conclude that acalabrutinib avoids major platelet dysfunction associated with ibrutinib therapy, and platelet function may be enhanced in patients with B-cell NHL by increasing plasma VWF and FVIII.

Clathrin-mediated integrin αIIbβ3 trafficking controls platelet spreading

Dynamic endocytic and exocytic trafficking of integrins is an important mechanism for cell migration, invasion, and cytokinesis. Endocytosis of integrin can be classified as clathrin dependent and clathrin independent manners. And rapid delivery of endocytic integrins back to the plasma membrane is key intracellular signals and is indispensable for cell movement. Integrin αIIbβ3 plays a critical role in thrombosis and hemostasis. Although previous studies have demonstrated that internalization of fibrinogen-bound αIIbβ3 may regulate platelet activation, the roles of endocytic and exocytic trafficking of integrin αIIbβ3 in platelet activation are unclear. In this study, we found that a selective inhibitor of clathrin-mediated endocytosis pitstop 2 inhibited human platelet spreading on immobilized fibrinogen (Fg). Mechanism studies revealed that pitstop 2 did not block the endocytosis of αIIbβ3 and Fg uptake, but inhibit the recycling of αIIbβ3 to plasma membrane during platelet or CHO cells bearing αIIbβ3 spreading on immobilized Fg. And pitstop 2 enhanced the association of αIIbβ3 with clathrin, and AP2 indicated that pitstop 2 inhibit platelet activation is probably due to disturbance of the dynamic dissociation of αIIbβ3 from clathrin and AP2. Further study demonstrated that Src/PLC/PKC was the key pathway to trigger the endocytosis of αIIbβ3 during platelet activation. Pitstop 2 also inhibited platelet aggregation and secretion. Our findings suggest integrin αIIbβ3 trafficking is clathrin dependent and plays a critical role in platelet spreading, and pitstop 2 may serve as an effective tool to address clathrin-mediated trafficking in platelets.