Trial of low doses of aspirin as prophylaxis in sickle cell disease

The Role of Antiplatelet in the Management of Sickle Cell Disease Patients

Sickle cell disease (SCD) is a genetic disorder characterized by abnormal hemoglobin, leading to red blood cell deformities and subsequent vaso-occlusive events. Platelet activation and adhesion play a significant role in the pathophysiology of SCD, contributing to the development of complications such as vaso-occlusive events, stroke, acute chest syndrome, and other manifestations. Antiplatelet therapy has emerged as a potential strategy to mitigate these complications by modulating the platelet function and reducing thrombotic events. This review article provides an overview of antiplatelet therapy's role in managing SCD patients. It discusses the pathophysiological abnormalities in the platelet function in SCD, the rationale for antiplatelet therapy, and the evidence supporting its use in various clinical scenarios. The article explores aspirin as the primary antiplatelet agent in SCD, including its mechanism of action, dosing considerations, and efficacy and safety data. Additionally, it highlights other antiplatelet agents, such as clopidogrel, prasugrel, ticagrelor, and emerging therapies under investigation. Clinical applications of antiplatelet therapy in primary and secondary prevention and the management of acute chest syndrome and other SCD complications are also discussed. Safety considerations are emphasized, including bleeding risk assessment, monitoring, and patient selection for antiplatelet therapy. Finally, the review highlights future research and clinical practice directions, including the development of novel antiplatelet agents, combination therapies, and the integration of antiplatelet therapy with other SCD treatments. Overall, this review provides a comprehensive understanding of the current role of antiplatelet therapy in SCD management, the challenges faced, and future directions for improving patient outcomes.

Thrombo-Inflammation in COVID-19 and Sickle Cell Disease: Two Faces of the Same Coin

People with sickle cell disease (SCD) are at greater risk of severe illness and death from respiratory infections, including COVID-19, than people without SCD (Centers for Disease Control and Prevention, USA). Vaso-occlusive crises (VOC) in SCD and severe SARS-CoV-2 infection are both characterized by thrombo-inflammation mediated by endothelial injury, complement activation, inflammatory lipid storm, platelet activation, platelet-leukocyte adhesion, and activation of the coagulation cascade. Notably, lipid mediators, including thromboxane A2, significantly increase in severe COVID-19 and SCD. In addition, the release of thromboxane A2 from endothelial cells and macrophages stimulates platelets to release microvesicles, which are harbingers of multicellular adhesion and thrombo-inflammation. Currently, there are limited therapeutic strategies targeting platelet-neutrophil activation and thrombo-inflammation in either SCD or COVID-19 during acute crisis. However, due to many similarities between the pathobiology of thrombo-inflammation in SCD and COVID-19, therapies targeting one disease may likely be effective in the other. Therefore, the preclinical and clinical research spurred by the COVID-19 pandemic, including clinical trials of anti-thrombotic agents, are potentially applicable to VOC. Here, we first outline the parallels between SCD and COVID-19; second, review the role of lipid mediators in the pathogenesis of these diseases; and lastly, examine the therapeutic targets and potential treatments for the two diseases.

Stroke in sickle cell disease and the promise of recent disease modifying agents

Sickle cell disease (SCD) is an inherited hemoglobinopathy affecting approximately 100,000 individuals in the United States. Cerebrovascular disease is among the most common and debilitating complications of SCA, with 53% experiencing silent cerebral infarct by age 30 and 3.8% experiencing overt stroke by age 40 years. This review highlights the burden of cerebrovascular disease in SCD, including both stroke and silent cerebral infarct (SCI). We then discuss the pathophysiology of stroke and cerebral fat embolism in the absence of a patent foramen ovale. This review also reveals that options for primary and secondary stroke prevention in SCD are still limited to hydroxyurea and blood transfusion, and that the role of aspirin and anticoagulation in SCD stroke has not been adequately studied. Limited data suggest that the novel disease-modifying agents for SCD management may improve renal dysfunction, leg ulcers, and lower the abnormally high TCD flow velocity. Further research is urgently needed to investigate their role in stroke prevention in SCD, as these novel agents target the main stroke contributors in SCD - hemolysis and vaso-occlusion. This literature review also explores the role of healthcare disparities in slowing progress in SCD management and research in the United States, highlighting the need for more investment in patient and clinician education, SCD management, and research.

Sickle cell vaso-occlusion: The dialectic between red cells and white cells

The pathophysiology of sickle cell anemia, a hereditary hemoglobinopathy, has fascinated clinicians and scientists alike since its description over 100 years ago. A single gene mutation in the HBB gene results in the production of abnormal hemoglobin (Hb) S, whose polymerization when deoxygenated alters the physiochemical properties of red blood cells, in turn triggering pan-cellular activation and pathological mechanisms that include hemolysis, vaso-occlusion, and ischemia-reperfusion to result in the varied and severe complications of the disease. Now widely regarded as an inflammatory disease, in recent years attention has included the role of leukocytes in vaso-occlusive processes in view of the part that these cells play in innate immune processes, their inherent ability to adhere to the endothelium when activated, and their sheer physical and potentially obstructive size. Here, we consider the role of sickle red blood cell populations in elucidating the importance of adhesion vis-a-vis polymerization in vaso-occlusion, review the direct adhesion of sickle red cells to the endothelium in vaso-occlusive processes, and discuss how red cell- and leukocyte-centered mechanisms are not mutually exclusive. Given the initial clinical success of crizanlizumab, a specific anti-P selectin therapy, we suggest that it is appropriate to take a holistic approach to understanding and exploring the complexity of vaso-occlusive mechanisms and the adhesive roles of the varied cell types, including endothelial cells, platelets, leukocytes, and red blood cells.

Culturable bacteria associated with Anopheles darlingi and their paratransgenesis potential

Background

Malaria remains a major public health problem in South America, mostly in the Amazon region. Among newly proposed ways of controlling malaria transmission to humans, paratransgenesis is a promising alternative. Paratransgenesis aims to inhibit the development of parasites within the vector through the action of genetically modified bacteria. The first step towards successful paratransgenesis in the Amazon is the identification of Anopheles darlingi symbiotic bacteria, which are transmitted vertically among mosquitoes, and are not pathogenic to humans.

Methods

Culturable bacteria associated with An. darlingi and their breeding sites were isolated by conventional microbiological techniques. Isolated strains were transformed with a GFP expressing plasmid, pSPT-1-GFP, and reintroduced in mosquitoes by feeding. Their survival and persistence in the next generation was assessed by the isolation of fluorescent bacteria from eggs, larvae, pupae and adult homogenates.

Results

A total of 179 bacterial strains were isolated from samples from two locations, Coari and Manaus. The predominant genera identified in this study were Acinetobacter, Enterobacter, Klebsiella, Serratia, Bacillus, Elizabethkingia, Stenotrophomonas and Pantoea. Two isolated strains, Serratia-Adu40 and Pantoea-Ovo3, were successfully transformed with the pSPT-1-GFP plasmid and expressed GFP. The fluorescent bacteria fed to adult females were transferred to their eggs, which persisted in larvae and throughout metamorphosis, and were detected in adult mosquitoes of the next generation.

Conclusion

Serratia-Adu40 and Pantoea-Ovo3 are promising candidates for paratransgenesis in An. darlingi. Further research is needed to determine if these bacteria are vertically transferred in nature.

Role of the coagulation system in the pathogenesis of sickle cell disease

Sickle cell disease (SCD) is an inherited monogenic red blood cell disorder affecting millions worldwide. SCD causes vascular occlusions, chronic hemolytic anemia, and cumulative organ damage such as nephropathy, pulmonary hypertension, pathologic heart remodeling, and liver necrosis. Coagulation system activation, a conspicuous feature of SCD that causes chronic inflammation, is an important component of SCD pathophysiology. The key coagulation factor, thrombin (factor IIa [FIIa]), is both a central protease in hemostasis and thrombosis and a key modifier of inflammation. Pharmacologic or genetic reduction of circulating prothrombin in Berkeley sickle mice significantly improves survival, ameliorates vascular inflammation, and results in markedly reduced end-organ damage. Accordingly, factors both upstream and downstream of thrombin, such as the tissue factor-FX complex, fibrinogen, platelets, von Willebrand factor, FXII, high-molecular-weight kininogen, etc, also play important roles in SCD pathogenesis. In this review, we discuss the various aspects of coagulation system activation and their roles in the pathophysiology of SCD.

Drug Therapies for the Management of Sickle Cell Disease

Sickle cell disease (SCD) afflicts millions of people worldwide but is referred to as an orphan disease in the United States. Over the past several decades, there has been an increasing understanding of the pathophysiology of SCD and its complications. While most individuals with SCD in resource-rich countries survive into adulthood, the life expectancy of patients with SCD remains substantially shorter than for the general African-American population. SCD can be cured using hematopoietic stem cell transplantation and possibly gene therapy, but these treatment approaches are not available to most patients, the majority of whom reside in low- and middle-income countries. Until relatively recently, only one drug, hydroxyurea, was approved by the US Food and Drug Administration to ameliorate disease severity. Multiple other drugs (L-glutamine, crizanlizumab, and voxelotor) have recently been approved for the treatment of SCD, with several others at various stages of clinical testing. The availability of multiple agents to treat SCD raises questions related to the choice of appropriate drug therapy, combination of multiple agents, and affordability of recently approved products. The enthusiasm for new drug development provides opportunities to involve patients in low- and middle-income nations in the testing of potentially disease-modifying therapies and has the potential to contribute to capacity building in these environments. Demonstration that these agents, alone or in combination, can prevent or decrease end-organ damage would provide additional evidence for the role of drug therapies in improving outcomes in SCD.

Emerging therapies in sickle cell disease

Despite sickle cell disease (SCD) being the most common and severe inherited condition worldwide, therapeutic options are limited. To date, hydroxyurea remains the main treatment option in SCD. However, in the last decade the numbers of interventional clinical trials focussing on therapies for SCD have increased significantly. Many new drugs with various pharmacological targets have emerged and, although the majority have failed to show benefit in clinical trials, some have produced encouraging results. It seems probable that more drugs will soon become available for the treatment of SCD. Furthermore, promising clinical trials with improved outcomes have recently changed the perspective of curative therapies in SCD. Nevertheless, the application of novel therapeutic agents and potential curative treatments will most likely be limited to high-income countries and, thus, will remain unavailable for the majority of people with SCD in the foreseeable future.

The Evolving Pharmacotherapeutic Landscape for the Treatment of Sickle Cell Disease

Sickle cell disease (SCD) is an extremely heterogeneous disease that has been associated with global morbidity and early mortality. More effective and inexpensive therapies are needed. During the last five years, the landscape of the pharmacotherapy of SCD has changed dramatically. Currently, 54 drugs have been used or under consideration to use for the treatment of SCD. These fall into 3 categories: the first category includes the four drugs (Hydroxyurea, L-Glutamine, Crizanlizumab tmca and Voxelotor) that have been approved by the United States Food and Drug Administration (FDA) based on successful clinical trials. The second category includes 22 drugs that failed, discontinued or terminated for now and the third category includes 28 drugs that are actively being considered for the treatment of SCD. Crizanlizumab and Voxelotor are included in the first and third categories because they have been used in more than one trial. New therapies targeting multiple pathways in the complex pathophysiology of SCD have been achieved or are under continued investigation. The emerging trend seems to be the use of multimodal drugs (i.e. drugs that have different mechanisms of action) to treat SCD similar to the use of multiple chemotherapeutic agents to treat cancer.

Advances in new drug therapies for the management of sickle cell disease

Introduction

Sickle cell disease (SCD) is an orphan disease in the United States, but is highly prevalent worldwide. Only two drugs, hydroxyurea and L-glutamine, are approved for this disease. With an improved understanding of the pathophysiology of SCD as well as the success of several recently approved drugs for other orphan diseases, there is an increased interest in the development of drugs for SCD.

Areas covered

This review summarizes published studies of drug therapies and ongoing trials of novel agents.

Expert opinion

The development of drugs with different mechanisms of action offers opportunities for combination and individualized therapy in SCD. In addition to acute pain crisis, the evaluation of other SCD-related complications, exercise capacity, patient reported outcomes and validated surrogate endpoints are necessary to advance drug development. It is important to involve sites in sub-Saharan Africa and India, which have the highest burden of SCD, in trials of novel therapies.

Novel Sickle Cell Disease Therapies: Targeting Pathways Downstream of Sickling

Sickle cell disease is an inherited hemoglobinopathy characterized by hemolytic anemia, frequent painful episodes, poor quality of life, end organ damage and a shortened lifespan. Although the seminal event is the polymerization of the abnormal hemoglobin, the downstream pathophysiology of vaso-occlusion results from heterotypic interactions between the altered, adhesive sickle cell RBCs, neutrophils, endothelium, and platelets. Ischemia reperfusion injury, hemolysis and oxidant damage all contribute to heightened inflammation and activation of the hemostatic system. These downstream targets are the focus of emerging treatments with considerable potential to ameliorate disease manifestations. This review summarizes the progress on development of these agents.

Pharmacotherapeutical strategies in the prevention of acute, vaso-occlusive pain in sickle cell disease: a systematic review

Sickle-cell disease (SCD) is characterized by frequent and painful vaso-occlusive crises (VOCs). Various treatments have been evaluated over the years. However, a clear overview is lacking. The objective of this study was to systematically review all pharmacotherapeutical strategies in the prevention of VOCs beyond hydroxyurea. We performed a systematic literature search (MEDLINE, Embase, CENTRAL). Eligible studies were controlled clinical trials evaluating pharmacotherapeutical interventions targeting the reduction of VOCs in patients with SCD. Primary outcomes were the number or duration of SCD-related pain days, VOCs, or hospital admissions for VOCs. Secondary outcomes included time to first VOC or hospital admission for a VOC. A standardized data extraction sheet was used. The methodological quality of studies was assessed using Cochrane's risk-of-bias tool. A total of 36 studies were included in this review, covering 26 different prophylactic interventions. The most promising interventions for reducing the frequency of either VOCs or hospitalizations were the oral antioxidants l-glutamine and ω-3 fatty acids and the IV antiadhesive agent crizanlizumab. Twenty-three studies did not show any beneficial effect of the intervention under investigation, and 6 studies were either too small or methodologically inadequate to draw conclusions. Because of the heterogeneity of interventions, no meta-analysis was performed. In conclusion, this review identified 3 promising pharmacotherapeutical strategies in the prevention of VOCs in SCD. Importantly, this study highlights the discrepancy between the significant burden of SCD worldwide and the low number of adequate trials performed. This review was registered at PROSPERO (CRD42015025250).

Prothrombotic aspects of sickle cell disease

Sickle cell disease (SCD) is a hematologic disorder caused by a well-characterized point mutation in the β-globin gene. Abnormal polymerization of hemoglobin tetramers results in the formation of sickle red blood cells that leads to vascular occlusions, hemolytic anemia, vascular inflammation and cumulative, multiple organ damage. Ongoing activation of coagulation is another hallmark of SCD. Recent studies strongly suggested that hypercoagulation in SCD is not just a secondary event but contributes directly to the disease pathophysiology. In this article we summarize mechanisms leading to the activation of coagulation, review data indicating direct contribution of coagulation to the pathology of SCD and, we discuss the anticoagulation as a possible treatment strategy to attenuate the disease progression.

Treating sickle cell disease by targeting HbS polymerization

Although the root cause of sickle cell disease is the polymerization of hemoglobin S (HbS) to form fibers that make red cells less flexible, most drugs currently being assessed in clinical trials are targeting the downstream sequelae of this primary event. Less attention has been devoted to investigation of the multiple ways in which fiber formation can be inhibited. In this article, we describe the molecular rationale for 5 distinct approaches to inhibiting polymerization and also discuss progress with the few antipolymerization drugs currently in clinical trials.

Coagulation abnormalities of sickle cell disease: Relationship with clinical outcomes and the effect of disease modifying therapies

Sickle cell disease (SCD) is a hypercoagulable state. Patients exhibit increased platelet activation, high plasma levels of markers of thrombin generation, depletion of natural anticoagulant proteins, abnormal activation of the fibrinolytic system, and increased tissue factor expression, even in the non-crisis "steady state." Furthermore, SCD is characterized by an increased risk of thrombotic complications. The pathogenesis of coagulation activation in SCD appears to be multi-factorial, with contributions from ischemia-reperfusion injury and inflammation, hemolysis and nitric oxide deficiency, and increased sickle RBC phosphatidylserine expression. Recent studies in animal models suggest that activation of coagulation may contribute to the pathogenesis of SCD, but the data on the contribution of coagulation and platelet activation to SCD-related complications in humans are limited. Clinical trials of new generations of anticoagulants and antiplatelet agents, using a variety of clinical endpoints are warranted.

Genetic diminution of circulating prothrombin ameliorates multiorgan pathologies in sickle cell disease mice

Sickle cell disease (SCD) results in vascular occlusions, chronic hemolytic anemia, and cumulative organ damage. A conspicuous feature of SCD is chronic inflammation and coagulation system activation. Thrombin (factor IIa [FIIa]) is both a central protease in hemostasis and a key modifier of inflammatory processes. To explore the hypothesis that reduced prothrombin (factor II [FII]) levels in SCD will limit vaso-occlusion, vasculopathy, and inflammation, we used 2 strategies to suppress FII in SCD mice. Weekly administration of FII antisense oligonucleotide "gapmer" to Berkeley SCD mice to selectively reduce circulating FII levels to ∼10% of normal for 15 weeks significantly diminished early mortality. More comprehensive, long-term comparative studies were done using mice with genetic diminution of circulating FII. Here, cohorts of FII(lox/-) mice (constitutively carrying ∼10% normal FII) and FII(WT) mice were tracked in parallel for a year following the imposition of SCD via hematopoietic stem cell transplantation. This genetically imposed suppression of FII levels resulted in an impressive reduction in inflammation (reduction in leukocytosis, thrombocytosis, and circulating interleukin-6 levels), reduced endothelial cell dysfunction (reduced endothelial activation and circulating soluble vascular cell adhesion molecule), and a significant improvement in SCD-associated end-organ damage (nephropathy, pulmonary hypertension, pulmonary inflammation, liver function, inflammatory infiltration, and microinfarctions). Notably, all of these benefits were achieved with a relatively modest 1.25-fold increase in prothrombin times, and in the absence of hemorrhagic complications. Taken together, these data establish that prothrombin is a powerful modifier of SCD-induced end-organ damage, and present a novel therapeutic target to ameliorate SCD pathologies.

Prasugrel in children with sickle cell disease: pharmacokinetic and pharmacodynamic data from an open-label, adaptive-design, dose-ranging study

INTRODUCTION

This phase 2 study was designed to characterize the relationship among prasugrel dose, prasugrel's active metabolite (Pras-AM), and platelet inhibition while evaluating safety in children with sickle cell disease. It was open-label, multicenter, adaptive design, dose ranging, and conducted in 2 parts. Part A: Patients received escalating single doses leading to corresponding increases in Pras-AM exposure and VerifyNow®P2Y12 (VN) platelet inhibition and decreases in VNP2Y12 reaction units and vasodilator-stimulated phosphoprotein platelet reactivity index. Part B: Patients were assigned daily doses (0.06, 0.08, and 0.12 mg/kg) based on VN pharmacodynamic measurements at the start of 2 dosing periods, each 14±4 days. Platelet inhibition was significantly higher at 0.12 mg/kg (56.3%±7.4%; least squares mean±SE) compared with 0.06 mg/kg (33.8%±7.4%) or 0.08 mg/kg (37.9%±5.6%). Patients receiving 0.12 mg/kg achieved ≥30% platelet inhibition; only 1 patient receiving 0.06 mg/kg exceeded 60% platelet inhibition. High interpatient variability in response to prasugrel and the small range of exposures precluded rigorous characterization of the relationship among dose, Pras-AM, and platelet inhibition.

SAFETY

No hemorrhagic events occurred in Part A; 3 occurred in Part B, all mild and self-limited.

CONCLUSIONS

Most children with sickle cell disease may achieve clinically relevant platelet inhibition with titration of daily-dose prasugrel.

Role of the hemostatic system on sickle cell disease pathophysiology and potential therapeutics

Recent studies suggest that sickle cell disease (SCD) is a hypercoagulable state contributing to vaso-occlusive events in the microcirculation, resulting in acute and chronic sickle cell-related organ damage. In this article, we review the existing evidence for contribution of hemostatic system perturbation to SCD pathophysiology. We also review the data showing increased risk of thromboembolic events, particularly newer information on the incidence of venous thromboembolism. Finally, the potential role of platelet inhibitors and anticoagulants in SCD is briefly reviewed.

A phase 1 study of prasugrel in patients with sickle cell disease: pharmacokinetics and effects on ex vivo platelet reactivity

AIMS

Prasugrel is a novel thienopyridine P2Y12 adenosine diphosphate (ADP) receptor antagonist that inhibits ADP-mediated platelet activation and aggregation. Accordingly, it may be useful in reducing platelet-related ischaemia in sickle cell disease (SCD). Exposure to prasugrel's active metabolite (Pras-AM) and its antiplatelet activity in SCD have not been investigated.

METHODS

Thirteen adult patients with SCD and an equal number of matched healthy control subjects were studied before and after 12 days of 5.0 or 7.5 mg day(-1) prasugrel treatment. Platelet reactivity was assessed by light transmission aggregometry (LTA), impedance aggregometry (MEA), VerifyNow® P2Y12, vasodilator-stimulated phosphoprotein (VASP) phosphorylation and Plateletworks. Exposure to Pras-AM was also assessed.

RESULTS

At baseline, patients with SCD showed increased platelet reactivity vs. healthy control subjects with VerifyNow (408 vs. 323 P2Y12 reaction units (PRU), respectively, P = 0.003) and MEA (106 vs. 77 area under the aggregation curve (AU.min), P = 0.002); lower platelet reactivity index with VASP flow cytometry (59 vs. 79% platelet reactivity index (PRI), P = 0.018); and no significant differences with LTA, VASP enzyme-linked immunosorbent assay or Plateletworks. Relative to baseline, prasugrel significantly reduced platelet reactivity by all assays in both populations (all P < 0.05). Prasugrel was well tolerated, with no bleeding-related events in patients with SCD. The mean concentration-time profiles of Pras-AM were comparable between healthy subjects and patients with SCD following a single 10 mg prasugrel dose and following the 12th dose of 7.5 or 5 mg prasugrel.

CONCLUSIONS

Results demonstrate that in response to prasugrel, patients with SCD and healthy subjects have similar degrees of platelet inhibition and exposure to Pras-AM, and provide a basis for further study of prasugrel in patients with SCD.

A pilot study of eptifibatide for treatment of acute pain episodes in sickle cell disease

INTRODUCTION

The contribution of platelet activation to the pathogenesis of sickle cell disease (SCD) remains uncertain. We evaluated the safety and efficacy of eptifibatide, a synthetic peptide inhibitor of the αIIbβ3 receptor, in SCD patients during acute painful episodes.

MATERIALS AND METHODS

In this single site, double-blind, placebo-controlled trial, eligible patients with SCD admitted for acute painful episodes were randomized to receive eptifibatide or placebo at a ratio of 2:1.

RESULTS

Thirteen patients (SS - 10, Sβ(0) - 2, SC - 1) were randomized to receive either eptifibatide (N=9; 6 females; median age - 25years) or placebo (N=4; 3 females; median age - 31years). In the intent-to-treat analysis, there were no major bleeding episodes in either the eptifibatide or placebo arms (point estimate of difference: 0.00, 95% CI; -0.604, 0.372). There was one minor bleeding episode in the eptifibatide arm (point estimate of difference for any bleeding: 0.11, 95% CI: -0.502, 0.494). There was no significant difference in the proportion of patients with thrombocytopenia between the treatment groups (point estimate of difference: 0.11, 95% CI: -0.587, 0.495). There were no differences in the median times to discharge, median times to crisis resolution or the median total opioid use.

CONCLUSIONS

In this small study, eptifibatide appeared to be safe, but did not improve the times to crisis resolution or hospital discharge. Adequately powered studies are required to evaluate the safety and efficacy of eptifibatide in SCD. Clinicaltrials.gov Identifier: NCT00834899.

Interplay between coagulation and vascular inflammation in sickle cell disease

Sickle cell disease is the most common inherited haematological disorder that leads to the irreversible damage of multiple organs. Although sickling of red blood cells and vaso-occlusion are central to the pathophysiology of sickle cell disease, the importance of haemolytic anaemia and vasculopathy has been recently recognized. A hypercoagulable state is another prominent feature of sickle cell disease and is mediated by activation of both intrinsic and extrinsic coagulation pathways. Growing evidence demonstrates that coagulation may not only contribute to the thrombotic complications, but also to vascular inflammation associated with this disease. This article summarizes the role of vascular inflammation and coagulation activation, discusses potential mechanisms responsible for activation of coagulation and reviews recent data demonstrating the crosstalk between coagulation and vascular inflammation in sickle cell disease.

Platelet activation and inhibition in sickle cell disease (pains) study

Platelet activation/aggregation in sickle cell disease (SCD) may promote tissue ischemia, suggesting that antiplatelet therapy may be useful. However, the assessment of platelet function and the effect of antiplatelet therapy in blood from SCD patients may be confounded by hemolysis with the release of adenosine 5'-diphosphate (ADP). Here we evaluate the levels of platelet activation markers in SCD adolescents vs. normal controls and compare, by multiple methods, the effect of in vitro blockade of the platelet ADP receptor P2Y₁₂ by prasugrel's active metabolite, R-138727. Platelet activation markers in blood from SCD adolescents (n = 15) and healthy adults (n = 10), and the effect of R-138727 (0.1-10 µM) added in vitro, were evaluated with and without ADP stimulation. The circulating levels of platelet-monocyte and platelet-neutrophil aggregates were significantly higher (p < 0.01) in SCD patients than in healthy controls. R-138727, in a concentration-dependent manner, inhibited platelet function in both SCD patients and healthy subjects as judged by ADP-stimulated light transmission aggregation, VerifyNow P2Y₁₂ assay, multiple electrode aggregometry, and flow cytometric analysis of platelet vasodilator-stimulated phosphoprotein, activated GPIIb-IIIa and P-selectin. The R-138727 IC₅₀s for each assay were not significantly different in SCD vs. healthy subjects. In summary: (1) The high circulating levels of platelet-monocyte and platelet-neutrophil aggregates demonstrate in vivo platelet activation in SCD and may be useful as markers of the in vivo pharmacodynamic efficacy of antiplatelet therapy in SCD. (2) The similar in vitro R-138727 IC50s in SCD and healthy subjects suggest that the prasugrel dose-dependence for platelet inhibition in SCD patients will be similar to that previously observed in healthy subjects.

A double-blind, randomized, multicenter phase 2 study of prasugrel versus placebo in adult patients with sickle cell disease

Background

Platelet activation has been implicated in the pathogenesis of sickle cell disease (SCD) suggesting antiplatelet agents may be therapeutic. To evaluate the safety of prasugrel, a thienopyridine antiplatelet agent, in adult patients with SCD, we conducted a double-blind, randomized, placebo-controlled study.

Methods

The primary endpoint, safety, was measured by hemorrhagic events requiring medical intervention. Patients were randomized to prasugrel 5 mg daily (n = 41) or placebo (n = 21) for 30 days. Platelet function by VerifyNow® P2Y12 and vasodilator-stimulated phosphoprotein assays at days 10 and 30 were significantly inhibited in prasugrel- compared with placebo-treated SCD patients.

Results

There were no hemorrhagic events requiring medical intervention in either study arm. Mean pain rate (percentage of days with pain) and intensity in the prasugrel arm were decreased compared with placebo. However, these decreases did not reach statistical significance. Platelet surface P-selectin and plasma soluble P-selectin, biomarkers of in vivo platelet activation, were significantly reduced in SCD patients receiving prasugrel compared with placebo. In sum, prasugrel was well tolerated and not associated with serious hemorrhagic events.

Conclusions

Despite the small size and short duration of this study, there was a decrease in platelet activation biomarkers and a trend toward decreased pain.

Platelets in pulmonary vascular physiology and pathology

Almost a trillion platelets pass through the pulmonary circulation every minute, yet little is known about how they support pulmonary physiology or contribute to the pathogenesis of lung diseases. When considering this conundrum, three questions jump out: Does platelet production in the lungs occur? Why does severe thrombocytopenia—which undercuts the principal physiological role of platelets to effect hemostasis—not lead to pulmonary hemorrhage? Why does atherothrombosis—which platelets initiate, maintain, and trigger is other critically important arterial beds—not develop in the pulmonary artery? The purpose of this review is to explore these and derivative questions by providing data within a conceptual framework that begins to organize a subject that is largely unassembled.

Pharmacology in childhood arterial ischemic stroke

In recent years, there has been increasing recognition of the impact of childhood stroke and interest in the role of drugs in the acute, chronic, and prophylactic management of this condition. Most treatment strategies are based on studies in adults with stroke, and the relative infrequency of stroke and the heterogeneity of etiologies in childhood compared with adults present significant challenges in study design for childhood stroke studies. The presence of thrombophilia has been associated with stroke in children, strengthening the concept that antithrombotic, antiplatelet, and even thrombolytic agents have a role in stroke treatment and prevention. There are several potential roles for drugs in the treatment of childhood stroke including hyperacute therapy, antithrombotic medication, antiplatelet medication, and disease-specific medications. Herein, we review the use and rationale of these medications in childhood arterial ischemic stroke.

P-selectin-mediated platelet-neutrophil aggregate formation activates neutrophils in mouse and human sickle cell disease

OBJECTIVE

To determine the role of platelets in stimulating mouse and human neutrophil activation and pulmonary injury in sickle cell disease (SCD).

METHODS AND RESULTS

Both platelet and neutrophil activation occur in SCD, but the interdependence of these events is unknown. Platelet activation and binding to leukocytes were measured in mice and patients with SCD and in controls. Relative to controls, blood obtained from mice or patients with SCD contained significantly elevated platelet-neutrophil aggregates (PNAs). Both platelets and neutrophils found in sickle PNAs were activated. Multispectral imaging (ImageStream) and conventional flow cytometry revealed a subpopulation of activated neutrophils with multiple adhered platelets that expressed significantly more CD11b and exhibited greater oxidative activity than single neutrophils. On average, wild-type and sickle PNAs contained 1.1 and 2.6 platelets per neutrophil, respectively. Hypoxia/reoxygenation induced a further increase in PNAs in mice with SCD and additional activation of both platelets and neutrophils. The pretreatment of mice with SCD with clopidogrel or P-selectin antibody reduced the formation of PNAs and neutrophil activation and decreased lung vascular permeability.

CONCLUSIONS

Our findings suggest that platelet binding activates neutrophils and contributes to a chronic inflammatory state and pulmonary dysfunction in SCD. The inhibition of platelet activation may be useful to decrease tissue injury in SCD, particularly during the early stages of vaso-occlusive crises.

Review: Management of Painful Vaso-Occlusive Crisis of Sickle-Cell Anemia: Consensus Opinion

Sickle-cell disease (SCD) is a wide-spread inherited hemolytic anemia that is due to a point mutation, leading to the substitution of valine for glutamic acid, causing a spectrum of clinical manifestations in addition to hemolysis and anemia. Acute painful crisis is a common sequela that can cause significant morbidity and negatively impact the patient’s quality of life. Remarkable improvements in the understanding of the pathogenesis of this clinical syndrome and the role of cell adhesion, inflammation, and coagulation in acute painful crisis have led to changes in the management of pain. Due to the endemic nature of SCD in various parts of the Middle East, a group of physicians and scientists from the United States and Middle East recently met to draw up a set of suggested guidelines for the management of acute painful crisis that are reflective of local and international experience. This review brings together a detailed etiology, the pathophysiology, and clinical presentation of SCD, including the differential diagnoses of pain associated with the disease, with evidence-based recommendations for pain management and the potential impact of low-molecular-weight heparin (LMWH), from the perspective of physicians and scientists with long-term experience in the management of a large number of patients with SCD.

Diagnosis and management of sickle cell disorders

Sickle cell disease (SCD) is a wide-spread inherited hemolytic anemia that is due to a point mutation leading to a valine/glutamic acid substitution in the beta-globin chain, causing a spectrum of clinical manifestations in addition to hemolysis and anemia. Acute painful crisis is a common sequela that can cause significant morbidity and negatively impact the patient's quality of life. Remarkable improvements in our understanding of the pathogenesis of this clinical syndrome and the role of cell adhesion, inflammation, and coagulation in acute painful crisis have led to changes in the management of pain. Due to the endemic nature of SCD in various parts of the Middle East, a group of physicians and scientists from the United States and Middle East recently met to draw up a set of suggested guidelines for the management of acute painful crisis that are reflective of local and international experience. This chapter brings together a detailed etiology, pathophysiology, and clinical presentation of SCD, including the differential diagnoses of pain associated with the disease, with evidence-based recommendations for pain management and the potential impact of low-molecular weight heparin (LMWH), from the perspective of physicians and scientists with long-term experience in the management of a large number of SCD patients.

Acute myocardial infarction in sickle cell disease: a systematic review

Myocardial infarction in young adults is, in practice, a diagnosis of exclusion. Given the fact that most of the patients with sickle cell disease are young and have predisposition to painful crisis, they are often overlooked for myocardial infarction. These patients often have few or no traditional risk factors for coronary artery disease, and risk stratification tools such as the Thrombolysis in Myocardial Infarction (TIMI) and Global Registry of Acute Coronary Events (GRACE) models place these patients at low risk. Nonspecific changes on electrocardiogram are of little diagnostic value. Myocardial infarction is very often a missed diagnosis in patients with sickle cell disease. Diagnostic criteria, potential mechanisms, and management for acute myocardial infarction in patients with sickle cell disease are discussed.

Morphological and functional platelet abnormalities in Berkeley sickle cell mice

Berkeley sickle cell mice are used as animal models of human sickle cell disease but there are no reports of platelet studies in this model. Since humans with sickle cell disease have platelet abnormalities, we studied platelet morphology and function in Berkeley mice (SS). We observed elevated mean platelet forward angle light scatter (FSC) values (an indirect measure of platelet volume) in SS compared to wild type (WT) (37+/-3.2 vs. 27+/-1.4, mean+/-SD; p<0.001), in association with moderate thrombocytopenia (505+/-49 x 10(3)/microl vs. 1151+/-162 x 10(3)/microl; p<0.001). Despite having marked splenomegaly, SS mice had elevated levels of Howell-Jolly bodies and "pocked" erythrocytes (p<0.001 for both) suggesting splenic dysfunction. SS mice also had elevated numbers of thiazole orange positive platelets (5+/-1% vs. 1+/-1%; p<0.001), normal to low plasma thrombopoietin levels, normal plasma glycocalicin levels, normal levels of platelet recovery, and near normal platelet life spans. Platelets from SS mice bound more fibrinogen and antibody to P-selectin following activation with a threshold concentration of a protease activated receptor (PAR)-4 peptide compared to WT mice. Enlarged platelets are associated with a predisposition to arterial thrombosis in humans and some humans with SCD have been reported to have large platelets. Thus, additional studies are needed to assess whether large platelets contribute either to pulmonary hypertension or the large vessel arterial occlusion that produces stroke in some children with sickle cell disease.

Beta-thalassaemia and sickle cell anaemia as paradigms of hypercoagulability

Thalassaemia and sickle cell disease (SCD) represent the most common forms of hereditary haemolytic anaemia and result from a partial or complete lack of synthesis of one of the major alpha- or beta-globin chains of haemoglobin A or from a single amino acid mutation (beta(6Glu-->Val)) of the beta-globin chain respectively. Although they have different pathophysiologies, patients with these conditions manifest both biochemical and clinical evidence of hypercoagulability. While the frequency of various thrombotic complications may vary in beta-thalassaemia and homozygous SCD [sickle cell anaemia (SCA)], patients with both diseases manifest decreased levels of natural anticoagulant proteins, as well as increased markers of thrombin generation and platelet activation. The abnormal phospholipid membrane assymetry present in the red blood cells of beta-thalassaemia and SCA patients, with resultant phosphatidylserine exposure appears to play a significant role in the aetiology of the observed hypercoagulable state. This review presents the available data on the aetiology and clinical manifestations of the coagulation and platelet activation that exist in both beta-thalassaemia and SCA, as well as the potential therapeutic implications resulting from this hypercoagulability.

Tramadol infusion for the pain management in sickle cell disease: a case report

We present the analgesic management of a 4-year-old child who suffered from severe abdominal and leg pain during his first vaso-occlusive crisis with sickle cell disease, diagnosed as beta/S disease when he was 1 year old. His mother and father were carriers of beta-thalassemia and hemoglobin S, respectively. He had an upper respiratory tract infection in which a vaso-occlusive crisis was precipitated. On admission to hospital, fever, severe abdominal and leg pain were noted. Hemoglobin was 4 g x dl(-1) with accompanying prominent reticulocytosis and acute spleen enlargement. These findings indicated a sequestration crisis as well as vaso-occlusive disease. He was transfused with packed red cells. Paracetamol (40-60 mg x kg(-1) x day(-1)) and ibuprofen (20 mg x kg(-1) x day(-1)) were administered to relieve pain. The child experienced moderate to severe pain (Oucher score 60-80) despite nonopioid analgesics, so a tramadol infusion (0.25 mg x kg(-1) x h(-1)) was started. During the tramadol infusion no morphine was required, the intensity of pain gradually decreased (Oucher score 20) and the child was able to move his legs. At the end of 3 days splenomegaly regressed, no fever and pain were observed and the infusion was stopped. In conclusion, tramadol infusion i.v. (0.25 mg x kg(-1) x h(-1)) combined with nonopioids was effective to relieve moderate to severe pain due to vaso-occlusive crisis and can be recommended before using morphine in a pediatric sickle cell crisis.

Hypercoagulability in Sickle Cell Disease: New Approaches to an Old Problem

Patients with sickle cell disease (SCD) exhibit high plasma levels of markers of thrombin generation, depletion of natural anticoagulant proteins, abnormal activation of the fibrinolytic system, and increased tissue factor expression, even in the non-crisis steady state. In addition, platelets and other cellular elements are chronically activated in the non-crisis state. Despite an abundance of evidence for coagulation and platelet activation, it remains uncertain whether these changes contribute to the pathophysiology of SCD or are, rather, simple epiphenomena. With the occurrence of macrovascular thrombotic complications in SCD, as well as the recognition that soluble CD40 ligand is biologically active in SCD, coagulation and platelet activation may indeed play a role in SCD pathophysiology. Defining a role for hypercoagulability in SCD requires further understanding of its pathogenesis. Furthermore, the conduct of well-controlled clinical trials using anticoagulants and antiplatelet agents and using a variety of clinical endpoints is warranted.

The prevention and management of stroke in sickle cell anaemia

Perhaps the most important clinical complication of sickle cell anaemia is stroke, an event that occurs in approximately 5-10% of children who inherit this disorder. To prevent recurrent or progressive CNS damage, the institution of regular red blood cell (RBC) transfusions is the standard of care. In addition, children at high risk of developing stroke, as screened by transcranial Doppler, also benefit from regular RBC transfusions for stroke prevention. In this review, standard and novel techniques of RBC transfusion, and also alternative therapies to treat children with or at risk for stroke are considered. In addition, haematopoietic cell transplantation, the only curative option for sickle cell anaemia, is considered, and speculation about its present and future application in this clinical setting is discussed.

Trials in sickle cell disease

Children with sickle cell disease are at risk of developing neurologic complications, including stroke, transient ischemic attack, seizures, coma, and a progressive reduction in cognitive function. Transcranial Doppler ultrasound, magnetic resonance imaging, and overnight pulse oximetry appear to predict, making prevention an achievable goal so that there is now a focus on randomized controlled trials. The Stroke Prevention Trial in Sickle Cell Anemia (STOP) reported a reduction in the number of overt clinical strokes experienced by those children with critically high transcranial Doppler velocities (>200 centimeters per second) who were chronically transfused. Two additional Phase III studies and two pilot trials have been funded. STOP II focused on whether it is safe to discontinue blood in prophylactically transfused children when their velocities had remained normal for at least 30 months. The Silent Infarct Transfusion trial is designed to determine whether children with sickle cell anemia and silent cerebral infarcts, approximately 20% of the population, will have a decrease in the progressive neurologic complications after receiving regular blood transfusion therapy. Pilot safety and feasibility trials of low-dose aspirin and overnight respiratory support are also beginning. The collaboration provides an infrastructure for future clinical trials in this vulnerable group of children.

Sickle Cell Vaso-Occlusion

The vaso-occlusion model has evolved impressively over the past several decades from polymerization-based concepts to a complex, wide-ranging schema that involves multistep, heterogeneous, and interdependent interactions among sickle erythrocytes (SSRBCs), adherent leukocytes, endothelial cells, plasma proteins, and other factors. Endothelial activation, induced directly or indirectly by the proinflammatory behavior of SSRBCs, is the most likely initiating step toward vaso-occlusion. Given the complexity and dynamic relationships of the potential mechanisms leading to vaso-occlusion, further in vivo studies in relevant sickle cell animal models will most likely yield the greatest advances and promote the development of novel, more effective therapeutic strategies.

Sickle-cell disease

With the global scope of sickle-cell disease, knowledge of the countless clinical presentations and treatment of this disorder need to be familiar to generalists, haematologists, internists, and paediatricians alike. Additionally, an underlying grasp of sickle-cell pathophysiology, which has rapidly accrued new knowledge in areas related to erythrocyte and extra-erythrocyte events, is crucial to an understanding of the complexity of this molecular disease with protean manifestations. We highlight studies from past decades related to such translational research as the use of hydroxyurea in treatment, as well as the therapeutic promise of red-cell ion-channel blockers, and antiadhesion and anti-inflammatory therapy. The novel role of nitric oxide in sickle-cell pathophysiology and the range of its potential use in treatment are also reviewed. Understanding of disease as the result of a continuing interaction between basic scientists and clinical researchers is best exemplified by this entity.

Hypercoagulability in sickle cell disease: a curious paradox

There is evidence of activation of both blood coagulation and platelets in sickle cell disease. For example, plasma samples obtained in the steady state and during painful crisis demonstrate high levels of thrombin generation, depletion of anticoagulant proteins, and abnormal activation of the fibrinolytic system. Similarly, exposure of surface markers such as CD62P and CD40L, along with increased circulating levels of thrombospondin, signal platelet activation. In addition to its effects on the cleavage of fibrinogen and its ability to activate platelets, the increase in circulating thrombin levels, with its wide-ranging effects on endothelial cells and blood vessels, may be important in the pathophysiology of sickle cell disease. Therefore, treatments that could decrease thrombin generation or platelet activation may be beneficial in both the treatment of sickle cell disease and the prevention of complications that characterize this genetic disorder. This review discusses hypercoagulability in the various forms of sickle cell disease, including homozygous sickle cell anemia, hemoglobin SC disease, hemoglobin SD disease, and sickle cell-beta-thalassemia.

Thrombogenesis in sickle cell disease

Thirty-three subjects with sickle cell disease (SCD), 11 during episodes of pain and 22 during periods without pain, were evaluated for in vivo thrombogenic activities as compared with 10 normal black control subjects. Measurements were performed for (1) platelet surface activation, assessing flow cytometric expression of activated integrin alpha(IIb)beta(3) receptor (GPIIb/IIIa, CD41a) and P-selectin (CD62p); (2) platelet and erythrocyte surface procoagulant activities, measuring flow cytometric binding of activated factor (FVa) and annexin V; (3) plasma levels of platelet-specific secreted proteins platelet factor 4 (PF4) and beta-thromboglobulin (betaTG); (4) plasma markers of thrombin generation, prothrombin activation fragment (F(1.2)), and thrombin: antithrombin complex (TAT); and (5) plasma markers of fibrinolysis, D -dimer, and plasmin:antiplasmin complex (PAP). As compared with control subjects, asymptomatic subjects with SCD demonstrated significantly increased platelet activation (P <.01 for P-selectin and annexin V binding), elevated plasma levels of PF4 and betaTG (P <.01 and P <.03, respectively), and increased plasma concentrations of F(1.2), TAT, PAP, and D -dimer (P <.05 in all cases). During episodes of SCD pain, platelet activation was increased as compared with periods without pain (P <.01 for expression of activated integrin alpha(IIb)beta(3) receptor and P-selectin and binding of FVa and annexin V), erythrocytes expressed procoagulant activities (P <.01 for FVa and annexin V binding), and platelet microparticles appeared in the circulation (3% to 30%; P <.001). SCD pain episodes were associated with elevated plasma levels of F(1.2), TAT, PAP, and D -dimer (P <.05 as compared with asymptomatic intervals). The frequency of pain episodes correlated with enhanced platelet procoagulant activity (r = 0.61, P <.05) and elevated plasma fibrinolytic activity (r = 0.74, P <.01) measured during periods without pain. Plasma fibrinolytic activity was inversely correlated with time to the next pain episode (r = -0.50, P <.05). Thus, asymptomatic subjects with SCD exhibit ongoing platelet activation, thrombin generation, and fibrinolysis that increases during episodes of pain. These changes are predictive of frequency of pain and interval to next pain episode, thereby implicating thrombogenic activity in the development of SCD pain episodes.