Clinical Pharmacology for Argatroban
Mechanism Of Action
Argatroban is a direct thrombin inhibitor that reversibly
binds to the thrombin active site. Argatroban does not require the co-factor
antithrombin III for antithrombotic activity. Argatroban exerts its anticoagulant
effects by inhibiting thrombin-catalyzed or â⬓induced reactions, including
fibrin formation; activation of coagulation factors V, VIII, and XIII;
activation of protein C; and platelet aggregation.
Argatroban inhibits thrombin with an inhibition constant
(Ki) of 0.04 mcM. At therapeutic concentrations, argatroban has little or no
effect on related serine proteases (trypsin, factor Xa, plasmin, and
kallikrein).
Argatroban is capable of inhibiting the action of both
free and clot-associated thrombin.
Pharmacodynamics
When argatroban is administered by continuous infusion,
anticoagulant effects and plasma concentrations of argatroban follow similar,
predictable temporal response profiles, with low intersubject variability. Immediately
upon initiation of argatroban infusion, anticoagulant effects are produced as
plasma argatroban concentrations begin to rise. Steady-state levels of both
drug and anticoagulant effect are typically attained within 1 to 3 hours and
are maintained until the infusion is discontinued or the dosage adjusted.
Steady-state plasma argatroban concentrations increase proportionally with dose
(for infusion doses up to 40 mcg/kg/min in healthy subjects) and are well
correlated with steady-state anticoagulant effects. For infusion doses up to 40
mcg/kg/min, argatroban increases in a dose-dependent fashion, the activated
partial thromboplastin time (aPTT), the activated clotting time (ACT), the
prothrombin time (PT), the International Normalized Ratio (INR), and the thrombin
time (TT) in healthy volunteers and cardiac patients. Representative
steady-state plasma argatroban concentrations and anticoagulant effects are
shown below for argatroban infusion doses up to 10 mcg/kg/min (see Figure 1).
Figure 1: Relationship at Steady State Between Argatroban
Dose, Plasma Argatroban Concentration and Anticoagulant Effect
Effect On International Normalized Ratio (INR)
Because argatroban is a direct thrombin inhibitor,
coadministration of argatroban and warfarin produces a combined effect on the
laboratory measurement of the INR. However, concurrent therapy, compared to
warfarin monotherapy, exerts no additional effect on vitamin K-dependent factor
Xa activity.
The relationship between INR on co-therapy and warfarin
alone is dependent on both the dose of argatroban and the thromboplastin
reagent used. This relationship is influenced by the International Sensitivity
Index (ISI) of the thromboplastin. Data for 2 commonly utilized thromboplastins
with ISI values of 0.88 (Innovin, Dade) and 1.78 (Thromboplastin C Plus, Dade)
are presented in Figure 2 for an argatroban dose of 2 mcg/kg/min.
Thromboplastins with higher ISI values than shown result in higher INRs on
combined therapy of warfarin and argatroban. These data are based on results
obtained in normal individuals [see DOSAGE AND ADMINISTRATION, WARNINGS
AND PRECAUTIONS].
Figure 2 : INR Relations hip of Argatroban Plus
Warfarin Versus Warfarin Alone
 |
Figure 2 demonstrates the relationship between INR for
warfarin alone and INR for warfarin coadministered with argatroban at a dose of
2 mcg/kg/min. To calculate INR for warfarin alone (INRW), based on
INR for co-therapy of warfarin and argatroban (INRWA), when the
argatroban dose is 2 mcg/kg/min, use the equation next to the appropriate
curve. Example: At a dose of 2 mcg/kg/min and an INR performed with
Thromboplastin A, the equation 0.19 + 0.57 (INRWA) = INR would allow
a prediction of the INR on warfarin alone (INRW). Thus, using an INR
value of 4obtained on combined therapy: INR = 0.19 + 0.57 (4) = 2.47 as the
value for INR on warfarin alone. The error (confidence interval) associated
with a prediction is ± 0.4 units. Similar linear relationships and prediction
errors exist for argatroban at a dose of 1 mcg/kg/min. Thus, for argatroban
doses of 1 or 2 mcg/kg/min, INRW can be predicted from INRWA.
For argatroban doses greater than 2 mcg/kg/min, the error associated with
predicting INRW from INRWA is ± 1. Thus INRW
cannot be reliably predicted from INRWA at doses greater than 2
mcg/kg/min.
Pharmacokinetics
Distribution
Argatroban distributes mainly in the extra cellular fluid
as evidenced by an apparent steady-state volume of distribution of 174 mL/kg
(12.18 L in a 70 kg adult). Argatroban is 54% bound to human serum proteins,
with binding to albumin and α1 â⬓ acid glycoprotein being 20%
and 34%, respectively.
Metabolism
The main route of argatroban metabolism is hydroxylation
and aromatization of the 3 methyltetrahydroquinoline ring in the liver. The
formation of each of the 4 known metabolites is catalyzed in vitro by the human
liver microsomal cytochrome P450 enzymes CYP3A4/5. The primary metabolite (M1)
exerts 3- to 5-fold weaker anticoagulant effects than argatroban. Unchanged
argatroban is the major component in plasma. The plasma concentrations of M1
range between 0% and 20% of that of the parent drug. The other metabolites (M2
to M4) are found only in very low quantities in the urine and have not been
detected in plasma or feces. These data, together with the lack of effect of erythromycin
(a potent CYP3A4/5 inhibitor) on argatroban pharmacokinetics, suggest that
CYP3A4/5 mediated metabolism is not an important elimination pathway in vivo.
Total body clearance is approximately 5.1mL/kg/min (0.31
L/kg/hr) for infusion doses up to 40 mcg/kg/min. The terminal elimination
half-life of argatroban ranges between 39 and 51 minutes.
There is no interconversion of the 21-(R):21-(S)
diastereoisomers. The plasma ratio of these diastereoisomers is unchanged by
metabolism or hepatic impairment, remaining constant at 65:35 (± 2%).
Excretion
Argatroban is excreted primarily in the feces, presumably
through biliary secretion. In a study in which 14C-argatroban (5
mcg/kg/min) was infused for 4 hours into healthy subjects, approximately 65% of
the radioactivity was recovered in the feces within 6 days of the start of
infusion with little or no radioactivity subsequently detected. Approximately
22% of the radioactivity appeared in the urine within 12 hours of the start of
infusion. Little or no additional urinary radioactivity was subsequently detected.
Average percent recovery of unchanged drug, relative to total dose, was 16% in
urine and at least 14% in feces.
Special Populations
Hepatic Impairment
The dosage of argatroban should be decreased in patients
with hepatic impairment [see DOSAGE AND ADMINISTRATION and WARNINGS
AND PRECAUTIONS]. Patients with hepatic impairment were not studied in
percutaneous coronary intervention (PCI) trials. At a dose of 2.5 mcg/kg/min,
hepatic impairment is associated with decreased clearance and increased
elimination halflife of argatroban (to 1.9 mL/kg/min and 181 minutes,
respectively, for patients with a Child-Pugh score >6).
Renal Impairment
No dosage adjustment is necessary in patients with renal
dysfunction. The effect of renal disease on the pharmacokinetics of argatroban
was studied in 6 subjects with normal renal function (mean Clcr = 95 ± 16
mL/min) and in 18 subjects with mild (mean Clcr = 64 ± 10 mL/min), moderate (mean
Clcr = 41 ± 5.8 mL/min), and severe (mean Clcr 5 ± 7 mL/min) renal impairment.
The pharmacokinetics and pharmacodynamics of argatroban at dosages up to 5
mcg/kg/min were not significantly affected by renal dysfunction.
Use of argatroban was evaluated in a study of 12 patients
with stable end-stage renal disease undergoing chronic intermittent
hemodialysis. Argatroban was administered at a rate of 2 to 3 mcg/kg/min (begun
at least 4 hours prior to dialysis) or as a bolus dose of 250 mcg/kg at the
start of dialysis followed by a continuous infusion of 2 mcg/kg/min. Although
these regimens did not achieve the goal of maintaining ACT values at 1.8 times
the baseline value throughout most of the hemodialysis period, the hemodialysis
sessions were successfully completed with both of these regimens. The mean ACTs
produced in this study ranged from 1.39 to 1.82 times baseline, and the mean
aPTTs ranged from 1.96 to 3.4 times baseline. When argatroban was administered
as a continuous infusion of 2 mcg/kg/min prior to and during a 4-hour
hemodialysis session, approximately 20% was cleared through dialysis.
Age, Gender
There are no clinically significant effects of age or
gender on the pharmacokinetics or pharmacodynamics (e.g. aPTT) of argatroban in
adults.
Drug-Drug Interactions
Digoxin
In 12 healthy volunteers, intravenous infusion of
argatroban (2 mcg/kg/min) over 5 days (study days 11 to 15) did not affect the
steady-state pharmacokinetics of oral digoxin (0.375 mg daily for 15 days).
Erythromycin
In 10 healthy subjects, orally administered erythromycin
(a potent inhibitor of CYP3A4/5) at 500 mg four times daily for 7 days had no
effect on the pharmacokinetics of argatroban at a dose of 1 mcg/kg/min for 5
hours. These data suggest oxidative metabolism by CYP3A4/5 is not an important elimination
pathway in-vivo for argatroban.
Aspirin And Acetaminophen
Drug-drug interactions have not been demonstrated between
argatroban and concomitantly administered aspirin (162.5 mg orally given 26 and
2 hours prior to initiation of argatroban 1 mcg/kg/min over 4 hours) or
acetaminophen (1,000 mg orally given 12, 6, and 0 hours prior to, and 6 and 12
hours subsequent to, initiation of argatroban 1.5 mcg/kg/min over 18 hours).
Clinical Studies
Heparin-Induced Thrombocytopenia
The safety and efficacy of argatroban were evaluated in
an historically controlled efficacy and safety study (Study 1) and a follow-on
efficacy and safety study (Study 2). These studies were comparable with regard
to study design, study objectives, dosing regimens as well as study outline,
conduct, and monitoring. In these studies, 568 adult patients were treated with
argatroban and 193 adult patients made up the historical control group.
Patients had a clinical diagnosis of heparin-induced thrombocytopenia, either
without thrombosis (HIT) or with thrombosis (HITTS [heparin-induced
thrombocytopenia and thrombosis syndrome]) and were males or non-pregnant females
between the ages of 18 and 80 years old. HIT/HITTS was defined by a fall in
platelet count to less than 100,000/μL or a 50% decrease in platelets
after the initiation of heparin therapy with no apparent explanation other that
HIT. Patients with HITTS also had an arterial or venous thrombosis documented
by appropriate imaging techniques or supported by clinical evidence such as
acute myocardial infarction, stroke, pulmonary embolism, or other clinical
indications of vascular occlusion. Patients who had documented histories of
positive heparin-dependent antibody tests without current thrombocytopenia or
heparin challenge (e.g., patients with latent disease) were also included if
they required anticoagulation.
These studies did not include patients with documented
unexplained aPTT greater than 200% of control at baseline, documented
coagulation disorder or bleeding diathesis unrelated to HIT, a lumbar puncture within
the past 7 days or a history of previous aneurysm, hemorrhagic stroke, or a
thrombotic stroke within the past 6 months unrelated to HIT.
The initial dose of argatroban was 2 mcg/kg/min. Two
hours after the start of the argatroban infusion, an aPTT level was obtained
and dose adjustments were made (up to a maximum of 10 mcg/kg/min) to achieve a
steady-state aPTT value that was 1.5 to 3.0 times the baseline value, not to
exceed 100 seconds. Overall the mean aPTT level for HIT and HITTS patients
during the argatroban infusion increased from baseline values of 34 and 38
seconds, respectively, to 62.5 and 64.5 seconds, respectively.
The primary efficacy analysis was based on a comparison
of event rates for a composite endpoint that included death (all causes),
amputation (all causes) or new thrombosis during the treatment and follow up
period (study days 0 to 37). Secondary analyses included evaluation of the
event rates for the components of the composite endpoint as well as
time-to-event analyses.
In study 1, a total of 304 patients were enrolled as
follows: active HIT (n=129), active HITTS (n=144), or latent disease (n=31).
Among the 193 historical controls, 139 (72%) had active HIT, 46 (24%) had active
HITTS, and 8 (4%) had latent disease. Within each group, those with active HIT
and those with latent disease were analyzed together. Positive laboratory
confirmation of HIT/HITTS by the heparininduced platelet aggregation test or
serotonin release assay was demonstrated in 174 of 304 (57%) argatroban treated
patients (i.e., in 80 with HIT or latent disease and 94 with HITTS) and in 149
of 193 (77%) historical controls (i.e., in 119 with HIT or latent disease and
30 with HITTS). The test results for the remainder of the patients and controls
were either negative or not determined.
There was a significant improvement in the composite
outcome in patients with HIT and HITTS treated with argatroban versus those in
the historical control group (see Table 9). The components of the composite
endpoint are shown in Table 9.
Table 9: Efficacy Results of Study 1: Composite
Endpointa and Individual Components, Ranked by ÃÂ Severityb
| Parameter, N (%) |
HIT |
HITTS |
HIT/HITTS |
Control
n = 147 |
Argatroban
n = 160 |
Control
n = 46 |
Argatroban
n = 144 |
Control
n = 193 |
Argatroban
n = 304 |
| Composite Endpoint |
57 (38.8) |
41 (25.6) |
26 (56.5) |
63 (43.8) |
83 (43) |
104 (34.2) |
| Individual Componentsb |
| Parameter, N (%) |
Control
n = 147 |
Argatroban
n = 160 |
Control
n = 46 |
Argatroban
n = 144 |
Control
n = 193 |
Argatroban
n = 304 |
| Death |
32 (21.8) |
27 (16.9) |
13 (28.3) |
26 (18.1) |
45 (23.3) |
53 (17.4) |
| Amputation |
3 (2) |
3 (1.9) |
4 (8.7) |
16 (11.1) |
7 (3.6) |
19 (6.2) |
| New Thrombosis |
22 (15) |
11 (6.9) |
9 (19.6) |
21 (14.6) |
31 (16.1) |
32 (10.5) |
a) Death (all causes), amputation (all causes), or new
thrombosis within 37-day study period.
b) Reported as the most severe outcome among the components of composite
endpoint (severity ranking: death>amputation>new thrombosis); patients
may have had multiple outcomes. |
Time-to-event analysis showed significant improvements in
the time-to-first event in patients with HIT or HITTS treated with argatroban
versus those in the historical control group. The between-group differences in
the proportion of patients who remained free of death, amputation, or new
thrombosis were statistically significant in favor of argatroban by these
analyses.
A time-to-event analysis for the composite endpoint is
shown in Figure 3 for patients with HIT and Figure 4 for patients with HITTS.
Figure 3 : Time-to-First Event for the Composite
Efficacy Endpoint: HIT Patients STUDY 1
Figure 4: Time-to-First Event for the Composite
Efficacy Endpoint: HITTS Patients STUDY 1
In Study 2, a total of 264 patients were enrolled as
follows: HIT (n=125) or HITTS (n=139). There was a significant improvement in
the composite efficacy outcome for argatroban-treated patients, versus the same
historical control group from Study 1, among patients having HIT (25.6% vs.
38.8%), patients having HITTS (41% vs. 56.5%), and patients having either HIT
or HITTS (33.7% vs. 43%). Time-toevent analyses showed significant improvements
in the time-to-first event in patients with HIT or HITTS treated with
argatroban versus those in the historical control group. The between-group
differences in the proportion of patients who remained free of death,
amputation, or new thrombosis were statistically significant in favor of
argatroban.
Anticoagulant Effect
In Study 1, the mean (± SE) dose of argatroban administered
was 2 ± 0.1 mcg/kg/min in the HIT arm and 1.9 ± 0.1 mcg/kg/min in the HITTS
arm. Seventy-six percent of patients with HIT and 81% of patients with HITTS
achieved a target aPTT at least 1.5-fold greater than the baseline aPTT at the
first assessment occurring on average at 4.6 hours (HIT) and 3.9 hours (HITTS)
following initiation of argatroban therapy.
No enhancement of aPTT response was observed in subjects
receiving repeated administration of argatroban.
Platelet Count Recovery
In Study 1, 53% of patients with HIT and 58% of patients
with HITTS, had a recovery of platelet count by Day 3. Platelet Count Recovery
was defined as an increase in platelet count to greater than 100,000/μL or
to at least 1.5- fold greater than the baseline count (platelet count at study
initiation) by Day 3 of the study.
Percutaneous Coronary Intervention (PCI) Patients With Or
At Risk For HIT
In 3 similarly designed trials, argatroban was
administered to 91 patients with current or previous clinical diagnosis of HIT
or heparin-dependent antibodies, who underwent a total of 112 percutaneous coronary
interventions (PCIs) including percutaneous transluminal coronary angioplasty
(PTCA), coronary stent placement, or atherectomy. Among the 91 patients
undergoing their first PCI with argatroban, notable ongoing or recent medical
history included myocardial infarction (n=35), unstable angina (n=23), and
chronic angina (n=34). There were 33 females and 58 males. The average age was 67.6
years (median 70.7, range 44 to 86), and the average weight was 82.5 kg (median
81.0 kg, range 49 to 141).
Twenty-one of the 91 patients had a repeat PCI using
argatroban an average of 150 days after their initial PCI. Seven of 91 patients
received glycoprotein IIb/IIIa inhibitors. Safety and efficacy were assessed
against historical control populations who had been anticoagulated with
heparin.
All patients received oral aspirin (325 mg) 2 to 24 hours
prior to the interventional procedure. After venous or arterial sheaths were in
place, anticoagulation was initiated with a bolus of argatroban of 350 mcg/kg
via a large-bore intravenous line or through the venous sheath over 3 to 5
minutes. Simultaneously, a maintenance infusion of 25 mcg/kg/min was initiated
to achieve a therapeutic activated clotting time (ACT) of 300 to 450 seconds.
If necessary to achieve this therapeutic range, the maintenance infusion dose
was titrated (15 to 40 mcg/kg/min) and/or an additional bolus dose of 150 mcg/kg
could be given. Each patient's ACT was checked 5 to 10 minutes following the
bolus dose. The ACT was checked as clinically indicated. Arterial and venous
sheaths were removed no sooner than 2 hours after discontinuation of argatroban
and when the ACT was less than 160 seconds.
If a patient required anticoagulation after the
procedure, argatroban could be continued, but at a lower infusion dose between
2.5 and 5 mcg/kg/min. An aPTT was drawn 2 hours after this dose reduction and the
dose of argatroban then was adjusted as clinically indicated (not to exceed 10
mcg/kg/min), to reach an aPTT between 1.5 and 3 times baseline value (not to
exceed 100 seconds).
In 92 of the 112 interventions (82%), the patient
received the initial bolus of 350 mcg/kg and an initial infusion dose of 25
mcg/kg/min. The majority of patients did not require additional bolus dosing
during the PCI procedure. The mean value for the initial ACT measurement after
the start of dosing for all interventions was 379 sec (median 338 sec; 5th
ÃÂ percentile-95th ÃÂ percentile 238 to 675 sec). The mean ACT value per
intervention over all measurements taken during the procedure was 416 sec
(median 390 sec; 5th ÃÂ percentile-95th ÃÂ percentile 261 to
698 sec). About 65% of patients had ACTs within the recommended range of 300 to
450 seconds throughout the procedure. The investigators did not achieve anticoagulation
within the recommended range in about 23% of patients. However, in this small
sample, patients with ACTs below 300 seconds did not have more coronary
thrombotic events, and patients with ACTs over 450 seconds did not have higher
bleeding rates.
Acute procedural success was defined as lack of death,
emergent coronary artery bypass graft (CABG), or Q-wave myocardial infarction.
Acute procedural success was reported in 98.2% of patients who underwent PCIs
with argatroban anticoagulation compared with 94.3% of historical control
patients anticoagulated with heparin (p=NS). Among the 112 interventions, 2
patients had emergency CABGs, 3 had repeat PTCAs, 4 had non-Q-wave myocardial
infarctions, 3 had myocardial ischemia, 1 had an abrupt closure, and 1 had an
impending closure (some patients may have experienced more than 1 event). No
patients died.