CLINICAL PHARMACOLOGY
Mechanism Of Action
AGGRASTAT is a reversible antagonist of fibrinogen
binding to the GP IIb/IIIa receptor, the major platelet surface receptor
involved in platelet aggregation. When administered intravenously, AGGRASTAT
inhibits ex vivo platelet aggregation in a dose- and concentration-dependent
manner.
When given according to the PRISM-PLUS regimen of 0.4
mcg/kg/min over 30 minutes followed by a 0.1 mcg/kg/min maintenance infusion,
> 90% inhibition of platelet aggregation is attained by the end of the
30-minute infusion. When given according to the recommended regimen of 25
mcg/kg followed by a 0.15 mcg/kg/min maintenance infusion, > 90% inhibition
of platelet aggregation is attained within 10 minutes. Platelet aggregation
inhibition is reversible following cessation of the infusion of AGGRASTAT.
Pharmacodynamics
AGGRASTAT inhibits platelet function, as demonstrated by
its ability to inhibit ex vivo adenosine phosphate (ADP)-induced platelet
aggregation and prolong bleeding time in healthy subjects and patients with
coronary artery disease. The time course of inhibition parallels the plasma concentration
profile of the drug.
Following discontinuation of an infusion of AGGRASTAT
0.10 mcg/kg/min, ex vivo platelet aggregation returns to near baseline in 4 to
8 hours in approximately 90% of patients with coronary artery disease. The
addition of heparin to this regimen does not significantly alter the percentage
of subjects with > 70% inhibition of platelet aggregation (IPA), but does
increase the average bleeding time, as well as the number of patients with
bleeding times prolonged to > 30 minutes. Similar platelet aggregation
recovery rates are observed following discontinuation of a 0.15 mcg/kg/min
infusion.
Pharmacokinetics
Tirofiban has a half-life of approximately 2 hours. It is
cleared from the plasma largely by renal excretion, with about 65% of an
administered dose appearing in urine and about 25% in feces, both largely as
unchanged tirofiban. Metabolism appears to be limited.
Tirofiban is not highly bound to plasma proteins and
protein binding is concentration independent over the range of 0.01 to 25
mcg/mL. The unbound fraction in human plasma is 35%. The steady state volume of
distribution of tirofiban ranges from 22 to 42 liters.
In healthy subjects, the plasma clearance of tirofiban
ranges from 213 to 314 mL/min. Renal clearance accounts for 39 to 69% of plasma
clearance.
Specific Populations
There is no effect on clearance of tirofiban by sex,
race, age, or hepatic impairment.
Renal Insufficiency
Plasma clearance of tirofiban is decreased about 40% in
subjects with creatinine clearance < 60 mL/min and > 50% in patients with
creatinine clearance < 30 mL/min, including patients requiring hemodialysis [see
DOSAGE AND ADMINISTRATION]. Tirofiban is removed by hemodialysis.
Clinical Studies
Two large-scale clinical studies established the efficacy
of AGGRASTAT in the treatment of patients with NSTE-ACS (unstable angina/non-ST
elevation MI). The two studies examined AGGRASTAT alone and added to heparin,
prior to and after percutaneous coronary revascularization (if indicated) (PRISM-PLUS)
and in comparison to heparin in a similar population (PRISM). These trials are discussed
in detail below.
PRISM-PLUS (Platelet Receptor Inhibition for Ischemic
Syndrome Management - Patients Limited by Unstable Signs and Symptoms)
In the double-blind PRISM-PLUS trial, 1570 patients with
documented NSTE-ACS within 12 hours of entry into the study were randomized to
AGGRASTAT (30 minute initial infusion of 0.4 mcg/kg/min followed by a
maintenance infusion of 0.10 mcg/kg/min) in combination with heparin (bolus of
5,000 U followed by an infusion of 1,000 U/h titrated to maintain an APTT of
approximately 2 times control) or to heparin alone. All patients received
concomitant aspirin unless contraindicated. Patients who were medically managed
or who underwent revascularization procedures were studied. Patients underwent
48 hours of medical stabilization on study drug therapy, and they were to
undergo angiography before 96 hours (and, if indicated,
angioplasty/atherectomy, while continuing on AGGRASTAT and heparin for 12-24
hours after the procedure). AGGRASTAT and heparin could be continued for up to
108 hours.
Exclusions included contraindications to anticoagulation,
decompensated heart failure, platelet count < 150,000/mm³, and serum
creatinine > 2.5 mg/dL. The mean age of the population was 63 years; 32% of patients
were female and approximately half of the population presented with non-ST
elevation myocardial infarction. On average, patients received AGGRASTAT for 71
hours.
A third group of patients was initially randomized to
AGGRASTAT alone (no heparin). This arm was stopped when the group was found, at
an interim look, to have greater mortality than the other two groups.
The primary endpoint of the study was a composite of
refractory ischemia, new MI and death within 7 days. There was a 32% risk
reduction in the overall composite primary endpoint. The components of the composite
were examined separately and the results are shown in Table 5. Note that the
sum of the individual components may be greater than the composite (if a
patient experiences multiple component events only one event counts towards the
composite).
Table 5 : Primary outcomes at 7 days in PRISM-PLUS
Endpoint |
AGGRASTAT + Heparin
(n=773) |
Heparin
(n=797) |
Risk Reduction |
p-value |
Death, new MI, and refractory ischemia at 7 days |
12.9% |
17.9% |
32% |
0.004 |
Death |
1.9% |
1.9% |
— |
— |
MI |
3.9% |
7.0% |
47% |
0.006 |
Refractory Ischemia |
9.3% |
12.7% |
30% |
0.023 |
The benefit seen at 7 days was maintained over time. The
risk reduction in the composite endpoint at 30 days and 6 months is shown in
the Kaplan-Meier curve below.
Figure 1: Time to first event of death, new MI, or
refractory is chemia in PRISM-PLUS
An analysis of the results by sex suggests that women who
are medically managed or who undergo subsequent PTCA/atherectomy may receive
less benefit from AGGRASTAT (95% confidence limits for relative risk of
0.61-1.74) than do men (0.43-0.89) (p=0.11). This difference may be a true
treatment difference, the effect of other differences in these subgroups, or a
chance occurrence.
Approximately 90% of patients in the PRISM-PLUS study
underwent coronary angiography and 30% underwent angioplasty/atherectomy during
the first 30 days of the study. The majority of these patients continued on
study drug throughout these procedures. AGGRASTAT was continued for 12-24 hours
(average 15 hours) after angioplasty/atherectomy. The effects of AGGRASTAT at
Day 30 did not appear to differ among sub-populations that did or did not
receive PTCA or CABG, both prior to and after the procedure.
PRISM (Platelet Receptor Inhibition for Ischemic
Syndrome Management)
In the PRISM study, a randomized, parallel, double-blind
study, 3232 patients with NSTE-ACS intended to be managed without coronary
intervention were randomized to AGGRASTAT (initial dose of 0.6 mcg/kg/min for
30 minutes followed by 0.15 mcg/kg/min for 47.5 hours) or heparin (5000-unit intravenous
bolus followed by an infusion of 1000 U/h for 48 hours). The mean age of the
population was 62 years; 32% of the population was female and 25% had non-ST
elevation MI on presentation. Thirty percent had no ECG evidence of cardiac
ischemia. Exclusion criteria were similar to PRISMPLUS. The primary endpoint
was the composite endpoint of refractory ischemia, MI or death at the end of
the 48-hour drug infusion. The results are shown in Table 6.
Table 6 : Primary outcomes in PRISM - Cardiac Ischemia
Events
Composite Endpoint (death, MI, or refractory ischemia) |
AGGRASTAT
(n=1616) |
Heparin
(n=1616) |
Risk Reduction |
p-value |
2 Days (end of drug infusion) |
3.8% |
5.6% |
33% |
0.015 |
7 Days |
10.3% |
11.3% |
10% |
0.33 |
In the PRISM study, no adverse effect of AGGRASTAT on
mortality at either 7 or 30 days was detected. This result is different from
that in the PRISM-PLUS study, where the arm that included AGGRASTAT without
heparin (n=345) was dropped at an interim analysis by the Data Safety Monitoring
Committee for increased mortality at 7 days.