Monitoring of Clopidogrel Action: Comparison of Methods

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Perioperative Management of Antithrombotic Therapy: An American College of Chest Physicians Clinical Practice Guideline

BACKGROUND

The American College of Chest Physicians Clinical Practice Guideline on the Perioperative Management of Antithrombotic Therapy addresses 43 Patients-Interventions-Comparators-Outcomes (PICO) questions related to the perioperative management of patients who are receiving long-term oral anticoagulant or antiplatelet therapy and require an elective surgery/procedure. This guideline is separated into four broad categories, encompassing the management of patients who are receiving: (1) a vitamin K antagonist (VKA), mainly warfarin; (2) if receiving a VKA, the use of perioperative heparin bridging, typically with a low-molecular-weight heparin; (3) a direct oral anticoagulant (DOAC); and (4) an antiplatelet drug.

METHODS

Strong or conditional practice recommendations are generated based on high, moderate, low, and very low certainty of evidence using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) methodology for clinical practice guidelines.

RESULTS

A multidisciplinary panel generated 44 guideline recommendations for the perioperative management of VKAs, heparin bridging, DOACs, and antiplatelet drugs, of which two are strong recommendations: (1) against the use of heparin bridging in patients with atrial fibrillation; and (2) continuation of VKA therapy in patients having a pacemaker or internal cardiac defibrillator implantation. There are separate recommendations on the perioperative management of patients who are undergoing minor procedures, comprising dental, dermatologic, ophthalmologic, pacemaker/internal cardiac defibrillator implantation, and GI (endoscopic) procedures.

CONCLUSIONS

Substantial new evidence has emerged since the 2012 iteration of these guidelines, especially to inform best practices for the perioperative management of patients who are receiving a VKA and may require heparin bridging, for the perioperative management of patients who are receiving a DOAC, and for patients who are receiving one or more antiplatelet drugs. Despite this new knowledge, uncertainty remains as to best practices for the majority of perioperative management questions.

On-Ticagrelor Platelet Reactivity and Clinical Outcome in Patients Undergoing Percutaneous Coronary Intervention for Acute Coronary Syndrome

BACKGROUND

 A strong association between on-thienopyridine platelet reactivity (PR) and the risk of both thrombotic and bleeding events in acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI) has been demonstrated. However, no study has analyzed the relationship between on-ticagrelor PR and clinical outcome in this clinical setting.

OBJECTIVES

 We aimed to investigate the relationship between on-ticagrelor PR, assessed by the vasodilator-stimulated phosphoprotein (VASP) index, and clinical outcome in patients with ACS undergoing PCI.

METHODS

 We performed a prospective, multicenter, observational study of patients undergoing PCI for ACS. PR was measured using the VASP index following ticagrelor loading dose. The primary study endpoint was the rate of Bleeding Academic Research Consortium (BARC) type ≥2 at 1 year. The key secondary endpoint was the rate of major adverse cardiovascular events (MACE) defined as the composite of cardiovascular death, myocardial infarction, stroke, and urgent revascularization.

RESULTS

 We included 570 ACS patients, among whom 33.9% had ST-elevation myocardial infarction. BARC type ≥2 bleeding occurred in 10.9% and MACE in 13.8%. PR was not associated with BARC ≥2 or with MACE (p = 0.12 and p = 0.56, respectively). No relationship between PR and outcomes was observed, neither when PR was analyzed quantitatively nor when it was analyzed qualitatively (low on-treatment PR [LTPR] vs. no LTPR).

CONCLUSION

 On-ticagrelor PR measured by the VASP was not associated with bleeding or thrombotic events in ACS patients undergoing PCI. PR measured by the VASP should not be used as a surrogate endpoint in studies on ticagrelor.

Pharmacokinetic, pharmacodynamic, and pharmacogenetic assays to monitor clopidogrel therapy

Clopidogrel is the most common and widely used antiplatelet agent for patients with coronary artery disease following confirmation by electrocardiographic studies. The nonresponsiveness of patients to clopidogrel and the possibility of testing for clopidogrel resistance by platelet function assays (PFA) are contentious issues. Light transmission aggregometry (LTA) is considered as the gold standard test among all PFA. In this review, the most commonly used PFA used for monitoring the effect of clopidogrel, LTA, vasodilator-stimulated phosphoprotein assay phosphorylation, rotational thromboelastometry (ROTEM) delta and ROTEM platelet, thromboelastography, PFA-100, VerifyNow P2Y12 assay, Multiplate analyzer, Plateletworks assay and pharmacogenetic studies, are comparatively discussed including their principles of action, advantages, and disadvantages. VerifyNow P2Y12 assay can be accepted as the ideal point of care test out of the discussed assays. However, modified assays are required which could overcome the limitations associated with currently available assays.

The Impact of Cold Storage on Adenosine Diphosphate-Mediated Platelet Responsiveness

Introduction  Cold storage of platelets is considered to contribute to lower risk of bacterial growth and to more efficient hemostatic capacity. For the optimization of storage strategies, it is required to further elucidate the influence of refrigeration on platelet integrity. This study focused on adenosine diphosphate (ADP)-related platelet responsiveness.

Materials and Methods  Platelets were prepared from apheresis-derived platelet concentrates or from peripheral whole blood, stored either at room temperature or at 4°C. ADP-induced aggregation was tested with light transmission. Activation markers, purinergic receptor expression, and P2Y12 receptor function were determined by flow cytometry. P2Y1 and P2X1 function was assessed by fluorescence assays, cyclic nucleotide concentrations by immunoassays, and vasodilator-stimulated phosphoprotein (VASP)-phosphorylation levels by Western blot analysis.

Results  In contrast to room temperature, ADP-induced aggregation was maintained under cold storage for 6 days, associated with elevated activation markers like fibrinogen binding or CD62P expression. Purinergic receptor expression was not essentially different, whereas P2Y1 function deteriorated rapidly at cold storage, but not P2Y12 activity. Inhibitory pathways of cold-stored platelets were characterized by reduced responses to nitric oxide and prostaglandin E1. Refrigeration of citrated whole blood also led to the attenuation of induced inhibition of platelet aggregation, detectable within 24 hours.

Conclusion  ADP responsiveness is preserved under cold storage for 6 days due to stable P2Y12 activity and concomitant disintegration of inhibitory pathways enabling a higher reactivity of stored platelets. The ideal storage time at cold temperature for the highest hemostatic effect of platelets should be evaluated in further studies.

Platelet Reactivity in Patients on Aspirin and Clopidogrel Therapy Measured by a New Bedside Whole-Blood Assay

Various tests are available for measuring on-treatment platelet reactivity. The pharmacologically most specific assays are time-consuming and elaborate. A highly specific and convenient assay would be desirable for clinical routine. In this pilot study, we aimed to examine the ability of a novel bedside whole-blood assay-ROTEM platelet-to evaluate platelet inhibition compared with established assays. Platelet reactivity was investigated in 93 patients. Forty-Seven patients were on permanent aspirin therapy and 46 on dual antiplatelet therapy (DAPT) with aspirin and clopidogrel. We used ROTEM platelet impedance aggregometry (ROTEM-PTL), light transmission aggregometry (LTA), Multiplate electrode aggregometry (MEA) and vasodilator-stimulated phosphoprotein flow cytometry. Receiver operating characteristic (ROC) analyses showed ROTEM-PTL differentiates well between patients on medication and healthy individuals: aspirin: ROCAUC 0.99 (95% confidence interval, 0.97-1.01); P < 0.0001; DAPT treatment: ROCAUC 0.80 (95% confidence interval, 0.69-0.91); P < 0.001. Pearson regression analyses showed moderate correlations between assays. Aspirin: MEA versus ROTEM-PTL r = 0.435, P ≤ 0.001; LTA versus ROTEM-PTL r = 0.048, P = 0.180. DAPT: MEA versus ROTEM-PTL r = 0.398, P = 0.001; LTA versus ROTEM-PTL r = 0.409, P = 0.001; vasodilator-stimulated phosphoprotein versus ROTEM-PTL r = 0.164, P = 0.055. ROTEM platelet distinguished well between treated and healthy individuals but correlated moderately with other assays. Clinical trials are needed to investigate the ability of this new assay to identify patients at risk of adverse events.

Platelet reactivity inhibition following ticagrelor loading dose in patients undergoing percutaneous coronary intervention for acute coronary syndrome

BACKGROUND

Ticagrelor induces more potent platelet reactivity (PR) inhibition with reduced interindividual variability compared to clopidogrel. Although on-clopidogrel PR was shown to correlate with ischemia and bleeding events, no study has investigated the relationship between on-ticagrelor PR and outcome.

OBJECTIVES

We aimed to evaluate the relationship between on-ticagrelor PR, assessed by the vasodilator-stimulated phosphoprotein index (VASP), and thrombotic and bleeding events in patients with acute coronary syndrome (ACS) treated by percutaneous coronary intervention (PCI).

METHODS

We performed a prospective, multicenter observational study on patients treated with PCI for ACS. The VASP index was used to assess PR after ticagrelor loading dose (LD). The primary endpoint was the link between major adverse cardiovascular events (MACE) and PR.

RESULTS

Among the 530 patients with ACS included, 183 (34.5%) were admitted for ST elevation myocardial infarction. We observed high potency and limited interindividual variability after the ticagrelor LD (VASP 19.1% ± 16.6%). At 1 month, 21 (3.8%) MACE and 29 (5.5%) bleedings ≥ 2 according to the Bleedings Academic Research Consortium (BARC) scale were recorded. Neither MACE nor bleeding was associated with PR (P = .34 and P = .78, respectively). However, there was a strong association between PR and the occurrence of definite acute stent thrombosis (P = .03). Platelet reactivity was the only factor associated with acute definite stent thrombosis.

CONCLUSION

In patients receiving a ticagrelor LD while undergoing PCI for ACS, PR using the VASP did not predict MACE or bleeding, but it was significantly associated with the occurrence of definite acute stent thrombosis.

Investigating the Effect of Demographics, Clinical Characteristics, and Polymorphism of MDR-1, CYP1A2, CYP3A4, and CYP3A5 on Clopidogrel Resistance

Clopidogrel is an antiplatelet agent that is indicated for cardiovascular emergencies and procedures. The drug, however, is subject to response variability leading to therapy resistance. In this research, we explored the demographic, clinical, and genetic factors associated with clopidogrel resistance. Data analysis among our 280 subjects receiving clopidogrel showed some risk factors that are significantly associated with clopidogrel resistance compared with responders. Those were: female sex (P = 0.021), advanced age (P = 0.011), obesity (P = 0.002), and higher body mass index (P = 0.008) and higher platelets count (P = 0.002). However, known polymorphisms of MDR-1, CYP1A2, CYP3A4, and CYP3A5 were not associated with treatment resistance when compared to responders to clopidogrel therapy. Knowledge about such risk factors might provide recommendation in the future about starting doses or monitoring recommendations.

Pharmacokinetics and competitive pharmacodynamics of ADP-induced platelet activation after oral administration of clopidogrel to horses

OBJECTIVE

To determine pharmacokinetics and pharmacodynamics after oral administration of a single dose of clopidogrel to horses.

ANIMALS

6 healthy adult horses.

PROCEDURES

Blood samples were collected before and at various times up to 24 hours after oral administration of clopidogrel (2 mg/kg). Reactivity of platelets from each blood sample was determined by optical aggregometry and phosphorylation of vasodilator-stimulated phosphoprotein (VASP). Concentrations of clopidogrel and the clopidogrel active metabolite derivative (CAMD) were measured in each blood sample by use of liquid chromatography-tandem mass spectrometry, and pharmacokinetic parameters were determined with a noncompartmental model.

RESULTS

Compared with results for preadministration samples, platelet aggregation in response to 12.5μM ADP decreased significantly within 4 hours after clopidogrel administration for 5 of 6 horses. After 24 hours, platelet aggregation was identical to that measured before administration. Platelet aggregation in response to 25μM ADP was identical between samples obtained before and after administration. Phosphorylation of VASP in response to ADP (20μM) and prostaglandin E₁ (3.3μM) was also unchanged by administration of clopidogrel. Time to maximum concentration of clopidogrel and CAMD was 0.54 and 0.71 hours, respectively, and calculated terminal-phase half-life of clopidogrel and CAMD was 1.81 and 0.97 hours, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Clopidogrel or CAMD caused competitive inhibition of ADP-induced platelet aggregation during the first 24 hours after clopidogrel administration. Because CAMD was rapidly eliminated from horses, clopidogrel administration may be needed more frequently than in other species in which clopidogrel causes irreversible platelet inhibition. (Am J Vet Res 2019;80:505-512).

Oxidative stress, aging, and diseases

Reactive oxygen and nitrogen species (RONS) are produced by several endogenous and exogenous processes, and their negative effects are neutralized by antioxidant defenses. Oxidative stress occurs from the imbalance between RONS production and these antioxidant defenses. Aging is a process characterized by the progressive loss of tissue and organ function. The oxidative stress theory of aging is based on the hypothesis that age-associated functional losses are due to the accumulation of RONS-induced damages. At the same time, oxidative stress is involved in several age-related conditions (ie, cardiovascular diseases [CVDs], chronic obstructive pulmonary disease, chronic kidney disease, neurodegenerative diseases, and cancer), including sarcopenia and frailty. Different types of oxidative stress biomarkers have been identified and may provide important information about the efficacy of the treatment, guiding the selection of the most effective drugs/dose regimens for patients and, if particularly relevant from a pathophysiological point of view, acting on a specific therapeutic target. Given the important role of oxidative stress in the pathogenesis of many clinical conditions and aging, antioxidant therapy could positively affect the natural history of several diseases, but further investigation is needed to evaluate the real efficacy of these therapeutic interventions. The purpose of this paper is to provide a review of literature on this complex topic of ever increasing interest.

Phosphorylation of vasodilator-stimulated phosphoprotein contributes to myocardial ischemic preconditioning

Ischemic preconditioning (IP) is a well-known strategy to protect organs against cell death following ischemia. The previous work has shown that vasodilator-stimulated phosphoprotein (VASP) is involved in cytoskeletal reorganization and that it holds significant importance for the extent of myocardial ischemia reperfusion injury. Yet, the role of VASP during myocardial IP is, to date, not known. We report here that VASP phosphorylation at serine¹⁵⁷ and serine²³⁹ is induced during hypoxia in vitro and during IP in vivo. The preconditioning-induced VASP phosphorylation inactivates the GP IIb/IIIa integrin receptor on platelets, which results in the reduced formation of organ compromising platelet neutrophil complexes. Experiments in chimeric mice confirmed the importance of VASP phosphorylation during myocardial IP. When studying this in VASP^-/- animals and in an isolated heart model, we were able to confirm the important role of VASP on myocardial IP. In conclusion, we were able to show that IP-induced VASP phosphorylation in platelets is a protective mechanism against the deleterious effects of ischemia.

Platelet function variability and non-genetic causes

Dual antiplatelet therapy (DAPT) has been established for the treatment of coronary artery disease, especially in and after acute coronary syndromes, and after coronary interventions. Data suggest that a significant percentage of individuals treated with clopidogrel do not receive the expected therapeutic benefit because of a decreased responsiveness of their platelets, which is caused by several extrinsic and intrinsic mechanisms. The clinical consequence of clopidogrel non-responsiveness is severe cardiovascular complications. Besides genetic variability in response to antiplatelet therapy, non-genetic causes such as drug interactions (proton-pump inhibitors, statins, calcium-channel blockers, coumarine derivates, antibiotics, antimycotics) and co-morbidities (diabetes mellitus, renal failure, obesity) are responsible for this phenomenon. Large clinical trials with standardised laboratory methods and hard clinical endpoints are needed to identify these interactions with clopidogrel and predictors for its non-responsiveness.

Prevalence of poor biological response to clopidogrel

The existence of poor biological response to clopidogrel has been shown in some patients. Despite the increasing number of studies, this phenomenon remains difficult to quantify. We performed a systematic review to estimate the prevalence of poor biological response to clopidogrel and investigate the factors known to modulate this. An exhaustive search was performed. Altogether 171 publications were identified, providing data for a total of 45,664 subjects. The estimated prevalence of poor biological response to clopidogrel ranged from 15.9% to 49.5% according to the platelet function assay employed. The assays most frequently used were light transmittance aggregometry (LTA), the vasodilator-stimulated phosphoprotein (VASP) assay and the Verify -now® assay. For all these assays, higher cut-off values were associated with a lower prevalence of poor biological response to clopidogrel. However, when choosing a fixed cut-off point for each assay, the prevalence of poor biological response to clopidogrel was highly variable suggesting that other factors could modulate poor biological response to clopidogrel. Finally, none of the studied factors could apparently explain the variability of poor biological response to clopidogrel. This meta-analysis shows that the prevalence of poor biological response depends on the assay employed, the cut-off value and on various unidentified additional factors.

High on-treatment platelet reactivity – definition and measurement

In the last decade, several studies revealed inter-patient response variability to antiplatelet agents: patients who display negligible or no responses to these drugs are considered poor responders, or “resistant” to treatment. In order to identify poor responders to an antiplatelet drug, laboratory tests of platelet function that specifically explore the platelet activation pathway that is targeted by the drug should be utilised. In addition, they should be performed both at baseline and during treatment: however, most studies explored platelet function during antiplatelet treatment, in order to identify those patients with “high on-treatment platelet reactivity” (HPR), which exposes them to increased risk of major adverse cardiovascular events (MACE). Many tests of platelet function have been used, most of which are able to identify patients at risk of MACE. Unfortunately, universal cut-off values for HPR have not been clearly established yet. In addition, the concordance among different tests in the identification of patients at risk is very poor and the most effective and safe treatment for patients at risk is still unknown.

Comparison of current platelet functional tests for the assessment of aspirin and clopidogrel response

The two most widely used antiplatelet drugs in the world are aspirin and clopidogrel. However, some patients on aspirin and/or clopidogrel therapy do not respond appropriately to either aspirin or clopidogrel. This phenomenon is usually called “aspirin/clopidogrel resistance”. Several platelet function tests have been used in various studies for the assessment of aspirin and clopidogrel resistance in healthy individuals and patients admitted in cardiology departments. An accurate assessment of platelet response to aspirin/clopidogrel could benefit patients by proposing tailored-antiplatelet therapy based on test results. However, there is a clear lack of standardisation of such techniques and their analytical variability may induce misinterpretation. After a quick report of the mechanisms responsible for aspirin/clopidogrel resistance, we describe the pre-analytical aspects and the analytical performances of current platelet function tests (Light-transmission aggregometry, whole-blood aggregometry, VerifyNow®, Platelet Function Analyzer®, thromboelastography, VASP assay) that are used for the assessment of aspirin/clopidogrel resistance in clinical studies. Considering the different variables that have to be taken into account with each of the platelet function tests, a particular attention should be paid when interpreting results.

Do antiplatelet drugs increase the risk of bleeding after tooth extraction? A case-crossover study

The aim of this study was to assess the risk of bleeding after tooth extraction in patients taking aspirin or clopidogrel. This case-crossover study evaluated patients taking aspirin (80mg/day) or clopidogrel (75mg/day) and undergoing tooth extraction. In the first session, extraction was performed without discontinuing aspirin (group 1) or clopidogrel (group 2). In the second session, patients ceased using antiplatelet drugs 5days prior to tooth extraction. Bleeding was evaluated using a visual analogue scale (VAS) for 72h after tooth extraction. The platelet function assay (PFA) was performed for group 1 and flow cytometry assessment of vasodilator-stimulated phosphoprotein (VASP) was performed for group 2, in both sessions. Thirty-eight patients were studied: 20 in group 1 and 18 in group 2. Analysis of the data did not demonstrate any difference in bleeding severity between sessions 1 and 2 in either group (P>0.05). There was a significant difference between sessions 1 and 2 in group 1 for the mean collagen/epinephrine membrane closure time (PFA) (P=0.001). A significant difference in platelet reactivity index (flow cytometry for VASP) was noted between sessions 1 and 2 in group 2 (P=0.001). According to this case-crossover study, dental extraction can be performed safely without withdrawal of aspirin or clopidogrel.

Effect of CYP3A4∗1G and CYP3A5∗3 Polymorphisms on Pharmacokinetics and Pharmacodynamics of Ticagrelor in Healthy Chinese Subjects

Ticagrelor is the first reversible, direct-acting, potent P2Y₁₂ receptor antagonist in management of acute coronary syndromes. It is rapidly absorbed and extensively metabolized. AR-C124910XX, the major active metabolite, antagonizes the P2Y₁₂ receptor at approximately equal potency. The metabolism of ticagrelor to AR-C124910XX involves CYP3A4 and CYP3A5. CYP3A polymorphisms have been well documented, and CYP3A4^∗1G (g.20230G>A, rs2242480) and CYP3A5^∗3 (g.6986A>G, rs776746) are the most important single nucleotide polymorphisms in Chinese. Genetic differences in CYP3A4 and CYP3A5 expression in human volunteers and patients might affect the clearance of ticagrelor or AR-C124910XX in vivo resulting in subsequent variable patient response. Thus, this study is designed to explore the effects of CYP3A4^∗1G and CYP3A5^∗3 polymorphisms on the pharmacokinetics and pharmcodynamics of ticagrelor in healthy Chinese subjects. The results indicated that the CYP3A4^∗1G polymorphism significantly influenced the pharmacokinetics of AR-C124910XX, and it may be more important than CYP3A5^∗3 with respect to influencing ticagrelor pharmacokinetics by increasing CYP3A4 activity. However, the significant effect of CYP3A4^∗1G polymorphism on AR-C124910XX plasma levels did not translate into detectable effect on inhibition of platelet aggregation. Therefore, it seems not necessary to adjust the dosage of ticagrelor according to the CYP3A4 or 3A5 genotype.

Validation of a New ELISA-Based Vasodilator-Associated Stimulated Phosphoprotein Phosphorylation Assay to Assess Platelet Reactivity Index in a Chinese Population

The level of platelet reactivity during P2Y12-adenosine diphosphate receptor antagonist is associated with ischemic and bleeding risks following percutaneous coronary intervention in acute coronary syndrome. Determining platelet reactivity inhibition may be valuable for confirming effective platelet inhibition for individual patients and identifying patients at risk of bleeding. The enzyme-linked immunosorbent assay (ELISA)-based vasodilator-stimulated phosphoprotein (VASP) assay offers unique advantages over other methods and has not been used in the Chinese population. We enrolled 10 healthy volunteers and 54 patients with acute coronary syndrome. The volunteers received no treatment, and patients were administered a loading dose of clopidogrel or ticagrelor. The platelet reactivity index (PRI) was measured using flow cytometry (FC)-VASP and ELISA-VASP at baseline and 8-hour postloading dose. Blood samples of healthy volunteers and clopidogrel- or ticagrelor-treated patients were frozen and stored for 1, 2, and 4 weeks after initial activation. All frozen samples were tested using ELISA-VASP. The PRI assessed by FC-VASP and ELISA-VASP correlated well showing a high degree of consistency in identifying high or low on-treatment platelet reactivity. No significant time-dependent changes in PRI results were observed in frozen samples stored up to 4 weeks compared to nonfrozen samples. The PRI of ticagrelor-treated patients was lower than that of clopidogrel-treated patients. The ELISA-VASP effectively assesses the PRI, and results obtained from frozen specimens are unaffected by storage and shipment prior to assay. Ticagrelor was superior to clopidogrel in decreasing the PRI.

Effects of clopidogrel therapy on whole blood platelet aggregation, the Plateletworks® assay and coagulation parameters in cats with asymptomatic hypertrophic cardiomyopathy: a pilot study

BACKGROUND

Although scientific evidence is limited, clopidogrel is frequently used as prophylaxis for arterial thromboembolism in cats with hypertrophic cardiomyopathy (HCM).

OBJECTIVES

Evaluating effects of clopidogrel therapy in asymptomatic cats with HCM on (1) conventional whole blood aggregation (WBA), (2) alternative platelet aggregation assessed with tubes of the Plateletworks® assay and (3) standard coagulation parameters.

ANIMALS AND METHODS

Prospective, randomized, double-blind, placebo-controlled pilot study. Fourteen asymptomatic HCM cats were randomly allocated to receive placebo (n = 5) or clopidogrel (18.75 mg/cat q24h, n = 9) as part of a larger study. Aggregation responses (to 20 µM adenosine diphosphate (ADP) and 10 µg/ml collagen) in WBA and the Plateletworks® assay and standard coagulation parameters were evaluated at baseline and after seven days of therapy.

RESULTS

Clopidogrel therapy significantly reduced aggregation responses to ADP and collagen in the Plateletworks® agonists tubes (ADP and collagen: P < 0.001), but did not significantly reduce aggregation responses to ADP and collagen in the WBA technique (ADP: P = 0.07, collagen: P = 0.30). Clopidogrel therapy did not show a significant effect on prothrombin time, activated partial thromboplastin time, antithrombin, D-dimers and fibrinogen concentrations.

CONCLUSION AND CLINICAL IMPORTANCE

Clopidogrel therapy at a dose of 18.75 mg/cat q24h for seven days causes a significant decrease in in vitro platelet aggregation evaluated with the Plateletworks® assay, without affecting standard coagulation parameters in cats with asymptomatic HCM.

Interleukin-10 does not modulate clopidogrel platelet response in mice

ESSENTIALS: It is unclear whether interleukin-10 (IL-10) could affect clopidogrel metabolism and response. The bioactivation of and response to clopidogrel were determined between mice with or without IL-10. Maximum clopidogrel active metabolite levels were the major driver of platelet response to clopidogrel. IL-10 did not modulate maximum levels of clopidogrel active metabolite and its antiplatelet effects.

SUMMARY

BACKGROUND

Elevated plasma interleukin-10 (IL-10) levels were observed in patients who responded less to clopidogrel (a prodrug that is required for further metabolic bioactivation in the liver). However, no data are currently available suggesting whether there is such an association.

OBJECTIVE

To systematically explore possible differences in the formation of and response to clopidogrel active metabolite (CAM) in mice with or without IL-10 gene expression.

METHODS

A single oral dose of clopidogrel (10 mg kg(-1)) was given to IL-10 knockout (KO) mice and wild-type (WT) control mice, respectively, and pharmacokinetic parameters of clopidogrel and CAM were calculated. Moreover, adenosine diphosphate-induced whole-blood platelet aggregation was measured in mice receiving 0, 5, 10, or 20 mg kg(-1) of clopidogrel, respectively.

RESULTS

Compared with IL-10 KO mice, WT mice had significantly lower area under the plasma concentration-time curve (AUC) of CAM as a result of a shorter mean elimination half-life but had significantly higher AUC of clopidogrel due to slower systemic clearance and smaller volume of distribution. Although AUC of CAM was significantly lower in WT mice than in KO mice, antiplatelet effects of clopidogrel did not differ significantly between the two mouse groups, as their maximum plasma concentrations (Cmax ) of CAM were not significantly different.

CONCLUSIONS

IL-10 expression level affects AUC rather than Cmax of CAM, but the Cmax of CAM is the major driver of antiplatelet effects of clopidogrel in mice.

Role of Purinergic Receptor Expression and Function for Reduced Responsiveness to Adenosine Diphosphate in Washed Human Platelets

Background

Washing of platelets is an important procedure commonly used for experimental studies, e.g. in cardiovascular research. As a known phenomenon, responsiveness to adenosine diphosphate (ADP) is reduced in washed platelets, although underlying molecular mechanisms—potentially interfering with experimental results—have not been thoroughly studied.

Objectives

Since ADP mediates its effects via three purinergic receptors P2Y1, P2X1 and P2Y12, their surface expression and function were investigated in washed platelets and, for comparison, in platelet-rich-plasma (PRP) at different time points for up to 2 hours after preparation.

Results

In contrast to PRP, flow cytometric analysis of surface expression in washed platelets revealed an increase of all receptors during the first 60 minutes after preparation followed by a significant reduction, which points to an initial preactivation of platelets and consecutive degeneration. The activity of the P2X1 receptor (measured by selectively induced calcium flux) was substantially maintained in both PRP and washed platelets. P2Y12 function (determined by flow cytometry as platelet reactivity index) was partially reduced after platelet washing compared to PRP, but remained stable in course of ongoing storage. However, the function of the P2Y1 receptor (measured by selectively induced calcium flux) continuously declined after preparation of washed platelets.

Conclusion

In conclusion, decreasing ADP responsiveness in washed platelets is particularly caused by impaired activity of the P2Y1 receptor associated with disturbed calcium regulation, which has to be considered in the design of experimental studies addressing ADP mediated platelet function.

Expression and function of purinergic receptors in platelets from apheresis-derived platelet concentrates

BACKGROUND

The storage of platelets affects platelet integrity and functionality, a process named platelet storage lesion (PSL). Reduced adenosine diphosphate (ADP)-induced platelet aggregation is a typical manifestation of PSL. However, the role of ADP receptors in this context has not been evaluated yet. The aim of this study was, therefore, to investigate surface expression and function of the purinergic receptors P2Y1, P2Y12 and P2X1 in stored platelet concentrates.

MATERIAL AND METHODS

Platelets were obtained from venous whole blood and from apheresis-derived platelet concentrates stored for 0, 2 and 5 days. Purinergic receptor expression was measured by flow cytometry and western blot analysis. Receptor function was determined by calcium-induced fluorescence (P2Y1 and P2X1) or by flow cytometric measurement of the platelet reactivity index (P2Y12).

RESULTS

The basal surface expression and total content of purinergic receptors remained unchanged throughout storage. After an initial reduction during apheresis, P2X1-mediated calcium flux was maintained, whereas the P2Y1-mediated increase of calcium flux gradually decreased during the course of storage. In contrast, the platelet reactivity index was comparable in freshly obtained and stored platelets.

DISCUSSION

The function of the P2Y12 receptor is maintained during storage of apheresis-derived platelet concentrates. However, the impairment of P2X1 and especially of P2Y1 receptor function indicated by decreased receptor-mediated calcium flux is an important mechanism contributing to reduced ADP responsiveness of stored platelets.

Clinical Relevance of Biomarkers of Oxidative Stress

SIGNIFICANCE

Oxidative stress is considered to be an important component of various diseases. A vast number of methods have been developed and used in virtually all diseases to measure the extent and nature of oxidative stress, ranging from oxidation of DNA to proteins, lipids, and free amino acids.

RECENT ADVANCES

An increased understanding of the biology behind diseases and redox biology has led to more specific and sensitive tools to measure oxidative stress markers, which are very diverse and sometimes very low in abundance.

CRITICAL ISSUES

The literature is very heterogeneous. It is often difficult to draw general conclusions on the significance of oxidative stress biomarkers, as only in a limited proportion of diseases have a range of different biomarkers been used, and different biomarkers have been used to study different diseases. In addition, biomarkers are often measured using nonspecific methods, while specific methodologies are often too sophisticated or laborious for routine clinical use.

FUTURE DIRECTIONS

Several markers of oxidative stress still represent a viable biomarker opportunity for clinical use. However, positive findings with currently used biomarkers still need to be validated in larger sample sizes and compared with current clinical standards to establish them as clinical diagnostics. It is important to realize that oxidative stress is a nuanced phenomenon that is difficult to characterize, and one biomarker is not necessarily better than others. The vast diversity in oxidative stress between diseases and conditions has to be taken into account when selecting the most appropriate biomarker.

A Randomized Study of the Safety, Tolerability, Pharmacodynamics, and Pharmacokinetics of Clopidogrel in Three Different CYP2C19 Genotype Groups of Healthy Japanese Subjects

AIM

We investigated the safety of 600/150 mg regimen of clopidogrel and the pharmacodynamics and pharmacokinetics of both 300/75 mg regimen and 600/150 mg regimen of clopidogrel in 72 Japanese subjects.

METHODS

A randomized study was conducted in healthy Japanese male subjects. Eligible subjects were stratified by dose regimen (300 mg loading dose of clopidogrel on day 1 followed by a 75 mg maintenance dose from days 2 to 7 or a 600 mg loading dose of clopidogrel on day 1 followed by a 150 mg maintenance dose from days 2 to 7) and CYP2C19 metabolizer group [extensive metabolizers (EMs), intermediate metabolizers (IMs), and poor metabolizers (PMs)]. Platelet aggregation and platelet reactivity were evaluated by measuring the maximum platelet aggregation intensity (MAI) induced by 5 and 20 μM ADP, phosphorylation of vasodilator-stimulated phosphoprotein (VASP), and P2Y12 reaction units (PRU) using the VerifyNow system, respectively. We also measured the plasma concentrations of clopidogrel and its active metabolite H4.

RESULTS

No treatment emergent adverse events in the 300/75 mg and 600/150 mg regimen were observed in EMs, IMs, and PMs. All CYP metabolizer groups exhibited a lower MAI (%) induced by ADP in the 300/75 mg and 600/150 mg clopidogrel regimens, and MAI (%) in IM group was equipotent to EM irrespective of the clopidogrel dosage. The double dose regimen decreased MAI in the PM group as equipotent to the IM group receiving the standard dose regimen without the extension of bleeding time. No clear relationship of exposure to clopidogrel and CYP2C19 function was observed, whereas active metabolite H4 exposure was likely to be related to CYP2C19 function.

CONCLUSION

Clopidogrel in the 600/150 mg regimen was well tolerated. All CYP metabolizer groups exhibited a lower MAI (%) induced by ADP and anti-platelet activities analyzed by VASP and VerifyNow test in the 300/75 mg and 600/150 mg regimens in healthy Japanese subjects.

An Assay of Measuring Platelet Reactivity Using Monoclonal Antibody against Activated Platelet Glycoprotein IIb/IIIa in Patients Taking Clopidogrel

Background and Objectives

Residual platelet reactivity in patients who are taking clopidogrel is commonly measured with VerifyNow assay, which is based on the principle of light transmission aggregometry. However, to evaluate the residual platelet reactivity, it would be more accurate if the reactivity of platelet glycoprotein (GP) IIb/IIIa is directly monitored. In this study, PAC1, a monoclonal antibody against activated platelet GP IIb/IIIa, was used to measure the residual platelet reactivity.

Subjects and Methods

Twenty seven patients with coronary artery disease taking clopidogrel were enrolled. Platelets in whole blood were stained with fluorescein isothiocyanate (FITC)-conjugated PAC1. Mean fluorescence intensity (MFI) and % positive platelets (PP) were measured with flow cytometry, and the binding index (BI; MFI × %PP/100) was calculated. P2Y12 reaction unit (PRU) and % inhibition of VerifyNow assay were also measured in the usual manner.

Results

PRU of VerifyNow assay correlated significantly with MFI, %PP, and BI at 10 µM (r=0.59, 0.73, and 0.60, respectively, all p<0.005) and 20 µM of adenosine diphosphate (ADP; r=0.61, 0.75, and 0.63, respectively, all p<0.005). The % inhibition also correlated significantly with MFI, %PP, and BI at 10 µM (r=-0.60, -0.69, and -0.59, respectively, all p<0.005) and 20 µM of ADP (r=-0.63, -0.71, and -0.62, respectively, all p<0.005).

Conclusion

Direct measurements of the reactivity of platelet GP IIb/IIIa were feasible using PAC1 and flow cytometry in patients taking clopidogrel. Further clinical studies are required to determine the cut-off values which would define high residual platelet reactivity in patients on this treatment protocol.

Efficacy of clopidogrel treatment and platelet responsiveness in peripheral arterial procedures

INTRODUCTION

Long-term antiplatelet therapy with clopidogrel has been recommended in patients undergoing peripheral arterial procedures. Poor antiplatelet effect of clopidogrel or high on-clopidogrel platelet reactivity (HCPR) has been recently identified in patients with peripheral arterial disease (PAD).

AREAS COVERED

This review focuses on the use of clopidogrel and the phenomenon of HCPR in PAD patients treated for intermittent claudication or critical limb ischaemia (CLI). The authors summarize current guidelines and recommendations for use of clopidogrel following peripheral arterial procedures and explore the prevalence and clinical impact of HCPR in the PAD population. Underlying mechanisms of HCPR and relevant clinical and genetic factors are analyzed with particular attention to the potential utility of point-of-care platelet function testing (PFT).

EXPERT OPINION

Clopidogrel is a safe, effective and well-tolerated antiplatelet agent in PAD patients following peripheral arterial revascularization. Dual-antiplatelet therapy could also be considered after complex endovascular procedures. HCPR has been identified in more than 50% of PAD patients on clopidogrel and has been related with significantly increased re-intervention rates. Incidence of HCPR is significantly higher in patients with CLI, diabetes mellitus and chronic renal disease. Personalized antiplatelet therapy on the basis of PFT is an elegant emerging concept for optimization of platelet inhibition and potential identification of patients at increased risk of bleeding and warrants investigation in future large-scale trials.

Reciprocal regulation of human platelet function by endogenous prostanoids and through multiple prostanoid receptors

Platelets are permanently exposed to a variety of prostanoids formed by blood cells or the vessel wall. The two major prostanoids, prostacyclin and thromboxane act through well established pathways mediated by their respective G-protein coupled receptors inhibiting or promoting platelet aggregation accordingly. Yet the role of other prostanoids and prostanoid receptors for platelet function regulation has not been thoroughly investigated. We aimed at a comprehensive analysis of prostanoid effects on platelets, the receptors and pathways involved and functional consequences. We analyzed cAMP formation and phosphorylation of proteins pivotal to platelet function as well as functional platelet responses such as secretion, aggregation and phosphorylation. The types of prostanoid receptors contributing and their individual share in signaling pathways were analyzed and indicated a major role for prostanoid IP1 and DP1 receptors followed by prostanoid EP4 and EP3 receptors while prostanoid EP2 receptors appear less relevant. We could show for the first time the reciprocal action of the endogenous prostaglandin PGE2 on platelets by functional responses and phosphorylation events. PGE2 evokes stimulatory as well as inhibitory effects in a concentration dependent manner in platelets via prostanoid EP3 or EP4 and prostanoid DP1 receptors. A mathematical model integrating the pathway components was established which successfully reproduces the observed platelet responses. Additionally we could show that human platelets themselves produce sufficient PGE2 to act in an autocrine or paracrine fashion. These mechanisms may provide a fine tuning of platelet responses in the circulating blood by either promoting or limiting endogenous platelet activation.

Ambient hemolysis and activation of coagulation is different between HeartMate II and HeartWare left ventricular assist devices

BACKGROUND

Thromboembolic and bleeding events in patients with a left ventricular assist device (LVAD) are still a major cause of complications. Therefore, the balance between anti-coagulant and pro-coagulant factors needs to be tightly controlled. The principle hypothesis of this study is that different pump designs may have an effect on hemolysis and activation of the coagulation system. Referring to this, the HeartMate II (HMII; Thoratec Corp, Pleasanton, CA) and the HeartWare HVAD (HeartWare International Inc, Framingham, MA) were investigated.

METHODS

For 20 patients with LVAD support (n = 10 each), plasma coagulation, full blood count, and clinical chemistry parameters were measured. Platelet function was monitored using platelet aggregometry, platelet function analyzer-100 system ( Siemens, Marburg, Germany), vasodilator-stimulated phosphoprotein phosphorylation assay, immature platelet fraction, platelet-derived microparticles, and von Willebrand diagnostic.

RESULTS

Acquired von Willebrand syndrome could be detected in all patients. Signs of hemolysis, as measured by lactate dehydrogenase levels (mean, 470 U/liter HMII, 250 U/liter HVAD; p < 0.001), were more pronounced in the HMII patients. In contrast, D-dimer analysis indicated a significantly higher activation of the coagulation system in HVAD patients (mean, 0.94 mg/liter HMII, 2.01 mg/liter HVAD; p < 0.01). The efficacy of anti-platelet therapy using clopidogrel was not sufficient in more than 50% of the patients.

CONCLUSIONS

Our results support the finding that all patients with rotary blood pumps suffered from von Willebrand syndrome. In addition, a distinct footprint of effects on hemolysis and the coagulation system can be attributed to different devices. As a consequence, the individual status of the coagulation system needs to be controlled in long-term patients.

Current diagnostic trends in coagulation disorders among dogs and cats

The diagnostic workup to differentiate hemorrhage caused by vascular injury from a systemic hemostatic imbalance typically involves a combination of broad screening tests and specific assays. The characterization of 3 overlapping phases of primary hemostasis, secondary hemostasis, and fibrinolysis provides a simple diagnostic framework for evaluating patients with clinical signs of hemorrhage. New techniques such as flow cytometry, thrombin-generation assays, thrombelastography, and anticoagulant drug monitoring are under investigation for veterinary patients; however, their ability to improve diagnosis or treatment requires further study in clinical trials.

Determination of ATP and ADP Secretion from Human and Mouse Platelets by an HPLC Assay

BACKGROUND

Secretion of ADP and ATP is an essential prerequisite for platelet aggregation. Impaired nucleotide secretion can cause aggregation defects and increased bleeding risk. Quantitative determination of platelet nucleotide content and exocytosis is thus of importance for the characterization and diagnosis of bleeding phenotypes. For transgenic animal models with hemostatic defects analysis of potential secretion defects is as well imperative.

METHODS

Supernatants of washed platelets and platelet-rich plasma were analyzed by HPLC for ADP and ATP concentration. Calibration of the HPLC data was accomplished with an internal standard compensating for loss of analyte, detection sensitivity, and interference of the biomatrix.

RESULTS

HPLC analysis of nucleotide secretion was carried out with human and mouse platelets. Detection limits were determined for washed platelet and platelet-rich plasma samples. In the physiological concentration range linearity with respect to the peak area is maintained.

CONCLUSION

The method combines reasonable sensitivity with robustness. The internal standard ensures reliable quantification of nucleotide concentrations even in presence of otherwise interfering substances. The low sample consumption renders possible the application to analysis of small samples like in mouse experiments.

Towards personalized medicine based on platelet function testing for stent thrombosis patients

Stent thrombosis (ST) is a severe and feared complication of coronary stenting. Patients who have suffered from ST are usually treated according to the “one-size-fits-all” dosing regimen of aspirin and clopidogrel. Many ST patients show high on-treatment platelet reactivity (HPR) despite this antiplatelet therapy (APT). It has been shown that HPR is a risk factor for major adverse cardiac events. Therefore, ST patients with HPR are at a high risk for recurrent atherothrombotic events. New insights into the variable response to clopidogrel and the advent of stronger P2Y12 inhibitors prasugrel and ticagrelor have changed the attention from a fixed APT treatment strategy towards “personalized APT strategies.” Strategies can be based on platelet function testing, which gives insight into the overall response of a patient to APT. At our outpatient ST clinic, we practice personalized APT based on platelet function testing to guide the cardiologist to a presumed optimal antiplatelet treatment of ST patients. Beside results of platelet function testing, comedication, clinical characteristics, and genetics have to be considered to decide on personalized APT. Ongoing studies have yet to reveal the optimal personalized APT strategy for cardiologists to prevent their patients from atherothrombotic and bleeding events.

P2Y12 receptor: platelet thrombus formation and medical interventions

Platelets express a wide range of receptors and proteins that play essential roles in thrombus formation. Among these, the P2Y(12) receptor, a member of the G protein-coupled receptor family, has attracted a significant amount of attention. Stimulation of the P2Y(12) receptor by ADP results in activation of various signaling pathways involved in amplification of platelet activation and aggregation. There have been extensive attempts to design an ideal antithrombotic agent to block P2Y(12), which shows selective expression, as an intervention for cardiovascular disease. Current inhibitors of the P2Y(12) receptor include indirect inhibitor members of the thienopyridine family (ticlopidine, clopidogrel, and prasugrel), and direct P2Y(12) inhibitors (ticagrelor, cangrelor and elinogrel). Of these, clopidogrel is the most commonly prescribed P2Y(12) blocker; however, this product does not fulfill the ideal therapeutic requirements. The main limitations of clopidogrel administration include slow onset, prevention of recovery of platelet functions, and interindividual variability. Hence, advanced studies have been carried out to achieve more efficient and safer P2Y(12) blockade. In this review, we provide a brief but comprehensive report on P2Y(12), its role on platelet thrombus formation, and the targeting of this receptor as an intervention for cardiovascular disease, for the benefit of basic science and clinical researchers.

Impact of dual antiplatelet therapy with proton pump inhibitors on the outcome of patients with acute coronary syndrome undergoing drug-eluting stent implantation

This study aimed to assess if proton pump inhibitors (PPIs) may reduce the effectiveness of clopidogrel, than H2 antagonist (anti-H2) in order to determine rehospitalization for acute coronary syndrome (re-ACS), target vessel revascularization (TVR) and cardiac death. This case-control study included 176 patients with ACS undergoing angioplasty (PCI) with drug-eluting stent implantation. The population was divided into two groups: PPI group (n = 121) consisting of patients receiving at discharge dual antiplatelet therapy (DAT) plus PPI and anti-H2 group (n = 55), consisting of patients receiving at discharge DAT + H2 receptor antagonist (H2RA). In a followup of 36 months the prevalence of ACS event (P = 0.014), TVR (P = 0.031) was higher in the PPI group than in the anti-H2 group; instead there was no statistically significant difference between groups for death. The variables independently associated with ACS were the diabetes, omeprazole, and esomeprazole; instead the variables independently associated with TVR were only omeprazole. Our data shows that the use of omeprazole and esomeprazole, with clopidogrel, is associated with increased risk of adverse outcomes after PCI with drug-eluting stent implantation.

Doubling the clopidogrel dose in patients with reduced responsiveness to the standard dose is associated with a limited effectiveness as evaluated by impedance aggregometry

BACKGROUND

Different methods are available for quantifying platelet function inhibition. Measuring vasodilator-stimulated phosphoprotein (VASP) phosphorylation is currently the most specific method for assessing the clopidogrel effect. The aim of our study was to compare different tests in view of a clinically applicable bedside test. Further, we examined whether doubling the clopidogrel dose to 150mg/d in clopidogrel low-responder would lead to a reduction in platelet reactivity.

METHODS AND RESULTS

ADP-, ADP Hs-, and TRAP-induced platelet aggregation were measured by impedance aggregometry in 100 patients with CAD and 18 healthy controls. Moreover, platelet aggregation was assessed by flow cytometrical detection of VASP-phosphorylation and surface P-selectin in a subgroup of 34 patients and in healthy controls. Another 10 patients with CAD, identified as low-responder, were treated with a clopidogrel dose of 150mg/d. Thereafter, ADP-induced platelet aggregation was assessed by impedance aggregometry. Significant correlations were observed between ADP-induced platelet aggregation assessed by VASP-phosphorylation and by impedance aggregometry. Doubling the dose of clopidogrel to 150mg/d was associated with a reduction of ADP-induced platelet aggregation in only 60% of the patients.

CONCLUSIONS

Impedance aggregometry is a valuable bedside test to assess platelet function inhibition. Doubling the clopidogrel dose is not effective to reduce high on-treatment platelet reactivity in almost half of these patients, pointing to the need of a more powerful platelet inhibitor.

Perioperative management of antithrombotic therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

This guideline addresses the management of patients who are receiving anticoagulant or antiplatelet therapy and require an elective surgery or procedure.

METHODS

The methods herein follow those discussed in the Methodology for the Development of Antithrombotic Therapy and Prevention of Thrombosis Guidelines. Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines article of this supplement.

RESULTS

In patients requiring vitamin K antagonist (VKA) interruption before surgery, we recommend stopping VKAs 5 days before surgery instead of a shorter time before surgery (Grade 1B). In patients with a mechanical heart valve, atrial fibrillation, or VTE at high risk for thromboembolism, we suggest bridging anticoagulation instead of no bridging during VKA interruption (Grade 2C); in patients at low risk, we suggest no bridging instead of bridging (Grade 2C). In patients who require a dental procedure, we suggest continuing VKAs with an oral prohemostatic agent or stopping VKAs 2 to 3 days before the procedure instead of alternative strategies (Grade 2C). In moderate- to high-risk patients who are receiving acetylsalicylic acid (ASA) and require noncardiac surgery, we suggest continuing ASA around the time of surgery instead of stopping ASA 7 to 10 days before surgery (Grade 2C). In patients with a coronary stent who require surgery, we recommend deferring surgery > 6 weeks after bare-metal stent placement and > 6 months after drug-eluting stent placement instead of undertaking surgery within these time periods (Grade 1C); in patients requiring surgery within 6 weeks of bare-metal stent placement or within 6 months of drug-eluting stent placement, we suggest continuing antiplatelet therapy perioperatively instead of stopping therapy 7 to 10 days before surgery (Grade 2C).

CONCLUSIONS

Perioperative antithrombotic management is based on risk assessment for thromboembolism and bleeding, and recommended approaches aim to simplify patient management and minimize adverse clinical outcomes.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Monitoring the effectiveness of antiplatelet therapy: opportunities and limitations

Previous clinical studies have shown heterogeneity in individual patient responses to antiplatelet therapy and high residual platelet reactivity is associated with increased risk of adverse clinical events. Monitoring response to antiplatelet therapy and tailoring treatment accordingly is currently not recommended in routine clinical practice largely due to the lack of a standardized definition of antiplatelet therapy hyporesponse and the need for a widely accepted point-of-care platelet function test that can be reliably utilized in frontline clinical practice. Recent data have shown that titrating the dose of clopidogrel in patients undergoing percutaneous coronary intervention significantly reduces the incidence of major adverse cardiovascular events and large-scale clinical trials are currently underway to investigate whether individually tailored treatment based on results of platelet function testing leads to improved clinical outcome. Furthermore, genetic testing has demonstrated a link between CYP2C19 genetic polymorphisms, altered clopidogrel metabolite concentrations and adverse clinical events. Clinical studies are currently underway to investigate the potential clinical benefit associated with genotype-guided tailoring of antiplatelet therapy. With the advent of newer, more potent antiplatelet agents and their associated increased bleeding risks, it will become imperative in the future to select the most appropriate, safe and effective drug.

The thrombin inhibitors hirudin and Refludan(®) activate the soluble guanylyl cyclase and the cGMP pathway in washed human platelets

A number of direct thrombin inhibitors are successfully used clinically and experimentally as novel antithrombotics and specific anticoagulants. They are also used as anticoagulants in certain blood collection tubes for the analysis of platelet function. A series of platelet function tests have emerged to measure adequate responses to antiplatelet therapy. For comparative and practical reasons, it would be of advantage to use the same anticoagulant in blood collection tubes for different methods, e.g. thrombin inhibitors. However, there are little data on the effects of thrombin inhibitors on platelet signalling pathways that could influence results. We examined the applicability of thrombin inhibitor containing blood for platelet reactivity index (PRI) measurements of the VASP assay and investigated the effects of two thrombin inhibitors (hirudin and lepirudin) on cAMP- and cGMP-mediated signalling pathways in washed human platelets. We show that induction of VASP phosphorylation by PGE1 is markedly reduced in lepirudin containing blood samples. In consequence, PRI levels were highly variable compared to routinely used citrated blood. Surprisingly, in vitro incubation of platelets with thrombin inhibitors increases platelet cGMP levels and induces NOS independent sGC/PKG-mediated VASP phosphorylation. We conclude that thrombin inhibitors activate sGC/PKG-dependent pathways resulting in an increase of VASP phosphorylation which contributes to deviations in PRI measurements. These effects of thrombin inhibitors on sGC- and cGMP-mediated pathways including increased VASP phosphorylation may indicate the presence of an important additional platelet-based mechanism for the reduction of thrombus formation and thromboembolism by thrombin inhibitors.