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

A critical review of therapeutic interventions in sickle cell disease: Progress and challenges

Sickle cell disease (SCD) is an autosomal recessive genetic disorder that occurs due to the point mutation in the β-globin gene, which results in the formation of sickle hemoglobin (HbS) in the red blood cells (RBCs). When HbS is exposed to an oxygen-depleted environment, it polymerizes, resulting in hemolysis, vaso-occlusion pain, and impaired blood flow. Still, there is no affordable cure for this inherited disease. Approved medications held promise but were met with challenges due to limited patient tolerance and undesired side effects, thereby inhibiting their ability to enhance the quality of life across various individuals with SCD. Progress has been made in understanding the pathophysiology of SCD during the past few decades, leading to the discovery of novel targets and therapies. However, there is a compelling need for research to discover medications with improved efficacy and reduced side effects. Also, more clinical investigations on various drug combinations with different mechanisms of action are needed. This review comprehensively presents therapeutic approaches for SCD, including those currently available or under investigation. It covers fundamental aspects of the disease, such as epidemiology and pathophysiology, and provides detailed discussions on various disease-modifying agents. Additionally, expert insights are offered on the future development of pharmacotherapy for SCD.

Platelet Aggregation Studies and Coagulation Profile in Sickle Cell Disease in Symptomatic and Steady State Patients

To determine whether there is higher degree of platelet and/ or coagulation activation in sickle cell anaemia (SS) patients in complications and with clinical risk factors. A cross sectional study was conducted at a tertiary health care centre in central India with study groups: sickle cell disease (SCD): sickle cell anaemia (SS) and sickle cell trait (AS) consisting of 100 subjects each and controls (AA) with 40 subjects. Platelet aggregation (PA) with ADP, collagen and epinephrine, PT and aPTT were performed in all subjects and PA with ristocetin in ten candidates of each group. ANOVA and student's unpaired t test were used to compare PA and coagulation profile of the three groups with respect to age groups, gender, present diagnosis, history of complications, frequency of hospital admissions (high ≥ 3/year) and frequency of blood transfusion (high > 2/year). The max PA% with ADP was significantly less in SS patients in steady state, which was even lesser in those having symptoms, complications in past/ present, high-frequency hospital admission and > 2 blood transfusions per year subgroups, as compared to all other groups and subgroups, but not consistently with collagen and epinephrine. The max PA % with ristocetin was least in SS with complications. No statistically significant difference in PT and aPTT values among the various clinical risk subgroups and groups was found. SCD patients can be monitored by using PA with ADP for their timely and better management. PA with ADP, PT and aPTT should be added to the workup of these patients for improved prognostication.

Effects of antiplatelet therapy on menstrual blood loss in reproductive-aged women: a systematic review

Background

The effects of antiplatelet therapy on menstrual bleeding have not been well characterized.

Objectives

To systematically review the effects of antiplatelet therapy on menstrual bleeding.

Methods

A literature search was performed for studies of reproductive-aged women who received antiplatelet therapy. Characteristics of menstrual bleeding both before and after initiation of antiplatelet therapy and from comparison groups were collected. Two reviewers independently assessed the risk of bias in individual studies.

Results

Thirteen studies with a total of 611 women who received antiplatelet therapy were included. Types of antiplatelet drugs used were aspirin (n = 8), aspirin and/or clopidogrel (n = 2), prasugrel (n = 1), and not specified (n = 2). Risk of bias was assessed at moderate (n = 1), serious (n = 8), critical (n = 2), and no information (n = 2). Three studies reported changes in menstrual blood loss volume. One of these showed no increase during antiplatelet therapy; the other 2 studies suggested that aspirin may increase menstrual blood loss volume. In 3 studies that assessed the duration of menstrual bleeding, up to 13% of women reported an increased duration of menstruation. In 5 studies that reported the intensity of menstrual flow, 13% to 38% of women experienced an increase in the intensity of flow. Five studies reported the prevalence of heavy menstrual bleeding in women who received antiplatelet therapy, with estimates ranging from 7% to 38%.

Conclusion

There is lack of high-quality data on the effects of antiplatelet therapy on menstrual bleeding. Aspirin may increase menstrual blood loss, at least in a minority of women, whereas the effects of P2Y12 inhibitors are unknown.

From Stress to Sick(le) and Back Again-Oxidative/Antioxidant Mechanisms, Genetic Modulation, and Cerebrovascular Disease in Children with Sickle Cell Anemia

Sickle cell anemia (SCA) is a genetic disease caused by the homozygosity of the HBB:c.20A>T mutation, which results in the production of hemoglobin S (HbS). In hypoxic conditions, HbS suffers autoxidation and polymerizes inside red blood cells, altering their morphology into a sickle shape, with increased rigidity and fragility. This triggers complex pathophysiological mechanisms, including inflammation, cell adhesion, oxidative stress, and vaso-occlusion, along with metabolic alterations and endocrine complications. SCA is phenotypically heterogeneous due to the modulation of both environmental and genetic factors. Pediatric cerebrovascular disease (CVD), namely ischemic stroke and silent cerebral infarctions, is one of the most impactful manifestations. In this review, we highlight the role of oxidative stress in the pathophysiology of pediatric CVD. Since oxidative stress is an interdependent mechanism in vasculopathy, occurring alongside (or as result of) endothelial dysfunction, cell adhesion, inflammation, chronic hemolysis, ischemia-reperfusion injury, and vaso-occlusion, a brief overview of the main mechanisms involved is included. Moreover, the genetic modulation of CVD in SCA is discussed. The knowledge of the intricate network of altered mechanisms in SCA, and how it is affected by different genetic factors, is fundamental for the identification of potential therapeutic targets, drug development, and patient-specific treatment alternatives.

Hope on the Horizon: New and Future Therapies for Sickle Cell Disease

This article provides an overview of conventional, new, and future treatment options for sickle cell disease (SCD), a genetic disorder affecting the production of hemoglobin. Current treatments include hydroxyurea, a conventional SCD treatment that increases the levels of fetal hemoglobin, and new treatments such as voxelotor, a recently approved SCD treatment that selectively binds hemoglobin, preventing formation of sickled red blood cells. In addition to discussing the mechanisms of action of current SCD treatments, potential side effects are also discussed, highlighting the need for new treatments that can address the limitations of current treatments and improve the quality of life for people with SCD. Future treatments, such as gene therapy, are also explored as promising treatment options for SCD patients.

Emerging drug targets for sickle cell disease: shedding light on new knowledge and advances at the molecular level

INTRODUCTION

In sickle cell disease (SCD), a single amino acid substitution at β6 of the hemoglobin (Hb) chain replaces glutamate with valine, forming HbS instead of the normal adult HbA. Loss of a negative charge, and the conformational change in deoxygenated HbS molecules, enables formation of HbS polymers. These not only distort red cell morphology but also have other profound effects so that this simple etiology belies a complex pathogenesis with multiple complications. Although SCD represents a common severe inherited disorder with life-long consequences, approved treatments remain inadequate. Hydroxyurea is currently the most effective, with a handful of newer treatments, but there remains a real need for novel, efficacious therapies.

AREAS COVERED

This review summarizes important early events in pathogenesis to highlight key targets for novel treatments.

EXPERT OPINION

A thorough understanding of early events in pathogenesis closely associated with the presence of HbS is the logical starting point for identification of new targets rather than concentrating on more downstream effects. We discuss ways of reducing HbS levels, reducing the impact of HbS polymers, and of membrane events perturbing cell function, and suggest using the unique permeability of sickle cells to target drugs specifically into those more severely compromised.

Emerging drugs for the treatment of sickle cell disease: a review of phase II/III trials

INTRODUCTION

The substitution of glutamic acid by valine on the ß-globin gene produces the hemoglobin S variant responsible for sickle cell disease (SCD), a disorder that affects millions of people worldwide and leads to acute and cumulative organ damage. Even though life expectancy has significantly improved where the best medical care is available, there are still few therapeutic options for SCD and those are limited by their availability, cost, and individual toxicities.

AREAS COVERED

This review summarizes the clinical data on current treatments for SCD and emerging therapies studied in the acute setting as well as potential disease-modifying agents, with an emphasis on the FDA-approved agents.

EXPERT OPINION

Hydroxyurea has been a gold standard for two decades, showing benefits in acute complications and overall survival in sickle cell anemia, although data is lacking for certain genotypes such as hemoglobin SC. As progress is made in our understanding of the pathophysiological networks characterizing SCD, numerous pathways appear to be targetable, with L-glutamine, Crizanlizumab and Voxelotor now approved by the FDA. Pursuing a multi-agent approach could alter the disease course in a more effective fashion and provide an alternative option to curative therapies, but longer clinical studies are needed.

Vaso-occlusive crisis in sickle cell disease: a vicious cycle of secondary events

Painful vaso-occlusive crisis (VOC) remains the most common reason for presenting to the Emergency Department and hospitalization in patients with sickle cell disease (SCD). Although two new agents have been approved by the Food and Drug Administration for treating SCD, they both target to reduce the frequency of VOC. Results from studies investigating various approaches to treat and shorten VOC have so far been generally disappointing. In this paper, we will summarize the complex pathophysiology and downstream events of VOC and discuss the likely reasons for the disappointing results using monotherapy. We will put forward the rationale for exploring some of the currently available agents to either protect erythrocytes un-involved in the hemoglobin polymerization process from sickling induced by the secondary events, or a multipronged combination approach that targets the complex downstream pathways of VOC.

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.

Crizanlizumab and comparators for adults with sickle cell disease: a systematic review and network meta-analysis

OBJECTIVES

Treatment options for preventing vaso-occlusive crises (VOC) among patients with sickle cell disease (SCD) are limited, especially if hydroxyurea treatment has failed or is contraindicated. A systematic literature review (SLR) and network meta-analysis (NMA) were conducted to evaluate the efficacy and safety of crizanlizumab for older adolescent and adult (≥16 years old) SCD patients.

METHODS

The SLR included randomised controlled trials (RCTs) and uncontrolled studies. Bayesian NMA of VOC, all-cause hospitalisation days and adverse events were conducted.

RESULTS

The SLR identified 51 studies and 9 RCTs on 14 treatments that met the NMA inclusion criteria. The NMA found that crizanlizumab 5.0 mg/kg was associated with a reduction in VOC (HR 0.55, 95% credible interval (0.43, 0.69); Bayesian probability of superiority >0.99), all-cause hospitalisation days (0.58 (0.50, 0.68); >0.99) and no evidence of difference on adverse events (0.91 (0.59, 1.43) 0.66) or serious adverse events (0.93 (0.47, 1.87); 0.59) compared with placebo. The HR for reduction in VOC for crizanlizumab relative to L-glutamine was (0.67 (0.50, 0.88); >0.99). These results were sensitive to assumptions regarding whether patient age is an effect modifier.

CONCLUSIONS

This NMA provides preliminary evidence comparing the efficacy of crizanlizumab with other treatments for VOC prevention.

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.

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.

Emerging disease-modifying therapies for sickle cell disease

Sickle cell disease afflicts millions of people worldwide and approximately 100,000 Americans. Complications are myriad and arise as a result of complex pathological pathways ‘downstream’ to a point mutation in DNA, and include red blood cell membrane damage, inflammation, chronic hemolytic anemia with episodic vaso-occlusion, ischemia and pain, and ultimately risk of cumulative organ damage with reduced lifespan of affected individuals. The National Heart, Lung, and Blood Institute’s 2014 evidence-based guideline for sickle cell disease management states that additional research is needed before investigational curative therapies will be widely available to most patients with sickle cell disease. To date, sickle cell disease has been cured by hematopoietic stem cell transplantation in approximately 1,000 people, most of whom were children, and significantly ameliorated by gene therapy in a handful of subjects who have only limited follow-up thus far. During a timespan in which over 20 agents were approved for the treatment of cystic fibrosis by the Food and Drug Administration, similar approval was granted for only two drugs for sickle cell disease (hydroxyurea and L-glutamine) despite the higher prevalence of sickle cell disease. This trajectory appears to be changing, as the lack of multimodal agent therapy in sickle cell disease has spurred engagement among many in academia and industry who, in the last decade, have developed new drugs poised to prevent complications and alleviate suffering. Identified therapeutic strategies include fetal hemoglobin induction, inhibition of intracellular HbS polymerization, inhibition of oxidant stress and inflammation, and perturbation of the activation of the endothelium and other blood components (e.g. platelets, white blood cells, coagulation proteins) involved in the pathophysiology of sickle cell disease. In this article, we present a crash-course review of disease-modifying approaches (minus hematopoietic stem cell transplant and gene therapy) for patients with sickle cell disease currently, or recently, tested in clinical trials in the era following approval of hydroxyurea.

Population Pharmacokinetic and Exposure-Response Analyses of Prasugrel in Pediatric Patients with Sickle Cell Anemia

BACKGROUND AND OBJECTIVE

Prasugrel, a P2Y₁₂ adenosine diphosphate (ADP) receptor antagonist, inhibits ADP-mediated platelet activation and aggregation in patients with sickle cell anemia (SCA). We developed a population pharmacokinetic (popPK) model in pediatric patients from 2 to <18 years of age with SCA, and performed exposure-response evaluations to characterize the effects of prasugrel in a subset of these patients who weighed 19 kg or more and experienced at least two episodes of vaso-occlusive crises (VOC) in the past year.

METHODS

A three-compartment popPK model adapted from that used in adults with acute coronary syndrome was used to describe the relationship between plasma concentrations of prasugrel's active metabolite (Pras-AM) and time using data from phase II and III clinical studies in children. A VOC event rate model was developed from the phase III study to explore the exposure-response relationship between Pras-AM exposure and VOC, and included evaluation of covariates.

RESULTS

The final popPK model for children with SCA provided a reasonable fit to Pras-AM plasma concentrations over time, with estimates of apparent clearance (CL/F) (172 L/h) and apparent volume of distribution (Vd/F) (51.7 L) that were comparable to previous studies in adults. The final model included weight as a covariate on both CL/F and Vd/F, and age as a covariate on CL/F. Analyses of safety (bleeding events requiring medical intervention) and efficacy (VOC event rate) variables showed no apparent relationship to model-predicted Pras-AM exposure quartiles, and no statistically significant effects of intrinsic or extrinsic factors on the VOC event rate were identified in the VOC event rate model. The effect of post hoc exposures on the VOC event rate did not reach statistical significance.

CONCLUSIONS

A popPK model was developed that provided reasonable parameter estimates, goodness-of-fit diagnostics, and visual predictive checks when applied to Pras-AM plasma concentrations in pediatric patients with SCA. Post hoc exposures obtained from this model did not correlate with measures of VOC or bleeding events in this population.

Therapeutic strategies for sickle cell disease: towards a multi-agent approach

For over 100 years, clinicians and scientists have been unravelling the consequences of the A to T substitution in the β-globin gene that produces haemoglobin S, which leads to the systemic manifestations of sickle cell disease (SCD), including vaso-occlusion, anaemia, haemolysis, organ injury and pain. However, despite growing understanding of the mechanisms of haemoglobin S polymerization and its effects on red blood cells, only two therapies for SCD - hydroxyurea and L-glutamine - are approved by the US Food and Drug Administration. Moreover, these treatment options do not fully address the manifestations of SCD, which arise from a complex network of interdependent pathophysiological processes. In this article, we review efforts to develop new drugs targeting these processes, including agents that reactivate fetal haemoglobin, anti-sickling agents, anti-adhesion agents, modulators of ischaemia-reperfusion and oxidative stress, agents that counteract free haemoglobin and haem, anti-inflammatory agents, anti-thrombotic agents and anti-platelet agents. We also discuss gene therapy, which holds promise of a cure, although its widespread application is currently limited by technical challenges and the expense of treatment. We thus propose that developing systems-oriented multi-agent strategies on the basis of SCD pathophysiology is needed to improve the quality of life and survival of people with SCD.

Ticagrelor does not impact patient-reported pain in young adults with sickle cell disease: a multicentre, randomised phase IIb study

Ticagrelor is an antiplatelet agent for adults with coronary artery disease. The inhibition of platelet activation may decrease the frequency of vaso-occlusion crisis (VOC) in sickle cell disease (SCD). The HESTIA2 study (NCT02482298) randomised 87 adults with SCD (aged 18-30 years) 1:1:1 to twice-daily ticagrelor 10, 45 mg or placebo for 12 weeks. Numerical decreases from baseline in mean proportion of days with patient-reported pain (primary endpoint) were seen in all three groups, as well as in pain intensity and analgesic use, with no significant differences between placebo and ticagrelor treatment groups. Plasma ticagrelor concentrations and platelet inhibition increased with dose. Adverse events were distributed evenly across groups and two non-major bleeding events occurred per group. Ticagrelor was well tolerated with a low bleeding risk, but no effect on diary-reported pain was detected. Potential effects on frequency of VOCs will need to be evaluated in a larger and longer study.

Inflammation in sickle cell disease

The primary β-globin gene mutation that causes sickle cell disease (SCD) has significant pathophysiological consequences that result in hemolytic events and the induction of the inflammatory processes that ultimately lead to vaso-occlusion. In addition to their role in the initiation of the acute painful vaso-occlusive episodes that are characteristic of SCD, inflammatory processes are also key components of many of the complications of the disease including autosplenectomy, acute chest syndrome, pulmonary hypertension, leg ulcers, nephropathy and stroke. We, herein, discuss the events that trigger inflammation in the disease, as well as the mechanisms, inflammatory molecules and cells that propagate these inflammatory processes. Given the central role that inflammation plays in SCD pathophysiology, many of the therapeutic approaches currently under pre-clinical and clinical development for the treatment of SCD endeavor to counter aspects or specific molecules of these inflammatory processes and it is possible that, in the future, we will see anti-inflammatory drugs being used either together with, or in place of, hydroxyurea in those SCD patients for whom hematopoietic stem cell transplants and evolving gene therapies are not a viable option.

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).

Prasugrel hydrochloride for the treatment of sickle cell disease

INTRODUCTION

Therapeutic options for sickle cell disease (SCD) are limited and, currently, only one drug (hydroxyurea) has FDA approval for the treatment of adult SCD. While this genetic disease is caused by hemoglobin polymerization, subsequent downstream events trigger platelet activation, vaso-occlusion and the disease's complex pathophysiology. Areas covered: The oral thienopyridine, prasugrel hydrochloride, irreversibly inhibits the P2Y12 receptors, inhibiting ADP-dependent platelet activation. We discuss recent clinical trials evaluating the pharmokinetics of prasugrel and its potential for use in SCD. Expert opinion: Prasugrel administration in SCD appears to be well tolerated and safe. However, although this drug modestly inhibits platelet activity in these patients, administration of prasugrel to a large group of children and adolescents for up to 24 months failed to convincingly reduce vaso-occlusive complications. Speculatively, prasugrel may be of occasional use for off-license purposes in patients unable or unwilling to take hydroxyurea (particularly in 12-17-year olds). Although there is currently no prospect of prasugrel being licensed for use in SCD, the success of on-going trials of other antiplatelet agents in SCD might lead to further trials of prasugrel in SCD.

Increased Vasoocclusive Crises in "O" Blood Group Sickle Cell Disease Patients: Association with Underlying Thrombospondin Levels

Objectives

To explore the incidence of vaso-occlusive crisis (VOC) in Blood Group “O” sickle cell disease (SCD) patients, and correlate it with the blood group and thrombospondin (TSP) levels.

Methods

In 89 consecutive SCD patients, blood samples were obtained for von Williebrand factor (vWF:Ag) antigen, collagen binding activity (CBA), ristocetin binding activity (RCo), blood group typing, C-reactive protein (CRP), high performance liquid chromatography (HPLC), Serum TSP 1 and TSP 2 levels, complete blood counts (CBC), lactic dehydrogenase (LDH) levels, liver function (LFT) and renal function tests (RFT) during VOC episodes and in steady state conditions.

Results

In steady state SCD patients (n=72), “O” blood group patients (n=37) showed a significantly higher median serum TSP 1 and TSP 2 levels as compared to non-O blood group patients [n=35] [p <0.05, Mann-Whitney test]; with an inverse relation between vWF:Ag, Factor VIII:C and TSP levels. Furthermore, the serum TSP 1 and TSP 2 levels were significantly higher in patients presenting with acute VOC [n=17], as well as in those with repeated VOC’s (group 1, n=16), especially amongst blood group “O” patients [p, <0.05, Mann-Whitney test].

Conclusions

The study demonstrates an inverse relation between TSP and vWF levels, in blood group “O” SCD patients, with an upregulation of the TSP levels. Expectedly, during active VOC crisis, the TSP 1 and TSP 2 levels were significantly elevated.

Real-time dose adjustment using point-of-care platelet reactivity testing in a double-blind study of prasugrel in children with sickle cell anaemia

Patients with sickle cell anaemia (SCA) have vaso-occlusive crises resulting from occlusive hypoxic-ischaemic injury. Prasugrel inhibits platelet activation and aggregation involved in SCA pathophysiology. Determining Effects of Platelet Inhibition on Vaso-Occlusive Events (DOVE) was a phase 3, double-blind, randomised, placebo-controlled trial assessing prasugrel efficacy. DOVE sought to bring patients' P2Y12 reaction unit (PRU) value within a targeted range via prasugrel dose adjustments using encrypted VerifyNow P2Y12^® (VN-P2Y12) point-of-care testing and an interactive voice-response system (IVRS). After PRU determination, randomised patients received 0.08 mg/kg/day prasugrel or placebo. Encrypted PRUs and IVRS provided double-blind dose adjustments to achieve a defined PRU target range of 136-231; placebo patients had mock titrations. Of 341 randomised patients, 166 placebo and 160 prasugrel patients reached the fully titrated dose (FTD). Most prasugrel patients (n=104, 65 %) remained on the initial 0.08 mg/kg dose; doses escalations occurred in 23 % of patients (n=36). Mean PRUs for the pharmacodynamic population at baseline were similar in the prasugrel (273 ± 44.9) and placebo groups (273 ± 51.7), with significant reductions in PRU (p<0.001) for prasugrel patients at the FTD and at 9 months. Concomitant use of hydroxyurea did not affect platelet reactivity at any time. The majority of prasugrel patients (n=135, 84.4 %) at the FTD were within the target range of 136-231 PRUs. Mean VN-P2Y12 percentage inhibition at baseline was similar in the prasugrel (2.8  ± 5.4 %) and placebo groups (2.0 ± 4.7 %); prasugrel patients had significant increases in inhibition (p<0.001) at FTD and at 9 months. Patients with higher PRU values at baseline required higher prasugrel doses to bring PRU within the prespecified range. DOVE is the first study to successfully employ double-blind, real-time, encrypted, point-of-care platelet testing and IVRS to dose-adjust antiplatelet therapy to a targeted range of platelet inhibition.

Sickle cell disease: an inherited thrombophilia

Activation of the hemostatic system occurs in patients with sickle cell disease. The extent to which this activation contributes to sickle cell pathophysiology is uncertain. Clinical trials of anticoagulants or platelet inhibitors have demonstrated the ability to decrease biomarkers of hemostatic activation, but this has generally not resulted in improvement in clinically relevant outcomes. Venous thromboembolism (VTE: deep venous thrombosis and pulmonary embolism) has been until recently an underappreciated complication of sickle cell disease, with incident event and recurrence rates consistent with a strong thrombophilia. There is no strong evidence that management should differ than for other patients with VTE, with the possible exception that secondary prophylaxis be extended regardless of provocation, given the persistent strong thrombophilic state.

Underlying mechanism and specific prevention of hemolysis-induced platelet activation

Thromboembolic complications significantly impair the outcome of hemolytic disorders. We hypothesized that red cell adenosine diphosphate (ADP) release results in significant platelet activation in hemolysis and that this prothrombotic state can be prevented by inhibition of the ADP P2Y₁₂ receptor. In the current study, we therefore sought to investigate the mechanism and inhibition of hemolysis-induced platelet activation. The expression of activated integrin αIIbß3 was determined by flow cytometry, and platelet aggregation was assessed by multiple electrode platelet aggregometry. We demonstrate platelet activation and increased platelet aggregation by adding hemolytic blood (lysates) to whole blood, similarly to that achieved by the platelet agonist ADP. Enhanced platelet activation and reactivity in the presence of hemolytic blood were significantly abolished by apyrase, which catalyzes ADP degradation, and inhibited by blockade of the platelet ADP P2Y₁₂ receptor with cangrelor. Platelets from patients treated with the ADP P2Y₁₂ receptor antagonist clopidogrel showed a reduced response to lysates compared to platelets from healthy controls without antiplatelet treatment. Further, in vitro blood group ABO incompatibility induced hemolysis and led to increased platelet activation. Finally, "spontaneous" platelet aggregation seen in patients with cold agglutinin disease was completely abolished by cangrelor. In conclusion, hemolysis is associated with increased platelet activation and aggregation due to red cell derived ADP, which can be prevented by ADP receptor blockade.

Precipitating factors and targeted therapies in combating the perils of sickle cell disease--- A special nutritional consideration

Nutritional research in sickle cell disease has been the focus in recent times owing to not only specific nutritional deficiencies, but also the improvements associated with less painful episodes. Though hydroxyurea remains the drug of choice, certain adverse health effects on long term supplementation makes room for researches of different compounds. Macro and micro nutrient deficiencies, along with vitamins, play an important role in not only meeting the calorific needs, but also reducing clinical complications and growth abnormalities. Symptoms of hyper protein metabolism, increased cell turnover, increased cardiac output, and appetite suppression due to enhanced cytokine production, might give us leads for better understanding of the mechanisms involved. Different nutritional approaches comprising of traditional herbal therapies, antioxidants, flavonoids, vitamins, minerals etc., reducing oxidative stress and blood aggregation, have been tried out to increase the health potential. Nutritional therapies may also serve complementary to the newer therapies using ozone, hematopoietic stem cell transplantation, antifungal medications, erythropoietin etc. Herein we try to present a holistic picture of the different patho-physiological mechanisms, and nutritional strategies adopted.

A Multinational Trial of Prasugrel for Sickle Cell Vaso-Occlusive Events

BACKGROUND

Sickle cell anemia is an inherited blood disorder that is characterized by painful vaso-occlusive crises, for which there are few treatment options. Platelets mediate intercellular adhesion and thrombosis during vaso-occlusion in sickle cell anemia, which suggests a role for antiplatelet agents in modifying disease events.

METHODS

Children and adolescents 2 through 17 years of age with sickle cell anemia were randomly assigned to receive oral prasugrel or placebo for 9 to 24 months. The primary end point was the rate of vaso-occlusive crisis, a composite of painful crisis or acute chest syndrome. The secondary end points were the rate of sickle cell-related pain and the intensity of pain, which were assessed daily with the use of pain diaries.

RESULTS

A total of 341 patients underwent randomization at 51 sites in 13 countries across the Americas, Europe, Asia, and Africa. The rate of vaso-occlusive crisis events per person-year was 2.30 in the prasugrel group and 2.77 in the placebo group (rate ratio, 0.83; 95% confidence interval, 0.66 to 1.05; P=0.12). There were no significant differences between the groups in the secondary end points of diary-reported events. The safety end points, including the frequency of bleeding events requiring medical intervention, of hemorrhagic and nonhemorrhagic adverse events that occurred while patients were taking prasugrel or placebo, and of discontinuations due to prasugrel or placebo, did not differ significantly between the groups.

CONCLUSIONS

Among children and adolescents with sickle cell anemia, the rate of vaso-occlusive crisis was not significantly lower among those who received prasugrel than among those who received placebo. There were no significant between-group differences in the safety findings. (Funded by Daiichi Sankyo and Eli Lilly; ClinicalTrials.gov number, NCT01794000.).

Pulmonary platelet thrombi and vascular pathology in acute chest syndrome in patients with sickle cell disease

A growing body of evidence suggests a role for platelets in sickle cell disease (SCD). Despite the proinflammatory, occlusive nature of platelets, a role for platelets in acute chest syndrome (ACS), however, remains understudied. To provide evidence and potentially describe contributory factors for a putative link between ACS and platelets, we performed an autopsy study of 20 SCD cases—10 of whom died from ACS and 10 whose deaths were not ACS‐related. Pulmonary histopathology and case history were collected. We discovered that disseminated pulmonary platelet thrombi were present in 3 out of 10 of cases with ACS, but none of the matched cases without ACS. Those cases with detected thrombi were associated with significant deposition of endothelial vWF and detection of large vWF aggregates adhered to endothelium. Potential clinical risk factors were younger age and higher platelet count at presentation. However, we also noted a sharp and significant decline in platelet count prior to death in each case with platelet thrombi in the lungs. In this study, neither hydroxyurea use nor perimortem transfusion was associated with platelet thrombi. Surprisingly, in all cases, there was profound pulmonary artery remodeling with both thrombotic and proliferative pulmonary plexiform lesions. The severity of remodeling was not associated with a severe history of ACS, or hydroxyurea use, but was inversely correlated with age. We thus provide evidence of undocumented presence of platelet thrombi in cases of fatal ACS and describe clinical correlates. We also provide novel correlates of pulmonary remodeling in SCD. Am. J. Hematol. 91:173–178, 2016. © 2015 The Authors. American Journal of Hematology Published by Wiley Periodicals, Inc.

Design of the DOVE (Determining Effects of Platelet Inhibition on Vaso-Occlusive Events) trial: A global Phase 3 double-blind, randomized, placebo-controlled, multicenter study of the efficacy and safety of prasugrel in pediatric patients with sickle cell anemia utilizing a dose titration strategy

BACKGROUND

Sickle cell disease (SCD) is an inherited blood disorder characterized by painful vaso-occlusive crises (VOC) with limited treatment options, particularly for children. Emerging knowledge of the pathophysiology of SCD suggests antiplatelet therapies may hold promise for treatment of VOC. Multiple small studies have evaluated antiplatelet agents on the frequency of VOC with varying results, but there has not been an adequately powered study to definitively determine the effect of antiplatelet agents on VOC. Prasugrel, a third-generation thienopyridine that irreversibly inhibits platelet activation and aggregation, is approved in adults with acute coronary syndrome managed with percutaneous coronary intervention.

PROCEDURE

Determining Effects of Platelet Inhibition on Vaso-Occlusive Events (DOVE) is a double-blind, randomized study with planned enrollment of >220 children from 14 countries across the Americas, Europe, Asia, and Africa, designed to test the hypothesis that prasugrel reduces the rate of VOC in children with sickle cell anemia (SCA) (homozygous hemoglobin S [HbSS] and hemoglobin Sβ(0) thalassemia [HbSβ(0)]). Secondary study endpoints include reductions in rate and intensity of vaso-occlusive pain as recorded in daily electronic diaries. Safety assessments include incidence of hemorrhagic events requiring medical intervention and treatment-emergent adverse events. DOVE incorporates a dose-titration strategy to reduce potential bleeding risks inherent with antiplatelet therapy while maintaining blinded treatment assignment.

CONCLUSIONS

DOVE presents a unique opportunity to determine whether antiplatelet therapy reduces frequency of patient-reported VOC and daily vaso-occlusive pain in a global study of children with SCA.

Neutrophils, platelets, and inflammatory pathways at the nexus of sickle cell disease pathophysiology

Sickle cell disease (SCD) is a severe genetic blood disorder characterized by hemolytic anemia, episodic vaso-occlusion, and progressive organ damage. Current management of the disease remains symptomatic or preventative. Specific treatment targeting major complications such as vaso-occlusion is still lacking. Recent studies have identified various cellular and molecular factors that contribute to the pathophysiology of SCD. Here, we review the role of these elements and discuss the opportunities for therapeutic intervention.

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.

Cation Homeostasis in Red Cells From Patients With Sickle Cell Disease Heterologous for HbS and HbC (HbSC Genotype)

Sickle cell disease (SCD) in patients of HbSC genotype is considered similar, albeit milder, to that in homozygous HbSS individuals--but with little justification. In SCD, elevated red cell cation permeability is critical as increased solute loss causes dehydration and encourages sickling. Recently, we showed that the KCl cotransporter (KCC) activity in red cells from HbSC patients correlated significantly with disease severity, but that in HbSS patients did not. Two transporters involved in red cell dehydration, the conductive channels Psickle and the Gardos channel, behaved similarly in red cells from the two genotypes, but were significantly less active in HbSC patients. By contrast, KCC activity was quantitatively greater in HbSC red cells. Results suggest that KCC is likely to have greater involvement in red cell dehydration in HbSC patients, which could explain its association with disease severity in this genotype. This work supports the hypothesis that SCD in HbSC patients is a distinct disease entity to that in HbSS patients. Results suggest the possibility of designing specific treatments of particular benefit to HbSC patients and a rationale for the development of prognostic markers, to inform early treatment of children likely to develop more severe complications of the disease.

Biomarkers and recent advances in the management and therapy of sickle cell disease

Although production of hemoglobin S, the genetic defect that causes sickle cell disease (SCD), directly affects only red blood cells, the manifestations of SCD are pervasive, and almost every cell type and organ system in the body can be involved. Today, the vast majority of patients with SCD who receive modern health care reach adulthood thanks to vaccine prophylaxis and improvements in supportive care, including transfusion. However, once patients reach adulthood, they commonly experience recurrent painful vaso-occlusive crises and frequently have widespread end-organ damage and severely shortened life expectancies. Over the last several decades, research has elucidated many of the mechanisms whereby abnormal red blood cells produce such ubiquitous organ damage. With these discoveries have come new ways to measure disease activity. In addition, new pharmaceutical interventions are now being developed to address what has been learned about disease mechanisms.

2015 Clinical trials update in sickle cell anemia

Polymerization of HbS and cell sickling are the prime pathophysiological events in sickle cell disease (SCD). Over the last 30 years, a substantial understanding at the molecular level has been acquired on how a single amino acid change in the structure of the beta chain of hemoglobin leads to the explosive growth of the HbS polymer and the associated changes in red cell morphology. O2 tension and intracellular HbS concentration are the primary molecular drivers of this process, and are obvious targets for developing new therapies. However, polymerization and sickling are driving a complex network of associated cellular changes inside and outside of the erythrocyte, which become essential components of the inflammatory vasculopathy and result in a large range of potential acute and chronic organ damages. In these areas, a multitude of new targets for therapeutic developments have emerged, with several ongoing or planned new therapeutic interventions. This review outlines the key points of SCD pathophysiology as they relate to the development of new therapies, both at the pre-clinical and clinical levels.

Hydroxyurea with AKT2 inhibition decreases vaso-occlusive events in sickle cell disease mice

Heterotypic cell-cell adhesion and aggregation mediate vaso-occlusive events in patients with sickle cell disease (SCD). Although hydroxyurea (HU), an inducer of fetal hemoglobin, is the main therapy for treatment of SCD, it is unclear whether it has immediate benefits in acute vaso-occlusive events in SCD patients. Using real-time fluorescence intravital microscopy, we demonstrated that short-term coadministration of HU and Akti XII, an AKT2 inhibitor, efficiently reduced neutrophil adhesion and platelet-neutrophil aggregation in venules of Berkeley (SCD) mice challenged with tumor necrosis factor α (TNF-α) or hypoxia/reoxygenation. Importantly, compared with HU or Akti XII treatment alone, short-term treatment with both agents significantly improved survival in those mice. We found that the level of plasma nitric oxide species was elevated by HU but not Akti XII, AKT2 phosphorylation levels in activated neutrophils and platelets were reduced by Akti XII but not HU, and the expression of endothelial E-selectin and intercellular adhesion molecule 1 was decreased by either agent. Our results suggest that short-term coadministration of HU and Akti XII has immediate benefits for acute vaso-occlusive events and survival in SCD mice exceeding those seen for single therapy.

Increased erythrocyte adhesion to VCAM-1 during pulsatile flow: Application of a microfluidic flow adhesion bioassay

Sickle cell disease (SCD) is characterized by microvascular occlusion mediated by adhesive interactions of sickle erythrocytes (SSRBCs) to the endothelium. Most in vitro flow adhesion assays measure SSRBC adhesion during continuous flow, although in vivo SSRBC adhesive interactions occur during pulsatile flow. Using a well-plate microfluidic flow adhesion system, we demonstrate that isolated SSRBCs adhere to vascular cell adhesion molecule (VCAM-1) at greater levels during pulsatile versus continuous flow. A significant increase in adhesive interactions was observed between all pulse frequencies 1 Hz to 2 Hz (60-120 beats/min) when compared to non-pulsatile flow. Adhesion of isolated SSRBCs and whole blood during pulsatile flow was unaffected by protein kinase A (PKA) inhibition, and exposure of SSRBCs to pulsatile flow did not affect the intrinsic adhesive properties of SSRBCs. The cell type responsible for increased adhesion of whole blood varied from patient to patient. We conclude that low flow periods of the pulse cycle allow more adhesive interactions between sickle erythrocytes and VCAM-1, and sickle erythrocyte adhesion in the context of whole blood may better reflect physiologic cellular interactions. The microfluidic flow adhesion bioassay used in this study may have applications for clinical assessment of sickle erythrocyte adhesion during pulsatile flow.

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.

Emerging drugs for sickle cell anemia

INTRODUCTION

The search for effective therapeutic interventions for sickle cell disease (SCD) has been an ongoing endeavor for over 50 years. During this period, only hydroxyurea (HU), which received US FDA approval in February 1998, was identified as an effective therapeutic agent in preventing or ameliorating the frequency of vaso-occlusive crises, acute chest syndrome and the need for blood transfusion. Approximately 25% of patients with sickle cell anemia (SCA), however, do not respond to HU and some patients experiencing serious side effects of this chemotherapeutic agent. Nevertheless, the success of HU opened the sluice gates to identify other effective drug therapies. The objective of this review is to describe the emerging drug therapies for SCA.

AREAS COVERED

In this review, we describe the pathophysiology of SCD and provide an in-depth analysis of the current and new pharmacologic therapies in the field. Literature searches involved multiple databases including Medline In-Process & Other Non-Indexed Citations, MEDLINE, Embase, Cochrane Database of Systematic Reviews, and Scopus.

EXPERT OPINION

SCA is a heterogeneous disease that has caused tremendous global morbidity and early mortality. More effective, individualized and inexpensive therapies are needed. New therapies targeting multiple pathways in its complex pathophysiology are under investigation.

Extracellular nucleotide and nucleoside signaling in vascular and blood disease

Nucleotides and nucleosides-such as adenosine triphosphate (ATP) and adenosine-are famous for their intracellular roles as building blocks for the genetic code or cellular energy currencies. In contrast, their function in the extracellular space is different. Here, they are primarily known as signaling molecules via activation of purinergic receptors, classified as P1 receptors for adenosine or P2 receptors for ATP. Because extracellular ATP is rapidly converted to adenosine by ectonucleotidase, nucleotide-phosphohydrolysis is important for controlling the balance between P2 and P1 signaling. Gene-targeted mice for P1, P2 receptors, or ectonucleotidase exhibit only very mild phenotypic manifestations at baseline. However, they demonstrate alterations in disease susceptibilities when exposed to a variety of vascular or blood diseases. Examples of phenotypic manifestations include vascular barrier dysfunction, graft-vs-host disease, platelet activation, ischemia, and reperfusion injury or sickle cell disease. Many of these studies highlight that purinergic signaling events can be targeted therapeutically.

Platelet activation biomarkers in Berkeley sickle cell mice and the response to prasugrel

Vaso-occlusive crisis (VOC) is a common complication that occurs in sickle cell disease (SCD) patients. Although underlying mechanisms of VOC remain unclear, platelet activation has been associated with VOC. In the present study, plasma adenine nucleotide measurements using LC-ESI-MS/MS showed that plasma ADP in the Berkeley murine model of SCD was significantly higher (applox. 2.7-fold increase) compared with control mice. Assessment of platelet activation markers using flow cytometry indicated that in SCD mice at steady state (8 weeks old), circulating platelets were partially activated and this tended to increase with age (15 weeks old). The administration of prasugrel, a thienopiridyl P2Y12 antagonist, did not affect the activation state of circulating platelets suggesting P2Y12 independent mechanism of activation. In this murine SCD model, ex vivo addition of ADP or PAR4 TRAP resulted in further platelet activation as assessed by expression of activated GPIIb/IIIa and P-selectin both at 8 and 15 weeks. In 15 weeks old SCD mice, agonist-induced increases in activation markers were enhanced compared to control mice. Oral administration of prasugrel effectively inhibited ex vivo platelet activation consistent with clinical data in patients with SCD. In conclusion, in the Berkeley murine model of SCD, we found evidence of basal and agonist-stimulated platelet activation which could in part be attenuated by prasugrel. These data are consistent with observations made in patients with SCD and suggest possible utility of this murine model and prasugrel therapy in exploring treatment options for patients with SCD.

Platelet bioenergetic screen in sickle cell patients reveals mitochondrial complex V inhibition, which contributes to platelet activation

Bioenergetic dysfunction, although central to the pathogenesis of numerous diseases, remains uncharacterized in many patient populations because of the invasiveness of obtaining tissue for mitochondrial studies. Although platelets are an accessible source of mitochondria, the role of bioenergetics in regulating platelet function remains unclear. Herein, we validate extracellular flux analysis in human platelets and use this technique to screen for mitochondrial dysfunction in sickle cell disease (SCD) patients, a population with aberrant platelet activation of an unknown mechanism and in which mitochondrial function has never been assessed. We identify a bioenergetic alteration in SCD patients characterized by deficient complex V activity, leading to decreased mitochondrial respiration, membrane hyperpolarization, and augmented oxidant production compared with healthy subjects. This dysfunction correlates with platelet activation and hemolysis in vivo and can be recapitulated in vitro by exposing healthy platelets to hemoglobin or a complex V inhibitor. Further, reproduction of this dysfunction in vitro activates healthy platelets, an effect prevented by attenuation of mitochondrial hyperpolarization or by scavenging mitochondrial oxidants. These data identify bioenergetic dysfunction in SCD patients for the first time and establish mitochondrial hyperpolarization and oxidant generation as potential pathogenic mechanism in SCD as well as a modulator of healthy platelet function.

Endothelial activation by platelets from sickle cell anemia patients

Sickle cell anemia (SCA) is associated with a hypercoagulable state. Increased platelet activation is reported in SCA and SCA platelets may present augmented adhesion to the vascular endothelium, potentially contributing to the vaso-occlusive process. We sought to observe the effects of platelets (PLTs) from healthy control (CON) individuals and SCA individuals on endothelial activation, in vitro. Human umbilical vein endothelial cells (HUVEC) were cultured, in the presence, or not, of washed PLTs from CON or steady-state SCA individuals. Supernatants were reserved for cytokine quantification, and endothelial adhesion molecules (EAM) were analyzed by flow cytometry; gene expressions of ICAM1 and genes of the NF-κB pathway were analyzed by qPCR. SCA PLTs were found to be more inflammatory, displaying increased adhesive properties, an increased production of IL-1β and a tendency towards elevated expressions of P-selectin and activated αIIbβ3. Following culture in the presence of SCA PLTs, HUVEC presented significant augmentations in the expressions of the EAM, ICAM-1 and E-selectin, as well as increased IL-8 production and increased ICAM1 and NFKB1 (encodes p50 subunit of NF-κB) gene expressions. Interestingly, transwell inserts abolished the effects of SCA PLTs on EAM expression. Furthermore, an inhibitor of the NF-κB pathway, BAY 11-7082, also prevented the induction of EAM expression on the HUVEC surface by SCA PLTs. In conclusion, we find further evidence to indicate that platelets circulate in an activated state in sickle cell disease and are capable of stimulating endothelial cell activation. This effect appears to be mediated by direct contact, or even adhesion, between the platelets and endothelial cells and via NFκB-dependent signaling. As such, activated platelets in SCD may contribute to endothelial activation and, therefore, to the vaso-occlusive process. Results provide further evidence to support the use of anti-platelet approaches in association with other therapies for SCD.

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: effects on biomarkers of platelet activation and coagulation

INTRODUCTION

Prasugrel, a P2Y₁₂ adenosine diphosphate (ADP) receptor antagonist effectively inhibits ADP-mediated platelet activation and aggregation, and may be useful in reducing vaso-occlusive crises in sickle cell disease (SCD). In this study, we assess the effect of prasugrel on biomarkers of platelet activation and coagulation in patients with SCD.

MATERIALS AND METHODS

Twelve adult patients with SCD and 13 healthy subjects were examined before and after 12 ± 2 days of 5.0 or 7.5 mg/day oral prasugrel. Assessed cellular biomarkers included monocyte- and neutrophil-platelet aggregates, activated glycoprotein IIb-IIIa (GPIIbIIIa), P-selectin, CD40 ligand (CD40L), tissue factor (TF) expression on circulating platelets and on monocyte-platelet aggregates, and platelet-erythrocyte aggregates. Soluble biomarkers included CD40L, prothrombin fragment 1.2 (F1.2), thromboxane B₂ (TXB₂), P-selectin, and TF.

RESULTS

Patients with SCD had increased platelet baseline activation compared to healthy subjects, as measured by percentages of monocyte-platelet aggregates, neutrophil-platelet aggregates, and platelets expressing CD40L. Likewise, baseline levels of soluble F1.2 and TXB₂ were elevated in patients with SCD compared to healthy subjects. After 12 days of prasugrel, patients with SCD had a significant reduction in platelet-monocyte aggregates that was not observed in healthy subjects. Following prasugrel administration, those with SCD maintained higher levels of monocyte-platelet aggregates and soluble F1.2, but had lower levels of platelet-erythrocyte aggregates and soluble TF compared to healthy subjects.

CONCLUSIONS

These results provide evidence for chronic platelet activation in the SCD steady state, activation that was in part attenuated by prasugrel, thereby suggesting that ADP may mediate platelet activation in SCD.