Interact CardioVasc Thorac Surg 2009;9:178-181. doi:10.1510/icvts.2009.206342
© 2009 European Association of Cardio-Thoracic Surgery
Institutional report - Coronary |
Aprotinin reduces the antiplatelet effect of clopidogrel
Gabriella Lindvalla,*,
Ulrik Sartipya,
Staffan Bjessmoa,
Peter Svenaruda,
Bo Lindvallb and
Jan van der Lindena
a From the Karolinska Institute, Department of Cardiothoracic Surgery and Anesthesiology, Karolinska University Hospital, SE-17176 Stockholm, Sweden
b From the Karolinska Institute, Department of Cardiology, Länssjukhuset i Sundsvall/Härnösand, Sundsvall, Sweden
Received 2 March 2009;
received in revised form 4 May 2009;
accepted 5 May 2009
 Financial support was provided by Karolinska Institute, and through regional agreement on medical training and clinical research (ALF) between Stockholm county council and the Karolinska Institute, Stockholm, Sweden. One of the authors (JvdL) has received an unconditional research grant from Bayer Sweden, which is supervised by the Karolinska University Hospital.
*Corresponding author. Tel.: +46-8-51770830; fax: +46-8-331931.
E-mail address: gabriella.lindvall{at}karolinska.se (G. Lindvall).
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Abstract
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Aprotinin reduces bleeding and transfusion rates in patients undergoing coronary surgery while on clopidogrel. However, safety studies have indicated that aprotinin may have a possible adverse effect related to an increased incidence of thromboembolic events. We therefore studied the adenosinediphosphate (ADP) mediated platelet aggregation before and after administration of aprotinin in patients on clopidogrel. Fifteen clopidogrel-treated patients with acute coronary syndrome undergoing coronary surgery were studied. ADP-mediated platelet aggregation and platelet count ratio (%) were measured before and after a bolus dose [2x106 kallikrein inhibiting units (KIU)] of aprotinin. Aprotinin induced an increased aggregation in 11 of 15 patients (73%), and a decrease was registered in two patients (13%). The median (25th/75th percentile) ADP-mediated platelet aggregation before and after aprotinin was 84% (76/91) and 94% (86/97, P<0.01). Clopidogrel non-responders with >90% aggregation (n=4) had a median aggregation of 94.5% (91.5/97.5) vs. 82% (73/87, P<0.01) in the responders (n=11). The median increase in platelet aggregation after aprotinin was 8% (5/20) in the responders vs. 0% (–5.25/3, P<0.01) in the non-responders. Aprotinin increased ADP induced platelet aggregation from 84 to 94% in patients on clopidogrel, which corresponds to a median decrease in relative platelet inhibition of >50%.
Key Words: Coronary artery bypass surgery; Pathology/pharmacology/physiology; Blood; Coagulation; Platelets
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1. Introduction
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Platelets are critically important in the acute setting of percutaneous coronary intervention (PCI) and coronary artery bypass graft surgery (CABG) including cardiopulmonary bypass (CPB), where both platelet count and function are affected. During the last decade, clopidogrel has, together with aspirin, become standard therapy in patients with unstable angina, i.e. acute coronary syndrome (ACS). However, when clopidogrel is discontinued <5 days before coronary surgery, the drug may still lead to increased perioperative bleeding and transfusion and reoperation rates [1]. The effect of clopidogrel is mainly due to its ability to inhibit platelet adenosinediphosphate (ADP)-receptors, which results in decreased aggregation of platelets after ADP-release. However, the clinical response to clopidogrel varies greatly, which may influence the volume of bleeding in patients undergoing coronary surgery while on clopidogrel. Several therapeutic strategies including prophylactic treatment with aprotinin and lysine analog have been suggested to reduce clopidogrel-induced perioperative bleeding. So far, only aprotinin has been proven effective [2]. Apart from the antifibrinolytic effect of aproptinin, which may partly be due to its platelet-protective properties that help to preserve platelet function after CPB, the effect of aprotinin on platelet function in patients on clopidogrel undergoing coronary surgery is still unclear.
The aim of this study was to investigate whether aprotinin influences ADP-mediated platelet aggregation in patients with varying degrees of response to clopidogrel.
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2. Material and methods
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Fifteen consecutive patients with ACS scheduled for CABG were included in the study. In all patients their last intake of clopidogrel had been <5 days before surgery. All patients had been given a loading dose of 300–600 mg of clopidogel orally, followed by 75 mg daily. In addition, they were on oral aspirin, 75 mg/day, and subcutaneous low molecular weight heparin. At the time of the study we routinely administered aprotinin according to the Hammersmith regime [2] to patients on clopidogrel undergoing CABG. Blood samples were drawn from the radial arterial cannula before and after administration of a bolus of two million kallikrein inhibiting units (KIU) just before start of surgery.
Platelet counts were determined in two steps immediately after sampling of 5 ml fresh whole blood, first in an ethylenediaminetetraacetic acid (EDTA) tube, and then in a tube containing 20 µM ADP (Plateletworks Helena Lab, Beamont, TX, USA). Each sample was analyzed in a cell-counter (ABX Micros 60, Diamond Diagnostics, Holliston, MA, USA). A simple formula was used to calculate the grade of inhibition before and after the patient received aprotinin (% inhibition=[ADP platelet count/EDTA platelet count]x 100). In principal, when aggregation occurs, the functional platelets aggregate to clumps that cannot be counted because of their increased size. Functional platelets will aggregate maximally after ADP-stimulation resulting in a platelet count close to zero, which corresponds to 0% inhibition (=100% aggregation) [3].
The study was approved by the regional Human Research Ethics Committee, Stockholm, Sweden (2006/272–31/2). Individual consent was obtained.
2.1. Peri- and postoperative considerations
All patients received the full Hammersmith regime of aprotinin, consisting of 2x106 KIU before start of surgery, 2x106 KIU in the CPB prime, and 0.5x106 KIU/h during surgery [2]. Aspirin and low molecular weight heparin treatment was continued until the day of surgery, but not given on the day of surgery. ACE inhibitors were omitted on the day of surgery.
2.2. Statistical analysis
Variables are reported as mean, S.D., and/or median and 25th/75th percentile. Comparisons between groups were performed with the Mann–Whitney U-test for continuous variables. Differences were considered significant at P< 0.05. Statistical analyses were performed using SPSS 15.0 (SPSS Inc, Chicago, IL, USA).
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3. Results
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The median (25th/75th percentile) ADP-mediated platelet aggregation before and after aprotinin was 84% (76/91) and 94% (86/97, P<0.01). As depicted in Fig. 1, aprotinin induced an increased aggregation in 11 of 15 patients (73%), whereas a decrease was registered in two patients (13%). EDTA platelet counts before (207±42) and after aprotinin (196±51) did not differ significantly (P=0.125). When applying the cut-off limit of <10% inhibition (>90% aggregation) for clopidogrel non-response to ADP, four out of 15 patients were classified as non-responders [4, 5]. The non-responders had a median aggregation of 94.5% (91.5/97.5, 25th/75th percentile) vs. 82% (73/87, P<0.01) in the responders. The median increase in platelet aggregation after aprotinin was 8% (5/20) in the responders vs. 0% (–5.25/3, P<0.01) in the non-responders. The median duration after the last intake of clopidogrel until start of surgery was very similar for non-responders and responders, 72 h (40/76) vs. 74 h (24/98, P=0.75). Preoperative patient characteristics are shown in Table 1. The mean last oral intake of clopidogrel was 63.7±28 h before surgery (range 24–103 h).
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Fig. 1. Platelet aggregation after ADP stimulation (platelet count ratio,%) before and after a bolus of 2,000,000 Kallikrein Inhibiting Units of aprotinin in 15 patients with acute coronary syndrome on clopidogrel undergoing primary CABG. Two patients have overlapping values before (90%) and after aprotinin.
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Table 1 Clinical data (mean±S.D., median and 25th75th percentiles, or percentages) in 15 patients with acute coronary syndrome on clopidogrel undergoing primary CABG
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4. Discussion
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The most important finding of this study was that aprotinin reduced the antiplatelet effect of clopidogrel. This effect was restricted to patients with a platelet inhibition of  10%.
Platelets play a critical role in the hemostasis mechanism by their formation of the primary hemostatic plug as well as by their contribution to the clotting cascade. There is a continuous activation of platelets and an aggravated clot formation related to the unstable condition of patients with ACS. To prevent coronary thrombosis, a potent anti-platelet drug, i.e. clopidogrel, has become gold standard for treatment of ACS, in addition to aspirin and low molecular weight heparin. These oral drugs are nowadays administered, whenever possible, even before admission to hospital and before coronary angiography, as well as before it is decided whether the individual patient needs acute PCI or coronary surgery. However, given the long half-life of clopidogrel, this approach increases the risk of excessive perioperative bleeding in ACS patients undergoing urgent coronary surgery [1]. Conversely, if surgery is postponed the necessary 5–7 days, there is an increased risk of ischemic events before revascularization [6]. To complicate matters further, the individual response to clopidogrel varies within a wide range. This is due to various mechanisms due to different mechanisms including genetic (receptor polymorphism), clinical (poor compliance and absorption, drug interactions, ACS, diabetes mellitus/insulin resistance, elevated body mass index), and cellular (up-regulation of the P2Y12 and P2Y-independent pathways, accelerated platelet turnover factors) [7]. The surgical dilemma may become even worse in the future if platelet inhibition is optimized in low- or non-responders. That may well be the case if the degree of platelet inhibition is routinely analyzed with a valid point-of-care method that narrows the variation after individualization of drug and dosage.
Previous clinical studies have shown reduced perioperative bleeding and transfusion rates when aprotinin was given indiscriminately to patients undergoing coronary surgery while on clopidogrel [2], i.e. with the last dose <5 days before surgery. Our study suggests an alternative option. If the degree of platelet aggregation in these patients would be tested preoperatively, the use of aprotinin could be restricted to clopidogrel responders. This would exclude administration of aprotinin to patients with none or a very limited effect of clopidogrel (<10% inhibition), where an already enhanced coagulation increases the risk of thromboembolic cardiac events [4, 5]. Aprotinin use in cardiac surgery has recently been studied in the large randomized Canadian BART study [8]. The results indicated that aprotinin may possibly be prothrombotic in patients without preoperative deranged hemostatic conditions. Indeed, any drug with a possible prothrombotic effect should be carefully restricted to patients with a, preferably, measurable defect in hemostasis. Thus, the administration should not be based on the subjective judgment of the individual surgeon and/or risks of patient groups (with a large variation). The latter approaches are exemplified in the observational studies by Mangano et al. [9] that have been contradicted by similar observational studies by other investigators [10, 11]. Even BART, the largest randomized trial so far, evaluated aprotinin vs. lysine analog in patients categorized as ‘high risk cardiac surgery patients’ [8]. Certainly, aprotinin had a more potent hemostatic effect as indicated by the fact that fewer patients in the aprotinin group received platelets and at least one unit of packed red blood cells. This more potent effect of aprotinin could also explain why the observed frequency of cardiac death was higher in aprotinin-treated patients. However, in the BART study 52% of eligible patients were excluded from the final analysis. If an exclusion rate of this magnitude occurs in a multi-center study it may as least be possible that involved doctors have directly or/and indirectly selected the patients least likely to bleed. Moreover, the definition of ‘high risk cardiac surgery’ may not at all be equal to a high risk of suffering a hemostatic deficiency perioperatively. These limitations may have affected the results in the BART trial. Finally, a recent reanalysis of the BART study data has indeed challenged the association between aprotinin and increased mortality [12].
The biochemical mechanism behind the interaction between aprotinin and clopidogrel is not completely understood. Nevertheless, the result of this small study suggests an influence on the level of ADP-receptors. The blockage of ADP-receptors of clopidogrel is thought to be irreversible and to last during the whole lifespan of the platelet, whereas functional platelets will aggregate maximally after addition of ADP. Since the platelet counts before and after aprotinin did not differ significantly, the drug itself is unlikely to activate the uninhibited ADP-receptors. This means that aprotinin does not increase platelet aggregation of normal functioning platelets. Consequently, aprotinin must interact with clopidogrel-blocked ADP-receptors, be it temporarily or permanently, and make the clopdiogrel inhibited platelet available to ADP stimulation. This may explain why aprotinin reduces bleeding in clopidogrel treated patients undergoing coronary surgery [2]. Screening of platelet function in patients on clopidogrel before coronary surgery may allow selection of patients with adequate inhibition of platelets. Thus, clopidogrel non- or low-responders will not be given aprotinin. This could minimize the risk of overshoot platelet aggregation with possible increased risk of thrombotic events after coronary surgery. Methods that are deemed to be closely related to platelet function, such as blood aggregometry or platelet count ratio, are considered appropriate for measurement of the effect of clopidogrel [13]. The used method meets this need and seems favorable for point-of-care measurements, since it is simple, inexpensive, quick to perform (2–10 min), and uses non-centrifuged whole blood samples [3].
Finally, Vilahur et al. argue that prophylactic transfusion of 10–12.5 units of platelets in clopidogrel responders undergoing coronary surgery may adequately reverse clopidogrel induced platelet disaggregation to facilitate postoperative hemostasis. This would make the alternative use of aprotinin redundant [14]. However, transfusion of platelets severely influences the immunological system with possibly both acute and long-term morbidity and mortality, including leukocyte and Rh-alloimmunization, transfusion related acute lung injury, graft vs. host disease, hemolysis, and anaphylaxis [15]. Further risks consist of transferred viral and bacterial infections or sepsis, the latter due to the storage of platelets in room temperature [15].
4.1. Limitations
The main limitation of this study was the small sample size. However, all patients were consecutive. The low number of patients in this study, the absence of a control group, and the risk of Type II errors did not allow for any conclusion regarding clinical outcome. Consequently, these data were omitted.
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5. Summary
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Aprotinin increased ADP induced platelet aggregation from 84% to 94% in patients on clopidogrel, which corresponds to a median decrease in relative platelet inhibition of >50%. This effect was restricted to clopidogrel responders, i.e. patients with platelet inhibition of 10%. Our findings suggest that aprotinin interacts with clopidogrel-blocked ADP-receptors, making the clopdiogrel inhibited platelet available to ADP stimulation. Screening of platelet function in patients on clopidogrel before acute or urgent coronary surgery may allow selection of patients for perioperative aprotinin treatment.
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Acknowledgements
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Thanks are due to registered nurse Kristina Kilsand for her help in compiling data for the study.
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Abstract
1. Introduction
