Reversibility of platelet P2Y12 inhibition by platelet supplementation: ex vivo and in vitro comparisons of prasugrel, clopidogrel and ticagrelor

Essentials Successful outcome of platelet transfusion depends on specific antiplatelet therapy in use. We assessed if ticagrelor, clopidogrel or prasugrel impacts on donor platelet activity ex vivo. Ticagrelor and/or its active metabolite in plasma or bound to platelets can inhibit donor platelets. This might compromise the effectiveness of platelet transfusion therapy.

SUMMARY

Background Platelet transfusion is the conventional approach to restore platelet function during acute bleeds or surgery, but successful outcome depends on the specific antiplatelet therapy. Notably ticagrelor is associated with inadequate recovery of platelet function after platelet transfusion. We examined whether plasma and/or platelets from ticagrelor-treated patients influence donor platelet function, in comparison with clopidogrel and prasugrel. Methods Platelet transfusion was mimicked ex vivo by mixing naïve donor platelet-rich plasma (PRP) or gel-filtered platelets (GFP) in defined proportions with PRP, plasma or GFP from cardiovascular patients receiving standard care including medication with prasugrel, clopidogrel or ticagrelor (n = 20 each). Blood was taken 4 h after the previous dose. HLA2/HLA28 haplotyping let us distinguish net (all platelet) and individual patient/donor platelet reactivity in mixtures of patient/donor platelets, measured by flow cytometry analysis of ADP-induced fibrinogen binding and CD62P expression. Results ADP responsiveness of donor platelets was dramatically reduced by even low (10%) concentrations of PRP or plasma from ticagrelor-treated patients. Clopidogrel and prasugrel were associated with more modest donor platelet inhibition. GFP from ticagrelor-treated patients but not patients receiving clopidogrel or prasugrel also suppressed donor GFP function upon mixing, suggesting the transfer of ticagrelor from patient platelets to donor platelets. This transfer did not lead to recovery of ADP responsiveness of patient's platelets. Conclusion Collectively, these observations support the concept that ticagrelor and/or its active metabolite in plasma or bound to platelets can inhibit donor platelets, which might compromise the effectiveness of platelet transfusion therapy.

Induction of Diabetes Abolishes the Antithrombotic Effect of Clopidogrel in Apolipoprotein E-Deficient Mice

Patients with acute coronary syndrome with diabetes mellitus (DM) exhibit an impaired platelet inhibitory response to clopidogrel which is only partially understood. DM was induced by the administration of streptozotocin (STZ) to 9-week-old mice. The antithrombotic effects of clopidogrel (10 mg/kg/d, orally × 5 days) were determined using a FeCl ₃ -induced thrombosis model employing wild-type (WT), apolipoprotein E (apoE)-deficient, and diabetic apoE-deficient mice at 21 weeks. Antiplatelet effects were determined using flow cytometry. The antithrombotic effects of clopidogrel were similar in WT and apoE-deficient mice but were attenuated in diabetic apoE-deficient mice with the percent inhibition of thrombus area (µm ² ) by clopidogrel being 85.5% (WT mice), 75.0% (apoE-deficient mice), and 1.9% (diabetic apoE-deficient mice). The time to first occlusion and lumen stenosis also reflected a significant loss of the antithrombotic effects of clopidogrel in diabetic apoE-deficient mice. Ex vivo platelet activation, which was assessed using ADP-induced expression of activated glycoprotein IIb/IIIa, was completely inhibited by clopidogrel in these three groups of mice. In contrast, the effect of clopidogrel on the ex vivo expression of platelet P-selectin induced by protease-activated receptor 4–activating peptide was diminished in diabetic apoE-deficient mice compared with that in WT and apoE-deficient mice. These data suggest that diabetic apoE-deficient mice may serve as a useful model to better understand the impaired responses to clopidogrel in patients with DM, which may partially reflect a reduction of the effect of clopidogrel on thrombin-induced platelet activation.

Incomplete reversibility of platelet inhibition following prolonged exposure to ticagrelor

Essentials Irreversible platelet inhibition persists after reversibly-binding ticagrelor is discontinued. Reversibility of platelet inhibition by ticagrelor and its active metabolite was assessed. Incomplete recovery was observed after prolonged exposure to ticagrelor. Activated GPIIb-IIIa and P-selectin, not platelet reactivity index, showed irreversibility.

SUMMARY

Introduction Ticagrelor is described as a reversible P2Y12 antagonist. However, residual platelet inhibition persists after discontinuation of ticagrelor when plasma levels are undetectable. We assessed the reversibility of platelet inhibition by ticagrelor and its active metabolite (T-AM) in comparison with cangrelor and prasugrel's active metabolite (P-AM). Methods Whole blood was treated in vitro with ~ 50% inhibitory concentrations of ticagrelor, T-AM, cangrelor, P-AM and assessed for ADP-stimulated activated GPIIb-IIIa and P-selectin and vasodilator-stimulated phosphoprotein (VASP) platelet reactivity index (PRI) before and after 100-fold dilution. Results Platelets exposed for 30 min to ticagrelor, T-AM or cangrelor showed full recovery of activated GPIIb-IIIa but only partial recovery of P-selectin. Longer exposure (24 h) to the drug decreased reversibility of activated GPIIb-IIIa by ticagrelor (65.1% [49.5-80.6], % of vehicle with 95% confidence interval [CI]) and T-AM (88.8% [79.2-98.3]), but not by cangrelor (101.4% [96.4-106.4]). Compared with 30 min exposure, the reversibility of P-selectin further decreased after 24 h exposure to ticagrelor (from 91.8% [82.1-101.5] to 51.8% [45.5-85.0]), but not T-AM (from 79.0% [67.8-90.3] to 77.4% [61.8-93.1]) or cangrelor (from 76.0% [67.6-84.4] to 76.2% [70.6-81.8]). In contrast, 24 h exposure to ticagrelor, T-AM and cangrelor resulted in full recovery of platelet reactivity as measured by PRI. Platelets exposed to P-AM showed no recovery of ADP reactivity. Conclusions Incomplete recovery after prolonged exposure to ticagrelor, observed by activated GPIIb-IIIa and P-selectin but not upstream VASP signaling, suggests that P2Y12 regains functionality and irreversible changes occur independent of VASP signaling.

Detecting a thienopyridine effect by platelet reactivity assessment and its implications for risk stratification

BACKGROUND

On-treatment platelet reactivity (OTR) is a predictor of clinical outcomes in patients receiving thienopyridine therapy.

OBJECTIVE

To assess whether point-of-care platelet reactivity testing can discriminate between patients who have and have not received a thienopyridine.

PATIENTS/METHODS

This was an analysis of a randomized, multicenter, pharmacodynamic trial. Subjects with coronary artery disease treated with aspirin were randomly assigned to clopidogrel 75 mg daily or prasugrel 10 mg daily for 7 days. Platelet reactivity assessment with the VerifyNow P2Y12 test was performed before study drug admistration and 24 h after the final dose. Optimal cut-offs for a detectable drug effect were identified by the use of receiver operating characteristic curve analysis.

RESULTS

A total of 54 subjects were enrolled and completed the study. The c-statistic for the identification of a thienopyridine effect was highly significant (0.93, P < 0.001), including for the clopidogrel and prasugrel groups considered separately (P < 0.001 for both). The optimal cut-off was < 213 P2Y12 reaction units (PRU), which provided a sensitivity of 80% and a specificity of 98%. This cut-off provided a sensitivity of 58% and a specificity of 100% for a clopidogrel effect, and a sensitivity of 100% and specificity of 96% for a prasugrel effect.

CONCLUSIONS

OTR of < 213 PRU is highly specific for exposure to either clopidogrel or prasugrel. This may be useful in the management of thienoypridine-treated patients who require surgery. Furthermore, this diagnostic cut-off is similar to levels of OTR that have been associated with ischemic events in thienopyridine-treated patients, supporting the contention that a lack of drug effect is the mechanistic basis for the prognostic relationship between OTR and clinical outcomes.

Platelet function normalization after a prasugrel loading-dose: time-dependent effect of platelet supplementation

BACKGROUND

Hemostatic benefits of platelet transfusions in thienopyridine-treated acute coronary syndrome (ACS) patients may be compromised by residual metabolite in circulation.

OBJECTIVES

To estimate the earliest time after a prasugrel loading-dose when added platelets are no longer inhibited by prasugrel's active metabolite.

METHODS

Baseline platelet reactivity of healthy subjects (n=25, 30 ± 5 years, 68% male) on ASA 325 mg was tested using maximum platelet aggregation (MPA, ADP 20 μm) and VerifyNow(®) P2Y12 and was followed by a 60 mg prasugrel loading-dose. At 2, 6, 12 and 24 h post-dose, fresh concentrated platelets from untreated donors were added ex-vivo to subjects' blood, raising platelet counts by 0% (control), 40%, 60% and 80%. To estimate the earliest time when prasugrel's active metabolite's inhibitory effect on the added platelets ceases, platelet function in supplemented samples was compared across time-points to identify the time when effect of supplementation on platelet function stabilized (i.e. the increase in platelet reactivity was statistically similar to that at the next time-point).

RESULTS

Supplemented samples showed concentration-dependent increases in platelet reactivity vs. respective controls by both MPA and VerifyNow(®) at all assessment time-points. For each supplementation level, platelet reactivity showed a sharp increase from 2 to 6 h but was stable (P=NS) between 6 and 12 h.

CONCLUSIONS

The earliest measured time when supplemented platelets were not inhibited by circulating active metabolite of prasugrel was 6 h after a prasugrel loading-dose. These findings may have important implications for prasugrel-treated ACS patients requiring platelet transfusions during surgery.

In the presence of strong P2Y12 receptor blockade, aspirin provides little additional inhibition of platelet aggregation

BACKGROUND

Aspirin and antagonists of platelet ADP P2Y(12) receptors are often coprescribed for protection against thrombotic events. However, blockade of platelet P2Y(12) receptors can inhibit thromboxane A(2) (TXA(2))-dependent pathways of platelet activation independently of aspirin.

OBJECTIVES

To assess in vitro whether aspirin adds additional antiaggregatory effects to strong P2Y(12) receptor blockade.

METHODS

With the use of platelet-rich plasma from healthy volunteers, determinations were made in 96-well plates of platelet aggregation, TXA(2) production and ADP/ATP release caused by ADP, arachidonic acid, collagen, epinephrine, TRAP-6 amide and U46619 (six concentrations of each) in the presence of prasugrel active metabolite (PAM; 0.1-10 μmol L(-1)), aspirin (30 μmol L(-1)), PAM + aspirin or vehicle. results: PAM concentration-dependently inhibited aggregation; for example, aggregation in response to all concentrations of ADP and U46619 was inhibited by ≥ 95% by PAM at > 3 μmol L(-1) . In further tests of PAM (3 μmol L(-1)), aspirin (30 μmol L(-1)) and PAM + aspirin, aspirin generally failed to produce more inhibition than PAM or additional inhibition to that caused by PAM. The antiaggregatory effects of PAM were associated with reductions in the platelet release of both TXA(2) and ATP + ADP. Similar effects were found when either citrate or lepirudin were used as anticoagulants, and when traditional light transmission aggregometry was conducted at low stirring speeds.

CONCLUSIONS

P2Y(12) receptors are critical to the generation of irreversible aggregation through the TXA(2) -dependent pathway. As a result, strong P2Y(12) receptor blockade alone causes inhibition of platelet aggregation that is little enhanced by aspirin. The clinical relevance of these observations remains to be determined.

Potentiation of clopidogrel active metabolite formation by rifampicin leads to greater P2Y12 receptor blockade and inhibition of platelet aggregation after clopidogrel

BACKGROUND

 The thienopyridine P2Y(12) receptor antagonist clopidogrel reduces the risk of arterial thrombosis and individual pharmacodynamic responses to clopidogrel are believed to reflect the levels of active metabolite (AM) generated. Rifampicin increases the inhibitory effect of clopidogrel on platelet aggregation (PA). We studied the response to clopidogrel before and during administration of rifampicin in order to study the relationship between individual AM levels and P2Y(12) blockade.

METHODS

 Healthy volunteers received a 600-mg loading dose of clopidogrel followed by 75 mg daily for 7 days and, after a washout period and treatment with rifampicin [300 mg twice a day (b.i.d.)], received the same regimen of clopidogrel. Clopidogrel AM levels were determined over 4 h after the clopidogrel loading dose and unblocked P2Y(12) receptor number was assessed using a (33) P-2MeSADP binding assay. PA was measured by optical aggregometry with ADP and TRAP.

RESULTS

 Rifampicin enhanced clopidogrel AM production [area-under-the-curve (AUC): clopidogrel 89±22 ng h mL(-1) , clopidogrel+rifampicin 335±86 ng h mL(-1) , P<0.0001], and P2Y(12) blockade (unblocked receptors: clopidogrel 48±24, clopidogrel+rifampicin 4±2, P<0.0001) and reduced PA (5 μmol L(-1) ADP: clopidogrel 20±4, clopidogrel+rifampicin 5±2, P<0.01). Increasing numbers of unblocked receptors were required for an aggregation response with a decreasing concentration of ADP. PA induced by ADP 2 μmol L(-1) was particularly sensitive to low levels of receptor blockade.

CONCLUSION

 Potentiation of clopidogrel AM production by rifampicin leads to greater P2Y(12) blockade and consequently greater inhibition of PA. PA responses to low concentrations of ADP are more sensitive to P2Y(12) blockade.

Effect of rifampin on the pharmacokinetics and pharmacodynamics of prasugrel in healthy male subjects

OBJECTIVE

Prasugrel is a thienopyridine antiplatelet agent for the prevention of atherothrombotic events in patients with acute coronary syndrome undergoing percutaneous coronary intervention. Since cytochrome P450 enzymes CYP3A4 and CYP2B6 play a major role in prasugrel's active metabolite formation, the effect of potent CYP induction by rifampin on the pharmacokinetics of prasugrel and on the pharmacodynamic response to prasugrel was evaluated in healthy male subjects.

RESEARCH DESIGN AND METHODS

This was an open-label, two-period, fixed-sequence study conducted at a single clinical research center. In the first treatment period, subjects received prasugrel as an oral 60-mg loading dose (LD) on the first day followed by ten oral, 10-mg daily maintenance doses. After a 2-week washout period, subjects received oral rifampin alone (600 mg once daily) for 8 days, followed by coadministration of oral rifampin with prasugrel, given as a 60-mg LD on the first day followed by five daily 10-mg MDs. Blood collection for pharmacokinetic and pharmacodynamic analyses occurred after the LD and fifth MD of prasugrel in both periods. CLINICAL TRIAL SYNOPSIS: clinicalstudyresults.org ID #8976

RESULTS

Rifampin coadministration (600 mg daily) did not affect exposure to prasugrel's active metabolite (R-138727). However, at 2 and 4 h after the prasugrel loading dose (60 mg), rifampicin coadministration was associated with a 6-9 percentage point decrease (p < 0.01) in the magnitude of platelet inhibition; similarly, a 5-17 percentage point decrease (p < 0.05) was observed with rifampin coadministration during the prasugrel maintenance dose (10 mg) period. Post hoc in vitro experiments demonstrated a dose-dependent R-138727-rifampin interaction at the P2Y(12) level unrelated to enzyme induction. A limitation of this study is that while results of the in vitro post hoc study indicate a pharmacodynamic interaction with rifampin, the mechanism underlying this interaction has not been elucidated.

CONCLUSIONS

Dose adjustment should not be necessary when prasugrel is administered with CYP inducers since formation of prasugrel's active metabolite is not affected by potent enzyme induction with rifampin.

Interaction of the active metabolite of prasugrel, R-138727, with cysteine 97 and cysteine 175 of the human P2Y12 receptor

BACKGROUND

The P2Y(12) receptor plays a crucial role in platelet aggregation and is the target of platelet aggregation inhibitors, including the thienopyridine compound prasugrel.

OBJECTIVE

The present study analyzed the effects of R-138727 (2-[1-[2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene]acetic acid), the active metabolite of prasugrel, on recombinant wild-type and mutant human P2Y(12) receptors in order to identify the molecular site of action of R-138727.

METHODS

The function of wild-type and mutant P2Y(12) receptors stably expressed in Chinese hamster ovary cells was assessed by measuring the 2-methylthio-ADP-mediated inhibition of forskolin-stimulated cellular cAMP production.

RESULTS

In cells expressing wild-type receptors, R-138727 potently inhibited receptor function with a half-maximal concentration below 1 microm. The mode of action was irreversible. The same effect of R-138727 was observed in cells expressing Cys17Ala/Cys270Ala constructs. In contrast, in cells expressing either a Cys97Ala construct or a Cys175Ala construct, R-138727 failed to inhibit the response to the agonist. When cells expressing wild-type receptors were pretreated with the P2 receptor antagonists ATP or suramin, no effect of R-138727 was observed. Similar experiments with N-acetylcysteine 10 microm showed no interference of N-acetylcysteine with R-138727.

CONCLUSIONS

The experiments demonstrate a potent and irreversible action of R-138727 at the recombinant human P2Y(12) receptor. The data suggest that R-138727 interacts with cysteine 97 (upper portion of the predicted third transmembrane region) and cysteine 175 (second extracellular loop) of the receptor, which are likely to form a disulfide bridge in native receptors. Moreover, the data also suggest that this site of action of R-138727 is close to the ligand-binding site of the receptor.

The reversible P2Y antagonist cangrelor influences the ability of the active metabolites of clopidogrel and prasugrel to produce irreversible inhibition of platelet function

BACKGROUND

Agents that act as antagonists at P2Y(12) ADP receptors on platelets are in use (clopidogrel), and in development for use (cangrelor and prasugrel), in patients with cardiovascular disease. Cangrelor is a direct-acting reversible antagonist being developed for short-term infusion; clopidogrel and prasugrel are oral prodrugs that provide irreversible inhibition via transient formation of active metabolites. At the cessation of cangrelor infusion, patients are likely to receive clopidogrel or prasugrel as a means of maintaining antiplatelet therapy.

OBJECTIVES

To apply an experimental in vitro approach to investigate the possibility that cangrelor influences the ability of the active metabolites of clopidogrel and prasugrel to inhibit ADP-mediated platelet function.

METHODS

The effects of cangrelor and the active metabolites of clopidogrel (C-AM) and prasugrel (P-AM) on platelet function were assessed by ADP-induced platelet P-selectin expression in whole blood. The method involved rapid removal of the antagonists by dilution, and measurement of residual platelet inhibition.

RESULTS

Cangrelor, C-AM and P-AM markedly inhibited P-selectin expression. The effect of cangrelor, but not of C-AM and P-AM, was reversible following antagonist removal. Preincubation of blood with cangrelor prior to addition of C-AM or P-AM reduced the ability of metabolites to irreversibly antagonize P2Y(12). Irreversible inhibition was maintained when blood was preincubated with metabolites prior to cangrelor.

CONCLUSIONS

Cangrelor influences the ability of the active metabolites of clopidogrel or prasugrel to inhibit platelet function irreversibly. Careful consideration should be given to the timing of administration of an oral P2Y(12) antagonist following cangrelor infusion.

The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

The greater in vivo antiplatelet effects of prasugrel as compared to clopidogrel reflect more efficient generation of its active metabolite with similar antiplatelet activity to that of clopidogrel's active metabolite

BACKGROUND AND METHODS

Prasugrel is a novel orally active thienopyridine prodrug with potent and long-lasting antiplatelet effects. Platelet inhibition reflects inhibition of P2Y(12) receptors by its active metabolite (AM). Previous studies have shown that the antiplatelet potency of prasugrel is at least 10 times higher than that of clopidogrel in rats and humans, but the mechanism of its higher potency has not yet been fully elucidated.

RESULTS

Oral administration of prasugrel to rats resulted in dose-related and time-related inhibition of ex vivo platelet aggregation, and its effect was about 10 times more potent than that of clopidogrel. The plasma concentration of prasugrel AM was higher than that of clopidogrel AM despite tenfold higher doses of clopidogrel, indicating more efficient in vivo production of prasugrel AM than of clopidogrel AM. In rat platelets, prasugrel AM inhibited in vitro platelet aggregation induced by adenosine 5'-diphosphate (ADP) (10 microm) with an IC(50) value of 1.8 microm. Clopidogrel AM similarly inhibited platelet aggregation with an IC(50) value of 2.4 microm. Similar results were also observed for ADP-induced (10 microm) decreases in prostaglandin E(1)-stimulated rat platelet cAMP levels. These results indicate that both AMs have similar in vitro antiplatelet activities.

CONCLUSIONS

The greater in vivo antiplatelet potency of prasugrel as compared to clopidogrel reflects more efficient in vivo generation of its AM, which demonstrates similar in vitro activity to clopidogrel AM.

Cytochrome P450 3A inhibition by ketoconazole affects prasugrel and clopidogrel pharmacokinetics and pharmacodynamics differently

Prasugrel and clopidogrel inhibit platelet aggregation through active metabolite formation. Prasugrel's active metabolite (R-138727) is formed primarily by cytochrome P450 (CYP) 3A and CYP2B6, with roles for CYP2C9 and CYP2C19. Clopidogrel's activation involves two sequential steps by CYP3A, CYP1A2, CYP2C9, CYP2C19, and/or CYP2B6. In a randomized crossover study, healthy subjects received a loading dose (LD) of prasugrel (60 mg) or clopidogrel (300 mg), followed by five daily maintenance doses (MDs) (15 and 75 mg, respectively) with or without the potent CYP3A inhibitor ketoconazole (400 mg/day). Subjects had a 2-week washout between periods. Ketoconazole decreased R-138727 and clopidogrel active metabolite Cmax (maximum plasma concentration) 34-61% after prasugrel and clopidogrel dosing. Ketoconazole did not affect R-138727 exposure or prasugrel's inhibition of platelet aggregation (IPA). Ketoconazole decreased clopidogrel's active metabolite AUC0-24 (area under the concentration-time curve to 24 h postdose) 22% (LD) to 29% (MD) and reduced IPA 28% (LD) to 33% (MD). We conclude that CYP3A4 and CYP3A5 inhibition by ketoconazole affects formation of clopidogrel's but not prasugrel's active metabolite. The decreased formation of clopidogrel's active metabolite is associated with reduced IPA.

Fused bicyclic Gly-Asp beta-turn mimics with potent affinity for GPIIb-IIIa. Exploration of the arginine isostere

6-[4-Amidinobenzoyl]amino]-tetralone-2-acetic acid is a potent antagonist of GPIIb-IIIa. Substitution in the meta position of the benzamidine, or replacement with a heteroaryl amidine was tolerated in this series. Use of an acyl-linked 4-alkyl piperidine as an arginine isostere also provided active compounds. Compounds from this series provided substantial systemic exposure in the rat following oral administration.

Development of dual-acting agents for thromboxane receptor antagonism and thromboxane synthase inhibition. 3. Synthesis and biological activities of oxazolecarboxamide-substituted omega-phenyl-omega-(3-pyridyl)alkenoic acid derivatives and related compounds

A novel series of oxazolecarboxamide-substituted omega-phenyl-omega-(3-pyridyl)alkenoic acid derivatives was discovered as potent dual-acting agents to block the TXA2 receptor and to inhibit the thromboxane synthase (TRA/TSI). Synthesis, structure-activity relationship (SAR), and in vitro and in vivo pharmacology of this series of compounds are described. Modification of the series revolved around the oxazole moiety to increase the hydrophilicity of the compounds and to correlate the biological activity with lipophilicity of the compounds. The most potent in the series was (E)-7-[4-[4-[[(4-cyclohexylbutyl)amino]carbonyl]-2-oxazolyl] phenyl]-7 -(3-pyridyl)hept-6-enoic acid (14) with Kd = 9.9 +/- 0.4 nM for the thromboxane receptor antagonism and IC50 = 55.0 +/- 17.9 nM for thromboxane synthase inhibition. The compound 14 was a selective TRA/TSI which exhibited desirable characteristics for oral activity, "shunt" effect to elevate PGI2 level, and absence of agonist activity.

Quantitative detection of platelet GPIIb-IIIa receptor antagonist activity using a flow cytometric method

Platelet-membrane surface receptors are important targets for pharmacologic intervention in cardiovascular disease. Among these, glycoprotein (GP) IIb-IIIa is dominant and integrally involved in platelet aggregation and thrombus formation. When activated, GPIIb-IIIa binds soluble fibrinogen (Fb) in a key, early step of this process. New drugs are under development that block Fb binding to GPIIb-IIIa and inhibit platelet aggregation. A thorough understanding of the relationship between circulating drug levels and the extent of GPIIb-IIIa receptor occupancy in humans is crucial for safe and efficacious use of these agents. Described here is the development of a new technique for measurement of GPIIb-IIIa receptor occupancy. In this assay, activated human platelets are incubated with biotinylated fibrinogen (Fb-biotin) followed by antibiotin-FITC.The extent of Fb binding is determined using flow cytometric analysis. Our results indicate that Fb-biotin binds rapidly to activated platelets and its detection is dependent on incubation temperature. Platelets that were pre-incubated with the GPIIb-IIIa antagonist echistatin were inhibited from binding Fb-biotin in a concentration-dependent manner. The fluorescence of processed samples was stable for two weeks in the cold. The assay described here is simple, cost effective, and can be adapted for use in clinical evaluations of these new drugs.

Effects of beta3-integrin blockade (c7E3) on the response to angioplasty and intra-arterial stenting in atherosclerotic nonhuman primates

Because the beta3-antagonist abciximab (c7E3 Fab) has significantly improved late outcomes after coronary angioplasty, the beta3 integrins have been implicated in the arterial response to injury. However, the mechanisms underlying this benefit are unknown. The observation that c7E3 binds beta3 integrins on vascular cells (alphavbeta3) with affinity equal to that for the platelet glycoprotein IIb/IIIa integrin has led to the hypothesis that c7E3 may act directly on the artery wall to prevent restenosis after angioplasty. To test this hypothesis, we studied the effects of c7E3 on structural changes within the artery wall after angioplasty or stent angioplasty in 23 male cynomolgus monkeys with established atherosclerosis. Animals were randomly assigned to receive either a bolus of c7E3 (0.4 mg/kg IV, n=11) followed by a 48-hour infusion (0. 2 microg. kg-1. min-1) or an equal volume of vehicle (n=12). Animals received weight-adjusted aspirin and heparin and then underwent unilateral iliac artery experimental angioplasty and subclavian artery stent angioplasty (Palmaz). Iliac artery lumen diameter (LD) was determined by angiography at baseline (LDPre), after angioplasty (LDPost), and 35 days later (LDDay35). Arteries were then fixed by perfusion and removed for analysis. Lumen, intima, media, and external elastic lamina (EEL) areas were measured in iliac artery cross sections. Values from each injured iliac artery were normalized to the contralateral uninjured iliac artery to control for interanimal variability in baseline artery size and atherosclerosis extent. Intimal area was also measured in subclavian stent cross sections. c7E3 blocked platelet aggregation and prolonged the bleeding time from 2.8+/-1.1 to 19.8+/-2.5 minutes, P<0.001. Experimental angioplasty increased LDPost an average of 28%, and the initial gain was similar in both groups (P=NS). Despite an anti-platelet effect, c7E3 did not inhibit iliac lumen narrowing (LDDay35-LDPost: c7E3, -0.69+/-0.17 versus vehicle, -0.99+/-.17 mm, P=0.35); intimal hyperplasia (neointima area: c7E3, 1.12+/-.28 versus vehicle, 1.22+/-.20 mm2, P=0.77); or decrease in artery wall size (EEL area [percent of uninjured control]: c7E3, 101+/-7% versus vehicle, 121+/-7%). Stent intimal hyperplasia was also unaltered by c7E3 treatment (neointimal area: c7E3, 1.09+/-0.16 versus vehicle, 1. 28+/-0.11 mm2, P=0.36). These results suggest that the benefits of c7E3 treatment in coronary angioplasty were not from inhibition of intimal hyperplasia or improved artery wall remodeling. Alternative mechanisms should be explored to explain improved late outcomes after angioplasty in patients treated with c7E3.

Development of dual-acting agents for thromboxane receptor antagonism and thromboxane synthase inhibition. 2. Design, synthesis, and evaluation of a novel series of phenyl oxazole derivatives

Synthesis and initial in vitro evaluation of a novel series of phenyl oxazole derivatives are described. An SAR study of the novel dual-acting TRA/TSI agent has revealed that the lipophilicity of the oxazole amide substituents greatly influences the TRA activity but not the TSI. The chain length of the alkenoic acid side chain affects both TRA and TSI. The optimal chain length for the combined activities was found to be n = 4 (heptenoic acid).

Intravenous administration of the glycoprotein IIb-IIIa receptor antagonist 7E3 induces reperfusion of an acute thrombotic occlusion of the canine coronary artery

The ability of the F(ab')2 fragment of the murine monoclonal antibody 7E3 directed against the platelet glycoprotein IIb-IIIa receptor complex, to cause reperfusion of a totally occluding coronary artery thrombus was examined alone and in combination with aspirin and heparin in a canine model of coronary artery thrombosis. A localized thrombus was produced in the left circumflex coronary artery in open-chest dogs by electrolytic injury of the endothelium. Intravenous administration of a single injection of 5.0 mg/kg aspirin and heparin (80 U/kg bolus plus 30 U/kg/hr x 2 hr) maintained vessel patency for approximately 101 +/- 15 minutes. After vessels had been completely occluded for 5 minutes (in the presence of aspirin + heparin), a single intravenous injection of saline (10 ml) or 0.8 mg/kg 7E3 was administered. Reperfusion was observed in all dogs (6 of 6) receiving 7E3; 4 of 6 dogs maintained vessel patency throughout the course of the 2 hour observation period. Activated partial thromboplastin and thrombin times were elevated 1.4 and 9 fold, respectively, in groups that received heparin. Template bleeding times were significantly elevated in the groups receiving 7E3. In the control group, 2 of 5 dogs reperfused briefly, however neither were patent at the end of the observation period. A third group of 4 dogs which did not receive the aspirin + heparin regimen was allowed to occlude and 5 minutes later received a single intravenous injection of 0.8 mg/kg 7E3. None of the 4 dogs in this group reperfused at any time during the study. There were no significant differences between groups in regards to hematological or hemodynamic measurements during the experiment. We concluded from these findings that the monoclonal antibody, 7E3 can promote the dissolution of friable coronary artery thrombi that evolve during standard anticoagulant and antiplatelet therapy.

Fibrinogen receptor (GPIIb-IIIa) antagonists derived from 5,6-bicyclic templates. Amidinoindoles, amidinoindazoles, and amidinobenzofurans containing the N-alpha-sulfonamide carboxylic acid function as potent platelet aggregation inhibitors

A series of highly potent and specific fibrinogen receptor antagonists have been discovered and optimized through structural modification of the novel amidinoindole and benzofuran compounds, I and II. Systematic linker optimization afforded the amidinobenzofuran-containing inhibitor 29, which displayed an IC50 value of 250 nM in platelet aggregation assays. Attempts to enhance activity by modification of the beta-position of the beta-alanyl carboxylate group of 29 had only a modest effect on inhibitory activity in aggregation assays. Analogues prepared to enhance the activity by conformational restriction were also found to be equally or less potent. In contrast, modification at the alpha-position of the beta-alanyl carboxylate group resulted in the identification of extremely potent and novel amidinobenzofuran-containing derivatives 46-49. Reexamination of 5,6-bicyclic aromatic nucleus led to the further identification of amidinoindole- and amidinoindazole-containing derivatives 53-55. These analogues, 46-49 and 53-55, exhibited potent in vitro activity with IC50 values of 25-65 nM in platelet aggregation assays and an IC50 value of 2 nM in fibrinogen binding assays and demonstrated a selectivity of > 50,000-fold for GPIIb-IIIa versus the most closely related integrin, the vitronectin receptor, alpha v beta 3.

Human group II 14 kDa phospholipase A2 activates human platelets

Recombinant human group II phospholipase A2 (sPLA2) added to human platelets in the low microg/ml range induced platelet activation, as demonstrated by measurement of platelet aggregation, thromboxane A2 generation and influx of intracellular free Ca2+ concentration and by detection of time-dependent tyrosine phosphorylation of platelet proteins. The presence of Ca2+ at low millimolar concentrations is a prerequisite for the activation of platelets by sPLA2. Mg2+ cannot replace Ca2+. Mg2+, given in addition to the necessary Ca2+, inhibits sPLA2-induced platelet activation. Pre-exposure to sPLA2 completely blocked the aggregating effect of a second dose of sPLA2. Albumin or indomethacin inhibited sPLA2-induced aggregation, similarly to the inhibition of arachidonic acid-induced aggregation. Platelets pre-treated with heparitinase or phosphatidylinositol-specific phospholipase C lost their ability to aggregate in response to sPLA2, although they still responded to other agonists. This suggests that a glycophosphatidylinositol-anchored platelet-membrane heparan sulphate proteoglycan is the binding site for sPLA2 on platelets. Previous reports have stated that sPLA2 is unable to activate platelets. The inhibitory effect of albumin and Mg2+, frequently used in aggregation studies, and the fact that isolated platelets lose their responsiveness to sPLA2 relatively quickly, may explain why the platelet-activating effects of sPLA2 have not been reported earlier.

Effect of Ca2+ on GP IIb-IIIa interactions with integrilin: enhanced GP IIb-IIIa binding and inhibition of platelet aggregation by reductions in the concentration of ionized calcium in plasma anticoagulated with citrate

BACKGROUND

Integrilin (eptifibatide), a potent inhibitor of the fibrinogen binding function of GP IIb-llla, has been shown to reduce the thrombotic complications of angioplasty and of acute coronary syndromes. The present study was designed to determine whether the reduced Ca2+ concentrations in plasma anticoagulated with citrate affect Integrilin binding to GP IIb-IIIa and the ex vivo pharmacodynamic measurements for this drug.

METHODS AND RESULTS

Lower concentrations of Integrilin were found to inhibit platelet aggregation in plasma anticoagulated with citrate (for ADP, mean+/-SD IC(50)=140+/-40 nmol/L, n=6; Ca2+ =40 to 50 micromol/L) than with PPACK (IC(50)=570+/-70 nmol/L, P<.0001, n=6; Ca2+ approximately 1 mmol/L). Chelation of Ca2+ with EDTA or citrate caused a similar degree of enhancement in the inhibitory activity of Integrilin. Measurements of D3 LIBS epitope expression showed that the enhanced inhibitory activity was caused by enhanced GP IIb-IIIa occupancy by Integrilin. Citrate anticoagulation decreased the amounts of Integrilin required to inhibit the binding of PAC1, a monoclonal antibody that mimics the GP IIb-IIIa binding activity of fibrinogen. Reduced Ca2+ also increased Integrilin inhibition of the binding of biotinylated fibrinogen to purified, immobilized GP IIb-IIIa.

CONCLUSIONS

These data suggest that citrate anticoagulation removes Ca2+ from GP IIb-IIIa and enhances the apparent inhibitory activity of Integrilin. This finding indicates that the inhibitory activity of Integrilin is overestimated in blood samples collected with citrate, suggesting that it may be possible to achieve greater antithrombotic efficacy beyond that observed in clinical trials to date with Integrilin.

Use of conformationally restricted benzamidines as arginine surrogates in the design of platelet GPIIb-IIIa receptor antagonists

The use of 5,6-bicyclic amidines as arginine surrogates in the design of a novel class of potent platelet glycoprotein IIb-IIIa receptor (GPIIb-IIIa) antagonists is described. The additional conformational restriction offered by the bicyclic nucleus results in 20-400-fold increases in potency compared to the freely flexible, acyclic benzamidine counterpart. The design, synthesis, structure-activity relationships (SAR), and in vitro activity of this novel class of GPIIb-IIIa antagonists are presented.

Non-peptide RGD surrogates which mimic a Gly-Asp beta-turn: potent antagonists of platelet glycoprotein IIb-IIIa

Cyclic heptapeptide 1, which contains an Arg-Gly-Asp sequence, has good affinity for the platelet receptor GPIIb-IIIa and was chosen for study by 1H NMR techniques. The key RGD sequence of this molecule was found to reside in a conformationally defined type II' Gly-Asp beta-turn, and this information was used in the design of simple non-peptide RGD mimics. Disubstituted isoquinolones, bearing an acidic side chain at position 2 and a basic side chain at position 6, were prepared and were found to have modest affinity for GPIIb-IIIa. Systematic modification of the basic residue contained in these molecules yielded compounds with high affinity for GPIIb-IIIa.

p38 mitogen-activated protein kinase phosphorylates cytosolic phospholipase A2 (cPLA2) in thrombin-stimulated platelets. Evidence that proline-directed phosphorylation is not required for mobilization of arachidonic acid by cPLA2

The Ca2+-sensitive 85-kDa cytosolic phospholipase A2 (cPLA2) is responsible for thrombin-stimulated mobilization of arachidonic acid for the synthesis of thromboxane A2 in human platelets. We have previously shown that thrombin activates p38 kinase, a recently discovered new member of the mitogen-activated protein kinase family (Kramer, R. M., Roberts, E. F., Strifler, B. A., and Johnstone, E. M. (1995) J. Biol. Chem. 270, 27395-27398) and also induces phosphorylation of cPLA2, thereby increasing its intrinsic catalytic activity. In the present study we have examined the role of p38 kinase in the phosphorylation and activation of cPLA2 in stimulated platelets. We have observed that activation of p38 kinase accompanies receptor-mediated events in platelets and coincides with cPLA2 phosphorylation. Furthermore, in the presence of inhibitors of p38 kinase, the proline-directed phosphorylation of cPLA2 was completely blocked in platelets stimulated with the thrombin receptor agonist peptide SFLLRN and was suppressed during the early (up to 2 min) phase of platelet stimulation caused by thrombin. Unexpectedly, we found that prevention of proline-directed phosphorylation of cPLA2 in stimulated platelets did not attenuate its ability to release arachidonic acid from platelet phospholipids. We conclude that: 1) cPLA2 is a physiological target of p38 kinase; 2) p38 kinase is involved in the early phosphorylation of cPLA2 in stimulated platelets; and 3) proline-directed phosphorylation of cPLA2 is not required for its receptor-mediated activation.

Differential activation of cytosolic phospholipase A2 (cPLA2) by thrombin and thrombin receptor agonist peptide in human platelets. Evidence for activation of cPLA2 independent of the mitogen-activated protein kinases ERK1/2

The thrombin receptor agonist peptide SFLLRN was less effective than thrombin in eliciting the liberation of arachidonic acid and the generation of thromboxane A2 by human platelets. We found that while SFLLRN evokes an initial transient increase in cystolic free calcium concentration ([Ca2+]i) of similar magnitude as that caused by thrombin, the SFLLRN-induced elevation of [Ca2+]i declines more rapidly to near resting levels than that evoked by thrombin, suggesting that disparate levels of [Ca2+]i may contribute to the attenuated arachidonic acid release. Furthermore, we observed that SFLLRN is less effective than thrombin in mediating the "activating" phosphorylation of cytolic phospholipase A2 (cPLA2). Both thrombin and SFLLRN rapidly and transiently activated kinases that phosphorylate the 21-residue synthetic peptide Thr669 derived from the epidermal growth factor receptor, but the maximal activation of proline-directed kinases by SFLLRN was less pronounced than that by thrombin. MonoQ chromatography and immunoblot analysis of extracts from stimulated platelets revealed that while thrombin induced a prominent activation of the mitogen-activated protein kinases ERK1 and ERK2, SFLLRN completely failed to do so. On the other hand, SFLLRN, like thrombin, stimulated the activity of a proline-directed kinase distinct from ERK1/2, but the activation of this kinase was less pronounced following stimulation of platelets with SFLLRN compared with thrombin. We conclude 1) that the partial activation of cPLA2 and the subsequent attenuated mobilization of arachidonic acid in response to SFLLRN may be the consequence of a less prolonged elevation of [Ca2+]i and insufficient activation of proline-directed kinase(s) by SFLLRN and 2) that the ability of SFLLRN to mediate the activating phosphorylation of cPLA2 in the absence of ERK1/2 stimulation suggest that, at least in human platelets, proline-directed kinases other than ERK1/2 may phosphorylate and activate cPLA2.

In vivo effects of a novel thromboxane A2/prostaglandin H2 (TXA2/PGH2) partial agonist, (+)5(Z)-7-[3-endo-phenylsulfonylamino[2.2.1]- bicyclohept-2-exo-yl]-heptenoic acid [(+)-S-145], on vascular, platelet and cardiac function

A novel thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor ligand, (+)5(Z)-7-[3-endo-phenylsulfonylamino[2.2.1]-bicyclohept-2-e xo- yl]-heptenoic acid [(+)-S-145], was evaluated in guinea pigs to assess the in vivo pharmacodynamic profile of this compound at vascular, cardiac and platelet TXA2/PGH2 receptors. Comparison was made to the TXA2/PGH2 receptor antagonist SQ29548. Upon i.v. injection, (+)-S-145, but not SQ29548, elicited transient (approximately 1 min) increases in mean arterial blood pressure (ED50 +/- 95% confidence limit = 6.1 + 4.0, -2.2 micrograms/kg). The potency of i.v. (+)-S-145 (ID50 = 6.3 + 2.3, -2.3 micrograms/kg) against the pressor response to subsequent i.v. TXA2/PGH2 mimetic, U44069, was 9.5-fold greater than that of SQ29548 (ID50 = 59.1 + 52.9, - 52.9 micrograms/kg). Intravenous (+)-S-145 inhibited U44069-induced decreases in circulating platelet count (ID50 = 4.2 + 4.1, - 2.0 micrograms/kg). In thoracotomized guinea pigs, i.v. (+)-S-145 (31.6 micrograms/kg) and increasing i.v. doses of U44069 increased mean arterial blood pressure, total peripheral resistance, left ventricular end-diastolic pressure and left ventricular peak positive dP/dt (LV + dP/dt) and depressed cardiac output (P < .05). Pretreatment with i.v. (+)-S-145 (31.6 micrograms/kg) abolished these U44069-induced effects. In thoracotomized guinea pigs in which left ventricular end-diastolic pressure and HR were held constant, U44069 again increased LV + dP/dt (P < .05), but (+)-S-145 decreased LV + dP/dt (P < .05), which indicates the lack of an (+)-S-145 direct inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of dual-acting agents for thromboxane receptor antagonism and thromboxane synthase inhibition--I. Synthesis, structure-activity relationship, and evaluation of substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids

A series of arylsulfonamido-substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids were synthesized and evaluated in vitro for their ability to act as both a thromboxane A2 receptor antagonist (TRA) and thromboxane synthase inhibitor (TSI). Variations of alkenoic acid chain length, olefin geometry, substituent effect on the benzenesulfonamido group, and conformational flexibility of the substituted arylsulfonamido group were examined. Among the various substituents, iodo-substitution gave the most potent compound. Conformational flexibility between the arylsulfonamido group and the phenyl ring attached to the alkenoic acid side chain significantly enhanced the dual activities. The compound (E)-21c was identified as the most potent TRA/TSI (TRA: Kd = 53 nM; TSI: IC50 = 23 nM) in the series studied. The compounds 9c and 10c have indicated that these series of compounds are orally active and are specific TSIs as exhibited by the so-called 'shunt' effect on prostacyclin synthesis in vitro.

Biochemical and pharmacological activity of arene-fused prostacyclin analogues on human platelets

Human platelets have been employed as an assay system to evaluate the pharmacological activity of a group of stable, arene-fused prostacyclin analogs. Prostacyclin (PGI2) is a highly active member of the eicosanoid family and is relatively unstable under physiological conditions. Prostacyclin's best characterized activities are those of inhibition of platelet aggregation and relaxation of vascular smooth muscle. These activities are mediated in large part via elevation of intracellular levels of cyclic AMP subsequent to receptor occupation and activation of adenylate cyclase. We previously described the synthesis of a series of arene-fused prostacyclin analogs with stability in aqueous media at physiological pH. Several of these compounds have prostacyclin-like activities, i.e., competitive binding at the platelet prostacyclin receptor, elevation of intraplatelet cyclic AMP levels and inhibition of human platelet aggregation. One compound in particular (11a) demonstrated these activities with potency similar to PGI2, i.e., Kd at platelet receptor of 3.7 nM and IC50 for inhibition of collagen-induced human platelet aggregation in plasma of 2.9 nM.

Thrombin-induced phosphorylation and activation of Ca(2+)-sensitive cytosolic phospholipase A2 in human platelets

Receptor-mediated activation of human platelets by thrombin initiates a series of rapid biochemical events that include activation of phospholipase A2 to liberate arachidonic acid for further conversion to thromboxane A2. The identity of the phospholipase A2 involved has not been clear. Here we show by immunochemical analysis that human platelets contain significant amounts (60 ng/10(9) platelets) of the recently identified Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2). Metabolic labeling of human platelets with 33Pi revealed that the extent of phosphorylation of cPLA2 was greatly increased after thrombin treatment. Upon stimulation of platelets with thrombin, cPLA2 exhibits enhanced catalytic activity, as well as a change in its electrophoretic and chromatographic properties compared with cPLA2 in resting platelets. These alterations of cPLA2 are reversed by treatment with phosphatase, demonstrating that they are the consequence of thrombin-stimulated phosphorylation. Thrombin-induced phosphorylation and activation of cPLA2 is rapid (half-maximal by 1 min at 1 unit/10(9) platelets) and dose-dependent. Agonist-induced phosphorylation of cPLA2 is more sensitive to thrombin than the generation of thromboxane A2, suggesting that it may be an early event in the sequence of steps leading to the mobilization and further metabolism of arachidonic acid. By comparing the functional properties of cPLA2 from control versus thrombin-stimulated platelets, we found that while activated cPLA2 exhibits the same Ca2+ requirement and apparent substrate affinity (Km), its catalytic activity (Vmax) is increased compared with control cPLA2. We conclude that 1) cPLA2 is likely to play an important role in agonist-induced mobilization of arachidonic acid and 2) thrombin elicits rapid and full activation of cPLA2 not only by promoting a rise in cytosolic free Ca2+ but also by inducing phosphorylation of cPLA2 thereby improving its catalytic activity.

Correlation of molecular shape with GPIIb-IIIa receptor antagonist activity in RGD peptides

Arg-Gly-Asp-Ser (RGDS) 1, Arg-Val-Asp-Ser (RVDS) 2, Arg-dVal-Asp-Ser (R[d]VDS) 3, and a cyclic RGD peptide, cyclo S,R [H-Pen-Arg-Gly-Asp-Pen-Gly-OH] 4, were tested for their ability to antagonize GPIIb-IIIa function. The activities were found to fall in the order 4 >> 1 >> 3 > 2. Simulated annealing and molecular dynamics studies were carried out to estimate the most populated conformations of each molecule. The acyclic molecules 1-3 were found to populate a much wider range of conformations than the cyclic molecule 4. The backbones of all four molecules were found to approximate beta-turn structures in the most populated conformations. In 4 the beta-turn intramolecular hydrogen bond between C = O of the i residue (Arg) and NH of the i + 3 residue (Ser) did not appear to be present. The distance between the beta-carbons of the critical Arg and Asp groups was found to be shorter in 4 (average 7.98 A) than in the less active acyclic molecules (averages of 8.65-9.33 A).

In vitro characterization of a novel TXA2/PGH2 receptor ligand (S-145) in platelets and vascular and airway smooth muscle

The stereoisomers of S-145, a novel thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor ligand, were compared to TXA2/PGH2 receptor antagonists, SQ29548 and BM13505 in guinea pig platelets, aortas and trachea. Equilibrium binding assays in platelets yielded Kd values (nanomolar) for (+)-S-145 (0.57 +/- 0.04), (-)-S-145 (9.2 +/- 1.3), SQ29548 (11.1 +/- 0.70) and BM13505 (118 +/- 16). In aortas, the corresponding Kb values (nanomolar) were (0.014 +/- 0.002), (1.90 +/- 0.31), (16.8 +/- 3.3) and (142 +/- 29), respectively, whereas in trachea, the Kd values (nanomolar) were (0.019 +/- 0.004), (1.12 +/- 0.18), (1.94 +/- 0.30) and (18.99 +/- 2.59), respectively. S-145 stereoisomers elicited platelet shape change stereoselectively that was characterized by EC50 values 8 to 16-fold higher than the EC50 values for these ligands to block aggregation induced by TXA2/PGH2 mimetic, U44069. S-145 (+)- and (-)-isomers stereoselectively induced transient aortic contraction at concentrations 214,000- and 16,000-fold higher, respectively, than the corresponding Kb values in this tissue. S-145-induced platelet shape change and aortic contraction were inhibitable by low concentrations of SQ29548. We postulate that S-145 may elicit partial agonist activity in platelets and aorta via lower affinity for the active than inactive state of the TXA2/PGH2 receptor in those tissues. S-145 had no agonist activity in isolated trachea possibly indicating different TXA2/PGH2 recognition sites in aorta and trachea or a smaller preligand ratio of active to inactive TXA2/PGH2 receptors in trachea than in aorta.

Pharmacological assessment of the antithrombotic activity of the peptide thrombin inhibitor, D-methyl-phenylalanyl-prolyl-arginal (GYKI-14766), in a canine model of coronary artery thrombosis

The antithrombotic activity of the tripeptide thrombin inhibitor, D-methyl-phenylalanyl-prolyl-arginal (GYKI-14766), was compared to heparin in a model of canine coronary artery thrombosis. Thrombogenesis was initiated by electrolytic injury of the intimal surface of the left circumflex coronary artery. Drug administration was started 15 min before initiation of intimal injury. Clotting times and ex vivo platelet aggregation were determined on citrated blood samples. Gingival template bleeding times were determined. Clotting times (thrombin time; activated partial thromboplastin time, APTT; prothrombin time, PT) increased in a dose-dependent manner with both anticoagulants. The two anticoagulants selectively inhibited thrombin-induced platelet aggregation. GYKI-14766 and heparin were found to delay thrombosis significantly when compared to vehicle-treated animals; minimum effective antithrombotic doses were 0.25 mg/kg/h and 80 U/kg + 30 U/kg/h, respectively. GYKI-14766 (0.25 mg/kg/h) had no effect on template bleeding time, APTT or PT. Heparin (80 U/kg + 30 U/kg/h), however, was associated with a 2.5- to 3.0-min increase in template bleeding time, a 1.8-fold and 1.7-fold increase in APTT and PT, respectively. Antithrombotic efficacy was achieved at doses of GYKI-14766 that did not affect APTT, PT or template bleeding time, whereas antithrombotic efficacy observed with heparin was associated with significant increases in APTT, PT and template bleeding time. These data demonstrate that the tripeptide thrombin inhibitor, GYKI-14766, could potentially prove to be a safer and more effective antithrombotic agent than heparin.

Minimal sequence requirement of thrombin receptor agonist peptide

A site-specific proteolytically generated neoamino terminus of the thrombin receptor having a sequence SFLLRNPNDKYEPF- has been reported to be a functional ligand of the receptor. This discovery raises question on the precise structural requirements of the "tethered ligand" responsible for receptor activation and signal transduction. By examining the agonist activity of a panel of synthetic sequence analogues of thrombin receptor agonist peptides (TRAP) on human platelet aggregation, we determined that the minimal sequence of the human platelet thrombin receptor ligand is SFLL-amide (TRAP1-4, EC50 = 300 uM). An extension of TRAP1-4 by an additional Arg-Asn segment yielded the most potent agonist among the series (TRAP1-6, EC50 = 1.3 microM). Based on the structure-activity relationships, we hypothesized a model of the ligand-binding site of the human platelet thrombin receptor that accommodates a hexapeptide structure. TRAP1-6, when administered intravenously, induced marked intravascular platelet aggregation in the anesthetized guinea pigs.

Effect of ethanol on low density lipoprotein and platelet composition

The present study was designed to investigate the effect of ethanol (EtOH) dose on low density lipoprotein (LDL) and platelet composition. Male squirrel monkeys were divided into three groups designated Control, Low, and High EtOH, and fed isocaloric liquid diets containing 0%, 12%, and 24% of calories as EtOH, respectively. After four months of treatment, monkeys fed the 12% alcohol dose had LDL and platelet cholesterol concentrations similar to Controls. By contrast, platelet membranes from High EtOH animals contained significantly more cholesterol which was associated with higher levels of plasma LDL cholesterol and apolipoprotein B. Blood platelet count, size, and mass were similar for all groups and circulating platelet aggregates were absent in the two alcohol cohorts. Despite elevations in platelet cholesterol mass and thromboxane A2 (TXA2) precursor, phospholipid arachidonate, platelet responsiveness, measured as thromboxane formed in response to a collagen challenge in vitro, and the cholesterol/phospholipid molar ratio, were not significantly altered by high dose alcohol. Normal platelet activity in High EtOH monkeys may have resulted from a significant increase in the platelet phospholipid polyunsaturated/saturated fatty acid ratio and a non-significant increase in platelet phospholipid mass, both of which would have a fluidizing effect on platelet membranes. Our data indicate that low EtOH intake has no effect on platelet composition and function while unfavorable platelet cholesterol enrichment following consumption of high dose ethanol may arise from elevations in plasma LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of serotonin (5HT2) receptor blockade in myocardial reperfusion injury: effects of LY53857 in a canine model of myocardial infarction

The potential protective effects of serotonin receptor antagonism during the process of acute myocardial infarction were studied in anesthetized male dogs, which were subjected to a 90-min left circumflex coronary artery occlusion followed by 5 h of reperfusion. Either vehicle (0.9% NaCl) or the serotonin (5HT2) receptor antagonist LY53857 was infused i.v. at a dose of 0.5 mg/kg, followed by a constant infusion of 2 mg/kg/min beginning 5 min before left circumflex coronary artery occlusion and continuing throughout the duration of the ischemia and subsequent reperfusion. Verification of functional 5HT2 receptor antagonism in the circulating blood of the LY53857-treated dogs was monitored throughout the experiments by periodic assessment of ex vivo platelet reactivity to exogenous serotonin. After 5 h of reperfusion, the hearts were excised and analyzed utilizing histochemical staining with triphenyltetrazolium, which demarcates myocardial infarct size and anatomical area of myocardium at risk of infarction. There was not a significant reduction of infarct size with LY53857 treatment: control infarct/area at risk = 38.6 +/- 4.7%, n = 9 LY53857 infarct/area at risk = 33.4 +/- 3.8%, n = 6. Similarly, when myocardial infarct size was analyzed as a function of myocardial collateral blood flow, there were no significant effects of drug treatment on the relationship between collateral blood flow and infarct size. The effects of 5HT on neutrophil activation were determined by measuring the potential ability of 5HT to enhance the chemotactic peptide-induced production of superoxide. 5HT did not activate human neutrophils in vitro and LY53857 had no effect on neutrophil superoxide production.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of coagulation and thrombin-induced platelet activities by a synthetic dodecapeptide modeled on the carboxy-terminus of hirudin

A synthetic, tyrosine-sulfated, dodecapeptide (BG8865) modeled on residues 53-64 of hirudin was found to elevate the activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT) of human plasma in a dose-dependent manner. The most sensitive assay was the TT, which was prolonged 2 and 3 times control values at 2.2 and 4.1 micrograms/mL hirudin peptide, respectively. The sulfated dodecapeptide exhibited no dependency on antithrombin III as monitored by the APTT in the presence of sheep anti-human antithrombin III antibodies, and its activity was not neutralized by platelet releasates or platelet factor 4. In studies of thrombin-induced platelet activation, the hirudin peptide was found to block aggregation, serotonin release and thromboxane A2 generation. At thrombin concentrations of 0.25 U/mL, the IC50 (concentration resulting in 50% inhibition) for inhibition of platelet aggregation was 0.72 micrograms/mL peptide. Inhibition of TXA2 generation and serotonin release correlated closely with inhibition of aggregation. Using platelets from patients with clinically documented heparin-induced thrombocytopenia anticoagulant doses of heparin were found to induce platelet aggregation and thromboxane A2 generation. In sharp contrast, anticoagulant-equivalent doses of hirudin peptide had no effect on patient platelets, as evidenced by a lack of platelet aggregation and thromboxane A2 generation. These data provide compelling in vitro evidence that the hirudin peptide has several potential advantages over heparin, namely effective inhibition of thrombin-induced platelet activities, co-factor independence, insensitivity to endogenous heparin-neutralizing factors, and an apparent lack of direct or immune-mediated platelet stimulating properties.

Contraction of diabetic rabbit aorta caused by endothelium-derived PGH2-TxA2

Endothelium-dependent relaxations and vasoactive prostanoid production caused by acetylcholine were determined in the aortas of rabbits with diabetes mellitus induced by alloxan. Aortas of diabetic rabbits, contracted submaximally by phenylephrine, showed significantly decreased endothelium-dependent relaxations induced by acetylcholine compared with the aortas of normal rabbits. Indomethacin, a cyclooxygenase inhibitor, and SQ 29548, a prostaglandin H2-thromboxane A2 (PGH2-TxA2) receptor antagonist, normalized the sensitivity of diabetic aortas to acetylcholine, whereas these agents had no effect on the response of normal aortas. The relaxations in response to a nonreceptor-mediated endothelium-dependent vasodilator, A23187, and an endothelium-independent vasodilator, sodium nitroprusside, were not different between normal and diabetic aortas. Acetylcholine also caused contractions of resting aortic rings with endothelium from diabetic, but not normal rabbits; these contractions were inhibited by indomethacin. Synthesis of TxA2, measured as immunoreactive TxB2, was significantly increased in diabetic aortic segments only when the endothelium was present. These results suggest that in the diabetic state, the endothelium releases a major vasoconstrictor cyclooxygenase product that either directly counteracts the relaxation caused by or selectively interferes with the release of endothelium-derived relaxing factor(s) induced by cholinergic receptor stimulation. The vasoconstrictor is most likely TxA2 or possibly its precursor, PGH2.

Agkistrodon piscivorus piscivorus platelet aggregation inhibitor: a potent inhibitor of platelet activation

Applaggin (Agkistrodon piscivorus piscivorus platelet aggregation inhibitor) is a potent inhibitor of platelet activation. The protein is isolated from the venom of the North American water moccasin snake in three steps, including gel filtration, cation exchange, and reverse-phase HPLC procedures. The purified protein migrates as a 17,700-Da polypeptide by SDS/PAGE under nonreducing conditions and as a 9800-Da peptide in the presence of thiol. The behavior of applaggin on SDS/PAGE would indicate that the protein is a disulfide-linked dimer. Applaggin has been completely sequenced by Edman degradation and consists of 71 amino acids. The sequence is rich in cysteine and contains Arg-Gly-Asp at residues 50-52. Applaggin blocks platelet aggregation induced by ADP, collagen, thrombin, or arachidonic acid with IC50 values ranging from 12 to 128 nM (0.2-2.3 micrograms/ml) depending on the agonist and its concentration. This inhibition is found to correlate with inhibition of thromboxane A2 generation and of dense granule release of serotonin. Inhibition by applaggin of serotonin release induced by ADP, gamma-thrombin, and collagen was monitored in plasma under stirred conditions with [3H]serotonin-loaded platelets, and IC50 values for inhibition are found to range from less than 10 to 145 nM. At saturating concentrations, 125I-labeled applaggin (125I-applaggin) binds to 28,500 sites per unstimulated, washed platelet with a Kd of 1.22 x 10(-7) M. Binding of 125I-applaggin to platelets is inhibited by the synthetic undecapeptide Arg8-Gly-Asp-Val at 200 microM.

Elevated glucose alters eicosanoid release from porcine aortic endothelial cells

Cultured porcine aortic endothelial cells were conditioned through two passages to mimic euglycemic and hyperglycemic conditions (5.2 mM, normal glucose; 15.6 mM, elevated glucose). After incubation with 1 microM [14C]arachidonic acid for 24 h, the cells were stimulated with 1 microM A23187 for times up to 30 min. Uptake of [14C]arachidonic acid and its distribution among cell lipids were unaffected by the increased glucose concentration. The release of eicosanoids from labeled cells and unlabeled cells was measured by reverse-phase HPLC and by RIA, respectively. Compared with cells stimulated in the presence of normal glucose concentrations, cells stimulated in the presence of elevated glucose released 62.6% less free [14C]arachidonic acid, but released 129% more 14C-labeled 15-hydroxyeicosatetraenoic acid (HETE). Increased release of 15-HETE in the presence of elevated glucose in response to A23187, bradykinin, and thrombin was confirmed by RIA. A similar increase in 5-HETE release was observed by RIA after A23187 treatment. The release of both radiolabeled and unlabeled prostanoids was equal at both glucose concentrations. The data indicate that glucose may play an important role in the regulation of release and metabolism of arachidonic acid after agonist stimulation. In the presence of elevated glucose concentrations, such as those associated with diabetes mellitus, the extent and pattern of eicosanoid release from endothelial cells is markedly altered.

Interaction of ethanol, prostacyclin, and aspirin in determining human platelet reactivity in vitro

Ethanol partitions into cellular membranes and alters membrane-associated phenomena in numerous cell types. Since platelet aggregation and its inhibition by prostacyclin are mediated by membrane-associated receptors and enzymes, we examined the interaction of ethanol, prostacyclin, and aspirin on human platelet reactivity. Using platelet-rich plasma, we examined the effect of increasing concentrations of ethanol (0.05% to 1.0%) on the platelet-inhibitory effects of a submaximal dose (5 x 10(-10) M) of prostacyclin, the concomitant production of cyclic 3',5'-adenosine monophosphate (AMP), and the release of thromboxane A2. Ethanol alone had little effect on platelet aggregation induced by 5 micrograms/ml collagen; however, it potentiated the inhibitory effect of prostacyclin on platelet aggregation in a dose-dependent manner in the range of 0.05% to 1.0% ethanol. Whereas prostacyclin increased platelet cyclic AMP levels, ethanol had no further effect on cyclic AMP levels. Ethanol alone reduced thromboxane A2 generation, but this effect could not totally account for the observed interaction of ethanol and prostacyclin on aggregation, since aspirin did not totally abolish the interaction. The dose range in which the ethanol/prostacyclin/aspirin interactions occur encompasses the plasma levels of ethanol that may be achieved by the consumption of alcoholic beverages. The results may, in part, explain the dose-related physiological and pathological consequences of chronic alcohol consumption on the cardiovascular system.

The pathophysiology and clinical relevance of platelet heterogeneity

Recent studies on platelet heterogeneity support the hypothesis that platelet production is regulated to maintain a constant functional platelet mass. In concept this form of regulation is analogous to the manner by which RBC production is controlled to maintain the oxygen-carrying capacity of blood. The platelet mass appears to correlate more closely with platelet function than the platelet count alone, since several factors in addition to the platelet count have been shown to influence the platelets' hemostatic function. These factors include platelet size, density, age, and previous hemostatic interactions. Application of these concepts to clinical problems has provided important insights into platelet physiology and reactivity. Failure to account for differences in platelet heterogeneity among individuals may introduce significant errors in the interpretation of data from laboratory and clinical investigations. However, despite advances, a number of practical issues remain to be resolved before measurements of platelet heterogeneity become accepted as routine clinical tests and are used in the diagnosis of pathologic states.

Hydrolysis of 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine, a common precursor of platelet-activating factor and eicosanoids, by human platelet phospholipase A2

The metabolism of platelet-activating factor (PAF) and arachidonic acid is linked through the common intermediate 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (alkylarachidonoyl-GPC). Hydrolysis of alkylarachidonoyl-GPC by phospholipase A2 may initiate the biosynthesis of both PAF and eicosanoids, since alkyllyso-GPC is formed for acetylation to PAF and arachidonic acid is liberated for conversion to biologically active metabolites. In order to elucidate the regulation and functional role of human platelet phospholipase A2 in the pathway leading to the formation of both classes of lipid mediators, we have characterized its action upon alkylarachidonoyl-GPC. Human platelet phospholipase A2 was solubilized and then partially purified in the presence of n-octyl-beta-D-glucopyranoside (octyl glucoside). Hexadecylarachidonoyl-GPC was prepared biosynthetically using platelet sonicates, purified by two-step high-performance liquid chromatography (HPLC) and suspended in buffer by sonication. Our results indicate that deacylation of alkylarachidonoyl-GPC by platelet phospholipase A2 has an absolute requirement for Ca2+. It occurs at submicromolar concentrations of free Ca2+ and exhibits a biphasic Ca2+-dependence with activity plateaus at 10 microM and 2 mM. Phospholipase A2-mediated hydrolysis of alkylarachidonoyl-GPC is increased 2-fold by albumin and is enhanced 5-fold if 1,2-dioleoylglycerol is incorporated into the substrate dispersion. The substrate dependence and specificity of platelet phospholipase A2 for 1-alkyl- vs. 1-acyl-linked subclasses of arachidonic acid containing phosphatidylcholine was examined with 1-O-hexadecyl-2-arachidonoyl-sn-glycero-3-phosphocholine (hexadecylarachidonoyl-GPC) and 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (palmitoylarachidonoyl-GPC). We found that the substrates were deacylated equivalently. We conclude that, in stimulated platelets, in the presence of increased levels of cytoplasmic free Ca2+ and newly generated diacylglycerol, alkylarachidonoyl-GPC may be rapidly hydrolyzed by phospholipase A2 and may serve as a precursor of both PAF and eicosanoids.

Vitamin E supplementation effect on human platelet function, arachidonic acid metabolism, and plasma prostacyclin levels

A randomized, placebo-controlled double-blind trial was conducted on 20 adults to assess the effect of vitamin E (800 IU/d 727 mg/d for 5 wk) on platelet function, arachidonic acid metabolism, and prostacyclin generation. Platelet aggregation was measured in response to collagen, arachidonic acid, and adenosine diphosphate. Thromboxane B2 was assayed in serum and in the supernatant plasma after platelet aggregation. Platelets were labeled with [3H]arachidonic acid to assess production and release of cyclooxygenase products (MDA, TXB2, and HHT), a lipoxygenase product (12-HETE), and arachidonic acid in response to stimulation by thrombin or collagen. Prostacyclin was measured in plasma and in blood collected from bleeding-time incisions by a sensitive HPLC-RIA procedure. Despite marked increases in plasma and erythrocyte vitamin E levels in the vitamin E group, there were no significant differences between the vitamin E and placebo groups in any of the variables measured.