In vivo efficacy of SM-20302, a GP IIb/IIIa receptor antagonist, correlates with ex vivo platelet inhibition in heparinized blood but not in citrated blood

Effects of anticoagulant on pH, ionized calcium concentration, and agonist-induced platelet aggregation in canine platelet-rich plasma

OBJECTIVE-To compare effects of 3.8% sodium citrate and anticoagulant citrate dextrose solution National Institutes of Health formula A (ACD-A) on pH, extracellular ionized calcium (iCa) concentration, and platelet aggregation in canine platelet-rich plasma (PRP). SAMPLE POPULATION-Samples from 12 dogs. PROCEDURES-Blood samples were collected into 3.8% sodium citrate (dilution, 1:9) and ACD-A (dilution, 1:5). Platelet function, pH, and iCa concentration were evaluated in PRP. Platelet agonists were ADP, gamma-thrombin, and convulxin; final concentrations of each were 20microm, 100nM, and 20nM, respectively. Washed platelets were used to evaluate effects of varying the pH and iCa concentration. RESULTS-Mean pH and iCa concentration were significantly greater in 3.8% sodium citrate PRP than ACD-A PRP. Platelet aggregation induced by ADP and gamma-thrombin was markedly diminished in ACD-A PRP, compared with results for 3.8% sodium citrate PRP. Anticoagulant had no effect on amplitude of convulxin-induced platelet aggregation. In washed platelet suspensions (pH, 7.4), there were no differences in amplitude of platelet aggregation induced by convulxin or gamma-thrombin at various iCa concentrations. Varying the pH had no effect on amplitude of aggregation induced by convulxin or gamma-thrombin, but the aggregation rate increased with increasing pH for both agonists. CONCLUSIONS AND CLINICAL RELEVANCE-Aggregation of canine platelets induced by ADP and gamma-thrombin was negligible in ACD-A PRP, which suggested an increase in extraplatelet hydrogen ion concentration inhibits signaling triggered by these agonists but not by convulxin. Choice of anticoagulant may influence results of in vitro evaluation of platelet function, which can lead to erroneous conclusions.

Disodium disuccinate astaxanthin prevents carotid artery rethrombosis and ex vivo platelet activation

BACKGROUND/AIMS

The disodium disuccinate derivative of astaxanthin (DDA) is a carotenoid antioxidant under development for the treatment of ischemic cardiovascular events. Recent evidence suggests that reactive oxygen species (ROS) play an important role in platelet activation. This study seeks to investigate the effects of a reactive oxygen species quencher, DDA, in a canine model of carotid artery thrombosis.

METHODS

After formation of an occlusive carotid thrombus, dogs were administered recombinant tissue plasminogen activator intra-arterially to achieve thrombolysis in the presence of either 0.9% NaCl solution or DDA (10-50 mg/kg i.v. infusion). Ex vivo platelet aggregation and tongue bleeding times were measured before and after drug administration. Residual thrombus mass was analyzed at the end of each experiment.

RESULTS

The data indicated a dose- dependent reduction in the incidence of carotid artery rethrombosis. In addition, platelet aggregation and thrombus weights were dose-dependently inhibited by DDA. No change was recorded in tongue bleeding time among the treatment groups.

CONCLUSIONS

The data demonstrate that at the doses used in this study, DDA significantly reduced the incidence of secondary thrombosis while maintaining normal hemostasis. The results suggest that upon further study, DDA may one day find utility in revascularization procedures.

Glycoprotein IIb/IIIa receptor antagonist (2S)-2-[(2-Naphthyl-sulfonyl)amino]-3-[[2-([4-(4-piperidinyl)-2-[2-(4-piperidinyl)ethyl] butanoyl]amino)acetyl]amino]propanoic acid dihydrochloride (CRL42796), in combination with aspirin and/or enoxaparin, prevents coronary artery rethrombosis after successful thrombolytic treatment by recombinant tissue plasminogen activator

The antithrombotic effect of the glycoprotein IIb/IIIa (GPIIb/IIIa) antagonist (2S)-2-[(2-naphthyl-sulfonyl)amino]-3-[[2-([4-(4-piperidinyl)-2-[2-(4-piperidinyl)ethyl] butanoyl]amino)acetyl]amino] propanoic acid dihydrochloride (CRL42796), administered alone, or in combination with aspirin, and/or enoxaparin, was examined in a canine left circumflex (LCX) coronary artery rethrombosis model. The electrolytic induction of arterial thrombosis was followed by intracoronary recombinant tissue plasminogen activator administration to achieve thrombolysis, and the adjunctive therapy was initiated 15 min earlier and maintained for 4 h. Thirty-five purpose-bred beagle dogs were randomized to receive one of the following treatments: group 0 (n = 6, placebo); group 1 (n = 6, CRL42796 15 microg/kg i.v. loading dose followed by 0.31 microg/kg/min i.v. infusion), group 2 (n = 6, aspirin 7 mg/kg, administered orally, at -47, -23, -17 h before entry into the experimental protocol); group 3 (n = 6, aspirin + CRL42796); group 4 (n = 6, aspirin + enoxaparin 0.6 microg/kg i.v. loading dose followed by 6.0 microg/kg/min i.v. infusion); and group 5 (n = 5, aspirin + CRL42796 + enoxaparin). The incidence of LCX reocclusion was as follows: group 0, 6/6; group 1, 3/6; group 2, 5/6; group 3, 2/6; group 4, 2/6; and group 5, 0/5. Aspirin pretreatment increased the tongue-bleeding time, whereas the addition of CRL42796 or enoxaparin did not prolong bleeding time to a further degree. However, the combination of the three drugs did increase bleeding time significantly, from 173.9 +/- 19.8 to 620.0 +/- 98.7 s. In conclusion, low-dose CRL42796 together with aspirin and enoxaparin prevented coronary artery rethrombosis, although bleeding time was prolonged. The latter may be of concern in the clinical use of combination therapy.

Prevention of carotid artery thrombosis after oral administration of the glycoprotein IIb/IIIa antagonist CRL42796

This study investigates the effect of the glycoprotein IIb/IIIa receptor antagonist CRL42796 in a canine model of carotid artery thrombosis. Both carotid arteries developed occlusive thrombosis in each of the five control animals (time to occlusion: right carotid artery, 92.6 minutes; left carotid artery, 89.0 minutes). A single oral dose of CRL42796 (3 mg/kg) prevented occlusive thrombosis in 4 of 6 vessels and increased time to thrombosis, albeit not significantly (right carotid artery, 134.1 minutes; left carotid artery, 145.0 minutes). When the initial dose of CRL42796 was followed by a second oral dose (3 mg/kg) 2 hours later, 10 of 10 carotid arteries remained patent throughout the period of electrolytic injury. CRL42796 reduced thrombus weight in both treatment protocols. Ex vivo platelet aggregation with arachidonic acid (AA) or adenosine diphosphate (ADP) was reduced at 120, 240, and 360 minutes after two doses of CRL42796. A single oral dose reduced ADP-induced responses at 240 and 360 minutes, but significant effects were not observed with AA. Bleeding time increased 360 minutes after two oral doses of CRL42796, but not at 120 minutes. Bleeding time was unchanged with the single dose of CRL42796. The results demonstrate that oral administration of CRL42796 prevents carotid artery thrombosis in response to deep vessel wall injury and may have potential value to be characterized in extended preclinical and clinical study.

Ethanol enhances the inhibitory effect of an oral GPIIb/IIIa antagonist on human platelet function

Ethanol is a commonly used substance that can significantly influence platelet responses when combined with therapeutic drugs. In in vitro studies, we combined ethanol with LY309562, a novel 2,6-disubstituted isoquinolone RGD mimic that competes for fibrinogen binding to GPIIb/IIIa. Ethanol inhibits aggregation and secretion, partly by inhibiting thromboxane A(2) formation. We measured aggregation and secretion of dense granule contents by platelets labeled with [(14)C] serotonin in plasma from blood anticoagulated with FPRCH(2)Cl (PPACK). Alone, LY309562 dose-dependently inhibited aggregation induced by 10 micromol/L adenosine diphosphate, 1 microg/mL collagen, 2 micromol/L U46619 (a thromboxane A(2) mimetic), or 15 micromol/L SFLLRN (protease-activated receptor-1-activating peptide); inhibition was complete at 1 micromol/L LY309562 and partial at 0.1 micromol/L (50% inhibitory concentration [IC(50)] 0.19-0.33 micromol/L). Secretion induced by collagen, U46619, and SFLLRN was also inhibited by LY309562 (IC(50) 0.08-0.31 micromol/L). At inhibitory concentrations of LY309562, ethanol (2 or 4 mg/mL) further inhibited responses to collagen, U46619, and SFLLRN (IC(50) for aggregation 0.12-0.16 micromol/L; for secretion 0.04-0.12 micromol/L). Responses of aspirin-treated platelets to U46619 were also inhibited, indicating that ethanol was not acting solely by inhibiting thromboxane A(2) formation. Because it is likely that our results with LY309562 are representative of results with other GPIIb/IIIa antagonists, our in vitro data suggest that the concomitant use of GPIIb/IIIa antagonists and consumption of alcoholic beverages may result in further impairment of platelet participation in hemostasis and thrombosis.

Prevention of experimental carotid and coronary artery thrombosis by the glycoprotein IIb/IIIa receptor antagonist CRL42796

1. The antithrombotic effect of the glycoprotein IIb/IIIa receptor antagonist, CRL42796, was examined in canine models of carotid and coronary artery thrombosis. 2. In the carotid artery thrombosis model, occlusion occurred in all control vessels (time to thrombosis 47.6+/-8.9 min). After treatment with low dose CRL42796 (15 microg kg(-1) loading dose +0.31 microg kg(-1) min(-1) i.v.), two of five vessels occluded. Time to thrombosis increased significantly to 155.2+/-23.1 min. When the drug infusion was increased (0.69 microg kg(-1) min(-1)), each of five vessels remained patent. 3. Ex vivo platelet aggregation in response to arachidonic acid (AA) and ADP was examined in platelet rich plasma (PRP) prepared from citrate or heparin anticoagulated blood. CRL42796 reduced platelet reactivity at low and high doses in PRP from citrate anticoagulated blood. However, in PRP from heparin anticoagulated blood, only the higher infusion dose produced a significant reduction in ex vivo platelet responses. 4. A combination of oral aspirin (4.6 mg kg(-1) -41, -17 h) and the low infusion dose of CRL42796 did not produce an additional benefit beyond that provided by CRL42796 alone. 5. Coronary artery thrombosis was inhibited in four of five vessels treated with the lower infusion dose of CRL42796 and in five of five vessels treated with the higher infusion. Time to thrombosis increased with both doses (Control, 90.8+/-10.4 min; low dose, 165.8+/-14.2 min; high dose, >180.0+/-0 min). 6. The results indicate that CRL42796 is an effective in vivo antithrombotic agent against experimentally-induced carotid and coronary artery thrombosis.

Protective effects of SM-20302, an orally active GPIIb/IIIa antagonist, in an ADP/epinephrine-induced guinea pig model of transient cerebral ischemia

The potential benefits of SM-20302, (2S)-3-(3-(4-amidinobenzoylamino)propanoylamino)-2-(4-ethyl)benzensulfonylaminopropionic acid hydrochloride, a nonpeptide GPIIb/IIIa receptor antagonist, were compared with those of aspirin and ticlopidine in a transient cerebral ischemia model in guinea pigs. Transient cerebral ischemia was induced in guinea pigs by an infusion of ADP/epinephrine into the left internal carotid artery. Each compound was orally administered 1 h (SM-20302 and aspirin) or 3 h (ticlopidine) before the ADP/epinephrine infusion. The ischemic area in coronal brain slices was assessed 1 min after the cessation of ADP/epinephrine infusion by a carbon black perfusion method. In a separate experiment, neurological deficits and lactate contents of ipsilateral hemispheres were evaluated 60 min after the cessation of ADP/epinephrine infusion by neurological scores and the standard enzymatic method, respectively. SM-20302 (0.3 and 1 mg/kg p.o.) significantly reduced the ischemic area, neurological deficits and lactate contents in comparison with the vehicle control. Aspirin (100 mg/kg po) had no significant effect on either parameter. Ticlopidine (300 mg/kg p.o.) reduced the lactate content. Although a combination of aspirin (100 mg/kg p.o.) and ticlopidine (300 mg/kg po) also reduced the lactate content, no additive effect was observed. These results suggest that SM-20302 is of potential clinical benefit in the treatment of thromboembolic diseases.

Human platelet alphaIIbeta3 integrin binding affinity and specificity of SJ874: antiplatelet efficacy versus aspirin

OBJECTIVE

To define the affinity and specificity of SJ874, a nonpeptide antiplatelet agent for platelet glycoprotein Ilb/IIIa integrin, and to determine the antiplatelet efficacy of SJ874 relative to those of glycoprotein IIbIIIa antagonists and aspirin.

METHODS

Binding affinity and specificity of SJ874 for platelet glycoprotein IIb/IIIa integrin were determined using integrin-mediated binding and adhesion assays with human cells. Additionally, the antiplatelet efficacy of SJ874 was determined and compared with those of other glycoprotein IIb/IIIa antagonists and aspirin using light-transmittance and laser-scattering aggregometry.

RESULTS

SJ874 inhibited aggregation of human platelets induced by 10 micromol/l adenosine diphosphate (ADP) with a concentration for half-maximal effect of 0.046 +/- 0.005 micromol/l using light-transmittance aggregometry. Using laser-scattering aggregometry, SJ874 was found to totally inhibit formation both of micro-aggregates and of macro-aggregates induced either by ADP or by epinephrine. In contrast, administration of 325 mg aspirin to normal healthy volunteers attenuated formation of macro-aggregates but not micro-aggregates. SJ874 inhibited binding of [125I]-fibrinogen to activated (by ADP, epinephrine, and arachidonic acid at concentrations of 100 micromol/l each) gel-filtered human platelets with a concentration for half-maximal effect of 0.0012 +/- 0.0005 micromol/l. SJ874 was demonstrated to associate more tightly with resting human platelets than did DMP754 [1] and slightly less tightly than did DMP802 [2]. SJ874 was demonstrated to exhibit a high degree of specificity for platelet glycoprotein IIb/IIIa (alphaIIb/beta3) integrin compared with other known integrins, including alphavbeta3, alphavbeta5, and alpha5beta1 (concentration for half-maximal effect > 100 micromol/l).

CONCLUSION

SJ874 is a potent and specific platelet glycoprotein IIb/IIIa antagonist with high affinity for and tight association with human platelets. These data suggest that SJ874 might have good antiplatelet utility for inhibiting formation both of platelet micro-aggregates and of macro-aggregates of platelets and a long duration of action in humans due to its slow dissociation from human platelets.

Antiplatelet and antithrombotic effects of orbofiban, a new orally active GPIIb/IIIa antagonist, in guinea pigs

Antiplatelet and antithrombotic effects of orbofiban, a new orally active glycoprotein IIb/IIIa antagonist, were evaluated in guinea pigs. SC-57101A (0.03-1 microM), the hydrochloride salt of the active form of orbofiban, inhibited in vitro ADP- and collagen-induced platelet aggregation in a concentration-dependent manner. Oral administration of orbofiban (3-30 mg/kg) resulted in dose-dependent inhibition of ADP- and collagen-induced platelet aggregation. The inhibition peaked at 1-2 hours postdose and then declined slowly. The agent also showed similar inhibition of platelet aggregation in guinea pigs with dietary-induced hypercholesterolemia. In contrast, the antiaggregatory effects of acetylsalicylic acid differed more widely between normal and hyperlipidemic animals compared to those of orbofiban. Plasma concentration of the active form, measured by a column-switching HPLC method, correlated well with the inhibition of platelet aggregation. Orbofiban (3-100 mg/kg, p.o.) caused dose-dependent inhibition of thrombus formation in an arteriovenous-shunt-thrombosis model. Orbofiban at high doses (> or =30 mg/kg) and acetylsalicylic acid (100 mg/kg) both prolonged cutaneous bleeding time measured by the template method. These results demonstrate that orbofiban is an orally active and potent inhibitor of platelet aggregation with an efficacy that correlates well with the plasma concentration of its active form.

Role of extracellular ionized calcium in the in vitro assessment of GPIIb/IIIa receptor antagonists

Several preclinical studies have found a poor correlation between the ex vivo platelet inhibitory potency and the in vivo antithrombotic efficacy of GPIIb/IIIa receptor antagonists. The present study was designed to examine the differential in vitro potencies of c7E3, MK-383, DMP-728, and SM-20302 in inhibiting ex vivo platelet aggregation under normocalcemic and hypocalcemic conditions. Human blood was collected in either trisodium citrate (0. 37%) or PPACK (20 microg/mL). Platelet aggregation assays were performed in platelet-rich plasma from citrate-anticoagulated blood (cPRP) and PPACK-anticoagulated blood (pPRP) using ADP (20 microM) and TRAP (10 microM) as agonists in the presence of c7E3, MK-383, DMP-728, or SM-20302. The concentration of ionized calcium in cPRP was 16-19 times lower than that in pPRP. The IC(50) of c7E3 for inhibiting ADP-induced platelet aggregation in cPRP (2.76 +/- 0.11 microg/mL) was 1.6 times lower than that in pPRP (4.46 +/- 0.48 microg/mL; P < 0.05). Similarly, the IC(50) for c7E3 for inhibiting TRAP-induced platelet aggregation in cPRP (4.52 +/- 0.34 microg/mL) was 1.7 times lower than that in pPRP (7.69 +/- 0.43 microg/mL; P < 0.05). MK-383, DMP-728, and SM-20302 also demonstrated 1.96-, 1.15-, and 1.43-fold lower IC(50) values, respectively, in cPRP as compared with pPRP. Chelation of ionized calcium in pPRP led to a progressive increase in platelet inhibition by all the antagonists. These results suggest that the observed in vitro inhibitory potency of a GPIIb/IIIa receptor antagonist is markedly enhanced when trisodium citrate is used as an anticoagulant to collect blood for ex vivo assay. These findings indicate that dosing regimens for GPIIb/IIIa receptor antagonists based on the platelet inhibition profile in citrate may provide misleading information with respect to their true in vivo antithrombotic efficacy.

Pharmacokinetics and pharmacodynamics of glycoprotein IIb-IIIa inhibitors

Antagonists of the platelet receptor glycoprotein (GP) IIb-IIIa are a novel class of antithrombotic agents that provide more comprehensive platelet blockade than the combination of aspirin and heparin. Studies in patients scheduled for percutaneous coronary intervention and those with unstable angina or non-Q-wave myocardial infarction have shown that a combination of intravenous GP IIb-IIIa inhibitors with aspirin and heparin is associated with a reduction in death or myocardial infarction compared with therapy with aspirin and heparin alone. As with other antithrombotic agents, the principal safety issue with GP IIb-IIIa inhibitors is bleeding, because the potent antiplatelet effect of these drugs may adversely affect hemostasis. Increased risk of hemorrhage is of particular concern for patients who subsequently require emergency or urgent bypass surgery because the rate of bleeding complications in these patients is considerable even in the absence of prior GP IIb-IIIa therapy. Additionally, antagonists of GP IIb-IIIa may increase the risk of thrombocytopenia. The safety profiles of various GP IIb-IIIa inhibitors are largely a function of their pharmacokinetic and pharmacodynamic properties, most notably the reversibility of platelet inhibition and the rate of plasma clearance. Knowledge of the pharmacokinetic and pharmacodynamic properties of the GP IIb-IIIa inhibitors is critical for the appropriate utilization of this new class of drugs.

Antithrombotic effect of SM-20302, a nonpeptide GPIIb/IIIa antagonist, in a photochemically induced thrombosis model in guinea pigs

SM-20302, a synthetic inhibitor of the fibrinogen receptor of platelets, has been shown to inhibit the platelet aggregation induced by various stimuli. In the present study, we performed ex vivo platelet aggregation studies by using heparinized platelet-rich plasma (PRP) as well as citrated PRP and compared the antiaggregatory activity with the in vivo antithrombotic efficacy of SM-20302. The oral administration of SM-20302 (0.3-10 mg/kg) to guinea pigs completely inhibited the ADP-induced ex vivo platelet aggregation in citrated PRP. In heparinized PRP, SM-20302 (1-10 mg/kg) showed a dose-dependent inhibition of ex vivo platelet aggregation, and it exhibited complete inhibition at a dose of 3 and 10 mg/kg, respectively. The concentration of ionized calcium in the citrated samples was approximately 35 times lower than that in heparinized samples. Chelation of ionized calcium caused an enhancement of the antiaggregatory activity of SM-20302 in guinea pig heparinized PRP in vitro. And addition of CaCl2 to citrated PRP reversed the enhancement. Citrate therefore appeared to enhance the inhibitory activity of SM-20302 by lowering the ionized calcium levels. We also examined the in vivo efficacy of SM-20302 in a photochemically induced femoral artery thrombosis model in guinea pigs. The photochemical injury of the endothelium of femoral artery resulted in a progressive decline in the blood flow. The oral administration of SM-20302 (0.1-3 mg/kg) produced a dose-dependent maintenance of the femoral artery patency and significantly prolonged the time to occlusive thrombus formation at a dose of 1 and 3 mg/kg, respectively. These results suggest that SM-20302 may be an orally active antithrombotic agent, and its in vivo antithrombotic efficacy appeared to correlate well with the ex vivo platelet inhibition in PRP prepared from heparinized blood but not in PRP anticoagulated with citrate.

Temporary and partial inhibition of platelets by SM-20302 prevents coronary artery thrombosis in a chronic canine model

We proposed that temporary and partial platelet inhibition by a GPIIb/IIIa receptor antagonist, SM-20302, would provide sustained antithrombotic efficacy in a chronic model of coronary artery thrombosis. Instrumented, conscious dogs received vehicle (Group I, n = 7), low dose SM-20302 (30 microg/kg bolus + 1 microg/kg/min infusion for 6 h) (Group II, n = 7), or high dose SM-20302 (100 microg/kg bolus + 1 microg/kg/min infusion for 6 h) (Group III, n = 7). Thrombosis was initiated by electrolytic injury to the circumflex coronary artery. Coronary blood flow was monitored for 6 h on day 1 and days 2-6. Platelet aggregation was performed in platelet-rich plasma prepared from citrated or heparinized blood. At 6 h, both doses of SM-20302 inhibited adenosine diphosphate-induced platelet aggregation completely (> 90%) in citrated platelet-rich plasma, but incompletely (57-59%) in heparinized platelet-rich plasma. Platelet reactivity returned to baseline values at 24 h. Control animals developed thrombotic occlusion on Day 1. Both doses of SM-20302 maintained vessel patency during the infusion period (Day 1) and the subsequent 5 days. Myocardial infarct size and mortality in the drug treated groups were reduced compared to the vehicle group. Thus, temporary inhibition of platelet reactivity by SM-20302 is associated with sustained prevention of primary thrombus formation, and reduction in infarct size and mortality.

Pharmacokinetics and pharmacodynamics of SM-20302, a GPIIb/IIIa receptor antagonist, in anesthetized dogs

We examined the pharmacokinetic and pharmacodynamic properties of SM-20302, a GPIIb/IIIa receptor antagonist, in anesthetized dogs. SM-20302 was administered intravenously in doses of 30 (n=2), 100 (n=4), 300 (n=4), and 1,000 microg/kg (n=4). The half-life of the initial phase was 4 min, and that of the terminal phase was 162-209 min. SM-20302 produced a dose-dependent increase in the initial plasma concentration and the area under concentration-time curve but did not alter the volume of distribution, mean residence time, or plasma clearance. Plasma clearance for SM-20302 ranged from 6.58 to 9.73 ml/min/kg. All doses of SM-20302 inhibited (> or =90%) the ex vivo platelet aggregation induced by adenosine diphosphate (ADP) or arachidonic acid (AA) in citrated platelet-rich plasma (cPRP). In heparinized PRP (hPRP), a dose-dependent (44-89%) inhibition was observed. By using a sigmoid Emax model, the in vivo median inhibitory concentration (IC50) for SM-20302 was estimated to be 14-19 ng/ml in cPRP and 79-89 ng/ml in hPRP. To validate the calculated parameters, an infusion regimen was designed for the prevention of coronary artery thrombosis. Infusion of SM-20302 produced 64-67% inhibition of platelets in hPRP and maintained vessel patency despite vessel wall injury. The results suggest that SM-20302 exhibits linear pharmacokinetics and that its ability to inhibit platelet aggregation in hPRP may correlate more accurately with its in vivo antithrombotic efficacy.