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PubMed2006

Antiviral therapy decreases GpIIb/IIIa activation of platelets in patients with chronic hepatitis C.

Interferon alpha (IFN-alpha) is used to treat haematological and solid malignancies and is the gold standard therapy for chronic hepatitis C infection in combination with ribavirin. It has a well known platelet lowering effect and was recently shown to impair platelet aggregation in the presence of various agonists and has been accused to increase patients' bleeding risk during IFN-alpha therapy. Thus, we hypothesised that antiviral treatment decreases GpIIb/IIIa activation and affects global platelet function. In a prospective clinical trial, we examined the effects of combination therapy with pegylated IFN-alpha 2a (PegIFN-alpha 2a) and ribavirin on platelet GpIIb/IIIa activation and platelet secretion in 20 patients with chronic hepatitis C at week 2, 4, 8 and 12 after the beginning of therapy. In addition, we determined global platelet function (CEPI-CT) with the PFA-100 and vWF-Ag levels. Antiviral therapy significantly decreased GpIIb/IIIa activation in a time dependent manner, whereas markers of platelet secretion (P-selectin, beta-thromboglobulin) remained unchanged. Despite a marked elevation of vWF-Ag levels, CEPI-CT did not change compared to baseline levels. Antiviral therapy significantly decreases GpIIb/IIIa activation in patients with chronic hepatitis C, while vWG-Antigen levels are markedly increased and alpha-granule secretion is not affected. This does not result in an alteration of global platelet function as assessed by the PFA-100, because elevated vWF-Antigen levels might compensate for the acquired defect.

Adult↗
PubMed1994

Randomized trial of a GPIIb/IIIa platelet receptor blocker in refractory unstable angina. European Cooperative Study Group.

BACKGROUND: Patients with unstable angina despite intensive medical therapy, ie, refractory angina, are at high risk for developing thrombotic complications: myocardial infarction or coronary occlusion during percutaneous transluminal coronary angioplasty (PTCA). Chimeric 7E3 (c7E3) Fab is an antibody fragment that blocks the platelet glycoprotein (GP) IIb/IIIa receptor and potently inhibits platelet aggregation. METHODS AND RESULTS: To evaluate whether potent platelet inhibition could reduce these complications, 60 patients with dynamic ST-T changes and recurrent pain despite intensive medical therapy were randomized to c7E3 Fab or placebo. After initial angiography had demonstrated a culprit lesion suitable for PTCA, placebo or c7E3 Fab was administered as 0.25 mg/kg bolus injection followed by 10 micrograms/min for 18 to 24 hours until 1 hour after completion of second angiography and PTCA. During study drug infusion, ischemia occurred in 9 c7E3 Fab and 16 placebo patients (P = .06). During hospital stay, 12 major events occurred in 7 placebo patients (23%), including 1 death, 4 infarcts, and 7 urgent interventions. In the c7E3 Fab group, only 1 event (an infarct) occurred (3%, P = .03). Angiography showed improved TIMI flow in 4 placebo and 6 c7E3 Fab patients and worsening of flow in 3 placebo patients but in none of the c7E3 Fab patients. Quantitative analysis showed significant improvement of the lesion in the patients treated with c7E3 Fab, which was not observed in the placebo group, although the difference between the two treatment groups was not significant. Measurement of platelet function and bleeding time demonstrated > 90% blockade of GPIIb/IIIa receptors, > 90% reduction of ex vivo platelet aggregation to ADP, and a significantly prolonged bleeding time during c7E3 Fab infusion, without excess bleeding. CONCLUSIONS: Combined therapy with c7E3 Fab, heparin, and aspirin appears safe. These pilot study results support the concept that effective blockade of the platelet GPIIb/IIIa receptors can reduce myocardial infarction and facilitate PTCA in patients with refractory unstable angina.

Abciximab↗
PubMed1999

Rapid platelet-function assay: an automated and quantitative cartridge-based method.

BACKGROUND: The platelet glycoprotein (GP) IIb/IIIa receptor is important in mediating platelet thrombus formation, and the GP IIb/IIIa antagonist abciximab (c7E3 Fab; ReoPro) is effective in preventing thrombotic ischemic cardiovascular complications of unstable angina and percutaneous coronary interventions. Small-molecule antagonists of GP IIb/IIIa based on the Arg-Gly-Asp (RGD) sequence show similar benefit, and some of these agents are orally active. However, there may be significant interindividual variation in response to such antagonists, especially with chronic oral therapy. It will be essential to balance the beneficial antithrombotic effect of these drugs with their potential for causing bleeding. In response to this need, we have developed a rapid platelet-function assay (RPFA), a point-of-care system that provides a quantitative measure of the competence of the GP IIb/IIIa receptor as reflected in the ability of platelets to agglutinate fibrinogen-coated beads. METHODS AND RESULTS: Polystyrene beads were coated with fibrinogen and placed in a cartridge along with a lyophilized peptide that activates the thrombin receptor. Anticoagulated whole blood was added to the cartridge, and then a microprocessor-controlled operation mixed the reagents and detected agglutination between platelets and coated beads. Quantitative digital results were displayed within 3 minutes. Because there is no dilution of the blood, the assay can be used to measure platelet activity in samples that have been treated with GP IIb/IIIa antagonists with high dissociation rates. RPFA results of whole-blood samples treated with different GP IIb/IIIa antagonists correlated well with both conventional turbidimetric platelet aggregation (r2=0.95) and the percentage of free GP IIb/IIIa molecules in the sample (r2=0.96). The mean difference in measurements between RPFA and aggregometry was -4% (+/-4% SD), and the mean difference in measurements between RPFA and free GP IIb/IIIa receptors was -2% (+/-6% SD). CONCLUSIONS: The RPFA provides rapid information on platelet function that mirrors turbidimetric platelet aggregation and reflects GP IIb/IIIa receptor blockade.

Abciximab↗
PubMed1990

Pharmacodynamic study of F(ab')2 fragments of murine monoclonal antibody 7E3 directed against human platelet glycoprotein IIb/IIIa in patients with unstable angina pectoris.

The pharmacodynamics of intravenous bolus injections of 0.05, 0.10, 0.15, and 0.20 mg/kg of F(ab')2 fragments of the murine monoclonal antibody 7E3, 7E3-F(ab')2, directed against the glycoprotein IIb/IIIa (GPIIb/IIIa) receptor of human platelets, were studied in groups of four patients with unstable angina pectoris. With 0.20 mg/kg, the template bleeding time prolonged from 6.3 +/- 1.9 (mean +/- SD) to greater than 30 min; it subsequently decreased to 13 +/- 7.8 min after 12 h and to 8.3 +/- 1.5 min after 24 h. The number of unblocked GPIIb/IIIa receptors (preinfusion value, 32,000 +/- 3,000 per platelet) decreased to 13 +/- 7% of the preinfusion value 1 h after infusion, and then increased to 33 +/- 10% at 12 h, 44 +/- 8% at 24 h and 67 +/- 7% at 72 h. The logarithm of the bleeding time was inversely proportional with the residual GPIIb/IIIa receptors (r = 0.73, P less than 0.0001). ADP-induced platelet aggregation (measured by changes in light transmittance in percent) decreased from 60 +/- 5% before infusion to 1.5 +/- 3% 1 h after infusion; it then increased to 29 +/- 3% after 24 h and 39 +/- 6% after 72 h. Platelet counts decreased by 16% at 1 h and returned to control values within 24 h. Proportionally smaller effects were seen at lower doses of 7E3-F(ab')2. Antibody injection did not induce spontaneous bleeding. Angina was not observed during the first 12 h when the bleeding time was significantly prolonged, but occurred in 6 of the 16 patients within the next 3 d. 2 of the 16 patients developed low titers of IgG antibodies specific for 7E3-F(ab')2. Thus 7E3-F(ab')2 induces dose-related inhibition of platelet function; at a dose of 0.20 mg/kg, it causes profound inhibition of platelet aggregation and prolongation of the bleeding time, but no spontaneous bleeding.

Adult↗
PubMed2000

Antiplatelet effects of MK-852, a platelet fibrinogen receptor antagonist, in healthy volunteers.

MK-852, a cyclic heptapeptide, is a potent platelet fibrinogen receptor antagonist. When administered to normal healthy male subjects by 1- and 4-hour constant rate intravenous infusions, it provides a generally well-tolerated and reversible means of inhibition of platelet function. At infusion rates of 1 microgram/kg/min for 1 hour and 0.44 microgram/kg/min for 4 hours, respectively, MK-852 extended baseline bleeding time by greater than 2.2-fold and 2.6-fold, inhibited ADP-induced platelet aggregation by 76% and 69%, and inhibited collagen-induced platelet aggregation by 65% and 67%, respectively. The pharmacokinetics of MK-852 include an elimination half-life of approximately 2 hours, total clearance of about 150 ml/min, and volume of distribution of about 18 liters. Examination of the relationship between MK-852 whole-blood concentration in vitro and inhibition of platelet aggregation showed an EC50 of about 55 ng/ml and a Hill coefficient of 1.55. The infusions were generally well tolerated, with no study drug-related changes in blood counts or biochemical profiles.

Adenosine Diphosphate↗
PubMed2001

Plasma and platelet von Willebrand factor abnormalities in patients with uremia: lack of correlation with uremic bleeding.

Chronic renal failure often is associated with abnormal bleeding that may represent an important complication of this disorder. The hemorrhagic tendency currently is attributed to altered primary hemostasis, mainly platelet dysfunction. However, von Willebrand factor (vWF) also seems to be involved, even though the nature of its abnormalities is still controversial. To gain insight into the role of vWF in determining uremic bleeding, we studied 11 patients with stable, chronic renal failure. We found a significant increase in plasma factor VIII (FVIII), vWF:antigen (Ag), and vWF:ristocetin cofactor (Rco) levels, associated with a mean decrease in platelet vWF:Ag. Plasma vWF multimer pattern was characterized by increased representation of all oligomers in all patients, but five patients also showed a slight decrease in large vWF multimers. In addition, platelet vWF multimer pattern displayed a decrease in all components, especially those with high molecular weight. Despite normal bleeding time, collagen-induced platelet aggregation was defective in almost all patients, whereas vWF collagen binding capacity was normal. The levels of glycocalicin, the circulating fragment of glycoprotein Ib-IX, the major platelet vWF receptor, were also normal. In six patients who also were studied after initiation of dialysis, collagen-induced platelet aggregation was impaired further. Moreover, plasma vWF, and especially FVIII levels, were increased additionally, in association with a normalized platelet vWF content and an improved vWF multimer pattern. The results suggest that vWF abnormalities are present in uremia. Moreover, thrombopathy caused by impaired collagen-induced platelet aggregation is constantly present and apparently not improved by dialytic treatment.

Adult↗
PubMed2003

Value of thromboelastography in the assessment of platelet function.

Thromboelastography (TEG) is a useful measure of coagulation. Modified TEG (that is with the addition of a GP IIb/IIIa receptor antagonist) has been used to assess the contribution of the fibrinogen-platelet interaction to TEG parameters (in particular the maximum amplitude, MA). Modified TEG was compared with other investigations of platelet function to assess its sensitivity in both normal subjects and in patients with peripheral arterial disease (PAD), a condition associated with activated platelets. Blood was collected from eight healthy subjects and 12 PAD patients. Platelet function was measured by TEG, flow cytometry (using PAC-1, P-selectin, GP IIIa and GP Ib murine antibodies) and platelet aggregometry (spontaneous and ADP-induced) in the presence and absence of tirofiban (a GP IIb/IIIa receptor antagonist). TEG showed a statistically significant reduction in MA with tirofiban at 0.4 mg/L and an increase in k (kinetic time; which indicates how fast clot strength is increasing once clotting starts) at 0.2 and 0.4 mg/L of tirofiban in both healthy subjects and PAD patients. Flow cytometry showed a significant decrease in the PAC-1 binding index (at 0.2 mg/L). This finding was compatible with the significant reductions found in spontaneous and ADP-induced platelet aggregation. However, aggregometry and flow cytometry were more sensitive indicators of platelet inhibition than the TEG parameters. TEG does not provide a comprehensive or sensitive reflection of impaired platelet function. If TEG is used as an index of severely impaired platelet function, we recommend that the k parameter should be used as well as MA. TEG should be supplemented by other methods of platelet function assessment wherever possible.

Adenosine Diphosphate↗
PubMed1999

The effect of antiplatelet drugs, heparin, and preanalytical variables on platelet function detected by the platelet function analyzer (PFA-100).

The platelet function analyzer (PFA)-100 is a newly developed instrument that provides a rapid, in vitro, quantitative measurement of platelet adhesion and aggregation in whole blood flowing through a small aperture under high shear conditions. Thirty patients undergoing percutaneous transluminal coronary angioplasty (PTCA) and ten normal individuals were included in this study. In vitro and in vivo studies were conducted to discern the effect of combinations of antiplatelet drugs (aspirin, ticlopidine, abciximab) and heparin on the performance of the device as well as the effects of preanalytical variables, such as method of sample collection and ex vivo anticoagulants. Studies were also conducted examining the effect of aperture size (standard 150 microns vs. smaller 120 microns) on the ability of the device to detect the effect of antiplatelet drugs. There was no difference in mean PFA-100 closure time with citrate versus PPACK anticoagulants or with venipuncture vs. sheath sampling. Closure times did not vary with heparin administration. Closure times were slightly longer for patients taking aspirin plus ticlopidine compared to aspirin alone (p = NS). In contrast adenosine disphosphate (ADP) induced platelet aggregation was significantly less in patients that took aspirin plus ticlopidine vs. aspirin alone (p = .0005). In vitro, there was a dose-dependent increase in closure time for both aperture sizes with increasing abciximab concentration. Although both cartridges showed infinite closure times at an abciximab concentration of 2.25 micrograms/mL, there was a slight benefit to using the 120 microns aperture cartridges at abciximab concentrations of 1.75 to 2.0 micrograms/mL. In ten patients who were followed during abciximab therapy to assess the effect of aperture size, the PFA-100 was able to detect in vivo platelet inhibition by abciximab, but detection of recovery from abciximab-induced platelet dysfunction was slightly better for the PFA-100 with the 120 microns aperture compared to the standard 150 microns aperture collagen/ADP cartridge.

Abciximab↗
PubMed2003

Metalloproteinase inhibitors improve the recovery and hemostatic function of in vitro-aged or -injured mouse platelets.

Platelet transfusions are a crucial component of support for patients with severe thrombocytopenia. Storage of platelet concentrates, however, is associated with a reduction in platelet posttransfusion recovery and hemostatic function. In this study, we established a model of mitochondrial injury that resembles platelet storage lesion. Mitochondrial injury, provoked by incubation of platelets with carbonyl cyanide m-chlorophenylhydrazone (CCCP), led to reduced posttransfusion recovery in mice, an effect that directly correlated with the duration of treatment. Damaged platelets were characterized by shape change, disruption of membrane asymmetry, surface expression of P-selectin, and profound proteolysis of GPIbalpha. Using our model, we identified a key role for endogenous metalloproteinase(s) in platelet clearance, as their inhibition markedly improved posttransfusion recovery of both the mitochondria-injured and in vitro-aged mouse platelets. Metalloproteinase inhibition also prevented proteolysis of GPIbalpha on damaged platelets, thereby improving the hemostatic function of these cells in vivo. We propose that inhibition of metalloproteinase activity during storage could significantly improve the effectiveness of platelet transfusions. Surface expression of GPIbalpha might be a powerful marker to determine the quality of platelet concentrates, because it reflects metalloproteinase activity in vitro.

Animals↗
PubMed2001

Platelet aggregability under shear is enhanced in patients with unstable angina pectoris who developed acute myocardial infarction.

The study investigated whether patients hospitalized for unstable angina pectoris (UAP), who subsequently develop complete coronary thrombosis (acute transmural myocardial infarction (AMI)) despite medical treatment, exhibit platelet hyperaggregability in an assay system that does not employ agonist stimulation. The study comprised 89 patients with UAP (Braunwald type B). Unfractionated heparin and nitrate were administered to all patients via continuous intravenous drip together with aspirin taken orally. Citrated platelet-rich plasma (230-250x 10(3)/microl) was obtained on admission and again, in some patients, following the AMI. Platelet aggregability was measured in an optically modified cone-plate viscometer that enables the detection of platelet aggregation without agonist stimulation. A continuous shear rate of 1,200/s was employed. Of the 89 patients, 85 were finally stabilized, while 4 developed an AMI accompanied by persistent ST-segment elevation with increased levels of plasma creatine kinase within 3 h after starting the treatment. The extent of platelet aggregation on admission was significantly greater in these 4 patients compared with the 85 who were stabilized (87.8+/-6.8%, n=4 vs 26.8+/-9.1%, n=85; mean+/-SD). These data suggest that platelet hyperaggregability mediated mainly by fibrinogen binding to the activated glycoprotein IIb/IIIa complex occurs before a complete thrombotic occlusion and this evaluation may provide important information before the onset of myocardial infarction.

Aged↗
PubMed2005

Platelet aggregation: involvement of thrombin and fibrin(ogen).

Platelets play a key role in hemostasis and thrombosis. The formation of a platelet plug is accompanied by the generation of thrombin, which results in the generation of fibrin required for stabilization of the platelet plug. Platelet plug formation and coagulation are closely linked processes. Thrombin is a potent platelet activator, which proceeds through proteolysis of the protease activated receptors (PARs). Furthermore, thrombin binds glycoprotein Ib(alpha), which amplifies platelet activation by accelerating PAR-1 activation, and possibly also by direct signaling events through glycoprotein Ib(alpha). Moreover, thrombin's specificity towards other substrates changes after binding glycoprotein Ib(alpha). Fibrinogen and fibrin, the end product of the coagulation cascade, are also involved in platelet aggregation. Both fibrinogen and fibrin bind the integrin alpha(IIb)beta(3), and another fibrin receptor involved in platelet aggregation has been postulated. This review will discuss the role of thrombin and fibrin(ogen) in platelet functioning, and will highlight pathways at the crossroad of coagulation and platelet functioning, which are potential targets for antithrombotic therapy.

Fibrin↗
PubMed1999

Higher prevalence of GPIIIa PlA2 polymorphism in siblings of patients with premature coronary heart disease.

BACKGROUND: The Pl(A2) polymorphism of GPIIIa has been associated with unstable coronary syndromes in some studies, but the association has remained debated. None of the previous studies have focused on families at high risk. Risk factors tend to cluster within kindreds with high prevalence of premature coronary heart disease (CHD). Therefore, a heightened prevalence of the Pl(A2) polymorphism among siblings of patients with CHD would support the hypothesis that Pl(A2) is linked, directly or indirectly, to CHD. OBJECTIVES: To measure the prevalence of the Pl(A2) polymorphism among siblings of patients with CHD before the age of 60 years and to seek an association between the Pl(A2) polymorphism and established atherosclerotic and thrombogenic risk factors. METHODS: From January 1994 to April 1996, we genotyped 116 asymptomatic siblings (60 Caucasians, 56 Afro-Caribbeans) of patients with CHD manifestations before the age of 60 years for the Pl(A) polymorphism (also called HPA-1). A control cohort was used for comparison, consisting of individuals that were matched for race and geographic area but were free of CHD (n = 268, 168 Caucasians and 100 Afro-Caribbeans). In addition, we have characterized the sibling cohort for other atherogenic and thrombogenic risk factors. RESULTS: The prevalence of Pl(A2)-positive individuals (Pl(A2)[+], Pl(A1/A2) heterozygotes plus Pl(A2/A2) homozygotes) in the sibling cohort was high: 41.4%. When analyzed separately, the prevalence of Pl(A2)(+) siblings was 53.3% among Caucasians and 28.6% among Afro-Caribbeans. There was no association between Pl(A2) and other established atherogenic or thrombogenic risk factors. Interestingly, the clustering of other risk factors was lesser among Pl(A2)(+) siblings than their Pl(A1) counterparts. CONCLUSIONS: This study supports the hypothesis that the prevalence of Pl(A2)(+) individuals is high in kindreds with premature CHD. Hence, like the established risk factors that tend to cluster in families with premature CHD and contribute strongly to the accelerated atherosclerotic process affecting these individuals, the Pl(A2) polymorphism of GPIIIa may represent an inherited risk that promotes the thromboembolic complications of CHD. That these asymptomatic Pl(A2)(+) siblings had overall less established risk factors than their Pl(A1) counterparts might represent an explanation for why they remained asymptomatic despite their Pl(A2) positivity.

Adult↗
PubMed1999

Heparin-induced thrombocytopenia: the role of platelet activation and therapeutic implications.

In the past, heparin has been the sole anticoagulant for interventional cardiovascular procedures. Today, several alternate approaches to anticoagulate patients with heparin-induced thrombocytopenia (HIT) are under consideration. Antiplatelet drugs, such as the ADP receptor antagonists and inhibitors of glycoprotein (GP) IIb/IIIa, are currently in development. We investigated the effect of two anti-platelet agents on platelet activation induced by HIT serum (n = 5 HIT positive sera, n = 5 HIT negative sera and n = 4 donor platelets) and heparin, using the traditional platelet aggregation assay, a Lumi-aggregation assay to also determine platelet release, and flow cytometry. By all methods, the GP IIb/IIIa inhibitor-GPI 562 (Novartis; Nürnberg, Germany)-produced a concentration dependent (6.25 to 125 ng/mL) decrease in platelet activation, as shown by platelet aggregation, platelet microparticle formation, P-selectin expression, and ATP release. Similar results were obtained with the thienopyridine ADP receptor antagonist ticlopidine (Sanofi Recherche; Toulouse, France) in vitro at high concentrations of 5.0 to 50 microg/mL and ex vivo in a patient dosed at 250 mg/day. These studies show that GP IIb/IIIa and ADP receptor inhibitors can block platelet activation induced by HIT serum/heparin, providing evidence that the mechanism of HIT may be multifactorial involving not only the generation of the heparin-PF4 or other antibodies but also involving platelet-specific processes and, potentially, the generation of proaggregatory substances. The new antiplatelet agents may be useful in the clinical management of HIT patients.

Adenosine Triphosphate↗
PubMed2001

Time and dose dependent augmentation of inhibitory effects of abciximab by aspirin.

Aspirin and abciximab independently decrease the incidence of cardiac events. To identify potential interactions, antiplatelet effects of abciximab were characterized in blood from healthy subjects given aspirin. Platelet activation was determined in whole blood with and without abciximab (2 microg/ml) added in vitro. Flow cytometry was used to quantify fibrinogen binding (glycoprotein IIb-IIIa activation). Binding of fluorochrome-labeled and 125I-labeled abciximab was determined before and after exposure to aspirin. In blood from subjects given aspirin for 5 days, abciximab-induced inhibition of the capacity to bind fibrinogen in response to 1 microM ADP was greater when the daily dose had been 325 mg compared with 81 mg (% inhibition: no aspirin 53 +/- 6; 81 mg daily 62 +/- 5; 325 mg daily 69 +/- 6). The effect of 5 daily doses of aspirin was greater than that of one. Larger single doses elicited larger effects (% inhibition 2 h after 325 mg 59 +/- 6; 2 h after 650 mg 78 +/- 5). Neither salicylsalicylic acid nor naproxen sodium potentiated the effect of abciximab. Exposure of platelets to 14C-acetylsalicylic acid led to acetylation of glycoprotein IIb and IIIa. Binding of 125I-abciximab to platelets was increased after 30 and 60 min. Acetylation of glycoprotein IIb-IIIa by aspirin augments inhibitory effects of abciximab in a dose- and time-dependent manner by increasing binding of abciximab to platelets.

Abciximab↗
PubMed2001

Development and assessment of enzyme immunoassay for platelet-derived microparticles.

Platelet-derived microparticles (PMPs) are released from platelets through the platelet activation by high shear stress, collagen, or calcium ionophore (A23187). PMPs are observed in patients with acute myocardial infarction, thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, heparin-induced thrombocytopenia and other thrombotic disorders, but the importance of circulating PMPs in the pathogenesis of these diseases is still debated. Numbers of PMPs are usually determined by flowcytometry (FCM), but easier and reproducible PMP assay systems are needed. To develop a better ELISA for PMPs, we used antibodies against the platelet antigens anti-GPIb (NNKY5-5), anti-GPIIb/IIIa (NNKY2-11, anti-CD41), anti-GPIX (KMP-9), and anti-CD9 (NNKY1-19). PMPs were detected with all combinations of these antibodies, but the ELISA having the highest and most specific absorbance was obtained with a combination of KMP-9 (capture antibody) and NNKY5-5 (detecting antibody). PMPs in blood samples were measured by ELISA and FCM. ELISA correlated with PMPs quantitated by FCM. By shaking ELISA plates during incubation, nonspecific binding of platelets was eliminated. The level of PMPs was not increased in diabetes mellitus, thrombotic thrombocytopenic purpura, antiphospholipid syndrome, or sepsis. The concentration of PMP was elevated in hemolytic uremic syndrome. Activated PMPs were absorbed to 0.8 microm filter, but circulating PMPs were not absorbed. These results suggest that activated PMPs are likely to adhere to leukocytes or endothelial cells at the activation site and that the circulating form of PMPs are likely to be a residue of activated PMPs. To detect only the activated form of PMPs, a new ELISA needs to be developed, and it will likely use a combination of antibodies that detect platelet activation markers such as P-selectin (CD62P) or activated GPIIb/IIIa.

Antibodies, Monoclonal↗
PubMed2001

Occupancy of the internal and external pools of glycoprotein IIb/IIIa following abciximab bolus and infusion.

The internal pool of GPIIb/IIIa, which is expressed upon platelet activation, may be inaccessible to inhibition by GPIIb/IIIa antagonists. To determine the occupancy of the internal and external pools of GPIIb/IIIa and platelet function following an abciximab bolus and infusion, 15 patients undergoing elective percutaneous transluminal coronary angioplasty were administered abciximab as a bolus and 36-h infusion. GPIIb/IIIa receptor number and occupancy in resting and TRAP-6 (20 microM)-activated samples (to expose the internal pool of GPIIb/IIIa) was quantified using a monoclonal antibody-based assay. Antibody binding was quantified by flow cytometry and platelet inhibition by light transmittance aggregation and by the rapid platelet function analyser (Accumetrics, San Diego, CA). The target of >80% receptor occupancy (range 82--99% occupancy) of the external pool of GPIIb/IIIa was achieved in all patients at 3 min. Receptor occupancy of the combined internal and external pools of GPIIb/IIIa was less, ranging from 75 to 93% and again was maximal at 3 min. Platelet aggregation was markedly inhibited to 20 microM ADP (maximal, 11 +/- 2% of baseline), but less so to 5 microM TRAP-6 (maximal, 36 +/- 25% of baseline). Following discontinuation of the drug, there was a gradual fall in receptor occupancy over 15 days coinciding with the disappearance of abciximab from the platelet surface. Maximum inhibition of platelet function and receptor occupancy of the external pool of GPIIb/IIIa occurs within 3 min of an abciximab bolus and infusion. However, some internal receptors that are expressed by potent agonists are not occupied, which may explain the incomplete inhibition of platelet aggregation.

Abciximab↗