(Circulation. 1997;96:1488-1494.)
© 1997 American Heart Association, Inc.
Articles |
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
From COR Therapeutics, South San Francisco, Calif (D.R.P., W.T., A.A., L.N.-A., A.R., R.M.S.); University of Tennessee, Memphis (M.M.W., C.L., L.K.J.); Lilly Research Laboratories, Indianapolis, Ind (J.A.J.); and Scripps Research Institute (S.J.S.).
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Abstract |
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Background Integrilin (eptifibatide), a potent inhibitor of the fibrinogen binding function of GP IIb-IIIa, 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 IC50=140±40 nmol/L, n=6;
Ca2+=40 to 50 µmol/L) than with PPACK
(IC50=570±70 nmol/L, P<.0001, n=6;
Ca2+ 
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.
Key Words: calcium • drugs • fibrinogen • glycoproteins • platelets
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Recent clinical trials have established that antagonists of fibrinogen binding to GP IIb-IIIa are effective in reducing thrombotic complications induced by coronary angioplasty.1 2 3 4 5 The most widely studied GP IIb-IIIa antagonists are parenteral agents that have been administered by infusion before coronary intervention and continuing for up to 36 hours after the procedure. Although all clinical trials show that GP IIb-IIIa inhibition reduces acute thrombotic complications, some variability in other end points has been observed. Thus, it is reasonable to question whether all trials have been performed with the same level and duration of GP IIb-IIIa antagonism and inhibition of platelet aggregation.
Integrilin, a cyclic heptapeptide, is a GP IIb-IIIa
antagonist that blocks platelet aggregation and has
been found to reduce the incidence of ischemic clinical events
in acute coronary indications.3 6 7 Integrilin,
like other antagonists of GP IIb-IIIa, functions by
blocking the binding of the adhesive proteins fibrinogen and von
Willebrand factor to GP IIb-IIIa on the surface of
activated platelets.1 7 8 GP IIb-IIIa
antagonists are effective antithrombotics because the
binding of adhesive proteins to GP IIb-IIIa is required for
platelet aggregation and arterial thrombus
formation.9 10 Pharmacodynamic measurements of the
potential in vivo activity of Integrilin and other parenteral
inhibitors have been based primarily on their effects on ex
vivo platelet aggregation, although receptor occupancy has been
determined in patients receiving the monoclonal antibody
abciximab.11 Ex vivo platelet aggregation
analyses are routinely performed in blood anticoagulated with
citrate,3 5 6 an agent that prevents in vitro clotting by
reducing the ionized calcium concentration from the 1
mmol/L level found in circulating blood to 40 to 50
µmol/L.12 Thus, the Ca2+
concentrations are markedly reduced in most routine pharmacodynamic
measurements of platelet aggregation inhibitors.
GP IIb-IIIa reversibly binds Ca2+ and other divalent cations at its five divalent cation binding sites; the affinities of these sites predict that all are occupied at physiological levels of Ca2+.13 14 Reductions in the amount of divalent cations bound to GP IIb-IIIa can dramatically affect its structure and function. For example, suspension of platelets in solutions containing ionized calcium concentrations of 40 to 50 µmol/L as achieved in citrate-anticoagulated blood can cause the removal of Ca2+ from GP IIb-IIIa15 16 and induce a loss of fibrinogen-binding activity.17 At <1 µmol/L Ca2+ at 37°C, GP IIb dissociates from GP IIIa within the plane of the plasma membrane,18 and both subunits lose their structure.19 Although it has been well established that the binding of adhesive proteins and peptide ligands to GP IIb-IIIa does not occur at ionized calcium concentrations <1 µmol/L, recent studies have established that peptide ligands harboring the Arg-Gly-Asp (RGD) sequence, which are also antagonists of GP IIb-IIIa, displace Mn2+ from GP IIb-IIIa when Mn2+ is used to occupy the divalent cation binding sites20 and that Ca2+ increases the rate of dissociation of RGD ligands from GP IIb-IIIa.21 From this finding, D'Souza and coworkers20 have suggested that divalent cations and competitive antagonists may compete for binding to GP IIb-IIIa. An extension of these observations is that partial reductions in the amount of divalent cations bound to GP IIb-IIIa may actually enhance the binding of some of the GP IIb-IIIa antagonists.22 The present study examines the effect of Ca2+ on the inhibitory activity of Integrilin and demonstrates that the ionized calcium concentrations achieved in blood anticoagulated with citrate markedly enhance the inhibitory activity of Integrilin. This finding indicates that pharmacodynamic measurements of the platelet aggregation inhibitory activity of Integrilin is enhanced by collection of blood samples with citrate as the anticoagulant.
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Platelet Aggregation
PRP was prepared from blood drawn from healthy human volunteers either into sodium citrate solution (51 mL of blood into 9 mL of 3.8% sodium citrate) to make citrate-anticoagulated blood or into PPACK (60 mL blood into 126 µL of 25 mg/mL PPACK, Calbiochem-Novabiochem) for PPACK-anticoagulated blood. Platelet aggregation in PRP samples was determined with a platelet aggregometer (Chrono-log Corp) as described,23 initiating aggregation either by the addition of ADP (20 µmol/L final concentration, Sigma Chemical Co) or TRAP (SFLLRN-NH2, 5 µmol/L final concentration, Peninsula Laboratories) to 0.50 mL of stirred PRP samples (1000 to 1200 rpm) at 37°C. Integrilin (solution for intravenous administration) was added 2 minutes before agonist. The Integrilin platelet aggregation IC50 was determined from full dose-response curves. Each value is reported as the average±SD from values determined on the platelet preparations from different donors.
Purified GP IIb-IIIa–Fibrinogen Binding
The binding of purified fibrinogen to purified GP IIb-IIIa,
immobilized on microtiter plates, was performed as
previously described.24 The indicated concentrations of
Integrilin were included in each incubation. The Integrilin ligand
binding IC50 was calculated as the average from replicate
dose-response curves.
Ca2+ Determinations
The ionized calcium concentration in PRP samples was determined
with an ion-selective electrode (Orion 93-20) coupled to a
single-junction reference electrode (Orion 90-01).12
Standard curves were generated with the Orion calcium activity standard
solution that were linear on a logarithmic scale from 10
mmol/L down to 80 µmol/L and curvilinear below
this value. All readings were made after a 2-minute incubation that was
required to achieve stability.
D3 Binding
Binding of the D3 antibody after Integrilin induction was
determined by a modification of a published procedure.25
Integrilin was added to 50-µL aliquots of PRP (platelet count
previously adjusted to 250 000/µL with autologous platelet-poor
plasma) and incubated for 2 minutes, followed by a 1-hour incubation
with PE-conjugated D3. Binding of PE-D3 was analyzed with a
Becton-Dickinson FACSCaliber. Baseline D3 binding was measured in the
absence of added Integrilin; nonspecific IgG binding was determined by
use of PE–mouse IgG. The Quantum Series (Flow Cytometry Standards
Corp) was used to quantify fluorescence intensity. The Quick
Cal (Flow Cytometry Standards Corp) program was used to standardize the
linear regressions for the construction of calibration plots. From a
full dose-response curve, the PE-D3 maximum mean fluorescence
intensity value was determined, converted to the corresponding number
of MESF, and designated 100% occupancy. For each dose of Integrilin
added, the mean fluorescence intensity value was also converted
to MESF units and the percent of maximum calculated and reported as
percent occupancy.
PAC1 Binding
PAC1 antibody was labeled with FITC. PAC1 binding was performed
in PRP samples treated with 20 µmol/L
ADP.26
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Aggregation
Fig 1
illustrates the platelet
aggregation inhibitory activity of 500 nmol/L
Integrilin in PRP treated with 20 µmol/L ADP from blood
anticoagulated with citrate or PPACK. Although this concentration of
drug resulted in complete inhibition of ADP-induced platelet
aggregation in the presence of citrate, only 15% to 30% inhibition
was observed in the PPACK-anticoagulated blood. To quantify the effect
of anticoagulant on the inhibitory activity of Integrilin,
the IC50 for Integrilin inhibition of platelet
aggregation was determined for each of the anticoagulants
(Table). The concentration required to
inhibit ADP-induced aggregation by 50% was more than 4-fold higher for
plasma anticoagulated with PPACK (mean±SD, IC50=570±70
nmol/L; n=6) than for plasma anticoagulated with citrate
(IC50=140±40 nmol/L, P<.0001; n=6). The
IC50 for heparin-anticoagulated PRP was similar to that of
PPACK-anticoagulated PRP (data not shown). It has been observed that
higher concentrations of GP IIb-IIIa antagonists are
required to inhibit platelet aggregation induced by thrombin, a
more potent platelet agonist.22 27 28 To determine
whether the anticoagulants also impact the inhibitory
activity of Integrilin on aggregation induced by activation of the
thrombin receptor, we measured the ability of Integrilin to inhibit
aggregation induced by TRAP, a synthetic peptide
(SFLLRN-NH2) that activates thrombin receptors to
induce platelet aggregation in PRP without clotting
fibrinogen.29 Although ADP-induced aggregation required a
4-fold- higher Integrilin concentration to inhibit aggregation by 50%
in PRP anticoagulated with PPACK compared with citrate
(P<.0001), TRAP-induced aggregation required a
7.5-fold-higher Integrilin concentration in PPACK-anticoagulated PRP
(P<.0001). TRAP-induced aggregation also required a
higher Integrilin concentration for inhibition than did ADP-induced
aggregation (1.4-fold for citrate, P=.095; 2.7-fold for
PPACK, P=.0002).
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Receptor Occupancy
To determine the effect of anticoagulant on Integrilin binding to
GP IIb-IIIa, Integrilin induction of the D3 epitope on the GP IIb-IIIa
of platelets in plasma was measured. Previous studies have
established that the binding of fibrinogen, RGD-containing peptides, or
Integrilin to GP IIb-IIIa induces the expression of an LIBS neoepitope
on GP IIb-IIIa that can be detected by the D3
antibody.25 30 As shown in the Table
, Integrilin induced
the expression of D3 binding in PRP anticoagulated by PPACK with an
IC50=420±37 nmol/L (n=3) and in PRP anticoagulated
by citrate with an EC50=105±5 nmol/L
(P=.0001, n=3). Because these data reflected the amount of
Integrilin bound to GP IIb-IIIa on unstimulated platelets, these
observations indicate that Integrilin has greater binding to GP
IIb-IIIa in blood anticoagulated with citrate than with PPACK.
PAC1 Binding
The monoclonal antibody PAC1 has binding properties that mimic the
binding of fibrinogen to GP IIb-IIIa in that (1) both require
platelet activation before binding can be observed, (2) both have
similar divalent cation requirements for binding, and (3) the binding
of both is inhibited by similar antagonists.26
The effect of anticoagulant on Integrilin inhibition of PAC1 binding
was measured in an effort to determine whether enhanced Integrilin
binding in citrate resulted in enhanced inhibition of ligand binding to
ADP-activated platelets. The Table shows that a higher
concentration of Integrilin was required to inhibit PAC1 binding to
platelets activated with ADP in PRP anticoagulated with
PPACK (EC50=143±87 nmol/L; n=3) than with citrate
(EC50=31.7±9.0 nmol/L, P=.092; n=3),
demonstrating that the enhanced Integrilin inhibition of aggregation in
citrate is due to enhanced inhibition of ligand binding to the
activated platelets.
Calcium Concentrations
Although blood contains 2.2 mmol/L total calcium,
the ionized calcium concentration is approximately one half this
amount, or 1.1 to 1.2 mmol/L. Although PPACK or heparin
anticoagulation maintains this concentration of Ca2+, the
Ca2+ concentration in citrate-anticoagulated PRP is reduced
to 40 to 50 µmol/L.12 To determine
whether the difference in the inhibitory activity of
Integrilin in citrate versus non–Ca2+-chelating
anticoagulants was due to the difference in Ca2+
concentration or to a specific effect of the anticoagulant, the
concentration of ionized calcium in PRP anticoagulated with heparin was
titrated with either citrate or EDTA. Fig 2 shows that chelation of
Ca2+ with either agent caused a similar degree of
enhancement of Integrilin inhibition of ADP to induced platelet
aggregation at all Ca2+ concentrations
(EC50=60 µmol/L). Because two different
Ca2+ chelators showed a similar enhancement of the
inhibitory activity of Integrilin, whereas the
inhibitory activity was similar in PRP anticoagulated by
two independent antithrombin strategies (eg, heparin and PPACK), these
data suggest that citrate enhances the apparent inhibitory
activity of Integrilin by reducing the concentration of ionized calcium
and not because of a nonchelating effect of citrate.
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Purified Protein Binding
We next determined whether the Ca2+ concentrations
achieved in PPACK- and citrate-anticoagulated PRP affected the ability
of Integrilin to inhibit direct fibrinogen–GP IIb-IIIa interactions.
This was determined by measuring Integrilin inhibition of the binding
of biotinylated fibrinogen to purified GP IIb-IIIa with a solid-phase
binding assay that measures the binding of fibrinogen to GP IIb-IIIa
immobilized on microtiter wells.24 Previous
studies have established that this assay, which uses purified proteins,
mimics the binding of soluble fibrinogen to GP IIb-IIIa on the surface
of activated platelets and correspondingly reflects the
binding of inhibitors to the activated form of GP
IIb-IIIa.7 24 As shown in Fig 3, Integrilin inhibits the binding of
fibrinogen to GP IIb-IIIa in 1 mmol/L Ca2+ with
an IC50 of 63±34 nmol/L (n=6). When this same
measurement is conducted in 50 µmol/L Ca2+,
Integrilin becomes a more potent inhibitor with an
IC50 of 8.7±1.2 nmol/L (P=.003, n=6).
Thus, reduction of the ionized calcium concentration enhances
Integrilin inhibition of fibrinogen binding to GP IIb-IIIa.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The present study demonstrates that a decrease in the ionized calcium from the
1 mmol/L concentration normally found
in blood to <100 µmol/L, as is found in
citrate-anticoagulated blood, enhances the GP IIb-IIIa binding of
Integrilin and increases its apparent platelet aggregation
inhibitory activity. Citrate is perhaps the most widely
used anticoagulant for analysis of the pharmacodynamics of
platelet aggregation inhibitors; the ionized calcium
concentrations achieved with this anticoagulant (40 to 50
µmol/L) are too low to support the catalytic activities of
coagulation factors yet are sufficient to support platelet
aggregation. The present data indicate, however, that use of
citrate-anticoagulated blood grossly overestimates the platelet
aggregation inhibitory activity of Integrilin.
Citrate Effect
The present study was designed to determine the mechanism by
which citrate enhances the platelet inhibitory activity
of Integrilin. The IC50 of 570 nmol/L for inhibition
of ADP-induced platelet aggregation in PPACK-anticoagulated PRP was
assumed to be indicative of the activity of Integrilin in PRP at normal
plasma concentrations of Ca2+, because similar
IC50 values were observed for ADP-induced platelet
aggregation in PRP anticoagulated with heparin or with hirudin (data
not shown). Because chelation of divalent cations with either citrate
or EDTA caused a similar enhancement of the inhibitory
activity of Integrilin, it would appear that the enhanced activity in
citrate was due to chelation of divalent cations and not to a direct
activity of citrate on platelets or on fibrinogen. In support of
this conclusion, Integrilin also showed enhanced inhibition of the
binding of fibrinogen to purified GP IIb-IIIa at 50
µmol/L Ca2+ compared with that observed at 1
mmol/L Ca2+, a binding reaction observed with
purified proteins in the absence of added chelator. We have also
observed that Integrilin has enhanced inhibition of ADP-induced
aggregation of washed platelets, with added fibrinogen suspended in
the absence of added Ca2+ compared with the same
platelet preparation suspended in the presence of 1
mmol/L Ca2+ (data not shown). When PAC1 binding to
activated platelets was measured, enhanced Integrilin
inhibition in PRP anticoagulated with citrate compared with PPACK was
observed. The relevance of this observation is that PAC1 binds to GP
IIb-IIIa in a manner similar to the binding of soluble fibrinogen in
that both require prior activation of platelets, have a similar
requirement on divalent cations, and have reduced binding in the
presence of GP IIb-IIIa antagonists.26 31
Finally, the presence of citrate was also found to lower the
EC50 for Integrilin-induced expression of the D3 LIBS
epitope compared with that observed with PPACK-anticoagulated PRP. For
certain ligands, expression of the D3 epitope is a function of GP
IIb-IIIa receptor occupancy.32 Taken together, these data
establish that the effect of the reduced concentration of divalent
cations caused by citrate chelation is to increase the binding of
Integrilin to both unactivated and activated forms of
GP IIb-IIIa and thus enhance the apparent platelet
inhibitory activity of Integrilin.
Ca2+–GP IIb-IIIa
Five divalent cation binding sites have been observed on GP
IIb-IIIa,13 14 and all of these are occupied when this
integrin is exposed to buffers containing 1 mmol/L
Ca2+. Four divalent cation binding sites have been
attributed to sequences encoding the classic calcium-binding domains
called the EF hands on GP IIb33 34 ; the remaining site is
believed to be at the MIDAS domain on GP IIIa.35 36 37
Although the precise affinities of each of the sites for divalent
cations have not been determined, it is known that suspension of
platelets in media containing 20 to 100 µmol/L
Ca2+ removes Ca2+ from GP IIb-IIIa on the
platelet surface.15 16 38 Previous studies have
established that divalent cations bound to GP IIb-IIIa are required for
GP IIb-IIIa to bind fibrinogen, enabling this bivalent adhesive protein
to mediate platelet aggregation.39 Furthermore, a
variety of experimental approaches, including cross-linking of peptide
ligands to GP IIb-IIIa, inhibition of GP IIb-IIIa binding activity by
peptides, fibrinogen-binding activities of GP IIb-IIIa peptides, and
blocking activities of GP IIb-IIIa monoclonal antibodies, have
established that the ligand binding sites on GP IIb-IIIa are in close
proximity to the divalent cation binding sites on this
protein.20 39 40 Therefore, it is somewhat paradoxical
that less than saturating Ca2+ concentrations would
actually enhance the binding and inhibitory activity of
Integrilin. Insight into this issue may be found in the observations of
D'Souza et al,20 who showed that peptide ligands
(Arg-Gly-Asp–containing peptides were used in their study) actually
displaced Ca2+ from GP IIb-IIIa upon binding, leading
these authors to postulate a "displacement hypothesis," which
predicts that selected ligands and divalent cations may actually
compete for overlapping sites on GP IIb-IIIa. In this capacity,
Ca2+ increases the rate of ligand dissociation from GP
IIb-IIIa.21 The importance of this model to the
present study is that it predicts that partial removal of divalent
cations from GP IIb-IIIa may enhance ligand binding.
An extension of these observations is that partial reductions in the
amount of divalent cations may enhance the binding of other ligands to
GP IIb-IIIa. Previous studies by Collen et al22 showed
that the cyclic nonapeptide TP9201, which contains an RGD sequence, has
>10-fold enhanced inhibition of either ADP- or collagen-induced
aggregation in human PRP anticoagulated with citrate versus with
heparin. On the basis of the observations reported here, it is possible
to speculate that the enhanced activity of TP9201 may also be due to
enhanced binding in the low-ionized divalent-cation environment
afforded by citrate anticoagulation. In another example, Hu et
al41 showed that Ca2+ will displace
vitronectin from 
Vß3, an integrin that has a
ß-subunit identical to that of GP IIb-IIIa and binds
vitronectin at its RGD sequence. It remains to be
determined whether other integrin inhibitors based on the
RGD binding motif also have enhanced binding after calcium
chelation.
Platelets are known to be activated by multiple agonists,
including ADP, thrombin, collagen, and high shear, all of which may be
involved in vivo in inducing platelet aggregation and thrombus
formation. The present study shows that platelet aggregation
induced by activation of the thrombin receptor required greater
concentrations of Integrilin to inhibit aggregation than did
aggregation induced by ADP. This was expected, because other GP
IIb-IIIa antagonists have shown similar properties when
platelets are activated by more potent
agonists.22 27 28 The GP IIb-IIIa within platelet
-granules is known to be surface expressed, partially with bound
fibrinogen, after platelet stimulation by thrombin but not with
ADP,42 43 and it is likely that this is the cause for
increased requirements on Integrilin to effectively inhibit
aggregation. Because TRAP-induced platelet aggregation showed a
similar citrate enhancement on the requirements for Integrilin
inhibition, we conclude that Ca2+ affects aggregation by
the two agonists similarly.
Mechanism
The above considerations provide a mechanism for the ability
of citrate and other calcium chelators (eg, EDTA) to enhance the
inhibitory activity of Integrilin in blocking platelet
aggregation in PRP. Citrate chelation reduces the Ca2+
concentration of PRP to 40 to 50 µmol/L, partially
removing Ca2+ from the divalent cation binding sites on GP
IIb-IIIa.16 38 Although the reduced Ca2+
lowers the affinity of fibrinogen for GP IIb-IIIa on the surface of
activated platelets,17 sufficient binding
persists to allow for platelet aggregation. Reduced
Ca2+ simultaneously increases the binding of
Integrilin, possibly because Integrilin and Ca2+ occupy
overlapping sites on GP IIb-IIIa. The increased binding of
Integrilin and the decreased binding of fibrinogen together serve to
increase the inhibitory activity of Integrilin, both in
blocking the binding of purified fibrinogen to purified GP IIb-IIIa in
buffers containing 50 µmol/L Ca2+ compared
with buffers containing 1 mmol/L Ca2+ and in
blocking fibrinogen binding to platelets and platelet
aggregation in PRP anticoagulated with citrate compared with PPACK.
Clinical Implications
Inherent in the analysis of the clinical activities
of GP IIb-IIIa antagonists is the development of ex vivo
analytical procedures to accurately reflect pharmacodynamics of this
class of compounds. Such information can be used to optimize
antagonist concentrations, determine differences between
classes of antagonists, optimize the duration of exposure
in the various clinical settings, and determine the impact of
thrombosis on the clinical disease. The recently completed IMPACT II
trial showed that Integrilin infusions during angioplasty caused a 40%
reduction in acute abrupt closure and reduced the clinical end points
of the trial at 30 days by 18%.44 The dosing
regimens of Integrilin used in IMPACT II included a bolus of 135
µg/kg followed by infusions of 0.5 or 0.75 µg ·
kg-1 · min-1
in the two treatment arms. These infusions yielded steady-state plasma
concentrations of 350 to 500 nmol/L, which are expected to
inhibit ADP-induced platelet aggregation by 70% to 80% in PRP
samples anticoagulated in citrate: greater inhibition of aggregation
was achieved immediately after the bolus (the present study and
References 3 and 6). In view of the data presented here,
it is expected that at normal plasma concentrations of
Ca2+, the steady-state plasma levels of Integrilin achieved
in IMPACT II would inhibit aggregation by <30% and would approach
only 50% receptor occupancy. It thus appears likely that collection
of blood in citrate as the anticoagulant increases the amount of
Integrilin bound to GP IIb-IIIa as measured by D3 binding and enhances
the apparent inhibitory activity of Integrilin over that
observed when plasma levels of Ca2+ are maintained. Safety
considerations played a major role in determining the dosing regimens
in early clinical trials with Integrilin and other GP IIb-IIIa
inhibitors.3 6 The doses studied in IMPACT II,
a study with more than 4000 patients, have been shown to be quite safe.
The further insights into the pharmacodynamic measurements of
Integrilin provided by the present data indicate that higher
Integrilin doses in patients may impart an increased therapeutic
benefit. It is currently unknown whether the observed effects of
citrate on the apparent potency of Integrilin will be observed with
other GP IIb-IIIa antagonists. The efficacy seen in IMPACT
II suggests that even at <80% receptor occupancy, antithrombotic
activity can be expected in this class of compounds.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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We gratefully acknowledge Ginny Wyss and Barbara Utterbach for initiating these studies.
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Footnotes |
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Reprint requests to David R. Phillips, PhD, Principal Research Scientist, COR Therapeutics, 256 E Grand Ave, South San Francisco, CA 94080.
Received November 26, 1996; revision received March 24, 1997; accepted March 26, 1997.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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