Jochen Graff

Clopidogrel but not aspirin reduces P‐selectin expression and formation of platelet‐leukocyte aggregates in patients with atherosclerotic vascular disease*

Formation of platelet‐leukocyte aggregates via the CD62 ligand represents an important mechanism by which leukocytes contribute to thrombotic events. In a cross‐sectional study, we investigated platelet‐leukocyte aggregate formation and markers indicative for platelet, leukocyte, and endothelial activation (CD62, activated fibrinogin receptor glycoprotein IIb/IIIA [PAC‐1], CD11b/CD18 [MAC‐1], and soluble intercellular adhesion molecule 1) in 44 patients with atherosclerotic vascular disease and peripheral occlusions receiving clopidogrel (n = 12), aspirin (n = 17), their combination (n = 8), or no treatment (n = 7), as well as in a group of healthy subjects (n = 9). Whole‐blood flow cytometry was performed before (baseline) and after stimulation with thrombin receptor‐activating peptide or adenosine diphosphate. Both at baseline and after stimulation, untreated patients and those receiving aspirin monotherapy exhibited significantly higher levels of platelet CD62 expression (baseline CD62: untreated, 22% [median]; with aspirin, 16%) and had higher rates of platelet‐leukocyte aggregate formation (monocyte‐platelet‐leukocyte aggregates at baseline: untreated, 27%; with aspirin, 16%) when compared with patients receiving clopidogrel alone (baseline CD62: 10% [P < .05]; monocyte‐platelet‐leukocyte aggregates: 13% [P < .05]) or combined with aspirin (baseline CD62: 5% [P < .05]; monocyte‐platelet‐leukocyte aggregates: 7% [P < .05]). Up‐regulation of MAC‐1 on monocytes after stimulation with thrombin receptor‐activating peptide and adenosine diphosphate was significantly lower in patients treated with clopidogrel and aspirin. Plasma levels of soluble intercellular adhesion molecule 1 were significantly lower in the group of healthy subjects (median, 186 ng/mL) when compared with those in untreated patients (median, 352 ng/mL) (P < .05), whereas intercellular adhesion molecule 1 levels in treated patients were similar for any antiplatelet regimen (aspirin, 262 ng/mL; clopidogrel, 274 ng/mL; combination therapy, 273 ng/mL) but significantly lower than those in untreated patients. This is the first report showing that platelet‐leukocyte aggregate formation is enhanced in atherosclerotic vascular disease but was found to be reduced in patients receiving clopidogrel.

Effects of the Oral, Direct Factor Xa Inhibitor Rivaroxaban on Platelet‐Induced Thrombin Generation and Prothrombinase Activity1

Rivaroxaban (BAY 59‐7939) is an oral, direct factor Xa inhibitor in advanced development. This study was undertaken to investigate its effects on thrombin generation. In this placebo‐controlled, randomized, crossover study, 12 healthy subjects received rivaroxaban (single 5‐ or 30‐mg dose) or placebo. Thrombin generation was investigated by measuring the endogenous thrombin potential and prothrombinase‐induced clotting time. Maximal effect of rivaroxaban was observed 2 hours after drug administration: prothrombinase‐induced clotting time was prolonged 1.8 and 2.3 times baseline after rivaroxaban 5 and 30 mg, respectively. Collagen‐induced endogenous thrombin potential was reduced by ∼80% and ∼90% compared with baseline after rivaroxaban 5 and 30 mg, respectively, and tissue factor‐induced endogenous thrombin potential was reduced by ∼40% (5 mg) and ∼65% (30 mg), respectively. Thrombin generation remained inhibited for 24 hours. There was a close correlation between plasma concentration of rivaroxaban and prolongation of prothrombinase‐induced clotting time and reduction in endogenous thrombin potential. Rivaroxaban strongly inhibits platelet‐induced thrombin generation, after activation of either platelets or the coagulation pathway, even in the presence of minimal factor Xa inhibition in plasma.

Clopidogrel, but not abciximab, reduces platelet leukocyte conjugates and P‐selectin expression in a human ex vivo in vitro model

Formation of platelet‐leukocyte aggregates (PLA) through the CD62 ligand is an important mechanism by which leukocytes contribute to thrombosis and inflammation. We investigated the formation of PLA in human subjects after stimulation with thrombin receptor activating peptide and adenosine diphosphate (ADP) after treatment with clopidogrel and after in vitro application of the platelet glycoprotein IIb/IIIa complex antagonist abciximab. Expression of CD62 was significantly reduced 30% to 50% with clopidogrel, depending on the type and concentration of the inducer, but addition of abciximab led to a significant approximately 30% increase in CD62 expression whenplatelets were stimulated by ADP. Formation of PLA decreased significantly with clopidogrel to 55% to 75% of the baseline value, whereas addition of abciximab caused a significant increase in PLA in ADP‐stimulated samples before but not after administration of clopidogrel. The increase in formation of PLA after in vitro addition of abciximab was not paralleled by a decrease in platelet microaggregates and is therefore presumed not caused by enhanced availability of platelets. To our knowledge, this is the first report showing that clopidogrel reduces formation of PLA. The findings also suggest intersection between an “outside‐in” signal generated by abciximab and stimulation of platelet P2T12 purinergic receptors that augments degranulation and increases formation of PLA but is inhibited by clopidogrel.

Novel oral anticoagulants: clinical pharmacology, indications and practical considerations.

BackgroundNovel oral anticoagulants are approved in several indications: rivaroxaban, apixaban, and dabigatran for the prevention of venous thromboembolism after elective hip or knee replacement surgery, and edoxaban for hip or knee replacement surgery and hip fracture surgery (in Japan only); rivaroxaban for the treatment of deep vein thrombosis (DVT) and pulmonary embolism (PE), and prevention of recurrent DVT and PE; and rivaroxaban, apixaban, and dabigatran for the prevention of stroke and systemic embolism in patients with non-valvular atrial fibrillation. These agents overcome some limitations of traditional anticoagulants, are suggested to have no requirement for routine coagulation monitoring, and are administered orally. Rivaroxaban, apixaban, and dabigatran have different pharmacological characteristics, and guidance is needed on optimum doses and dosing intervals and the effects of renal or hepatic impairment, age, food, and other drugs. Dabigatran has stricter prescribing advice than rivaroxaban or apixaban for patients with moderate-to-severe renal impairment. All three drugs have restrictions on use in patients with hepatic impairment. Apixaban requires twice-daily dosing in all indications, whereas rivaroxaban and dabigatran are dosed once- or twice-daily depending on indication. Although head-to-head comparisons are lacking, the novel oral anticoagulants may show favorable cost–benefit relations compared with traditional vitamin K antagonists or no therapy.AimThis review summarizes the pharmacology of rivaroxaban, apixaban, edoxaban, and dabigatran, and the indications for which they are approved. Issues regarding the optimization of the use of these anticoagulants for the management of thromboembolic disorders will also be discussed.

Anti-inflammatory effects of clopidogrel intake in renal transplant patients: Effects on platelet-leukocyte interactions, platelet CD40 ligand expression, and proinflammatory biomarkers

Increased platelet activation has been described during treatment with various immunosuppressive agents and may contribute to the high cardiovascular mortality rate in renal transplant patients. Platelets are thought to propagate the inflammatory process of atherosclerosis by interaction with leukocytes.

Effects of parecoxib and dipyrone on platelet aggregation in patients undergoing meniscectomy: a double-blind, randomized, parallel-group study.

BACKGROUND Based on a PubMed search of the literature using the terms parecoxib, platelets, thromboxane, bleeding, and platelet aggregation, the effects of parecoxib on platelet function have not fully been established under clinical conditions. OBJECTIVE The aim of this study was to determine platelet aggregation, thromboxane B(2) (TxB(2)) formation, and plasma concentrations with the use of parecoxib in postoperative pain management. METHODS This double-blind, randomized, parallel-group trial was conducted at the University Hospital for Orthopedic Surgery, Friedrichsheim, Frankfurt, Germany. Male and female patients aged 18 to 55 years and scheduled to undergo routine partial meniscectomy (or a similar arthroscopic procedure) were eligible. All patients received dose-adjusted enoxaparin before surgery and parecoxib 40 mg BID or dipyrone 1000 mg QID. Blood samples were drawn before first injection (predose) and at 0.5, 2, and 6 hours after injection. Platelet aggregation (expressed as percentage of the maximal light transmittance [A(max)]) was induced with arachidonic acid (A(max)AA) and collagen (A(max)CO). TxB(2) formation was determined using enzyme-linked immunosorbent assay. RESULTS This study included 26 patients. In both treatment groups, 8 males and 5 females, all white, were enrolled. In the dipyrone group, the mean age was 48 years (range, 32-61 years) and the mean weight was 85 kg (range, 63-122 kg); in the parecoxib group, the mean age was 47 years (range, 31-61 years) and the mean weight was 81 kg (range, 57-100 kg). Median (interquartile range [IQR]) predose values for A(max)AA were 76% (65%-83%) in the parecoxib group and 87% (80%-89%) in the dipyrone group. At 0.5 hour after injection, A(max)AA was 52% (5%-77%) with parecoxib and 8% (0%-11%) with dipyrone (P=0.004). At 2 hours after injection, A(max)AA was 78% (72%-80%) in the parecoxib group versus 7% (5%-11%) in the dipyrone group (P<0.001). At 6 hours after study drug administration, no treatment differences were found. For A(max)CO, no statistically significant differences were found. Consistent with the stronger inhibition of aggregation, patients who received dipyrone had lower TxB(2) formation values. Six hours after parecoxib administration, mean TxB(2) formation was significantly enhanced compared with predose values (132 ng/mL [IQR, 62-228 ng/mL] vs 185 ng/mL [IQR, 135-239 ng/mL]; P=0.05). CONCLUSIONS Platelet aggregation and TxB(2) formation were significantly lower for 6 hours in dipyronetreated patients compared with parecoxib-treated patients. In contrast, TxB(2) formation was increased with parecoxib 6 hours after administration compared with pretreatment values. In this small study, parecoxib did not affect platelet aggregation in a population of patients undergoing routine partial meniscectomy (or a similar arthroscopic procedure) under clinical conditions.

Effects of antiplatelet agents on platelet-induced thrombin generation.

OBJECTIVES The influence of antiplatelet agents on platelet-induced thrombin generation may increase the risk of bleeding. Assessment of the endogenous thrombin potential (ETP), is therefore a parameter deserving attention in early pharmacodynamic studies with antiplatelet drugs. The aim ofthis study was to assess whether an automated ETP-assay can be used to determine possible inhibitory effects of antiplatelet drugs on platelet-associated thrombin generation. METHODS We first characterized the in vitro dose-response relationship of several platelet agonists (ADP, collagen, U46619, TRAP (amino acid sequence: SFLLRNP) and tissue factor (TF) using the generation of ETP. One submaximal concentration of each agonist was then used to assess the influence of in vivo treatment with aspirin (single oral dose of 500 mg as inhibitor of thromboxane synthesis) and clopidogrel (given orally for 6 days, as an inhibitor of the purinergic P2Y12-receptor on platelets) and in vitro treatment with abciximab (which inhibits the platelet glycoprotein IIb/IIIa-receptor for fibrinogen), on the ETP. RESULTS The effect of TF and the other platelet inducers on thrombin generation was dose-dependent. Repeat measurements on samples from the same subject, with the same inducer concentration on 2 different occasions showed a variability of approx. 22% (absolute difference between 2 measurements as % of mean). The coefficient on variation of repeat measurements of one sample varied between 7% and 17%, depending on the inducer. After a single dose of aspirin, ETP was reduced by 25-40%, depending on the platelet activating agent used. The reduction in ETP with abciximab in vitro was more pronounced. In contrast, TF-induced ETP was not influenced by aspirin or abciximab. Clopidogrel, administered for 6 days, reduced the ETP by 60% when platelets were stimulated using 20 microM ADP, whereas collagen-induced ETP and TF-induced ETP remained unchanged. CONCLUSIONS The ETP-method is a sensitive and reproducible method for the detection of drug effects on platelet-induced thrombin generation of high throughput, and can be recommended for studies on the pharmacodynamic profile of drugs interfering with platelet function.

Differential in Vitro Effects of the Platelet Glycoprotein IIb/IIIa Inhibitors abixicimab or SR121566A on Platelet Aggregation, Fibrinogen Binding and Platelet Secretory Parameters

The aim of this study was to compare fibrinogen binding, inhibition of platelet aggregation and secretory potential of the MAb abciximab (0.5-5 microg/mL) and the peptidomimetic compound SR121566A (15-250 ng/mL) in vitro in whole blood. Fibrinogen binding was followed by flow cytometry; platelet function was evaluated by light transmittance and by impedance aggregometry. Secretory functions of platelets were evaluated using ATP as marker for early secretion by dense granulae and P-selectin (CD62) for alpha-granular secretion as well as CD63 for lysosomal degranulation. Results showed that fibrinogen binding induced by 5 microM TRAP was maximally inhibited greater than 80% at 3 microg/mL abciximab or at 250 ng/mL SR121566A. At these concentrations of antagonists, platelet aggregation induced by 5 microM ADP or 2 microg/mL collagen was inhibited completely. Expression of CD62 was reduced 34% with abciximab or 15% with SR121566A; CD63 expression was reduced 22% with both agents. With both agents, the EC50 for inhibition of CD62 and CD63 expressions was in similar magnitudes than the EC50 for fibrinogen binding inhibition. With 3 microg/mL abciximab, ATP secretion was maximally reduced to 50% of the control, whereas SR121566A at 250 ng/mL had no inhibitory effect on this parameter. A slight increase in ATP secretion was seen with 0.5 microg/mL abciximab and with SR121566A in concentrations of less than 45 ng/mL. The data suggest a discoupling between the anti-aggregatory and the antisecretory effects of IIb/IIIa antagonists. Because it is not established to what extend CD62 or CD63 expression can be reduced by any means, the reduction by 20-30% obtained by 3 microg/mL abciximab or 250 ng/mL SR121566A might already be the maximum possible inhibition by these agents.

Immunosuppressive therapy regimen and platelet activation in renal transplant patients

Increased platelet activation caused by an immunosuppressive therapy regimen may contribute to the high incidence of death from cardiovascular disease in renal transplant patients. Cyclosporine (INN, ciclosporin) and azathioprine are reported to activate platelets, but data are rare and controversial for tacrolimus and mycophenolate mofetil.

Induction of central signalling pathways and select functional effects in human platelets by β-boswellic acid

We have recently shown that in polymorphonuclear leukocytes, 11‐keto boswellic acids (KBAs) induce Ca2+ mobilisation and activation of mitogen‐activated protein kinases (MAPK). Here we addressed the effects of BAs on central signalling pathways in human platelets and on various platelet functions. We found that β‐BA (10 μM), the 11‐methylene analogue of KBA, caused a pronounced mobilisation of Ca2+ from internal stores and induced the phosphorylation of p38 MAPK, extracellular signal‐regulated kinase (ERK)2, and Akt. These effects of β‐BA were concentration dependent, and the magnitude of the responses was comparable to those obtained after platelet stimulation with thrombin or collagen. Based on inhibitor studies, β‐BA triggers Ca2+ mobilisation via the phospholipase (PL)C/inositol‐1,4,5‐trisphosphate pathway, and involves Src family kinase signalling. Investigation of platelet functions revealed that β‐BA (10 μM) strongly stimulates the platelet‐induced generation of thrombin in an ex‐vivo in‐vitro model, the liberation of arachidonic acid (AA), and induces platelet aggregation in a Ca2+‐dependent manner. In contrast to β‐BA, the 11‐keto‐BAs (KBA or AKBA) evoke only moderate Ca2+ mobilisation and activate p38 MAPK, but fail to induce phosphorylation of ERK2 or Akt, and do not cause aggregation or significant generation of thrombin. In summary, β‐BA potently induces Ca2+ mobilisation as well as the activation of pivotal protein kinases, and elicits functional platelet responses such as thrombin generation, liberation of AA, and aggregation.