Article

In Vitro Inhibition of Platelet Aggregation in Response to Increasing Concentrations of Tirofiban in Patients with Significant Renal Insufficiency

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Abstract

Background: Patients with impaired renal function are a growing subset at higher risk for cardiovascular complications due to vasculopathic state, inducing accelerated atherosclerosis and arteriosclerosis. These patients are at increased risk for complications after coronary interventions, especially major bleeding events. As a result, this at-risk population of patients has not been well studied in most of the major clinical trials evaluating coronary interventions. Of particular interest is the optimal dosing of glycoprotein IIb–IIIa inhibitors in the setting of acute myocardial infarction. In this study,we attempted to find the in vitro concentration of tirofiban required to inhibit platelet aggregation to <10% in patients with moderate to severe renal insufficiency. Methods: A total of 21 patients were divided into two groups based on estimated creatinine clearance (group 1 <46ml/min; group 2 >46ml/min). Platelet-rich plasma from each subject was then incubated in vitro with increasing concentrations of tirofiban (25, 37.5, and 50ng/ml), and light transmission aggregometry assay was used to assess the degree of platelet aggregation in response to adenosine diphosphate (ADP). Results: Patients in group 1 had a baseline platelet aggregation of 45%, which decreased to 10% at a 25.0ng/ml concentration of tirofiban; the effect was enhanced to a platelet aggregation of <5% at higher doses. In contrast, subjects in group 2 with creatinine clearance ≥46ml/min had an average platelet aggregation inhibition of 12% with 50ng/ml of tirofiban.We found a significant decrease in platelet aggregation in group 2 at 25, 37.5, and 50ng/ml of tirofiban (p<0.05) in comparison with group 1. Conclusions: Our data indicate that patients with moderate to severe renal dysfunction suppress their platelet aggregation to <10% with 25ng/ml of tirofiban, one-third of the standard effective dose for patients with normal renal function.We suggest further clinical trials to define an objective means to calculate proper renal dosing of glycoprotein IIb–IIIa inhibitors in these patients to prevent potentially fatal complication of major hemorrhagic events.

DOI:https://doi.org/10.15420/ahhj.2009.7.1.17

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Platelet aggregation plays a central role in primary hemostasis.1–3 Chronic renal failure is associated with the occurrence of excessive bleeding.4,5 The bleeding tendency of uremia is considered to represent an acquired defect in primary hemostasis.6 Uremic platelets show a reduced adhesion to vascular sub-endothelium7,8 and an impaired aggregation response to various stimuli,9,10 which is reversible with improvement in renal function or hemodialysis.11 Renal insufficiency was found to be a significant risk factor for bleeding in the PRISM-PLUS trial.12 The addition of heparin to tirofiban increased bleeding risk irrespective of renal function.14 Glycoprotein IIb–IIIa inhibitors are potent inhibitors that can prevent platelet aggregation in a dose- and concentration-dependent fashion. They are indicated for treatment of acute coronary syndrome (ACS) and patients undergoing percutaneous coronary intervention (PCI).13–16 Renal impairment is common in patients with coronary artery disease and often requires dose adjustment for drugs that are eliminated by the kidneys. Many investigators have suggested that a lower dose of tirofiban be used in patients with chronic renal insufficiency (CRI) to decrease adverse outcomes, the most common being bleeding. The purpose of this in vitro study is to determine the optimal serum concentration of tirofiban to inhibit platelet aggregation to <10% in patients with moderate to severe renal insufficiency.

Material and Methods

An informed consent, approved by the Institutional Review Board of Baylor College of Medicine, was obtained from each patient before venous blood sample was drawn. None of the subjects was on clopidogrel. Subjects were divided into two groups based on estimated creatinine clearance (CrCl) using the Cockcroft-Gault equation.17
Men: CrCl = (140 – age) x (bodyweight in kg) / 72 x serum Cr concentration
Women: CrCl = Estimated CrCl (above equation) x 0.85

Subjects in group 1 had CrCl ≥46ml/min and those in group 2 had a CrCl <46ml/min. A creatinine clearance of 46ml/min was chosen as a dividing point as patients with CrCl <46.2ml/min who were not yet on dialysis were found to have an early hazard with respect to survival, which stabilized initially while on hemodialysis.18 Twenty milliliters of blood were drawn from an antecubital vein using a 19-guage needle and collected into polypropylene tubes. Volume dilution was performed using a standardized tirofiban solution (25ng/ml). Since in previous studies a blood concentration of 67–82ng/ml of tirofiban had been shown to effectively reduce turbidometric platelet aggregation to <10% in response to 20μ-mole/liter of adenosine diphosphate (ADP), we aimed to assess the effects of lower concentrations of tirofiban in the setting of renal insufficiency.19 Final concentrations of 50, 37.5, and 25ng/ml of tirofiban were obtained in the diluted blood samples. Blood was centrifuged at 1,200rpm for 15 minutes to obtain platelet-rich plasma (PRP) and then at 10,000rpm for another 15 minutes to obtain platelet-poor plasma (PPP) at 25ºC. Both PRP and PPP were incubated at 37ºC before platelet aggregation assay using ex vivo light transmission aggregometry (LTA) in the presence of D-phenylalanyl-L-propyl- L-arginine chloromethyl ketone as anticoagulant to ensure accurate estimation of the inhibitory properties of tirofiban, and 20μ-mole/l of adenosine diphosphate as a platelet agonist, as previously described18–21 on PACKS-4 four-channel light transmission aggregometer (Helena Laboratories, Beaumont, Texas). Platelet aggregation was determined as the percent change in light transmission using PPP as control. No platelet count adjustments were made. LTA assay was performed for 10 minutes.

Statistical analysis was performed using independent sample student’s t-test and Chi square test to determine the difference between groups 1 and 2 in terms of platelet aggregation response. A p-value of <0.05 was considered to be statistically significant.

Results

We included a total of 21 patients admitted to Ben Taub General Hospital for non-cardiac problems and divided them into two groups as described above. Fifteen patients were included in group 1 (CrCl <46ml/min) and six in group 2 (CrCl ≥46ml/min). CrCl for group 1 ranged from 3 to 35ml/min (mean 14 and median 7.5ml/min) and for group 2 ranged from 46 to 125ml/min (mean 94 and median 75ml/min). The mean age of patients in groups 1 and 2 was 49 and 50 years, respectively (p=0.7). The male-to- female ratio favored males for group 1, but the numbers were too small to draw clinically relevant conclusion from this finding (Chi2 0.3).

More than half (12 of 21) of the subjects were Latin American, while four were African-American, four were Caucasian, and one was Asian. The two groups did not differ in terms of their plasma platelet concentrations (256K/ml for group 1 and 263K/ml for group 2; p=0.2). Table 1 shows the patients’ various clinical and biochemical variables that were evaluated. Table 2 shows the mean platelet aggregation at three different concentrations of tirofiban in groups 1 and 2. Patients in group 1 had a baseline platelet aggregation of 45%, which decreased to 10% at 25.0ng/ml of tirofiban; the effect was further enhanced to a platelet aggregation of <5% at higher doses. In contrast, subjects in group 2 with creatinine clearance ≥46ml/min had an average platelet aggregation inhibition of 12% with 50ng/ml of tirofiban. We found a significant decrease in platelet aggregation in group 2 at 25, 37.5, and 50ng/ml of tirofiban concentration (p<0.05) in comparison with group 1 (see Table 2 and Figure 2).

Discussion

Cardiovascular disease is the predominant cause of morbidity and mortality in patients with CRI and end-stage renal disease (ESRD). Cardiovascular disease is the leading cause of morbidity and mortality in ESRD patients on dialysis, accounting for approximately 50% of deaths.22 Overall, dialysis patients have a 10–20-fold increased risk for death compared with the general population, which in the US corresponds to an annual cardiovascular mortality rate of 9%.23 In a prospective coronary care unit (CCU) registry analysis, 45% of the patients with ESRD had history of CHF.18 A two-year mortality rate of 52–65% has been reported in patients with ESRD who are discharged from the CCU after an episode of acute myocardial infarction.18,24 The risk for stent thrombosis appears to be higher with worsening renal function. Attallah et al. reported bare-metal stent thrombosis rates in the setting of worsening renal function ranging from 11.2 to 19.7% at six months.25 Since major clinical trials evaluating the effects of glycoprotein IIb–IIIa inhibitors have excluded patients with CRI and ESRD due to an increased risk for complications, with the most common being hemorrhage, the guidelines regarding dosing of these agents in this subset of patients are not well defined. Several authors have suggested using 50% of the dose in these patients to avoid potential bleeding complications. Patients with CRI form an increasing proportion of the population at higher risk for coronary artery disease thus requiring PCI. The renal dosing of glycoprotein IIb–IIIa agents has always been an issue for these patients. We have attempted to objectively define the optimal dose of tirofiban by quantifying platelet aggregation inhibition in those patients with sub-optimal renal function.

Our findings suggest that patients with impaired renal function achieve optimal (>90%) platelet aggregation inhibition with much lower doses than required by healthy individuals. In contrast to the previously reported tirofiban plasma concentration of 67–82ng/ml, 25ng/ml provides clinically significant in vitro platelet aggregation inhibition (>90%) in the setting of CRI and ESRD.

These findings suggest that patients with CRI should receive a much lower dose of tirofiban when given for ACS or PCI. In vivo studies are required to verify these findings in patients with ACS or those undergoing PCI, as platelets may be more activated than baseline in these settings. Moreover, other studies are needed on eptifibatide and abciximab to investigate whether the above findings are also applicable to these other agents.

Limitations

There are several major limitations to this study. First, it was an in vitro study and thus does not completely represent the in vivo milieu including platelet and endothelium interaction (including von Willibrand factor). Second, the number of subjects included is too small to draw clinically generalized conclusions.

Conclusions

In this small in vitro study, a plasma concentration of 25ng/ml (approximately one-third of the effective standard plasma concentration) of tirofiban appears to adequately inhibit platelet aggregation in patients with significant renal dysfunction as opposed to patients with relatively preserved renal function. Adopting a lower than recommended 50% dosing in patients with CRI who suffer from acute myocardial infarction requiring PCI may reduce the incidence of ischemic events while decreasing the risk for significant bleeding. We suggest that glycoprotein IIb–IIIa agents are an important class of drugs that have not been used adequately in the subset of patients who are at increased risk for complications, and that further clinical trials are needed to confirm our findings in a larger number of patients.

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