Measuring antiplatelet drug effects in the laboratory

Assessment of platelet function on whole blood by multiple electrode aggregometry in high-risk patients with coronary artery disease receiving antiplatelet therapy

This study sought to compare Multiplate impedance platelet aggregometry (IPA) with light transmission aggregometry (LTA) and the PFA-100 for determining the prevalence of residual platelet reactivity (RPR) by the Multiplate IPA in 297 patients with acute coronary syndrome receiving dual antiplatelet therapy. Aggregations were induced by adenosine-5 diphosphate (ADP), arachidonic acid, and collagen. PFA-100 closure times were measured by collagen and ADP and epinephrine (CEPI) cartridges. Significant correlations were observed between Multiplate IPA and LTA after all stimulations (P < .0001) and between Multiplate IPA (arachidonate and collagen) and PFA-100 CEPI closure time (P < .0001 for both). Cutoff values of Multiplate IPA (for all stimulations) were calculated for the identification of RPR. Between the Multiplate IPA and LTA good agreement was found with all 3 agonists (P < .0001 for all). Multiplate IPA might represent a reliable, handy, rapid tool to monitor antiplatelet therapy in clinical practice and for clinical investigations.

Pharmacodynamic assessment of platelet inhibition by prasugrel vs. clopidogrel in the TRITON-TIMI 38 trial

AIMS

To examine the extent of platelet inhibition by prasugrel vs. clopidogrel in a TRITON-TIMI 38 substudy.

METHODS AND RESULTS

TRITON-TIMI 38 randomized acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI) to prasugrel or standard dose clopidogrel. Selected sites prospectively enrolled TRITON-TIMI 38 patients to evaluate adenosine diphosphate (ADP)-attenuated phosphorylation of platelet vasodilator-stimulated phosphoprotein (VASP) (n = 125 patients) and, in a subset (n = 31 patients), ADP-stimulated platelet aggregation. VASP platelet reactivity index (PRI) was lower in prasugrel-treated patients than in clopidogrel-treated patients at 1-2 h post-PCI (>or=1 h after loading dose) (P < 0.001) and at 30 days (P < 0.001). Maximal platelet aggregation to 20 microM ADP was lower in prasugrel-treated patients than in clopidogrel-treated patients at 1-2 h (P = 0.004) and 30 days (P = 0.03). Results were similar with 5 microM ADP. Thienopyridine hyporesponsiveness, prespecified as VASP PRI >50%, was more frequent in clopidogrel-treated patients than in prasugrel-treated patients at 1-2 h (P < 0.001) and 30 days (P = 0.03).

CONCLUSIONS

The TRITON-TIMI 38 platelet substudy shows that prasugrel results in greater inhibition of ADP-mediated platelet function in ACS patients than clopidogrel, supporting the hypothesis that greater platelet inhibition leads to a lower incidence of ischaemic events and more bleeding both early and late following PCI.

Laboratory assessment and perioperative management of patients on antiplatelet therapy: from the bench to the bedside

The contribution of platelets in the pathophysiology of thromboses has established antiplatelet therapy as a cornerstone for prevention or treatment of these disorders. However, patients on antiplatelet drugs undergoing surgery face the life-threatening dilemma between the risk of perioperative thrombosis by ceasing therapy and restoring platelet function versus the risk of surgical bleeding by its continuation. According to their mechanism of action, antiplatelet drugs can be conventionally classified as agents that inhibit cyclooxygenase, block the platelet adenosine diphosphate P2Y12 receptor, inhibit phosphodiesterase, or block platelet glycoprotein IIb/IIIa. Although several tests have been developed to assess platelet inhibition by most of these compounds, studies to date have not been able to reliably evaluate the diagnostic efficiency of these tests to predict hemorrhage and/or blood loss, and accordingly perioperative assessment of drug-induced platelet inhibition cannot be recommended as yet. Although several management options are available to counteract the hemorrhagic risk of surgical patients using antiplatelet agents, perioperative discontinuation of these drugs is the preferable choice wherever possible. The use of platelet transfusions should be limited where necessary to the treatment of major, life-threatening bleeding. The contribution of newer hemostatic agents, such as desmopressin and recombinant activated factor VII, is yet to be fully determined, and there remain many challenges and unresolved issues in the clinical care of these patients.

Methods for the measurement of platelet function

This article discusses the advantages and disadvantages of methods for the measurement of platelet function. The focus is on tests that can be used to monitor antiplatelet activity in the setting of cardiovascular disease and potentially predict thrombosis and bleeding. The tests described are platelet aggregometry; impedance aggregometry; VerifyNow (Accumetrics, San Diego, CA); Plateletworks (Helena Laboratories, Beaumont, TX); platelet surface P-selectin, platelet surface-activated glycoprotein IIb/IIIa, and leukocyte-platelet aggregates; TEG Platelet Mapping system (Haemoscope, Niles, IL); Impact cone and plate(let) analyzer (DiaMed, Cressier, Switzerland); Platelet Function Analyzer-100 (Siemens Healthcare Diagnostics, Inc., Deerfield, IL); phosphorylation of vasodilator-stimulated phosphoprotein; serum thromboxane B(2); and urinary 11-dehydro thromboxane B(2). Some of the factors that differentiate these tests are sample volume requirements, the use of whole blood, the presence of shear, point-of-care status, need for a technician, and expense.

Point-of-care whole blood impedance aggregometry versus classical light transmission aggregometry for detecting aspirin and clopidogrel: the results of a pilot study

BACKGROUND

We determined whether whole blood impedance aggregometry using the Multiplate detects the effects of antiplatelet drugs as reliably as does classical light transmission aggregometry (LTA) or the platelet function analyzer PFA-100(R).

METHODS

Multiplate (M) assays, measuring changes in electrical resistance as aggregation units over time (AU*min), and LTA assays induced by collagen (COL), adenosine diphosphate (ADP) or arachidonic acid (AA) and PFA-100 testing, using epinephrine (PFA100-EPI) or ADP (PFA100-ADP) cartridges, were performed simultaneously using arterial blood samples obtained before induction of anesthesia in 70 consecutive patients scheduled for elective coronary artery bypass grafting. Patients in group A (n = 48) served as controls, patients in group B (n = 11) received aspirin 100 mg/d and those in group C (n = 11) aspirin 100 mg/d and clopidogrel 75 mg/d until the day before surgery.

RESULTS

In controls the median (1st, 3rd quartiles) change in impedance AU*min for M-COL (374 [231-469]) was significantly greater than in patients receiving aspirin (164 [86-211], P = 0.0009) or receiving aspirin and clopidogrel (118 [101-244], P = 0.004). M-ADP values in controls were 258 (158-389), in patients receiving aspirin 261 (159-393), and in patients receiving aspirin and clopidogrel 88 (48-231, P = 0.054). M-AA values were significantly lower in patients receiving aspirin alone (45 [28-60], P = 0.0004) or aspirin and clopidogrel (44 [26-221], P = 0.008) than in controls (200 [86-345]). The areas under the receiver operating characteristic curves indicating the ability to discriminate patients taking aspirin from those not taking aspirin were comparable for COL and AA assays using whole blood impedance aggregometry or classical LTA (M-COL 0.84 [P = 0.001], LTA-COL 0.85 [P = < .001], M-AA 0.84 [P = < .001] and LTA-AA 0.87 [P = < .001]), but only 0.74 for PFA-100-EPI (P = 0.03). Similarly, for discrimination of patients not taking antiplatelet drugs from patients taking clopidogrel and aspirin the areas under the receiver operating characteristic curve were also comparable for both aggregometry methods M-COL 0.77 (P = 0.006), LTA-COL 0.78 (P = 0.004), M-ADP 0.74 (P = 0.015), LTA-ADP 0.73 (P = 0.018).

CONCLUSION

Results achieved with the bedside Multiplate assays were not different than those obtained with classical aggregometry for detecting the effects of aspirin and clopidogrel in preoperative patients scheduled for elective cardiac surgery.

Aspirin and the in vitro linear relationship between thromboxane A2-mediated platelet aggregation and platelet production of thromboxane A2

BACKGROUND

Currently, 'aspirin resistance', the anti-platelet effects of non-steroid anti-inflammatory drugs (NSAIDs) and NSAID-aspirin interactions are hot topics of debate. It is often held in this debate that the relationship between platelet activation and thromboxane (TX) A(2) formation is non-linear and TXA(2) generation must be inhibited by at least 95% to inhibit TXA(2)-dependent aggregation. This relationship, however, has never been rigorously tested.

OBJECTIVES

To characterize, in vitro and ex vivo, the concentration-dependent relationships between TXA(2) generation and platelet activity.

METHOD

Platelet aggregation, thrombi adhesion and TXA(2) production in response to arachidonic acid (0.03-1 mmol L(-1)), collagen (0.1-30 microg mL(-1)), epinephrine (0.001-100 micromol L(-1)), ADP, TRAP-6 amide and U46619 (all 0.1-30 micromol L(-1)), in the presence of aspirin or vehicle, were determined in 96-well plates using blood taken from naïve individuals or those that had taken aspirin (75 mg, o.d.) for 7 days.

RESULTS

Platelet aggregation, adhesion and TXA(2) production induced by either arachidonic acid or collagen were inhibited in concentration-dependent manners by aspirin, with logIC(50) values that did not differ. A linear relationship existed between aggregation and TXA(2) production for all combinations of arachidonic acid or collagen and aspirin (P < 0.01; R(2) 0.92; n = 224). The same relationships were seen in combinations of aspirin-treated and naïve platelets, and in blood from individuals taking an anti-thrombotic dose of aspirin.

CONCLUSIONS

These studies demonstrate a linear relationship between inhibition of platelet TXA(2) generation and TXA(2)-mediated aggregation. This finding is important for our understanding of the anti-platelet effects of aspirin and NSAIDs, NSAID-aspirin interactions and 'aspirin resistance'.

The GPIIb/IIIa (integrin alphaIIbbeta3) odyssey: a technology-driven saga of a receptor with twists, turns, and even a bend

Starting 90 years ago with a clinical description by Glanzmann of a bleeding disorder associated with a defect in platelet function, technologic advances helped investigators identify the defect as a mutation(s) in the integrin family receptor, alphaIIbbeta3, which has the capacity to bind fibrinogen (and other ligands) and support platelet-platelet interactions (aggregation). The receptor's activation state was found to be under exquisite control, with activators, inhibitors, and elaborate inside-out signaling mechanisms controlling its conformation. Structural biology has produced high-resolution images defining the ligand binding site at the atomic level. Research on alphaIIbbeta3 has been bidirectional, with basic insights resulting in improved Glanzmann thrombasthenia carrier detection and prenatal diagnosis, assays to identify single nucleotide polymorphisms responsible for alloimmune neonatal thrombocytopenia, and the development of alphaIIbbeta3 antagonists, the first rationally designed antiplatelet agents, to prevent and treat thrombotic cardiovascular disease. The future looks equally bright, with the potential for improved drugs and the application of gene therapy and stem cell biology to address the genetic abnormalities. The alphaIIbbeta3 saga serves as a paradigm of rigorous science growing out of careful clinical observations of a rare disorder yielding both important new scientific information and improved diagnosis, therapy, and prevention of other disorders.

[Platelets "Toll-like receptor" engagement stimulates the release of immunomodulating molecules]

Platelets are nonnucleated cellular elements that play a role in the process of haemostasis, and also in various ways in innate immunity and in inflammation. Platelets also contain numerous secretory products and can exert critical roles in several aspects of haemostasis. In addition, they house and secrete a variety of cytokines, chemokines and associated molecules which behave as ligands for receptors/counterparts displayed by endothelial cells lining tissue vessels and most leukocyte subsets. These latter studies show that platelets have an important role in innate as well as adaptive immunity; thus platelets can take part in an immune directive response. Moreover, platelets display receptors for several types of cytokines/chemokines along with FcgammaRII receptors. Finally, platelets not only express a variety of Toll-like receptors, with recently identified functions or not as-yet fully identified, but have also been demonstrated to express the key tandem pair of inflammatory and antigen presentation molecules (CD40 and CD40-ligand/CD154), this latter function making them the major purveyors of soluble CD40L in the plasma. It appears that platelets may be regarded as one of the neglected components of immune cell regulators, and platelets contribute to some interesting aspects in bridging innate and adaptive immunity. We propose that platelets discriminate danger signals and adapt the subsequent responses, with polarized cytokine secretion. Platelets may recognize several types of infectious pathogens and limit microbial colonization by sequestering these pathogens and releasing immunomodulatory factors. This review allows us to re-explore indications that platelets exert direct anti-infection immunity and we will present experimentally-driven arguments in favour of a role of platelet TLR in regulating certain immune activities.

Comparison of four tests to assess inhibition of platelet function by clopidogrel in stable coronary artery disease patients

AIMS

We investigated the comparability of platelet function tests in quantifying platelet inhibition achieved by clopidogrel.

METHODS AND RESULTS

This pre-specified substudy of a randomized, double-blind trial included 116 patients with stable coronary artery disease requiring diagnostic angiography. Patients received clopidogrel for 1 (300 or 600 mg) or 7 days (300 + 75 or 150 mg daily) before the procedure. Blood samples obtained before clopidogrel initiation and before diagnostic coronary angiography were assayed using light transmission aggregometry [adenosine diphosphate (ADP) 5 and 20 microM as the agonist], whole-blood aggregometry (ADP 5 and 20 microM), PFA-100 (Collagen-ADP cartridge), and VerifyNow P2Y12. Although all assays studied were found sensitive to clopidogrel ingestion, none could distinguish categorically between patients who had, or not, ingested clopidogrel. Agreement between assays to identify patients with insufficient inhibition of platelet aggregation by clopidogrel was low.

CONCLUSION

The assessment of platelet function inhibition by clopidogrel is highly test-specific. Decision to increase clopidogrel dosage may vary on the basis of the assay used, thus highlighting the need for unambiguous guidelines with respect to assay selection, as platelet function assays are not interchangeable. At present, platelet function testing evaluating clopidogrel efficacy cannot be recommended in routine clinical practice.

Utility of the PFA-100 instrument and the novel multiplate analyzer for the assessment of aspirin and clopidogrel effects on platelet function in patients with cardiovascular disease

This study evaluated the utility of the PFA-100 and the Multiplate analyzer for the assessment of aspirin and clopidogrel effects on platelet function in patients with cardiovascular disease. Platelet function was determined with the PFA-100 using collagen+epinephrine (CEPI) and collagen+adenosine-5'-diphosphate (CADP) cartridges, and with whole blood impedance aggregometry using the Multiplate ASPI and ADP+PG tests (aggregation triggered with arachidonic acid and ADP+ prostaglandin E1, respectively). Four study groups were identified from the 154 patients enrolled: patients without antiplatelet therapy, patients with 100 mg aspirin daily but without clopidogrel treatment, patients with 75 mg clopidogrel daily but without aspirin treatment, and patients with both 100 mg aspirin daily plus 75 mg clopidogrel daily. It was found that the PFA-100 instrument is useful for detection of aspirin but not for detection of a clopidogrel effect, while the Multiplate analyzer is useful for specific detection of both aspirin and clopidogrel effects on platelet function.

Aspirin resistance in atherosclerosis

Clinically, aspirin resistance is defined as the failure of aspirin therapy to prevent an acute vascular thrombotic event despite regular intake of appropriate doses. In the laboratory, aspirin resistance encompasses the drug's failure to attain a particular level of platelet inhibition. From a clinical standpoint, the inability of aspirin to prevent a thrombotic event, despite appropriate cyclooxygenase-1 inhibition, implies the involvement of other factors. Evidence is emerging that aspirin resistance, as defined by residual platelet activity, merely reflects an individual's enhanced basal platelet function and suggests a hereditary component. Due to the multifactorial nature of cardiovascular disease, it is likely that a single therapy like aspirin cannot fully treat and prevent all thrombotic complications in the setting of atherosclerosis.

Elevated Risk of Thrombosis in Neonates Undergoing Initial Palliative Cardiac Surgery

BACKGROUND

Thrombotic events cause significant morbidity and mortality in children who undergo surgery for complex congenital cardiac disease. We prospectively evaluated the incidence of thrombosis and examined preoperative and postoperative laboratory tests of coagulation and inflammation in neonates experiencing initial surgical palliation for variations of single ventricle physiology.

METHODS

Neonates (<30 days) requiring initial surgical palliation were studied. All subjects received aspirin from postoperative day 1 onward. Thromboses were diagnosed by serial transthoracic echocardiograms, vascular imaging, and interstage cardiac catheterizations according to predefined criteria.

RESULTS

Twenty-two neonates, age 1 to 11 days (mean 4 +/- 2.5) were studied. Follow-up ranged from three hours to 18 months (median, 212 days). Eight infants died. Four of the 14 subjects who survived (28%), and one of the eight who died (12.5%), had evidence of thrombosis identified over a range of four hours to nine months postoperatively (median 14 days). When compared with reference values established in healthy children, preoperative subject hematocrit (Hct), platelet count, factors II, V, VII, VIII, and X, antithrombin, protein C, and soluble CD40 ligand measures were significantly lower, and the prothrombin time and partial thromboplastin time were significantly higher. Postoperative C reactive protein (CRP) was significantly higher, and Hct and platelet count significantly lower, than preoperative values. Thrombotic events were significantly related to high preoperative CRP (p = 0.02).

CONCLUSION

Thrombotic complications occur frequently in neonates undergoing initial palliative surgery, suggesting that aspirin therapy alone may constitute inadequate protection. Elevated preoperative CRP appears to be associated with increased thrombotic risk.

Different methodologies for evaluating the effect of clopidogrel on platelet function in high-risk coronary artery disease patients

BACKGROUND

Two point-of-care (POC) systems have been recently proposed as rapid tools with which to evaluate residual platelet reactivity (RPR) in coronary artery disease (CAD) patients.

OBJECTIVES AND METHODS

We compared Platelet Function Analyzer-100 (PFA-100) closure times (CTs) by collagen/adenosine 5'-diphosphate (ADP) (C/ADP CT) cartridge and the VerifyNow P2Y12 Assay (VerifyNow) with light transmission aggregation (LTA) induced by 2 and 10 micromol L(-1) ADP in 1267 CAD patients on dual antiplatelet therapy who underwent percutaneous coronary intervention. We also performed the vasodilator-stimulated phosphoprotein (VASP) phosphorylation assay by cytofluorimetric analysis in a subgroup of 115 patients.

RESULTS

Cut-off values for identifying RPR were: > or = 54% and > or = 66% for LTA induced by 2 and 10 micromol L(-1) ADP respectively, and > or = 264 P2Y12 Reaction Units (PRU) for VerifyNow. The cut-off for PFA-100 C/ADP CT was > or = 68 s. RPR was detected in 25.1% of patients by 2 mumol L(-1) ADP-induced LTA (ADP-LTA), in 23.2% by 10 micromol L(-1) ADP-LTA, in 24.4% by PFA-100, and in 24.7% by VerifyNow. PFA-100 results did not parallel those obtained with LTA. VerifyNow showed a significant correlation (rho = 0.62, P < 0.001) and significant agreement (k = 0.34, P < 0.001) with LTA induced by 2 micromol L(-1) ADP. The correlation was similar but the agreement was better between VerifyNow and 10 micromol L(-1) ADP-LTA (rho = 0.64, P < 0.0001; k = 0.43, P < 0.001). Significant relationships were found between VASP platelet reactivity index and both ADP-LTA and VerifyNow. PFA-100 C/ADP CT did not significantly correlate with any of the other assays.

CONCLUSIONS

Our results show a significant correlation between LTA and VerifyNow but not the PFA-100 C/ADP assay. Clinical validation studies for POC systems are necessary.