Increased platelet turnover in patients with previous definite stent thrombosis

Role of Reticulated Platelets in Cardiovascular Disease

Human platelets differ considerably with regard to their size, RNA content and thrombogenicity. Reticulated platelets (RPs) are young, hyper-reactive platelets that are newly released from the bone marrow. They are larger and contain more RNA compared to older platelets. In comparison to more mature platelets, they exhibit a significantly higher thrombogenicity and are known to be elevated in patients with an increased platelet turnover such as, diabetics and after acute myocardial infarction. Several studies have shown that RPs correlate with an insufficient antiplatelet response to aspirin and specific P2Y₁₂ receptor inhibitors. In addition, RPs are promising novel biomarkers for the prediction of adverse cardiovascular events in cardiovascular disease. However, the reason for RPs intrinsic hyper-reactivity and their association with ischemic events is not completely understood and the biology of RPs is still under investigation. We here present a structured review of preclinical and clinical findings concerning the role of RPs in cardiovascular disease.

Sphingosine-1-phosphate modulates PAR1-mediated human platelet activation in a concentration-dependent biphasic manner

Sphingosine 1-phosphate (S1P) is a bioactive signalling sphingolipid that is increased in diseases such as obesity and diabetes. S1P can modulate platelet function, however the direction of effect and S1P receptors (S1PRs) involved are controversial. Here we describe the role of S1P in regulating human platelet function and identify the receptor subtypes responsible for S1P priming. Human platelets were treated with protease-activated receptor 1 (PAR-1)-activating peptide in the presence or absence of S1P, S1PR agonists or antagonists, and sphingosine kinases inhibitors. S1P alone did not induce platelet aggregation but at low concentrations S1P enhanced PAR1-mediated platelet responses, whereas PAR1 responses were inhibited by high concentrations of S1P. This biphasic effect was mimicked by pan-S1PR agonists. Specific agonists revealed that S1PR₁ receptor activation has a positive priming effect, S1PR₂ and S1PR₃ have no effect on platelet function, whereas S1PR₄ and S1PR₅ receptor activation have an inhibitory effect on PAR-1 mediated platelet function. Although platelets express both sphingosine kinase 1/2, enzymes which phosphorylate sphingosine to produce S1P, only dual and SphK2 inhibition reduced platelet function. These results support a role for SphK2-mediated S1P generation in concentration-dependent positive and negative priming of platelet function, through S1PR1 and S1PR4/5 receptors, respectively.

Lower Antiplatelet Effect of Aspirin in Essential Thrombocythemia than in Coronary Artery Disease

Background  Patients with essential thrombocythemia (ET) and coronary artery disease (CAD) have increased risk of thromboembolic complications. In addition, a reduced antiplatelet effect of aspirin has been demonstrated in both patient groups. As ET is a platelet disorder, platelets may be more important for the thromboembolic risk in ET than in CAD. We aimed to investigate the antiplatelet effect of aspirin and platelet turnover in ET versus CAD patients.

Methods  We included 48 ET patients and an age-matched group of 48 CAD patients. The effect of aspirin was evaluated by thromboxane B ₂ (TXB ₂ ) levels and platelet aggregation. Platelet turnover was assessed by immature platelet count (IPC) and immature platelet fraction (IPF).

Results  ET patients had reduced effect of aspirin compared with CAD patients, demonstrated by significantly higher TXB ₂ levels (median of differences = 22.3 ng/mL, p  < 0.0001) and platelet aggregation (median of differences = 131.0 AU*min, p  = 0.0003). Furthermore, ET patients had significantly higher IPC ( p  < 0.0001) and IPF ( p  = 0.0004) than CAD patients.

Conclusion  ET patients have lower 24-hour antiplatelet effect of aspirin than CAD patients. This may be explained by an increased platelet production and turnover counteracting the antiplatelet effect of aspirin. These findings strengthen the rationale for exploring novel antiplatelet regimens in ET patients to reduce the risk of cardiovascular events.

Platelet aggregation and response to aspirin therapy in cardiac allograft vasculopathy

BACKGROUND

Long-term survival after heart transplantation (HTx) is compromised by cardiac allograft vasculopathy (CAV) characterized by coronary macro- and microvascular disease. The pathogenesis of CAV is unclear and may involve coronary thrombosis. We investigated whether HTx patients with CAV had higher platelet aggregation and turnover than HTx patients without CAV and healthy controls. Furthermore, we investigated the anti-platelet effect of low-dose aspirin in HTx patients.

METHODS

We included 57 patients who had undergone HTx (median 8.3 years from HTx) and 57 healthy controls. Platelet aggregation was measured on-aspirin and off-aspirin using impedance aggregometry with adenosine diphosphate (ADP) and arachidonic acid (AA). We evaluated platelet turnover by flow cytometry, CAV burden by coronary angiography and echocardiography, and microvascular function by echocardiographic coronary flow velocity reserve (CFVR).

RESULTS

Off-aspirin, HTx patients with CAV (n = 21) had higher ADP-induced platelet aggregation than healthy controls (p < 0.01) and HTx patients without CAV (n = 36) (p < 0.05). Aspirin treatment reduced AA-induced platelet aggregation in both HTx groups, but HTx patients with CAV had higher platelet aggregation on-aspirin than HTx patients without CAV (p < 0.05). Platelet turnover did not differ between HTx patients with CAV and HTx patients without CAV (p > 0.34). HTx patients with lower CFVR values had higher platelet aggregation than HTx patients with higher CFVR values (p < 0.05).

CONCLUSIONS

Off-aspirin, platelet aggregation was higher in HTx patients with CAV than in HTx patients without CAV and healthy controls. On-aspirin, platelet aggregation was higher in HTx patients with CAV than in HTx patients without CAV. Aspirin monotherapy may not provide sufficient platelet inhibition in HTx patients with CAV.

Oral antiplatelet agents in cardiovascular disease

Antiplatelet agents significantly reduce mortality and morbidity in ischemic heart disease, cerebrovascular disease and peripheral artery disease (PAD), and are therefore part of guideline-driven daily medical treatment in these patients. Due to its beneficial effects in the secondary prevention of atherothrombotic events, aspirin remains the most frequently prescribed antiplatelet agent in cardiovascular disease. In patients with acute coronary syndromes (ACS) and in those undergoing angioplasty with stent implantation dual antiplatelet therapy with aspirin and an adenosine diphosphate (ADP) receptor antagonist is indicated. The development of the newer ADP P2Y₁₂ inhibitors prasugrel and ticagrelor has further improved prognosis in ACS patients compared to clopidogrel. Moreover, vorapaxar allows the inhibition of platelet activation by thrombin via protease-activated receptor-1 and has been approved for the use in patients with PAD and in those with a history of myocardial infarction. This review article summarizes the current evidence on oral antiplatelet agents in cardiovascular disease. Keywords: Aspirin, clopidogrel, prasugrel, ticagrelor, vorapaxar, cardiovascular disease.

Platelet characteristics in patients with essential thrombocytosis

BACKGROUND

Essential thrombocytosis (ET) is a myeloproliferative disorder characterized by an increased platelet count. ET is associated with an increased risk of thrombosis, and procoagulant features of the disease may include an increased number of reactive reticulated platelets and an increased aggregation potential. We aimed to explore the association between platelet count, platelet turnover, and platelet aggregation in patients with ET.

METHODS

We included 24 ET patients who discontinued antiplatelet therapy prior to blood sampling. Reticulated platelets were assessed as immature platelet count (IPC) and immature platelet fraction by automated flow cytometry (Sysmex XE-5000). Platelet aggregation was investigated by impedance aggregometry (Multiplate^® Analyzer) and aggregation potential by flow cytometry (NAVIOS).

RESULTS

Our results showed that ET patients had increased IPC compared to healthy individuals (median 12.3 vs. median 6.9, P < 0.0001). Furthermore, a positive correlation between platelet count and impedance aggregation was demonstrated using arachidonic acid (r = 0.48, P = 0.02), thrombin-receptor-activating-peptide (r = 0.46, P = 0.03) and adenosine diphosphate (r = 0.56, P = 0.007) as agonists. Finally, an increased aggregation potential was demonstrated in ET patients compared to healthy individuals.

CONCLUSIONS

The study showed that ET patients compared to healthy individuals have an increased amount of reticulated platelets and increased aggregation potential. These findings might in part explain the increased thromboembolic risk in patients with ET. © 2018 International Clinical Cytometry Society.

Antiplatelet Effect Durability of a Novel, 24-Hour, Extended-Release Prescription Formulation of Acetylsalicylic Acid in Patients With Type 2 Diabetes Mellitus

High platelet reactivity and high platelet turnover have been implicated in incomplete platelet inhibition during immediate-release acetylsalicylic acid therapy in patients with type 2 diabetes mellitus (DM). An extended-release acetylsalicylic acid (ER-ASA; Durlaza) formulation was developed to provide 24-hour antithrombotic effects with once-daily dosing. The objective of the study was to evaluate the antiplatelet effects of ER-ASA in patients with DM. In this open-label, single-center study, patients with DM (n = 40) and multiple cardiovascular risk factors received ER-ASA 162.5 mg/day for 14 ± 4 days. Multiple platelet function tests, serum and urinary thromboxane B₂ metabolites, prostacyclin metabolite, and high-sensitive C-reactive protein levels were assessed at 1, 12, 16, and 24 hours post-dose. Patients with high platelet turnover and/or high platelet reactivity were treated with ER-ASA 325 mg/day for 14 ± 4 days, and laboratory analyses were repeated. All patients responded to ER-ASA 162.5 mg/day as measured by arachidonic acid-induced aggregation, and there was no loss of the platelet inhibitory effect of ER-ASA 162.5 mg/day over 24 hours post-dose (p = not significant). The antiplatelet effect was sustained over 24 hours for all platelet function measurements. Mean 1- to 24-hour serum thromboxane B₂ levels were low with both doses and were lower with ER-ASA 325 mg/day compared with 162.5 mg/day therapy (p = 0.002). In conclusion, ER-ASA 162.5 mg daily dose provided sustained antiplatelet effects over 24 hours in patients with type 2 DM and multiple cardiovascular risk factors and had a favorable tolerability profile.

Acetylsalicylic Acid Daily vs Acetylsalicylic Acid Every 3 Days in Healthy Volunteers: Effect on Platelet Aggregation, Gastric Mucosa, and Prostaglandin E2 Synthesis

Substantial platelet inhibition was observed 3 days after a single administration of acetylsalicylic acid 81 mg to healthy volunteers. Here we investigate prostaglandin E2 (PGE2 ) antrum concentrations and gastrointestinal symptoms in two treatment groups: one receiving losartan and acetylsalicylic acid every day and the other receiving losartan every day and acetylsalicylic acid every 3 days. Twenty-eight healthy volunteers from both sexes received either 50 mg losartan and acetylsalicylic acid 81 mg daily or 50 mg losartan and acetylsalicylic acid 81 every 3 days with placebo on the other days. Therapy was delivered for 30 days for both groups. Gastric endoscopy was performed before and after treatment period. Biopsies were collected for PGE2 quantification. Platelet function tests were carried out before and during treatment and TXB2 release on platelet rich plasma was measured. The every 3 day low-dose acetylsalicylic acid regimen produced complete inhibition of platelet aggregation compared to the daily treatment. Thromboxane B2 release was substantially abolished for both groups during treatment. There was no significant difference on the endoscopic score of both treatment groups after the 30-day treatment (P = .215). There was over 50% suppression of antrum PGE2 content on volunteers receiving acetylsalicylic acid daily (P = .0016), while for the every 3 day dose regimen there was no significant difference between pre and post-treatment antrum PGE2 dosages (P = .4193). Since PGE2 is involved in gastric healing, we understand that this new approach could be safer and as efficient as the standard daily therapy on a long-term basis.

Reduced Effect of Aspirin and Clopidogrel Following Hybrid Coronary Revascularization

OBJECTIVE

Reduced effect of antiplatelet therapy following coronary artery bypass grafting is associated with reduced graft patency. We sought to evaluate the impact of hybrid coronary revascularization on the effect of aspirin and clopidogrel and whether high baseline platelet aggregation, high postoperative levels of platelet turnover, and acute-phase response may contribute to the effect.

METHODS

We prospectively studied platelet aggregation (VerifyNow and Multiplate Analyzer), platelet turnover (immature platelets, mean platelet volume, and thrombopoietin), and acute-phase reactants (C-reactive protein, von Willebrand factor, and coagulation factor VIII) in 40 patients undergoing elective hybrid coronary revascularization (off-pump surgical revascularization through J-hemisternotomy followed by percutaneous coronary intervention [PCI]). Preoperative blood samples on- and off-aspirin were compared with blood samples obtained postoperatively, following PCI when dual antiplatelet therapy had been initiated and at 1-year follow-up.

RESULTS

The antiplatelet effect of aspirin was significantly reduced in the early postoperative period as measured by VerifyNow Aspirin but not by Multiplate ASPI test. The antiplatelet effect of clopidogrel was significantly reduced following PCI as measured by VerifyNow P2Y12 and Multiplate ADP test. The level of baseline aggregation did not predict the antiplatelet effect of aspirin or clopidogrel, and no association was found between platelet aggregation and postoperative platelet turnover or acute-phase reaction.

CONCLUSIONS

A transient reduction in the antiplatelet effect of aspirin and clopidogrel was observed after hybrid coronary revascularization despite limited surgical trauma and off-pump technique. Neither baseline platelet aggregation nor postoperatively increased platelet turnover and acute-phase response could explain this finding. ClinicalTrials.gov identifier, NCT02293928.

24-hour antiplatelet effect of aspirin in patients with previous definite stent thrombosis

OBJECTIVE

Once-daily aspirin is standard treatment, but recent studies point towards increased platelet function at the end of the dosing interval. Stent thrombosis (ST) has been linked with reduced antiplatelet effect of aspirin, so we investigated if platelet inhibition by aspirin declines through 24 h in patients with previous definite ST. Furthermore, we explored whether increased levels of immature platelets and thrombopoietin are associated with a particularly rapid recovery of platelet function.

METHODS

This case-control study included 50 patients with previous definite ST matched with 100 patients with stable coronary artery disease and 50 healthy volunteers. All participants were on aspirin 75 mg/day mono antiplatelet therapy. Platelet aggregation was measured 1 and 24 h after aspirin intake using platelet aggregometry (Multiplate® Analyzer). Cyclooxygenase-1 activity, platelet activation, immature platelets, and thrombopoietin were measured.

RESULTS

Platelet aggregation increased by 109±150 (arachidonic acid) and 47±155 (collagen) aggregation units per minute from 1 to 24 h after aspirin intake (p-values <0.0001) with corresponding increases in thromboxane B2 (5.6±5.1 ng/ml, p<0.0001) and soluble P-selectin (6.2±15.5 ng/ml, p<0.0001). Platelet aggregation increased equally in all groups, but patients with previous ST displayed the highest levels of platelet aggregation at 24 h (p-values≤0.05) and the highest levels of immature platelets (p<0.01) and thrombopoietin (p<0.0001).

CONCLUSIONS

Platelet inhibition declined significantly during the 24-hour dosing interval in aspirin-treated patients with previous definite ST or stable coronary artery disease and in healthy individuals. Increased levels of immature platelets and thrombopoietin were observed in patients with previous definite ST.

Platelet aggregation is dependent on platelet count in patients with coronary artery disease

INTRODUCTION

Platelet function testing in whole blood is widely used to evaluate the effect of antiplatelet agents, but it is not known whether results are affected by whole blood parameters. This study investigated the importance of platelet count, haematocrit, red blood cells (RBC), and white blood cells in whole blood platelet aggregometry.

MATERIALS AND METHODS

We included 417 patients with coronary artery disease on aspirin mono-therapy and 21 aspirin-naïve healthy individuals. Blood sampling was performed one hour after aspirin ingestion. The antiplatelet effect of aspirin was evaluated using the VerifyNow® Aspirin assay and multiple electrode aggregometry (MEA, Multiplate®) induced by collagen (1.0 μg/mL) and arachidonic acid (1.0 or 0.75 mmol/L). Measurements of whole blood parameters were performed to evaluate the three major cell lines in circulating blood.

RESULTS

In patients, platelet count correlated significantly with platelet aggregation (MEA(collagen), p<0.0001; MEA(arachidonic acid), p<0.0001; VerifyNow®, p=0.03). Haematocrit and RBC correlated inversely with MEA induced by collagen (p(haematocrit)<0.001; p(RBC)=0.07) and with VerifyNow® (p(haematocrit)<0.0001; p(RBC)<0.0001), but not with MEA induced by arachidonic acid (p(haematocrit)=1; p(RBC)=0.87). White blood cells correlated significantly with platelet aggregation (MEA(collagen), p<0.001; MEA(arachidonic acid), p<0.0001; VerifyNow®, p=0.05). Similar associations were observed in aspirin-naïve healthy individuals.

CONCLUSIONS

Whole blood aggregometry is dependent on all major cell lines in whole blood. Importantly, platelet aggregation is significantly associated with platelet count even within the normal range.