© 1997 American Heart Association, Inc.
Articles |
Continued Thromboxane A2 Formation Despite Administration of a Platelet Glycoprotein IIb/IIIa Antagonist in Patients Undergoing Coronary Angioplasty
From Centre for Cardiovascular Science, Royal College of Surgeons in Ireland and Beaumont Hospital, Dublin, and Department of Cardiology (N.W., P.C.), St. James Hospital, Dublin.
Correspondence to Dr. Anthony Byrne, Centre for Cardiovascular Science, Royal College of Surgeons in Ireland, St Stephens Green, Dublin 2, Ireland. E-mail dfitzgerald{at}rcsi.ie
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Abstract Experimental data suggest that formation of thromboxane A2 may be suppressed during administration of a glycoprotein IIb/IIIa antagonist. We determined the dose of one such compound, fradafiban, required to provide >80% occupancy of the platelet glycoprotein IIb/IIIa and examined its effects on thromboxane A2 formation in patients undergoing PTCA. The dose response to fradafiban and additional effects of aspirin were explored initially in patients with stable coronary artery disease. Fradafiban induced a dose-dependent inhibition of platelet aggregation that correlated with fibrinogen receptor occupancy and plasma drug concentration. Addition of aspirin 300 mg had no effect on these parameters. At the highest dose, mean fibrinogen receptor occupancy was 89.7±1.2% (n=3) at 4 hours and platelet aggregation had decreased by 93.4±2.7%. Eighteen patients undergoing coronary angioplasty were randomized to receive either aspirin 330 mg or that dose of fradafiban producing >80% fibrinogen receptor occupancy. Platelet aggregation was suppressed throughout the infusion of fradafiban to a greater extent than with aspirin. However, there was a marked increase in urinary excretion of 11-dehydrothromboxane B2 in patients treated with fradafiban: from 1973±889 to a peak of 9760±3509 pg/mg creatinine (P=.0046). Despite this evidence of continued platelet activation in vivo, there were no cases of coronary thrombosis. In conclusion, fradafiban suppresses platelet aggregation and may be a useful alternative to aspirin in the prevention of thrombotic events in patients undergoing PTCA. However, there is continued formation of thromboxane A2, which may continue to exert its effects as a potent vasoconstrictor and vascular smooth muscle mitogen.
Key Words: platelet activation • thromboxane • glycoprotein IIb • glycoprotein IIIa • coronary angioplasty
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Introduction |
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One of the earliest events in the development of arterial thrombosis is the adhesion and aggregation of platelets at a site of vascular injury. The accumulated platelet mass expresses binding sites for coagulant proteins including factors V and VIII, and acts to localize procoagulant activity and the generation of thrombin.1,2 Platelet aggregation and to some extent platelet adhesion are mediated through the binding of fibrinogen to an integrin on the platelet surface.3 Integrins are adhesion receptors consisting of an heterodimeric complex of two glycoproteins (
and ß). In the case of the platelet fibrinogen receptor
these subunits are IIb (IIb) and ß3 (IIIa).4 Under
resting conditions the platelet glycoprotein (GP)
IIb/IIIa has a low affinity for soluble fibrinogen. However, upon
activation of the cell, the receptor undergoes a conformational change
and expresses a high affinity for several ligands including fibrinogen
and von Willebrand factor.5,6 Expression
of a fibrinogen receptor is a common response to most platelet
agonists including thrombin, thromboxane
A2 (TxA2), and ADP, all of
which have been implicated in the development of arterial
thrombosis in vivo.7–9 Consequently, compounds
that antagonize this receptor and prevent platelet aggregation
would have a broader spectrum of activity than both aspirin or
ticlopidine, which inhibit thromboxane
A2 and ADP, respectively. Several classes of platelet fibrinogen receptor antagonists have been developed10 and this approach has been shown to be effective in preventing thrombosis and restenosis when combined with aspirin during coronary angioplasty.11,12 It is not clear, however, whether there is a need to coadminister aspirin with GPIIb/IIIa antagonists. Suppression of platelet thromboxane A2 may add to the antithrombotic activity of these compounds.13 Moreover, thromboxane A2 is a potent vasoconstrictor14 and vascular smooth muscle mitogen.15 Consequently, suppression of thromboxane A2 biosynthesis may be desirable particularly in the setting of coronary angioplasty. Thromboxane A2 formation is stimulated by most platelet agonists through transmembrane receptors that activate phospholipases either directly or as a consequence of raised intracellular Ca2+. However, thromboxane A2 formation is also dependent on secondary signaling that occurs following fibrinogen-GPIIb/IIIa interactions and subsequent platelet aggregation.16 Indeed, there is evidence that at least some antagonists of the platelet GPIIb/IIIa suppress thromboxane A2 formation in vivo,17 potentially obviating the need for aspirin.
We have examined the pharmacology of a novel, synthetic antagonist of the platelet GPIIb/IIIa receptor (fradafiban) in patients with coronary artery disease. Fradafiban has both a prolonged half-life and oral bioavailability so that it may be suitable for chronic administration.18 The purpose of the study was to characterize the relationship between occupancy of the platelet fibrinogen receptor and platelet aggregation, to identify a dose of fradafiban that would result in a >80% occupancy of the fibrinogen receptor and to examine the generation of thromboxane A2 in the presence of fradafiban alone in patients undergoing coronary angioplasty.
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Methods |
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Fradafiban (3S,5S)-5-[(4'-amidino-4-biphenylyl)oxymethyl]-3-carboxymethyl-2-pyrrolidinone) was supplied by Boehringer Ingelheim UK Ltd. The study was approved by the Ethics Committees of Beaumont, Blackrock, and St. James hospitals. Each patient gave written, informed consent before treatment.
Dose Finding Study
Initially 10 patients with stable coronary artery
disease (7 males and 3 females, mean age 56±2.7 yr) were evaluated in
a dose-ranging study. Those with a diagnosis of unstable angina or
recent (within 14 days) acute myocardial infarction were excluded. No
patient had contraindications for antiplatelet therapy and baseline
tests of hematological, renal, and hepatic function were normal. Before
study commencement, all subjects were required to avoid nonsteroidal
anti-inflammatory drugs, including aspirin, for at least 10 days. No
patient was receiving regular anticoagulant therapy.
The patients received one of three doses of fradafiban: 8.8 mg, 17.7 mg, or 26.5 mg by intravenous infusion. The drug was prepared in a total volume of 500 mL of 5% dextrose. A bolus of 5 mg, 10 mg, or 15 mg, respectively, was given over 30 minutes with the remainder given over 6 hours. Four hours into the infusion, aspirin 300 mg was administered orally to assess whether it had any additive antiplatelet effect. Samples were obtained and bleeding times performed at baseline and 4, 6, and 24 hours after commencement of infusion. The blood was drawn from an indwelling catheter in the arm opposite to the one used for the drug infusion and the catheter was flushed with 5 mL of 0.9% saline. A discard of 10 mL was taken before the sample collection.
Fradafiban Compared With Aspirin During PTCA
In a second study, patients undergoing
percutaneous transluminal coronary angioplasty
(PTCA) were randomized in a double-blind, placebo controlled fashion to
receive either fradafiban or aspirin. Patients received
fradafiban+heparin+ aspirin placebo or intravenous placebo
fradafiban+heparin+oral aspirin 330 mg. The ratio of active to placebo
fradafiban treatments was 4:1. Eighteen patients were enrolled at the 2
hospitals (15 males and 3 females, mean age 56±1.9 years). Two
patients started on fradafiban were withdrawn from the study without
completing the protocol, in one because the angioplasty failed and in a
second due to coronary artery dissection. The latter underwent
coronary artery bypass grafting 24 hours later, without any
adverse hemostatic events. Their data are not included in the
analysis.
Based on the findings of the dose-ranging study, to achieve >80% occupancy of platelet fibrinogen receptors, fradafiban was administered as a bolus dose of 15 mg over 30 minutes followed by a continuous infusion rate of 2.09 mg/h for 23.5 hours. Infusion of drug commenced at least 1 hour before the start of PTCA. Patients also received acetylsalicylic acid 330 mg or placebo orally immediately before commencement of placebo/fradafiban infusion. At the beginning of the procedure a bolus dose of heparin (10 000 IU) was administered followed by a continuous infusion for a minimum of 6 hours. The heparin infusion was adjusted to achieve a therapeutic response as measured by a PPT. Bleeding times and blood samples for platelet studies and drug levels were obtained at timed intervals for up to 72 hours after drug commencement. In addition, timed urine collections were obtained for measurement of urinary 11-dehydro thromboxane B2, the major enzymatic metabolite of thromboxane.19
Platelet Aggregation
Platelet aggregation was measured ex vivo using an optical
aggregometer (Biodata PAP 4, Biodata Corporation, Horesham, Pa, USA).
Samples were collected into syringes containing 3.8% sodium citrate at
a final dilution of 1:10. Platelet-rich plasma (PRP) was
immediately prepared by centrifugation at
150g for 10 minutes. Following removal of the PRP, the
remaining plasma was centrifuged at 1500g for 10
minutes to obtain platelet-poor plasma (PPP). This was then used
for calibration of the aggregometer. Platelet aggregation was
measured in response to thrombin receptor activator peptide
(TRAP): Ser-Phe-Leu-Leu-Arg-Asn-Pro-Asn-Asp 20 µmol/L (Dr
Brian Walker, Queen's University, Belfast), collagen 2 µg/mL
(Helena Laboratories, Beaumont, TX, USA), or a combination of both.
Agonists were prepared at 10X concentration and stored on ice during
use. Platelet responses were measured as percent aggregation at 4
minutes and are reported as such in the text. Percent inhibition was
determined by expressing the platelet aggregation on drug as a
percent of the baseline aggregation and subtracting that from 100.
Fibrinogen Receptor Occupancy
Fibrinogen receptor occupancy was measured by competitive assay
using 3H fradafiban (Dr. Hans Weisenberger, Dr
Karl Thomae GmbH). Platelet-rich plasma was prepared and 200 µL
of this was mixed with 10 µL of 14C-sucrose (2.7 kBq) and 10 µL of
3H-fradafiban (5 nmol/L final
concentration). The mixture was incubated at room temperature for 20
minutes, centrifuged at 2000g for 5 minutes with
subsequent removal of the supernatant. A 100 µL aliquot of this was
counted for free ligand. The platelet pellet was dissolved in 200
µL of 0.2N NaOH and 180 µL of this mixed with 10 µL 5N HCl and
counted in 2 mL of scintillation cocktail. As
14C-sucrose does not enter cells, and the ratio
of 3H to 14C is constant,
the amount of free 3H fradafiban could be calculated. The amount of
fradafiban bound in the pretreatment samples of each patient was taken
as 100% binding for that patient, with lower binding in subsequent
samples expressed as a percentage of this. Binding was unaffected by
heparin or aspirin.
Ligand-Induced Binding Site (LIBS) Expression
LIBS expression was assessed ex vivo by flow cytometric
analysis (FACScan, Becton Dickenson, Oxford, UK). Samples were
analyzed for platelet associated fluorescence
following incubation with a monoclonal antibody to the D3 epitope (kind
gift of Dr. L. Jennings, University of Tennessee, Memphis). Results
were presented as a percentage of maximal EDTA induction of
LIBS. Samples were collected in 3.8% sodium citrate (1:10 dilution)
and three aliquots of 200 µL of PRP prepared. EDTA (5
mmol/L final concentration) was added to the first and an equal
volume of vehicle (phosphate buffered saline) to the other two. Samples
were incubated at 37°C for 30 minutes and fixed in an equal volume of
formaldehyde 2%. The samples were stored at 4°C before the assay.
The fixed samples were washed in PBS containing 1% BSA and resuspended
in 1 mL of PBS/BSA. Aliquots (100 µL) were incubated with 4 µL of
the D3 monoclonal antibody (4 ng/mL) for 30 minutes at room
temperature. The samples were washed and r resuspended on 100 µL of
fluorescein isothiocyanate goat anti-mouse IgG (FITC GM)
(Becton-Dickinson, Oxford, UK) for a further 30 minutes at room
temperature. For each time point, a control sample was incubated with
FITC GM) alone to determine background fluorescence. Flow
cytometric analysis was performed with a Becton-Dickinson
FACScan and the data analyzed by Becton-Dickinson Lysis II.
GMP40 Expression
Ex vivo and in vitro expression of GMP140 was determined also by
flow cytometry. Briefly, aliquots of PRP (50 µL) were incubated with
4 µL of anti-CD62 monoclonal antibody (Immunotech SA, Marseilles, FR)
for 20 minutes at room temperature. The samples were washed and
resuspended in 100 µL of FITC GM. Background fluorescence
was determined as described above and the samples analyzed by
flow cytometry.
Biochemical Analysis
For serum thromboxane B2
measurement, whole blood was placed in nonsiliconized glass vials and
incubated at 37°C for 45 minutes. Following
centrifugation at 1500g for 10 minutes
supernatants were stored at -20°C until analyzed.
Thromboxane B2 was derivatized to the
trimethylsilyl ether, pentafluorobenzyl ester before quantification by
gas chromatography and negative-ion, chemical
ionization mass spectrometry (INCOS-XL, Finnigan Mat, Hemel Hempstead).
Plasma samples for drug levels were collected in 3.8% sodium citrate
onto ice and centrifuged at 1500g for 10 minutes.
Supernatants were combined with an equal volume of 0.5 N HCl and stored
at -20°C. Plasma drug levels were measured by automated column
switching high-performance liquid
chromatography. Samples were applied to Bondapak
C18/Corasil-filled columns and separation achieved on a column filled
with ODS-Hypersil, 5 µmol/L. The mobile phase consisted
of an equal volume of solution A (ammonium acetate, SDS, and acetic
acid) and solution B (acetonitrile and methanol). Fluorescence
detection, with excitation at 310 nm, was used for quantitation. The
coefficient of variation was <2% and there was an overall imprecision
of approximately 6% for plasma samples.
Bleeding Time
The bleeding time was measured on the volar aspect of the
forearm. A blood pressure cuff was positioned and inflated to 40
mm Hg. An incision, 5 mm in length and 1 mm in depth,
was made distally using a Simplate bleeding device (Organon Teknika,
Durham, NC, USA). Blood was absorbed onto Whatman filter paper every 30
seconds until the bleeding stopped.
Statistical Analysis
The data are expressed as the mean±SEM. The data were
analyzed by Kruskal-Wallis one way analysis of variance
with subsequent Mann-Whitney test for comparison between groups. This
analysis makes no assumptions about the distribution of the
data. Paired data, where appropriate, were compared by t
test.
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Results |
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In Vitro Studies
To explore whether fradafiban exhibited any agonist activity, platelet-rich plasma from 2 subjects was incubated at increasing concentrations of fradafiban (0.33 ng/ml to 330 µg/mL) for 10 minutes and serum thromboxane B2 determined. There was no change in either subject (0.21 and 0.24 ng/mL to 0.26 and 0.19 ng/mL, respectively, at the highest concentration of drug). In one subject, GMP140 expression was assessed and fell from 13% of cells to 9% following incubation for 10 minutes at the highest concentration of drug, suggesting that in vitro there was no agonist activity.
Dose Finding Study
No serious adverse effects were reported in the dose-finding
group. One patient receiving the highest dose of fradafiban had
evidence of localized gingival bleeding at 3 hours of infusion that
resolved within 30 minutes. No other evidence of bleeding was noted.
There was no significant change in platelet counts (x109/L) in the
9 subjects at 6 hours (250±18) or 12 hours (250±18) of infusion
compared with baseline (268±18).
Fibrinogen Receptor Occupancy and Platelet Aggregation
Plasma fradafiban concentration was similar at 4 hours (121±18,
232±14, and 319±37 ng/mL) and at 6 hours (114±16, 228±11.5,
and 301±44 ng/mL), with low, intermediate, and high doses,
respectively, of fradafiban. There was a dose-dependent increase in
fibrinogen receptor occupancy (Table
). A primary end
point of the study was to identify a dose of fradafiban capable of
achieving greater than 80% fibrinogen receptor occupancy. The highest
dose achieved this criterion in the 3 subjects studied, with an average
fibrinogen receptor occupancy of 89.7±1.2% at 4 hours. Fibrinogen
receptor occupancy was unchanged (88.6±1.4%) following the
administration of aspirin (Table).
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Fradafiban also induced a dose-dependent inhibition of platelet
aggregation to the thrombin receptor activator peptide
(TRAP) or to a combination of TRAP and collagen in the dose-ranging
group. Platelet aggregation to the combination of collagen and TRAP
at baseline was 79±2.6% and fell at 4 hours of infusion to 18±7%
(P=.0081), 14±7.6% (P=.007), and 4.7±2.2%
(P=.017) at low, intermediate, and high doses of fradafiban,
respectively. The addition of aspirin had no further effect on
platelet aggregation, which at 6 hours was 23.3±1.1%, 9.3±5.6%,
and 5.3±0.8% at low, intermediate, and high doses of fradafiban,
respectively. Inhibition of platelet aggregation and fibrinogen
receptor occupancy were closely related to the plasma concentration of
fradafiban (Fig 1). Inhibition of
platelet aggregation occurred only at a fibrinogen receptor
occupancy of >20% and was maximal at a receptor occupancy of >80%.
At 24 hours, 17.5 hours after discontinuation of the drug infusion,
fibrinogen receptor occupancy was 50.9±3.6% in the highest dose
group, and platelet aggregation had recovered to 74±1% of
baseline.
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Effect of Fradafiban on Bleeding Time
Bleeding time at 4 hours of infusion before the administration of
aspirin was prolonged to >30 minutes in 9 of 10 patients in the
dose-finding study. Bleeding times were also >30 minutes after the
administration of aspirin. In 1 patient receiving the lowest dose,
bleeding time increased to 17 minutes at 4 hours. The bleeding time
returned to within 20% of baseline at 24 hours in 7 patients and by 28
hours in the remainder.
Effects of Fradafiban and Aspirin on Serum Thromboxane
B2
Fradafiban infusion had no significant effect on serum
thromboxane B2 levels at 4 hours
compared to baseline in the dose-ranging study: 359±38 ng/mL
versus 423±17 ng/mL (P=NS). In contrast, aspirin
suppressed serum thromboxane B2 by
99%, to 2.5±0.4 ng/mL.
Fradifiban Compared With Aspirin During Coronary
Angioplasty
Sixteen of the 18 patients completed the study. Eleven had
proximal left anterior descending (LAD) coronary artery
lesions, 4 had mid LAD lesions, and 1 had mid right coronary
artery disease. Thirteen patients were receiving calcium channel
blockers, 6 patients were receiving beta blockers, 2 received
angiotensin converting enzyme inhibitors, and 1
patient was on a diuretic. Three patients developed small
hematomas at the femoral puncture site, 2 had received fradafiban and 1
had received aspirin. Transient oozing was noted at femoral and/or
subclavian line sites during drug infusion in 9 patients (8 fradafiban
and 1 placebo). The most serious bleed occurred in a patient treated
with aspirin only, who had an episode of blood loss at the time of
removal of the femoral sheath, estimated at 200 to 300 mL. There was a
fall in hemoglobin (Hb) of 2.8 g/dL in this patient over the
study period, which did not require treatment, and the patient
completed the study protocol without further complication. One patient
receiving active fradafiban suffered a significant fall in hemoglobin
over the study period, a decrease of 3.6 gr/dL (15.0 gr/dL to 11.4
gr/dL). This was associated with a moderate femoral hematoma and oozing
at catheter sites at 4 and 6 hours. No other sites of bleeding were
noted and no blood transfusion was required. One patient treated with
fradafiban also had mild bleeding from the buccal mucosa after 10 hours
of infusion. This lasted for 1 hour and resolved without treatment.
There were no acute thrombotic events.
There was no significant change in platelet count (x109/L) in either the aspirin-treated patients (236±1, 214±14, 204±11, and 207±11 at baseline, 6 hours, 24 hours, and 72 hours, respectively) or in the patients receiving fradafiban (253±15, 231±15, 224±11, and 231±17, at baseline, 6 hours, 24 hours, and 72 hours, respectively).
Administration of fradafiban resulted in a steady state plasma
concentration of 295±38 ng/mL, whereas none was detected in
patients treated with aspirin alone. Peak fibrinogen receptor occupancy
was 95±0.5% (n=16) and at 24 hours (end of infusion) was 87±1.2%
(n=16). Note that in all cases, fibrinogen receptor occupancy exceeded
80% throughout the fradafiban infusion (Fig 2). In contrast, fibrinogen receptor
occupancy reached a peak of 13.3±14% in those patients receiving
aspirin, reflecting a high, possibly aberrant value in 1 patient, as it
had returned to normal at 1.5 hours. As for the patients in the
dose-finding study, fibrinogen receptor occupancy correlated with
plasma drug level (Fig 3). Platelet
aggregation was markedly suppressed in the patients receiving
fradafiban and again this was closely related to fibrinogen receptor
occupancy. Baseline aggregation to TRAP/collagen was 77.5±2.8% and
fell to 7.2±1.8% (P=.001) at 6 hours and 6.3±1.7%
(P<.0039) at 24 hours. In contrast, platelet
aggregation was inhibited only marginally in patients treated with
aspirin (77±1.2% to 60±11.7% at 6 hours). Inhibition of
platelet aggregation by fradafiban was maximum at 80% receptor
occupancy and had a threshold of 20% to 30% receptor occupancy (Fig 3). Following discontinuation of fradafiban, platelet aggregation
to the combination of TRAP and collagen recovered over the next 24
hours as fibrinogen receptor occupancy decreased (Fig 2). The delayed
recovery of fibrinogen receptor occupancy was due to the long-terminal
half-life of the drug (mean 14 hours, range 10.2 to 20.2 hours). In the
patients receiving fradafiban, bleeding times increased significantly,
to 19.9±2.2 minutes from a baseline of 4.9±0.5 minutes.
(P=.0017). Bleeding times had returned to less than 10
minutes in all patients 24 hours after discontinuing the infusion, with
a mean of 7.2±0.4 minutes. (Fig 4). In
the placebo group receiving aspirin, baseline and peak bleeding times
were 4.2±0.2 and 5.5±1.6 minutes (P=NS), respectively.
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Patients undergoing coronary angioplasty who received
fradafiban and no aspirin had a significant increase in
11-dehydrothromboxane B2 levels
(P=0.0046) during the study period (Fig 5). In contrast, urinary
11-dehydrothromboxane B2 was
suppressed in the patients treated with aspirin (data not shown). These
findings demonstrate persistent platelet activation in the patients
receiving fradafiban. Fradafiban did not induce expression of the D3
epitope during administration compared to baseline in either arm of the
study. In the dose-ranging study baseline (measured as % maximal EDTA
expression) was 8.2±1.3% and was 7.1±2.5% during the fradafiban
infusion. In the PTCA group, those receiving fradafiban had a
pretreatment expression of 9.4±1.6% and a level 11.5±1.8% during
the fradafiban infusion, compared to 4.3±2.6% and 8.3±2.4%,
respectively, in those receiving placebo. Similarly, ex vivo GMP140
expression on resting platelets was unaltered in 3 patients
receiving fradafiban (3.7±1.2% of platelets before treatment
versus 3±0.8% during the fradafiban infusion).
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Discussion |
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Antagonism of the Platelet GPIIb/IIIa
Several novel antagonists of platelet GPIIb/IIIa are being developed as potential antiplatelet agents. Large-scale clinical trials have been completed with two of these agents, the chimeric antibody 7E311,12 and the peptide antagonist integrelin.20 Whereas the chimeric antibody was effective in preventing events in high-risk angioplasty, integrelin had little protective effect in patients with unstable angina undergoing the same procedure.20 It is possible that this reflected different levels of receptor antagonism by the two agents. 7E3 was administered in a dose that achieved >80% receptor occupancy21 and complete suppression of platelet aggregation. With a lower dose of 7E3, suppression of platelet aggregation was incomplete and no benefit was seen. These data suggest that the level of receptor occupancy is critical for clinical effect, at least in the setting of coronary angioplasty. It is possible that the dose of integrelin was inadequate, as complete inhibition of aggregation has been shown only at higher doses.22
Based on the findings of EPIC, namely that a dose of 7E3 that occupied 80% of receptors was effective in preventing acute thrombotic events during coronary angioplasty, we performed a dose-escalating study to identify a dose of fradafiban that would provide complete suppression of platelet aggregation and >80% occupancy of the platelet GPIIb/IIIa. Fradafiban induced a dose-dependent fibrinogen receptor occupancy and inhibition of platelet aggregation ex vivo. Both fibrinogen receptor occupancy and inhibition of aggregation correlated closely with the plasma concentration of the drug. Moreover, the degree of receptor occupancy was a major determinant of the platelet response. Thus, the antiplatelet activity of fradafiban is explained by its known biological activity.
Although there was a good correlation between platelet aggregation and receptor occupancy, a minimum of 20% fibrinogen receptor occupancy was required before an effect was seen. Consequently, during the washout phase platelet aggregation had recovered considerably at 24 hours despite continued fibrinogen receptor occupancy and detectable plasma drug levels. There was a similar discrepancy with the bleeding time, which recovered rapidly following discontinuation of the drug. A similar disparity between recovery of bleeding time and platelet aggregation following discontinuation of integrelin has been reported.23 Indeed, there are data suggesting that a minimum of 70% to 80% fibrinogen receptor occupancy is required before bleeding time is altered.24
Thromboxane A2 Formation and GPIIb/IIIa
Antagonism
During coronary angioplasty there was a marked increase in
thromboxane A2 biosynthesis measured
as excretion of its enzymatic metabolite,
11-dehydrothromboxane B2, despite
substantial suppression of platelet aggregation with fradafiban.
One explanation, that fradafiban is acting as a partial agonist and
therefore stimulating platelet thromboxane formation,
is unlikely for several reasons. Although for ethical reasons there was
no "untreated" group for comparison, the magnitude of increase in
thromboxane biosynthesis was similar to that reported
previously in aspirin-sensitive subjects receiving no antiplatelet
therapy during coronary angioplasty.25
Moreover, fradafiban did not induce thromboxane
A2 formation in vitro or the expression of LIBS
or GMP140 in vitro or ex vivo. The more likely explanation is that
platelets continue to be activated by agonists acting on
transmembrane receptors linked to phospholipases via G-proteins.
Although fibrinogen-GPIIb/IIIa interactions and platelet
aggregation provides additional thromboxane
A2, this mechanism appears to be important only
for weak agonists, such as
epinephrine.16
In conclusion, fradafiban suppresses platelet aggregation and may be a useful alternative to aspirin in the prevention of thrombotic events in patients undergoing PTCA. However, even at high (>80%) receptor occupancy and suppression of platelet aggregation, there is continued formation of thromboxane A2, a potent vasoconstrictor and vascular smooth muscle mitogen.
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Acknowledgments |
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Supported in part by grants from the Wellcome Trust and the Irish Heart Foundation. Dr Byrne is a Health Research Board of Ireland Research Fellow.
Received May 6, 1997; accepted July 29, 1997.
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