CLINICAL PHARMACOLOGY
Mechanism Of Action
Epoprostenol has 2 major pharmacological actions: (1) direct vasodilation of pulmonary and
systemic arterial vascular beds and (2) inhibition of platelet aggregation.
Pharmacodynamics
Acute Hemodynamic Effects
Acute intravenous infusions of FLOLAN for up to 15 minutes in patients with idiopathic or
heritable PAH or PAH/SSD produce dose-related increases in cardiac index (CI) and stroke
volume (SV) and dose-related decreases in pulmonary vascular resistance (PVR), total
pulmonary resistance (TPR), and mean systemic arterial pressure (SAPm). The effects of
FLOLAN on mean pulmonary artery pressure (PAPm) were variable and minor.
In humans, hemodynamic changes due to epoprostenol (e.g., increased heart rate, facial flushing)
returned to baseline within 10 minutes of termination of 60-minute infusions of 1 to 16
ng/kg/min. This pharmacodynamic behavior is consistent with a short in vivo half-life and rapid
clearance in humans, as suggested by the results of animal and in vitro studies.
In animals, the vasodilatory effects reduce right- and left-ventricular afterload and increase
cardiac output and stroke volume. The effect of epoprostenol on heart rate in animals varies with
dose. At low doses, there is vagally-mediated bradycardia, but at higher doses, epoprostenol
causes reflex tachycardia in response to direct vasodilation and hypotension. No major effects on
cardiac conduction have been observed. Additional pharmacologic effects of epoprostenol in
animals include bronchodilation, inhibition of gastric acid secretion, and decreased gastric
emptying.
Drug Interaction Studies
Additional reductions in blood pressure may occur when FLOLAN is administered with
diuretics, antihypertensive agents, or other vasodilators.
When other antiplatelet agents or anticoagulants are used concomitantly, there is a potential for
FLOLAN to increase the risk of bleeding. However, patients receiving infusions of FLOLAN in
clinical trials were maintained on anticoagulants without evidence of increased bleeding.
Pharmacokinetics
Absorption/Distribution
Epoprostenol is rapidly hydrolyzed at neutral pH in blood and is also subject to enzymatic
degradation. No available chemical assay is sufficiently sensitive and specific to assess the in
vivo human pharmacokinetics of epoprostenol. Animal studies using tritium-labeled
epoprostenol have indicated a high clearance (93 mL/kg/min), small volume of distribution
(357 mL/kg), and a short half-life (2.7 minutes). During infusions in animals, steady-state plasma
concentrations of tritium-labeled epoprostenol were reached within 15 minutes and were
proportional to infusion rates.
Metabolism
Tritium-labeled epoprostenol has been administered to humans in order to identify the metabolic
products of epoprostenol. Epoprostenol is metabolized to 2 primary metabolites: 6-keto-PGF1α
(formed by spontaneous degradation) and 6,15-diketo-13,14-dihydro-PGF1α (enzymatically
formed), both of which have pharmacological activity orders of magnitude less than
epoprostenol in animal test systems. The recovery of radioactivity in urine and feces over a
1-week period was 82% and 4% of the administered dose, respectively. Fourteen additional
minor metabolites have been isolated from urine, indicating that epoprostenol is extensively
metabolized in humans.
Elimination
The in vitro half-life of epoprostenol in human blood at 37°C and pH 7.4 is approximately
6 minutes; therefore, the in vivo half-life of epoprostenol in humans is expected to be no greater
than 6 minutes.
Drug Interaction Studies
In a pharmacokinetic substudy in patients with congestive heart failure receiving furosemide in
whom therapy with FLOLAN was initiated, apparent oral clearance values for furosemide
(n = 23) were decreased by 13% on the second day of therapy and returned to baseline values by
Day 87. The change in furosemide clearance value is not likely to be clinically significant.
In a pharmacokinetic substudy in patients with congestive heart failure receiving digoxin in
whom therapy with FLOLAN was initiated, apparent oral clearance values for digoxin (n = 30)
were decreased by 15% on the second day of therapy and returned to baseline values by Day 87.
Clinical significance of this interaction is not known.
Clinical Studies
Chronic Infusion In Idiopathic Or Heritable PAH
Hemodynamic Effects
Chronic continuous infusions of FLOLAN in patients with idiopathic or heritable PAH were
studied in 2 prospective, open, randomized trials of 8 and 12 weeks' duration comparing
FLOLAN plus conventional therapy with conventional therapy alone. Dosage of FLOLAN was
determined as described in Dosage and Administration (2) and averaged 9.2 ng/kg/min at trials'
end. Conventional therapy varied among patients and included some or all of the following:
anticoagulants in essentially all patients; oral vasodilators, diuretics, and digoxin in one-half to
two-thirds of patients; and supplemental oxygen in about half the patients. Except for 2 NYHA
Functional Class II patients, all patients were either functional Class III or Class IV. As results
were similar in the 2 trials, the pooled results are described.
Chronic hemodynamic effects were generally similar to acute effects. Increases in CI, SV, and
arterial oxygen saturation and decreases in PAPm, mean right atrial pressure (RAPm), TPR, and
systemic vascular resistance (SVR) were observed in patients who received FLOLAN
chronically compared with those who did not. Table 4 illustrates the treatment-related
hemodynamic changes in these patients after 8 or 12 weeks of treatment.
Table 4. Hemodynamics during Chronic Administration of FLOLAN in Patients with
Idiopathic or Heritable PAH
Hemodynamic
Parameter |
Baseline |
Mean Change from
Baseline at End of
Treatment Perioda |
FLOLAN
(n = 52) |
Standard
Therapy
(n = 54) |
FLOLAN
(n = 48) |
Standard
Therapy
(n = 41) |
CI
(L/min/m2) |
2.0 |
2.0 |
0.3b |
-0.1 |
PAPm
(mm Hg) |
60 |
60 |
-5b |
1 |
PVR
(Wood U) |
16 |
17 |
-4b |
1 |
SAPm
(mm Hg) |
89 |
91 |
-4 |
-3 |
SV
(mL/beat) |
44 |
43 |
6b |
-1 |
TPR
(Wood U) |
20 |
21 |
-5b |
1 |
a At 8 weeks: FLOLAN n = 10, conventional therapy n = 11 (n is the number of patients with
hemodynamic data).
At 12 weeks: FLOLAN n = 38, conventional therapy n = 30 (n is the number of patients with
hemodynamic data).
b Denotes statistically significant difference between group receiving FLOLAN and group
receiving conventional therapy.
CI = Cardiac index, PAPm = Mean pulmonary arterial pressure, PVR = Pulmonary vascular
resistance, SAPm = Mean systemic arterial pressure, SV = Stroke volume, TPR = Total
pulmonary resistance. |
These hemodynamic improvements appeared to persist when FLOLAN was administered for at
least 36 months in an open, nonrandomized trial.
The acute hemodynamic response to FLOLAN did not correlate well with improvement in
exercise tolerance or survival during chronic use of FLOLAN.
Clinical Effects
A statistically significant improvement was observed in exercise capacity, as measured by the 6-
minute walk test in patients receiving continuous intravenous FLOLAN plus conventional
therapy (n = 52) for 8 or 12 weeks compared with those receiving conventional therapy alone
(n = 54). Improvements were apparent as early as the first week of therapy. Increases in exercise
capacity were accompanied by statistically significant improvement in dyspnea and fatigue, as
measured by the Chronic Heart Failure Questionnaire and the Dyspnea Fatigue Index,
respectively.
Survival was improved in NYHA Functional Class III and Class IV patients with idiopathic or
heritable PAH treated with FLOLAN for 12 weeks in a multicenter, open, randomized, parallel
trial. At the end of the treatment period, 8 of 40 (20%) patients receiving conventional therapy
alone died, whereas none of the 41 patients receiving FLOLAN died (P = 0.003).
Chronic Infusion In PAH/SSD
Hemodynamic Effects
Chronic continuous infusions of FLOLAN in patients with PAH/SSD were studied in a
prospective, open, randomized trial of 12 weeks’ duration comparing FLOLAN plus
conventional therapy (n = 56) with conventional therapy alone (n = 55). Except for 5 NYHA
Functional Class II patients, all patients were either functional Class III or Class IV. In the
controlled 12-week trial in PAH/SSD, for example, the dose increased from a mean starting dose
of 2.2 ng/kg/min. During the first 7 days of treatment, the dose was increased daily to a mean
dose of 4.1 ng/kg/min on Day 7 of treatment. At the end of Week 12, the mean dose was
11.2 ng/kg/min. The mean incremental increase was 2 to 3 ng/kg/min every 3 weeks.
Conventional therapy varied among patients and included some or all of the following:
anticoagulants in essentially all patients, supplemental oxygen and diuretics in two-thirds of the
patients, oral vasodilators in 40% of the patients, and digoxin in a third of the patients. A
statistically significant increase in CI and statistically significant decreases in PAPm, RAPm,
PVR, and SAPm after 12 weeks of treatment were observed in patients who received FLOLAN
chronically compared with those who did not. Table 5 illustrates the treatment-related
hemodynamic changes in these patients after 12 weeks of treatment.
Table 5. Hemodynamics during Chronic Administration of FLOLAN in Patients with
PAH/SSD
Hemodynamic
Parameter |
Baseline |
Mean Change from Baseline at
12 Weeks |
FLOLAN
(n = 56) |
Conventional
Therapy
(n = 55) |
FLOLAN
(n = 50) |
Conventional
Therapy
(n = 48) |
CI
(L/min/m2) |
1.9 |
2.2 |
0.5a |
-0.1 |
PAPm
(mm Hg) |
51 |
49 |
-5a |
1 |
RAPm
(mm Hg) |
13 |
11 |
-1a |
1 |
PVR
(Wood U) |
14 |
11 |
-5a |
1 |
SAPm
(mm Hg) |
93 |
89 |
-8a |
-1 |
a Denotes statistically significant difference between group receiving FLOLAN and group
receiving conventional therapy (n is the number of patients with hemodynamic data).
CI = Cardiac index, PAPm = Mean pulmonary arterial pressure, RAPm = Mean right arterial
pressure, PVR = Pulmonary vascular resistance, SAPm = Mean systemic arterial pressure. |
Clinical Effects
Statistically significant improvement was observed in exercise capacity, as measured by the
6-minute walk, in patients receiving continuous intravenous FLOLAN plus conventional therapy
for 12 weeks compared with those receiving conventional therapy alone. Improvements were
apparent in some patients at the end of the first week of therapy. Increases in exercise capacity
were accompanied by statistically significant improvements in dyspnea and fatigue, as measured
by the Borg Dyspnea Index and Dyspnea Fatigue Index. At Week 12, NYHA functional class
improved in 21 of 51 (41%) patients treated with FLOLAN compared with none of the 48
patients treated with conventional therapy alone. However, more patients in both treatment
groups (28/51 [55%] with FLOLAN and 35/48 [73%] with conventional therapy alone) showed
no change in functional class, and 2/51 (4%) with FLOLAN and 13/48 (27%) with conventional
therapy alone worsened.
No statistical difference in survival over 12 weeks was observed in patients with PAH/SSD
treated with FLOLAN as compared with those receiving conventional therapy alone. At the end
of the treatment period, 4 of 56 (7%) patients receiving FLOLAN died, whereas 5 of 55 (9%)
patients receiving conventional therapy alone died.
Increased Mortality In Patients With Heart Failure Caused By Severe Left Ventricular
Systolic Dysfunction
A large trial evaluating the effect of FLOLAN on survival in NYHA Class III and IV patients
with congestive heart failure due to severe left ventricular systolic dysfunction was terminated
after an interim analysis of 471 patients revealed a higher mortality in patients receiving
FLOLAN plus conventional therapy than in those receiving conventional therapy alone. The
chronic use of FLOLAN in patients with heart failure due to severe left ventricular systolic
dysfunction is therefore contraindicated.