P2Y12 receptor blockade synergizes strongly with nitric oxide and prostacyclin to inhibit platelet activation.

AIMS: In vivo platelet function is a product of intrinsic platelet reactivity, modifiable by dual antiplatelet therapy (DAPT), and the extrinsic inhibitory endothelial mediators, nitric oxide (NO) and prostacyclin (PGI2 ), that are powerfully potentiated by P2Y12 receptor blockade. This implies that for individual patients endothelial mediator production is an important determinant of DAPT effectiveness. Here, we have investigated this idea using platelets taken from healthy volunteers treated with anti-platelet drugs. METHODS: Three groups of male volunteers (n = 8) received either prasugrel (10 mg), aspirin (75 mg) or DAPT (prasugrel + aspirin) once daily for 7 days. Platelet reactivity in the presence of diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA/NONOate) and PGI2 was studied before and following treatment. RESULTS: Ex vivo, PGI2 and/or DEA/NONOate had little inhibitory effect on TRAP-6-induced platelet reactivity in control conditions. However, in the presence of DAPT, combination of DEA/NONOate + PGI2 reduced platelet aggregation (74 ± 3% to 19 ± 6%, P < 0.05). In vitro studies showed even partial (25%) P2Y12 receptor blockade produced a significant (67 ± 2% to 39 ± 10%, P < 0.05) inhibition when DEA/NONOate + PGI2 was present. CONCLUSIONS: We have demonstrated that PGI2 and NO synergize with P2Y12 receptor antagonists to produce powerful platelet inhibition. Furthermore, even with submaximal P2Y12 blockade the presence of PGI2 and NO greatly enhances platelet inhibition. Our findings highlight the importance of endothelial mediator in vivo modulation of P2Y12 inhibition and introduces the concept of refining ex vivo platelet function testing by incorporating an assessment of endothelial function to predict thrombotic outcomes better and adjust therapy to prevent adverse outcomes in individual patients.

Novel whole blood assay for phenotyping platelet reactivity in mice identifies ICAM-1 as a mediator of platelet-monocyte interaction.

Testing of platelet function is central to the cardiovascular phenotyping of genetically modified mice. Traditional platelet function tests have been developed primarily for testing human samples and the volumes required make them highly unsuitable for the testing of mouse platelets. This limits research in this area. To address this problem, we have developed a miniaturized whole blood aggregometry assay, based on a readily accessible 96-well plate format coupled with quantification of single platelet depletion by flow cytometric analysis. Using this approach, we observed a concentration-dependent loss of single platelets in blood exposed to arachidonic acid, collagen, U46619 or protease activated receptor 4 activating peptide. This loss was sensitive to well-established antiplatelet agents and genetic manipulation of platelet activation pathways. Observations were more deeply analyzed by flow cytometric imaging, confocal imaging, and measurement of platelet releasates. Phenotypic analysis of the reactivity of platelets taken from mice lacking intercellular adhesion molecule (ICAM)-1 identified a marked decrease in fibrinogen-dependent platelet-monocyte interactions, especially under inflammatory conditions. Such findings exemplify the value of screening platelet phenotypes of genetically modified mice and shed further light upon the roles and interactions of platelets in inflammation.

Prostaglandin E1 potentiates the effects of P2Y12 blockade on ADP-mediated platelet aggregation in vitro: Insights using short thromboelastography.

In addition to adenosine diphosphate (ADP), a number of platelet function tests including the VerifyNow P2Y12 assay (VN-P2Y12) employ prostaglandin E1 (PGE1) to improve specificity for P2Y12 blockade by mitigating the contribution of the P2Y1 pathway on ADP-mediated platelet aggregation. Using short thromboelastography (s-TEG), we have previously shown that VN-P2Y12 overestimates the functional effect of clopidogrel in some individuals. We investigated whether PGE1 systematically increases the inhibitory effects of P2Y12 blockade on ADP-mediated platelet aggregation in an in vitro model. Using s-TEG, we measured ADP-induced platelet aggregation either in the presence or absence of PGE1 (11 or 22 nM) in blood samples taken from healthy volunteers pre-incubated with prasugrel active metabolite (PAM; 0, 1, 3 or 10 µM). Individually, both PGE1 (p < 0.02) and PAM (p < 0.0001) inhibited ADP-mediated platelet aggregation in a dose-dependent manner, as expected. Furthermore, inclusion of PGE1 augmented inhibition of ADP-mediated platelet aggregation in response to PAM (p < 0.02) in a dose-dependent manner such that a 10-fold higher dose of PAM was required to attain equivalent inhibition of ADP-mediated platelet aggregation to that achieved by 1 µM PAM in the presence of 11 nM PGE1. In conclusion, PGE1 potentiates the anti-aggregatory effects of P2Y12 blockade on ADP-mediated platelet aggregation. Assays that employ PGE1 with ADP may therefore overestimate therapeutic response to prasugrel in a proportion of individuals, potentially making them unsuitable candidates for guiding delivery of personalized antiplatelet therapy.

Evidence that links loss of cyclooxygenase-2 with increased asymmetric dimethylarginine: novel explanation of cardiovascular side effects associated with anti-inflammatory drugs.

BACKGROUND: Cardiovascular side effects associated with cyclooxygenase-2 inhibitor drugs dominate clinical concern. Cyclooxygenase-2 is expressed in the renal medulla where inhibition causes fluid retention and increased blood pressure. However, the mechanisms linking cyclooxygenase-2 inhibition and cardiovascular events are unknown and no biomarkers have been identified. METHODS AND RESULTS: Transcriptome analysis of wild-type and cyclooxygenase-2(-/-) mouse tissues revealed 1 gene altered in the heart and aorta, but >1000 genes altered in the renal medulla, including those regulating the endogenous nitric oxide synthase inhibitors asymmetrical dimethylarginine (ADMA) and monomethyl-l-arginine. Cyclo-oxygenase-2(-/-) mice had increased plasma levels of ADMA and monomethyl-l-arginine and reduced endothelial nitric oxide responses. These genes and methylarginines were not similarly altered in mice lacking prostacyclin receptors. Wild-type mice or human volunteers taking cyclooxygenase-2 inhibitors also showed increased plasma ADMA. Endothelial nitric oxide is cardio-protective, reducing thrombosis and atherosclerosis. Consequently, increased ADMA is associated with cardiovascular disease. Thus, our study identifies ADMA as a biomarker and mechanistic bridge between renal cyclooxygenase-2 inhibition and systemic vascular dysfunction with nonsteroidal anti-inflammatory drug usage. CONCLUSIONS: We identify the endogenous endothelial nitric oxide synthase inhibitor ADMA as a biomarker and mechanistic bridge between renal cyclooxygenase-2 inhibition and systemic vascular dysfunction.

Optical multichannel (optimul) platelet aggregometry in 96-well plates as an additional method of platelet reactivity testing.

Platelet reactivity testing is important for the diagnosis of bleeding disorders, and increasingly to optimise anti-platelet therapy. Traditional light transmission aggregometry is considered the gold standard, whilst 96-well plate aggregometry, founded on similar principles, provides a higher throughput screening method. Despite the widespread use of both, methodologies and outputs vary widely between laboratories. We report a methodological approach towards providing a standardised optical detection of platelet aggregation (optimul method) based upon 96-well plate aggregometry. Individual wells of half-area 96-well plates were coated with gelatine and one of seven concentrations of arachidonic acid (AA), adenosine diphosphate (ADP), collagen, epinephrine (EPI), ristocetin, TRAP-6 amide or U46619, before being lyophilised, vacuum-sealed, foil-packed and stored at room temperature for up to 24 weeks. For platelet testing, 40 µl of platelet-rich plasma was added to each well. Platelet aggregation was determined by changes in light absorbance, release of ATP/ADP by luminescence and release of thromboxane (TX) A(2) by ELISA. Some experiments were conducted in the presence of aspirin (30 µM) or prasugrel active metabolite (PAM; 3 µM). Optimul plates stored for up to 12 weeks permitted reliable detection of concentration-dependent platelet aggregation, ATP/ADP release and TXA2 production. PAM caused reductions in platelet responses to AA, ADP, collagen, EPI, TRAP-6 and U46619, whilst aspirin inhibited responses to AA, collagen and EPI. We conclude that the optimul method offers a viable, standardised approach, allowing platelet reactivity testing and could provide a broad platelet function analysis without the need for dedicated equipment.

GPVI and GPIbα mediate staphylococcal superantigen-like protein 5 (SSL5) induced platelet activation and direct toward glycans as potential inhibitors.

BACKGROUND: Staphylococcus aureus (S. aureus) is a common pathogen capable of causing life-threatening infections. Staphylococcal superantigen-like protein 5 (SSL5) has recently been shown to bind to platelet glycoproteins and induce platelet activation. This study investigates further the interaction between SSL5 and platelet glycoproteins. Moreover, using a glycan discovery approach, we aim to identify potential glycans to therapeutically target this interaction and prevent SSL5-induced effects. METHODOLOGY/PRINCIPAL FINDINGS: In addition to platelet activation experiments, flow cytometry, immunoprecipitation, surface plasmon resonance and a glycan binding array, were used to identify specific SSL5 binding regions and mediators. We independently confirm SSL5 to interact with platelets via GPIbα and identify the sulphated-tyrosine residues as an important region for SSL5 binding. We also identify the novel direct interaction between SSL5 and the platelet collagen receptor GPVI. Together, these receptors offer one mechanistic explanation for the unique functional influences SSL5 exerts on platelets. A role for specific families of platelet glycans in mediating SSL5-platelet interactions was also discovered and used to identify and demonstrate effectiveness of potential glycan based inhibitors in vitro. CONCLUSIONS/SIGNIFICANCE: These findings further elucidate the functional interactions between SSL5 and platelets, including the novel finding of a role for the GPVI receptor. We demonstrate efficacy of possible glycan-based approaches to inhibit the SSL5-induced platelet activation. Our data warrant further work to prove SSL5-platelet effects in vivo.

Dual antiplatelet therapy in cardiovascular disease: does aspirin increase clinical risk in the presence of potent P2Y12 receptor antagonists?

Aspirin is now widely accepted as the first-line antithrombotic platelet therapy for at-risk individuals. During the last decade or so it has also become established that co-administering antagonists of the ADP receptor P2Y(12) with aspirin further reduces the risk of acute thrombotic events. By the nature of its evolution, this therapeutic approach assumes that P2Y(12) receptor antagonists will be added to aspirin, and this therefore dominates the design of clinical trials. This strategy has resulted in the generation of a large body of clinical evidence showing the benefit of aspirin plus P2Y(12) receptor antagonists, largely from studies with clopidogrel and more recently from those with prasugrel and ticagrelor, but with obvious limitations in terms of residual ischaemic event rates and bleeding complications. It is our hypothesis, however, that when administered in the presence of potent P2Y(12) receptor antagonists, aspirin could actually increase total cardiovascular risk, although this has never been tested in large outcome studies. Clearly, this potentially negative interaction could be of relevance to millions of patients.

Utility of 96-well plate aggregometry and measurement of thrombi adhesion to determine aspirin and clopidogrel effectiveness.

Aspirin and clopidogrel are key anti-thrombotic therapies. Results from platelet reactivity testing during therapy, have been shown to correlate with future events and would allow for the optimisation of therapy. However, there is little agreement among current tests and there remains a clear clinical need for a universal standardised test. It was the objective of this study to explore the potential of 96-well plate aggregometry as a definitive clinical test of platelet reactivity with respect to aspirin and clopidogrel. A small non-blinded trial of 16 healthy male volunteers assigned to seven days of aspirin (75mg/day) or clopidogrel (75mg/day) therapy. Blood was collected before and on day 7 of treatment. Platelet aggregation was measured using a 96-well plate based aggregation method, and thrombi adhesion measured by colourimetric assay. Platelet agonists used were ADP (0.1-30microM), arachidonic acid (0.03-1.3mM), collagen (0.1-30microg/ml), adrenaline (0.001-100microM), ristocetin (0.2-3mg/ml), TRAP6 amide (0.130microM) and U46619 (0.130microM). Concentration response curves were constructed to each agonist under the various conditions and used to extract data such as log EC(50), Hill slope, and area under the curve. These demonstrated low intra- and inter-assay variability and strong discrimination of drug effects. This study demonstrates the ability of the 96-well plate based aggregation and adhesion method to detect and differentiate between stable aspirin and clopidogrel treatment in healthy volunteers. Moreover, this assay marries the ability to test subjects or patients using a range of platelet agonists with more rapidity and ease than the current gold standard platelet assay, traditional light transmission aggregometry, making it a serious alternative assay for use in clinical settings.

Antiplatelet actions of statins and fibrates are mediated by PPARs.

OBJECTIVES: Statins and fibrates are hypolipidemic drugs which decrease cardiac events in individuals without raised levels of cholesterol. These drugs inhibit platelet function, but the mechanisms by which this pleiotropic effect is exerted are not known. METHODS AND RESULTS: We used a range of approaches to show statins inhibit human platelet activation in vitro while engaging PPARalpha and PPARgamma. The effects of simvastatin were prevented by the PPARgamma antagonist GW9662 or the PPARalpha antagonist GW6471. In a small-scale human study fluvastatin activated PPARalpha and PPARgamma in platelets and reduced aggregation in response to arachidonic acid ex vivo. The effects of fenofibrate were prevented by PPARalpha antagonism with GW6471. Fenofibrate increased bleeding time in wild-type, but not in PPARalpha-/- mice. The inhibitory effect of fenofibrate, but not simvastatin, on aggregation was prevented by deletion of PPARalpha in murine platelets. PKCalpha, which influences platelet activation, associated and immune-precipitated with PPARgamma in platelets stimulated with statins and with PPARalpha in platelets stimulated with fenofibrate. CONCLUSIONS: This study is the first to provide a unifying explanation of how fibrates and statins reduce thrombotic and cardiovascular risk. Our findings that PPARs associate with PKCalpha in platelets also provide a mechanism by which these effects are mediated.