Complete Inhibition Of Platelet Aggregation Research Articles

A New Shear-Stress-Based Point-of-Care Technology for Evaluation of the Hemostatic Pattern in Whole Blood.

The currently available point-of-care hemostasis tests are burdened by criticisms concerning the use of different activators and inhibitors and the lack of dynamic flow. These operating conditions may constitute an impediment to the determination of the patient's hemostatic condition. Hence, the diffusion of these tests in clinical practice is still limited to specific scenarios. In this work, we present a new method for analyzing the patient's global hemostasis based on the visualization of the main components of the coagulation process and its computerized quantitative image analysis. The automated "Smart Clot" point-of-care system presents a micro-fluidic chamber in which whole blood flows, without the addition of any activator or inhibitor. In this micro-channel, platelet adhesion, activation and aggregation to the type I collagen-coated surface take place (primary hemostasis), leading to the production of endogenous thrombin on the surface of platelet aggregates and the consequent fibrin mesh and thrombus formation (secondary hemostasis). These observations are verified by inhibiting primary hemostasis with the antiplatelet drugs Indomethacin (-70% on platelet aggregation, -60% on fibrin(ogen) formation) and Tirofiban (complete inhibition of platelet aggregation and fibrin(ogen) formation) and secondary hemostasis with the antithrombin drugs Heparin (-70% on platelet aggregation, -80% on fibrin(ogen) formation) and Lepirudin (-80% on platelet aggregation, -90% on fibrin(ogen) formation). Smart Clot, through a single test, provides quantitative results concerning platelet aggregation and fibrin formation and is suitable for undergoing comparative studies with other coagulation point-of-care devices.

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

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

Pharmacokinetics and pharmacodynamics of acetylsalicylic acid after intravenous and oral administration to healthy volunteers

BackgroundThe pharmacology of single doses of acetylsalicylic acid (ASA) administered intravenously (250 or 500 mg) or orally (100, 300, or 500 mg) was evaluated in a randomized, placebo-controlled, crossover study.MethodsBlood and urine samples were collected before and up to 24 hours after administration of ASA in 22 healthy volunteers. Pharmacokinetic parameters and measurements of platelet aggregation were determined using validated techniques.ResultsA comparison between administration routes showed that the geometric mean dose-corrected peak concentrations (Cmax/D) and the geometric mean dose-corrected area under the curve (AUC0–∞/D) were higher following intravenous administration of ASA 500 mg compared with oral administration (estimated ratios were 11.23 and 2.03, respectively). Complete inhibition of platelet aggregation was achieved within 5 minutes with both intravenous ASA doses, reflecting a rapid onset of inhibition that was not observed with oral dosing. At 5 minutes after administration, the mean reduction in arachidonic acid-induced thromboxane B2 synthesis ex vivo was 99.3% with ASA 250 mg intravenously and 99.7% with ASA 500 mg intravenously. In exploratory analyses, thromboxane B2 synthesis was significantly lower after intravenous versus oral ASA 500 mg (P<0.0001) at each observed time point up to the first hour after administration. Concentrations of 6-keto-prostaglandin1α at 5 and 20 minutes after dosing were also significantly lower with ASA 500 mg intravenously than with ASA 500 mg orally.ConclusionThis study demonstrates that intravenous ASA provides more rapid and consistent platelet inhibition than oral ASA within the first hour after dosing.

Pharmacology of the New P2Y12 Receptor Inhibitors: Insights on Pharmacokinetic and Pharmacodynamic Properties

The P2Y(12) receptor is a key player in platelet activation and represents an effective pharmacological target for the inhibition of platelet aggregation and prevention of atherothrombotic events. Indeed, the clinical use of the P2Y(12) receptor inhibitor clopidogrel is an effective strategy for inhibiting platelet activity in patients with acute coronary syndrome, and for preventing thrombotic events in those undergoing percutaneous coronary intervention with stenting. However, clopidogrel has several drawbacks, which include delayed onset of action, large inter-individual variability in platelet response, genetic polymorphism of the metabolizing enzyme, drug-drug interactions (DDIs), and the two-step activation process catalyzed by a series of cytochrome P450 (CYP) isoenzymes. For these reasons, new P2Y(12) receptor inhibitors have been developed in an attempt to improve on the pharmacological and clinical profile of clopidogrel. Three new P2Y(12) receptor inhibitors--prasugrel, cangrelor, and ticagrelor--have arrived, and more are coming into clinical use. Each of these antagonists has individual properties and, according to their mechanism of inhibition, can be divided into irreversible (prasugrel) and reversible inhibitors (ticagrelor, cangrelor). These agents also have different metabolic pathways: prasugrel is a prodrug that requires metabolic activation through a cytochrome-dependent pathway, while ticagrelor and cangrelor do not require metabolic conversion. However, ticagrelor is a CYP3A4 substrate/inhibitor and thus it can be involved in DDIs. Indeed, ticagrelor significantly increases the plasma levels of CYP3A4 substrates such as statins. Moreover, concomitant use with strong CYP3A4 inhibitors (such as ketoconazole, itraconazole, clarithromycin, ritonavir, telithromycin, etc.) is contraindicated, while the co-administration of ticagrelor with potent CYP3A inducers (carbamazepine, rifampicin, phenytoin, phenobarbital) is discouraged. Prasugrel and ticagrelor determine a faster, greater, and more consistent adenosine diphosphate (ADP)-receptor inhibition than clopidogrel, with a near complete inhibition of platelet aggregation between 1-2 h after administration of an oral loading dose, while cangrelor shows a rapid and potent platelet inhibitory effect with intravenous infusion. Thus, the different pharmacokinetic and pharmacodynamic characteristics of the P2Y(12) receptor inhibitors enable clinicians to personalize therapy according to patient-specific medical requirements for better prevention of atherothrombotic events. In the present review, we describe the pharmacological properties, the pharmacokinetic and pharmacodynamic differences, and the clinical efficacy of the currently available P2Y(12) receptor inhibitors.

261 P2Y12 INHIBITION GREATLY POTENTIATES THE ANTI-PLATELET EFFECTS OF PROSTACYCLIN AND NITRIC OXIDE

When P2Y₁₂ receptors on platelets are blocked by commonly used anti-platelet drugs such as clopidogrel and prasugrel, the inhibitory brake on adenylate cyclase (AC) activity is lifted and the anti-platelet effects of prostacyclin (PGI₂) and other agents that activate platelet AC are synergistically enhanced. We have recently demonstrated that blockade of the P2Y₁₂ receptor also enhances the anti-platelet effect of nitric oxide (NO). The aim of this study was to study the interaction between PGI₂, NO and P2Y₁₂ receptor inhibition on platelet aggregation and to determine pharmacologically, using isobolographic analysis, if this interaction constitutes a true synergy or simply an additive effect. <h3>Methods</h3> Blood was collected by venepuncture into 0.32% trisodium citrate. Platelet rich plasma (PRP) was isolated by centrifugation and either tested directly or further washed with Tyrode’s-Hepes buffer. The PRP or washed platelets (WP) were incubated with the P2Y₁₂ inhibitor prasugrel active metabolite (PAM; 3 µM) or vehicle (0.5% DMSO) for 30 min followed by 1 min incubation with the NO donor DEA/NONOate (10 nM–1 mM) and/or PGI₂ (0.2 nM–100 nM) and/or vehicle (0.01M NaOH). WP platelet aggregation to thrombin (1U/ml) was measured by 96-well aggregometry and PRP platelet aggregation to TRAP-6 (Thrombin Receptor Activating Peptide-6, 30 µM) or collagen (30 µg/ml) was measured by light transmission aggregometry (LTA). Isobolograms were constructed by plotting the IC₅₀ values for DEA-NONOate and PGI₂ in vehicle or PAM treated WP. Data represents mean±SEM % final platelet aggregation from 4–5 healthy volunteers. <h3>Results</h3> Thrombin (1U/ml), TRAP-6 (30 µM) or collagen (30 µg/ml) all produced robust aggregation responses in WP and PRP respectively which was largely unaffected by the addition of 10nM DEA-NONOate and 4nM PGI₂ or 3 µM PAM. However, the combination of all three (NO, PGI₂ and PAM) resulted in almost complete inhibition of platelet aggregation. Isobolographic analysis of the data showed that the interaction between DEA/NONOate, PGI₂ and PAM in WP was strongly synergistic (isoboles curved away from the predicted linear line for an additive relationship). <h3>Conclusions</h3> These data confirm that activation of platelet P2Y₁₂ receptors by secreted ADP limits the anti-platelet effects of both NO and PGI_2, suggesting that P2Y₁₂ activation may be an important mechanism for haemostasis. In addition, we have demonstrated that the interaction between P2Y₁₂ receptors and vascular mediators is strongly synergistic. Potentiation of the effects of endogenous NO and PGI₂ may represent an important mechanism for how P2Y₁₂ inhibitors produce anti-thrombotic protection.

No differences in the pharmacodynamics and pharmacokinetics of the thrombin receptor antagonist vorapaxar between healthy Japanese and Caucasian subjects

Vorapaxar, a novel antiplatelet agent in advanced clinical development for the prevention and treatment of atherothrombotic disease, is a potent, orally bioavailable thrombin receptor antagonist selective for the protease-activated receptor 1 (PAR-1). Since race/ethnicity may affect the safety, efficacy and dosage of drugs, this study was conducted to evaluate potential differences in the pharmacodynamics, pharmacokinetics and safety of vorapaxar after single (5, 10, 20, or 40mg) or multiple (0.5, 1, or 2.5mg once daily) doses in healthy Japanese and matched (gender, age, height, and weight) Caucasian volunteers. Vorapaxar was well tolerated in both Japanese and Caucasian subjects. Pharmacodynamic and pharmacokinetic profiles of vorapaxar in the two racial/ethnic groups were similar. In both racial groups, complete inhibition of platelet aggregation was achieved most rapidly with vorapaxar 40mg and was consistently achieved and maintained with a 2.5mg daily maintenance dose. There were no substantial differences in the safety, pharmacokinetics or pharmacodynamics of vorapaxar between Japanese and Caucasian subjects.

ACCF/AHA Clopidogrel Clinical Alert: Approaches to the FDA “Boxed Warning”

The recent US Food and Drug Administration (FDA) “boxed warning” on clopidogrel raises important questions for practitioners and patients. The warning addresses the need for pharmacogenomic testing to identify patients’ altered clopidogrel metabolism and thus their risk for a suboptimal clinical response to clopidogrel. Although there is an expanding database on genetic polymorphisms that may affect clopidogrel metabolism and thus clinical outcomes, there are no evidence-based data upon which todevelop specific recommendations on the role of genetic testing in routine care nor strategies proven to improve the safety/efficacy of specific pharmacologic approaches. To provide guidance on this issue, the American College of Cardiology Foundation (ACCF) and the American Heart Association (AHA) convened a writing committee. The ACCF and AHA adhere to a rigorous policy regarding relationships with industry and other entities (RWI) of authors and peer reviewers for clinical document development (see http://www.cardiosource.org/Science-And-Quality/Practice-Guidelines-and-Quality-Standards/Relationships-With-Industry-Policy.aspx). This policy requires that a majority of writing committee members have no relevant relationships with industry to this topic, a standard that has been achieved for this document as indicated in Appendix 1. In the spirit of full disclosure, comprehensive RWI (RWI not relevant to this document) for all authors is available online for public view. RWI …

Population Pharmacodynamic Modelling of Aspirin- and Ibuprofen-Induced Inhibition of Platelet Aggregation in Healthy Subjects

The objective of this study was to develop a mechanism-based pharmacodynamic model that characterizes the antiplatelet effects of aspirin (acetylsalicylic acid) and ibuprofen alone and in combination. Ten healthy volunteers were enrolled in a single-blinded, randomized, three-way crossover study. Treatments consisted of single doses of oral aspirin (325 mg) and ibuprofen (400 mg) and concomitant administration of aspirin (325 mg) and ibuprofen (400 mg). Ex vivo whole blood platelet aggregation induced by collagen (1 microg/mL) or arachidonic acid (0.5 mmol/L) was measured by impedance aggregometry. Model development and population parameter estimation were performed using nonlinear mixed-effects modelling implemented in NONMEM. Relatively complete inhibition of platelet aggregation was achieved following aspirin treatment (approximately 77% inhibition within 2 hours), and return to baseline values occurred within 72-96 hours after dosing. In contrast, treatment with ibuprofen alone or in combination with aspirin produced transient inhibition of platelet aggregation, with complete recovery observed in 6-8 hours. The final pharmacodynamic model was based on the turnover of cyclo-oxygenase-1 (COX-1) enzyme, and incorporated irreversible inhibition by aspirin and reversible binding and antiplatelet effects of ibuprofen. The temporal response profiles from all three study arms were well described by the final model, and the parameters were estimated with good precision. The apparent turnover rate constant for COX-1 (kout) and the irreversible inhibition rate constant for aspirin (K) were estimated to be 0.0209 h(-1) and 0.152 (mg/L)(-1).h(-1), with interindividual variability of 30.6% and 26.2%, respectively. Simulations were used to evaluate the influence of clinically relevant ibuprofen regimens on the antiplatelet effect of aspirin, confirming clinical reports that the antiplatelet effect of aspirin would be blocked when multiple daily doses of ibuprofen are given, even if taken after aspirin administration. A mechanism-based pharmacodynamic model has been developed that characterizes the antiplatelet effects of aspirin and ibuprofen, alone and concomitantly, and predicts a significant inhibition of aspirin antiplatelet effects in the presence of a typical ibuprofen dosing regimen.

Aspirin Resistance: Definitions, Mechanisms, Prevalence, and Clinical Significance

Aspirin is the most commonly used therapeutic agent in prevention of vascular ischemic events. Aspirin exerts its antithrombotic effect primarily by interfering with the biosynthesis of thromboxane A2 (TXA2) and inhibition of TXA2 -dependent platelet aggregation. A meta-analysis of secondary prevention trials indicated that aspirin reduced major cardiovascular or cerebral events by 25%. This led to the widespread use of aspirin for prevention of cardiovascular events. However, it appears that aspirin antiplatelet effect is not uniform in all patients and previous studies estimated that 8-45% of the population were aspirin resistant. Furthermore, (i) the optimal dosage of aspirin for complete inhibition of platelet aggregation by physiological agonists (i.e arachidonic acid) is subject to great interindividual variability, (ii) the tests to detect aspirin resistance in vitro are subject to debate and (iii) the mechanisms by which some patients are resistant to aspirin in vitro remain to be determined. Despite these unresolved questions, recent clinical studies provide the reliable evidence that aspirin resistance correlates with confirmed clinical unresponsiveness, highlighting the clinical interest of determining the aspirin inhibitory effects on patients' platelets. In conclusion, discovery of aspirin resistance in individuals might be important in order to devise better anti-platelet strategies and improve our ability to prevent acute thrombotic complication.

Effects of BM-573, a thromboxane A 2 modulator on systemic hemodynamics perturbations induced by U-46619 in the pig

The aim of our study was to evaluate the effects of thromboxane A 2 (TXA 2) agonist, U-46619, on systemic circulatory parameters in the pigs before and after administration of a novel TXA 2 receptor antagonist and synthase inhibitor (BM-573). Twelve anesthetized pigs were randomly assigned in two groups: in Ago group ( n = 6), the animals received six consecutive injections of U-46619 at 30 min interval, while in Anta group ( n = 6) they received an increasing dosage regimen of BM-573 10 min before each U-46619 injection. The effects of each dose of BM-573 on ex vivo platelet aggregation induced by arachidonic acid, collagen or ADP were also evaluated. Vascular properties such as characteristic impedance, peripheral resistance, compliance, arterial elastance were estimated using a windkessel model. Intravenous injections of 0.500 mg/ml of BM-573 and higher doses resulted in a complete inhibition of platelet aggregation induced by arachidonic acid. In the same conditions, BM-573 completely blocked the increase of arterial elastance, and stabilized both mean aortic blood pressure and mean systemic blood flow. In conclusion, BM-573 could therefore be a promising therapeutic approach in pathophysiological states where TXA 2 plays a main role in the increase of vascular resistance like in pathologies such as systemic hypertension.

TMVA, a novel GPIb-binding protein, significantly prevents platelet microthrombi formation and prolongs discordant cardiac xenograft survival.

In xenotransplantation, donor endothelium is the first target of immunological attack. Activation of the endothelial cell by preformed natural antibodies leads to platelet binding via the interaction of the glycoprotein (GP) Ib and von Willebrand factor (vWF). TMVA is a novel GPIb-binding protein purified from the venom of Trimeresurus mucrosquamatus. In this study, the inhibitory effect of TMVA on platelet aggregation in rats and the effect on discordant guinea pig-to-rat cardiac xenograft survival were investigated. Three doses (8, 20 or 40 microg/kg) of TMVA were infused intravenously to 30 rats respectively. Platelet aggregation rate was assayed 0.5, 12, and 24 h after TMVA administration. Wister rats underwent guinea pig cardiac cervical heterotopic transplantation using single dosing of TMVA (20 microg/kg, i.v., 0.5 h before reperfusion). Additionally, levels of TXB(2) and 6-keto-PGF(1alpha) within rejected graft tissues were determined by radioimmunoassay. Treatment with TMVA at a dose of 20 or 40 microg/kg resulted in complete inhibition of platelet aggregation 0.5 h after TMVA administration. Rats receiving guinea pig cardiac xenografts after TMVA therapy had significantly prolonged xenograft survival. Histologic and immunopathologic analysis of cardiac xenografts in TMVA treatment group showed no intragraft platelet microthrombi formation and fibrin deposition. Additionally, the ratio of 6-keto-PGF(1alpha) to TXB(2) in TMVA treatment group was significantly higher than those in control group. We conclude that the use of this novel GPIb-binding protein was very effective in preventing platelet microthrombi formation and fibrin deposition in a guinea pig-to-rat model and resulted in prolongation of xenograft survival. The increased ratio of PGI(2)/TXA(2) in TMVA treatment group may protect xenografts from the endothelial cell activation and contribute to the prolongation of xenograft survival.

The role of platelet factor 4 in platelet aggregation induced by the antibodies implicated in heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is a severe side effect of heparin treatment. Recent studies using immunological methods demonstrated that antibodies contained in plasma, or in purified total immunoglobulin (Ig)G from patients suffering HIT, recognize as target antigen the complex heparin/platelet factor (PF4). In the present study, the role of PF4 in in-vitro platelet aggregation induced by purified total IgG or platelet-poor plasma from patients suffering HIT was investigated. In order to demonstrate the functional role of PF4, an anti-PF4 antibody that specifically blocked PF4 was used. In an experimental system composed of washed platelet suspension, incubation of F(ab')2 fragments (0.125 microg/ml) of the polyclonal anti-PF4 antibody resulted in complete inhibition of platelet aggregation triggered by purified total IgG from patients suffering HIT and heparin. In platelet-rich plasma, a significantly higher concentration (4.25 microg/ml) of the anti-PF4 F(ab')2 was required to inhibit platelet aggregation induced by HIT-PPP and heparin. Intermediate concentrations of the anti-PF4 antibody partially inhibited platelet aggregation. In plasma milieu, the concentration of PF4 was about five-fold higher in comparison with that measured in the purified system. The intensity of platelet aggregation depended on the concentration of HIT-IgG. Platelet aggregation was abolished in the presence of high concentrations of heparin (superior or equal to 10 IU/ml). The present study shows that PF4 is essential for platelet aggregation triggered by the antibodies related to HIT in the presence of heparin. The concentration of PF4 that is available to bind with heparin or with the HIT-related antibodies is critical for platelet aggregation induced by HIT antibodies.

A population pharmacokinetic-pharmacodynamic and logistic regression analysis of lotrafiban in patients

Our objective was to assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of lotrafiban, an oral glycoprotein IIb/IIIa inhibitor, in patients with a recent myocardial infarction, unstable angina, transient ischemic attack, or stroke. A 12-week, double-blind, multi-center, placebo-controlled, parallel-group, phase II study of lotrafiban (the Anti-platelet Useful Dose Study) was conducted in patients. Lotrafiban or placebo was administered as a twice daily oral dose at four dose levels (5-100 mg) for 12 weeks with daily doses of aspirin (300-325 mg). The pharmacokinetics of lotrafiban were characterized with the use of a population approach and were described by a two-compartment model with first order absorption and first order elimination. The pharmacodynamic data, ex vivo platelet aggregation, were described with the use of a direct effect inhibitory sigmoidal model with a baseline. The relationship between the severity of bleeding episodes and predicted steady-state lotrafiban exposure was characterized by logistic regression. Pharmacokinetic analysis showed that increasing age and decreasing creatinine clearance resulted in increased exposure to lotrafiban. The concentration-effect relationship was steep, with near complete inhibition of platelet aggregation at lotrafiban concentrations in excess of 20 ng/mL. Logistic regression showed that at exposures that exceeded approximately 835 ng. h/mL, the severity of adverse bleeding events increased considerably; this suggested that dosing recommendations should be generated to minimize the likelihood of patients having an area under the plasma concentration-time curve from 0 to 24 hours in excess of this value. Patients whose age exceeded 65 years or whose creatinine clearance was less than 60 mL/min should be given a lower dose of lotrafiban than younger patients with good renal function.

Evidence for Platelet αIIbβ3 Activation despite Elevated Cytosolic cAMP

Cyclic nucleotides, such as cAMP, are known to inhibit the multistep cascade that results in platelet aggregation. In the present study we provide evidence that it is possible to bypass cAMP inhibitory effect on fibrinogen binding site exposure induced by the thromboxane A2 synthetic analogue U46619, the snake venom toxin convulxin, or by the direct PKC activator OAG, by concomitantly activating a G1-coupled receptor by means of epinephrine or by inducing cytosolic calcium influx by means of ionomycin. In fact, in our study we demonstrate that, in iloprost-treated platelets, the inhibition of both platelet aggregation and fibrinogen binding was overcome by adding epinephrine or ionomycin. To further confirm this, we used the cAMP analogue dibutyryl cAMP and we obtained platelet aggregation in response to U46619, convulxin or OAG plus epinephrine. Moreover, a complete inhibition of platelet aggregation in the presence of high concentrations of cAMP was observed only in the case of U46619, while a small percentage of aggregation persisted when convulxin or OAG were used, due to the small amount of ADP that both convulxin and OAG are able to release. Since PKC inhibition didn't allow platelet aggregation to occur in response to the concomitant activation of U46619 or convulxin, plus epinephrine or ionomycin, we can conclude that cAMP-induced inhibition of aggregation can be counteracted by the simultaneous activation of PKC in the presence of an activated G1-coupled receptor or of an induced calcium influx.

Abciximab treatment in vitro after aspirin treatment in vivo has additive effects on platelet aggregation, ATP release, and P-selectin expression

To prevent arterial thrombosis, abciximab is administered together with aspirin. However, whether or not there are benefits to combine abciximab with aspirin is not yet well defined. Healthy volunteers were studied for the effect of aspirin+abciximab using sodium arachidonate and adenosine diphosphate (ADP) alone or in combination to induce platelet activation/aggregation. Abciximab produced complete inhibition of platelet aggregation induced with ADP but only 40% inhibition of aggregation induced by 0.75-mmol/l sodium arachidonate. Abciximab added in vitro to platelet-rich plasma (PRP) from platelets from aspirin-treated donors produced an almost complete inhibition of platelet aggregation. Aspirin, and abciximab alone, did not inhibit adenosine triphosphate (ATP) release as thoroughly as aspirin+abciximab did. Abciximab (3–5 μg/ml) produced inhibition of P-selectin expression induced with 5 (from 46.2±6.0% to 27.4±7.0%, P=.002) and 20-μmol/l ADP (from 53.1±8.1% to 35.1±11.0%, P=.019), but no effect was observed when 0.75-mmol/l sodium arachidonate was used ( P=.721). Aspirin diminished P-selectin expression in sodium arachidonate-stimulated platelets (from 77.7±11.8% to 40.2±3.6%, P<.0001) in non-aspirinated and platelets from aspirin-treated donors, respectively. Abciximab (3, 4, and 5 μg/ml) added to platelets from aspirin-treated donors decreased P-selectin expression in platelets stimulated with sodium arachidonate from 40.2±8.6% to 25.6±11.5% ( P=.027), to 20.5±3.5% ( P<.0001), and to 22.5±1.8% ( P<.0001). We concluded that the antiplatelet effect of abciximab is greatly increased by aspirin.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation.

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models. Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer. Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments. Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

Superiority of platelet integrin GPIIb–IIIa receptor antagonist over aspirin in preventing cyclic flow reductions in the guinea pig middle cerebral artery

We established a photothrombotic occlusion model in the guinea pig middle cerebral artery. In this model, the middle cerebral artery was recanalized within 10 to 20 min after thrombotic occlusion, with subsequent cyclic flow reductions. Cyclic flow reductions in the middle cerebral artery are expected to manage cerebral infarction by modulating arterial patency. Therefore, we evaluated the effect of several antiplatelet agents on the frequency of cyclic flow reductions and subsequent cerebral infarction using this model. A platelet integrin GPIIb–IIIa receptor antagonist, ME3277 (sodium hydrogen [4-[(4,5,6,7-tetrahydrothieno[3,2 c]pyridin-2-yl) carbonylamino] acetyl- o-phenylene] dioxydiacetate, 0.3, 1 and 3 mg/kg i.v.) dose-dependently inhibited the ex vivo platelet aggregation induced by ADP (5 μM), collagen (0.8 and 20 μg/ml) and arachidonic acid (100 μM). While the same doses of ME3277 reduced the frequency of the cyclic flow reductions and increased the total patency time of the middle cerebral artery, time to thrombotic occlusion was prolonged only at the highest dose, 3 mg/kg. ME3277 (0.3–3 mg/kg) significantly reduced the infarct volume and improved the neurological deficit at 24 h. In contrast, aspirin (30 mg/kg) did not affect these variables in spite of complete inhibition of platelet aggregation induced by arachidonic acid and collagen (0.8 μg/ml). A thromboxane A 2 synthetase inhibitor, sodium ozagrel, significantly increased the total patency time and reduced the infarct volume at 30 mg/kg. Inhibition of prostaglandin I 2 generation could explain the effectiveness of sodium ozagrel but not aspirin in this model. These results suggest that platelet integrin GPIIb–IIIa receptor antagonists are more beneficial than aspirin for the treatment of cerebral thrombosis.

YC-1 potentiates nitric oxide- and carbon monoxide-induced cyclic GMP effects in human platelets.

Nitric oxide (NO), the physiological activator of soluble guanylyl cyclase (sGC), induces inhibitory effects on platelet activation via elevation of cGMP levels and stimulation of the cGMP-dependent protein kinase. YC-1, a benzylindazole derivative, was shown to activate sGC in intact platelets, resulting in inhibition of platelet aggregation. In a previous study, we demonstrated that YC-1 not only stimulates purified sGC but also potentiates the stimulatory action of submaximally effective NO and carbon monoxide (CO) concentrations. Here, we investigated the potentiating effect of YC-1 in intact platelets. YC-1 together with NO or CO led to complete inhibition of platelet aggregation at concentrations that were ineffective by themselves. Maximally effective 2, 2-diethyl-1-nitroso-oxyhydrazine (3 microM) and YC-1 (100 microM) concentrations each elevated the cGMP levels in intact platelets approximately 13-fold, and administration of the two drugs together resulted in enormous potentiation of cGMP formation, which greatly exceeded the effect on the purified enzyme and yielded a >1300-fold increase in cGMP levels. Similar results were obtained using CO instead of NO. Furthermore, YC-1 not only stimulated sGC but also inhibited cGMP-hydrolyzing phosphodiesterases in platelets. The enormous elevation of cGMP levels led to enhanced phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein. Thus, by the combination of two effects (i.e., potentiation of NO-induced sGC stimulation and phosphodiesterase inhibition), YC-1-like substances are potent activators of the sGC/cGMP pathways and are therefore interesting candidates to act as modulators of cGMP-mediated effects, especially within the cardiovascular system.

SR 121787, a New Orally Active Fibrinogen Receptor Antagonist

The aim of this study was to describe the pharmacological properties of SR 121787, a new antiaggregating drug which is metabolized in vivo into SR 121566, a potent non-peptide antagonist of Gp IIb/IIIa. In vitro, SR 121566 antagonized the binding of [125I]-fibrinogen (IC50 = 19.8+/-6.3 nM) and of [125I]-L-692,884, an RGD-containing peptide (IC50 = 291+/-96 nM) to activated human platelets. SR 121566 inhibited the aggregation of human platelets induced by ADP, collagen, thrombin, arachidonic acid and PAF at concentrations lower than 0.1 microM. Adhesion of human platelets to adhesive proteins was inhibited by SR 121566 (IC50 = 40.3+/-2.5 nM) only when Gp IIb/IIIa and fibrinogen were involved. No effect was found with regard to other adhesive proteins and/or other integrins. SR 121787 demonstrated a potent and sustained antiaggregating effect when administered intravenously to baboons at a dose 50 microg/kg, and eight hours after the administration of 100 microg/kg, ADP-induced aggregation was still strongly inhibited (more than 80%). A single oral administration of 2 mg/kg of SR 121787 produced a nearly complete inhibition of platelet aggregation for up to 8 h (ED50 at 8 h = 193+/-20 microg/kg), a significant residual antiaggregating activity being still observed 24h after the administration. When administered orally to rabbits, SR 121787 exhibited a potent antiaggregating (ED50 = 2.3+/-0.3 mg/kg) and antithrombotic activity in an arterio-venous shunt thrombosis model (ED50 = 10.4+/-0.8 mg/kg). After oral and IV administration, SR 121787 was well tolerated suggesting that SR 121787, the most potent and long lasting orally active Gp IIb/IIIa antagonist described to date, is a promising antithrombotic compound.

TP-9201, a glycoprotein IIb/IIIa platelet receptor antagonist, prevents rethrombosis after successful arterial thrombolysis in the dog.

We examined the ability of TP-9201, a platelet glycoprotein IIb/IIIa receptor antagonist, to prevent carotid artery rethrombosis in the anesthetized dog. Occlusive thrombosis was induced by electrolytic injury of the left carotid artery. Thirty minutes later, 0.05 U/kg of anisoylated plasminogen streptokinase activator complex (APSAC) was infused locally to achieve clot lysis. Carotid artery recanalization was followed immediately by the infusion of either saline (10 mL/h, 240 minutes; n = 9), low-dose TP-9201 (120 micrograms/kg plus 3 micrograms.kg-1.min-1, 240 minutes; n = 7), or high-dose TP-9201 (185 micrograms/kg plus 5 micrograms.kg-1.min-1, 240 minutes; n = 7). Ex vivo platelet aggregation responses to ADP or arachidonic acid were determined. TP-9201 produced complete inhibition of platelet aggregation in citrated platelet-rich plasma but a partial and dose-dependent inhibition in heparinized platelet-rich plasma. A twofold and eightfold increase in the template bleeding time was associated with the infusion of low-dose and high-dose TP-9201, respectively. There were frequent cyclic flow reductions in both the saline and low-dose TP-9201-treated groups after thrombolysis. However, the high-dose TP-9201-treated group exhibited a sustained flow with minimal evidence of cyclic flow reductions. At the conclusion of the protocol, patent vessels were found more frequently in the high-dose TP-9201 (5/7; P = .0048) than in the low-dose TP-9201 treatment group (2/7; P = .17) when compared with the saline group (0/9). Infusion of high-dose TP-9201 was associated with a significant reduction in the thrombus mass as compared with the control vessels. Administration of TP-9201 in conjunction with successful thrombolysis inhibited ex vivo platelet aggregation and prevented rethrombosis of the canine carotid artery. This study demonstrates that TP-9201, an inhibitor of the platelet GPIIb/IIIa receptor, can inhibit platelet-vessel wall interaction and thus prevent rethrombosis.