CLINICAL PHARMACOLOGY
Mechanism Of Action
Pramipexole is a non-ergot dopamine agonist with high
relative in vitro specificity and full intrinsic activity at the D subfamily of
dopamine receptors, binding with higher affinity to D3 than to D2 or D4 receptor
subtypes.
Parkinson’s Disease
The precise mechanism of action of pramipexole as a
treatment for Parkinson's disease is unknown, although it is believed to be
related to its ability to stimulate dopamine receptors in the striatum. This conclusion
is supported by electrophysiologic studies in animals that have demonstrated
that pramipexole influences striatal neuronal firing rates via activation of
dopamine receptors in the striatum and the substantia nigra, the site of
neurons that send projections to the striatum. The relevance of D3 receptor
binding in Parkinson's disease is unknown.
Restless Legs Syndrome (RLS)
The precise mechanism of action of MIRAPEX tablets as a
treatment for RLS is unknown. Although the pathophysiology of RLS is largely
unknown, neuropharmacological evidence suggests primary dopaminergic system
involvement. Positron Emission Tomographic (PET) studies suggest that a mild striatal
presynaptic dopaminergic dysfunction may be involved in the pathogenesis of
RLS.
Pharmacodynamics
The effect of pramipexole on the QT interval of the ECG
was investigated in a clinical study in 60 healthy male and female volunteers.
All subjects initiated treatment with 0.375 mg extended release pramipexole
tablets administered once daily, and were up-titrated every 3 days to 2.25 mg
and 4.5 mg daily, a faster rate of titration than recommended in the label. No
dose- or exposure-related effect on mean QT intervals was observed; however,
the study did not have a valid assessment of assay sensitivity. The effect of
pramipexole on QTc intervals at higher exposures achieved either due to drug
interactions (e.g., with cimetidine), renal impairment, or at higher doses has
not been systematically evaluated.
Although mean values remained within normal reference
ranges throughout the study, supine systolic blood pressure (SBP), diastolic
blood pressure (DBP), and pulse rate for subjects treated with pramipexole
generally increased during the rapid up-titration phase, by 10 mmHg, 7 mmHg,
and 10 bpm higher than placebo, respectively. Higher SBP, DBP, and pulse rates
compared to placebo were maintained until the pramipexole doses were tapered;
values on the last day of tapering were generally similar to baseline values.
Such effects have not been observed in clinical studies with Parkinson's disease
patients, who were titrated according to labeled recommendations.
Pharmacokinetics
Pramipexole displays linear pharmacokinetics over the
clinical dosage range. Its terminal half-life is about 8 hours in young healthy
volunteers and about 12 hours in elderly volunteers. Steady-state concentrations
are achieved within 2 days of dosing.
Absorption
Pramipexole is rapidly absorbed, reaching peak
concentrations in approximately 2 hours. The absolute bioavailability of
pramipexole is greater than 90%, indicating that it is well absorbed and
undergoes little presystemic metabolism. Food does not affect the extent of
pramipexole absorption, although the time of maximum plasma concentration (Tmax)
is increased by about 1 hour when the drug is taken with a meal.
Distribution
Pramipexole is extensively distributed, having a volume
of distribution of about 500 L (coefficient of variation [CV]=20%). It is about
15% bound to plasma proteins. Pramipexole distributes into red blood cells as
indicated by an erythrocyte-to-plasma ratio of approximately 2.
Metabolism
Pramipexole is metabolized only to a negligible extent
( < 10%). No specific active metabolite has been identified in human plasma or
urine.
Elimination
Urinary excretion is the major route of pramipexole
elimination, with 90% of a pramipexole dose recovered in urine, almost all as
unchanged drug. The renal clearance of pramipexole is approximately 400 mL/min
(CV=25%), approximately three times higher than the glomerular filtration rate.
Thus, pramipexole is secreted by the renal tubules, probably by the organic
cation transport system.
Pharmacokinetics In Specific Populations
Because therapy with MIRAPEX tablets is initiated at a
low dose and gradually titrated upward according to clinical tolerability to
obtain the optimum therapeutic effect, adjustment of the initial dose based on
gender, weight, race, or age is not necessary. However, renal insufficiency,
which can cause a large decrease in the ability to eliminate pramipexole, may
necessitate dosage adjustment [see DOSAGE AND ADMINISTRATION].
Gender
Pramipexole clearance is about 30% lower in women than in
men, but this difference can be accounted for by differences in body weight.
There is no difference in half-life between males and females.
Age
Pramipexole clearance decreases with age as the half-life
and clearance are about 40% longer and 30% lower, respectively, in elderly
(aged 65 years or older) compared with young healthy volunteers (aged less than
40 years). This difference is most likely due to the reduction in renal
function with age, since pramipexole clearance is correlated with renal
function, as measured by creatinine clearance.
Race
No racial differences in metabolism and elimination have
been identified.
Parkinson's Disease Patients
A cross-study comparison of data suggests that the
clearance of pramipexole may be reduced by about 30% in Parkinson's disease
patients compared with healthy elderly volunteers. The reason for this difference
appears to be reduced renal function in Parkinson's disease patients, which may
be related to their poorer general health. The pharmacokinetics of pramipexole
were comparable between early and advanced Parkinson's disease patients.
Restless Legs Syndrome Patients
A cross-study comparison of data suggests that the pharmacokinetic
profile of pramipexole administered once daily in RLS patients is similar to
the pharmacokinetic profile of pramipexole in healthy volunteers.
Hepatic Impairment
The influence of hepatic insufficiency on pramipexole
pharmacokinetics has not been evaluated. Because approximately 90% of the
recovered dose is excreted in the urine as unchanged drug, hepatic impairment
would not be expected to have a significant effect on pramipexole elimination.
Renal Impairment
Clearance of pramipexole was about 75% lower in patients
with severe renal impairment (creatinine clearance approximately 20 mL/min) and
about 60% lower in patients with moderate impairment (creatinine clearance
approximately 40 mL/min) compared with healthy volunteers [see WARNINGS AND
PRECAUTIONS and DOSAGE AND ADMINISTRATION]. In patients with varying
degrees of renal impairment, pramipexole clearance correlates well with
creatinine clearance. Therefore, creatinine clearance can be used as a
predictor of the extent of decrease in pramipexole clearance.
Drug Interactions
Carbidopa/levodopa: Carbidopa/levodopa did not influence
the pharmacokinetics of pramipexole in healthy volunteers (N=10). Pramipexole
did not alter the extent of absorption (AUC) or the elimination of carbidopa/levodopa,
although it caused an increase in levodopa Cmax by about 40% and a decrease in Tmax
from 2.5 to 0.5 hours.
Selegiline: In healthy volunteers (N=11),
selegiline did not influence the pharmacokinetics of pramipexole.
Amantadine: Population pharmacokinetic analyses
suggest that amantadine may slightly decrease the oral clearance of
pramipexole.
Cimetidine: Cimetidine, a known inhibitor of renal
tubular secretion of organic bases via the cationic transport system, caused a
50% increase in pramipexole AUC and a 40% increase in half-life (N=12).
Probenecid: Probenecid, a known inhibitor of renal
tubular secretion of organic acids via the anionic transporter, did not
noticeably influence pramipexole pharmacokinetics (N=12).
Other drugs eliminated via renal secretion: Population
pharmacokinetic analysis suggests that coadministration of drugs that are
secreted by the cationic transport system (e.g., cimetidine, ranitidine, diltiazem,
triamterene, verapamil, quinidine, and quinine) decreases the oral clearance of
pramipexole by about 20%, while those secreted by the anionic transport system
(e.g., cephalosporins, penicillins, indomethacin, hydrochlorothiazide, and
chlorpropamide) are likely to have little effect on the oral clearance of
pramipexole. Other known organic cation transport substrates and/or inhibitors
(e.g., cisplatin and procainamide) may also decrease the clearance of
pramipexole.
CYP interactions: Inhibitors of cytochrome P450
enzymes would not be expected to affect pramipexole elimination because
pramipexole is not appreciably metabolized by these enzymes in vivo or in vitro.
Pramipexole does not inhibit CYP enzymes CYP1A2, CYP2C9, CYP2C19, CYP2E1, and
CYP3A4. Inhibition of CYP2D6 was observed with an apparent Ki of 30 μM,
indicating that pramipexole will not inhibit CYP enzymes at plasma
concentrations observed following the clinical dose of 4.5 mg/day (1.5 mg TID).
Animal Toxicology And/Or Pharmacology
Retinal Pathology In Rats
Pathologic changes (degeneration and loss of
photoreceptor cells) were observed in the retina of albino rats in the 2-year
carcinogenicity study with pramipexole. These findings were first observed during
week 76 and were dose-dependent in animals receiving 2 or 8 mg/kg/day (plasma
AUCs equal to 2.5 and 12.5 times that in humans at the MRHD). In a similar
study of pigmented rats with 2 years exposure to pramipexole at 2 or 8
mg/kg/day, retinal degeneration was not observed. Animals given drug had
thinning in the outer nuclear layer of the retina that was only slightly
greater (by morphometric analysis) than that seen in control rats.
Investigative studies demonstrated that pramipexole
reduced the rate of disk shedding from the photoreceptor rod cells of the
retina in albino rats, which was associated with enhanced sensitivity to the
damaging effects of light. In a comparative study, degeneration and loss of
photoreceptor cells occurred in albino rats after 13 weeks of treatment with 25
mg/kg/day of pramipexole (54 times the occurred in albino rats after 13 weeks
of treatment with 25 mg/kg/day of pramipexole (54 times the MRHD on a mg/m²
basis) and constant light (100 lux) but not in pigmented rats exposed to the
same dose and higher light intensities (500 lux). Thus, the retina of albino
rats is considered to be uniquely sensitive to the damaging effects of
pramipexole and light. Similar changes in the retina did not occur in a 2-year
carcinogenicity study in albino mice treated with 0.3, 2, or 10 mg/kg/day (0.3,
2.2 and 11 times the MRHD on a mg/m² basis). Evaluation of the retinas of
monkeys given 0.1, 0.5, or 2.0 mg/kg/day of pramipexole (0.4, 2.2, and 8.6
times the MRHD on a mg/m² basis) for 12 months and minipigs given 0.3, 1, or 5
mg/kg/day of pramipexole for 13 weeks also detected no changes.
The potential significance of this effect in humans has
not been established, but cannot be disregarded because disruption of a
mechanism that is universally present in vertebrates (i.e., disk shedding) may
be involved.
Fibro-osseous Proliferative Lesions In Mice
An increased incidence of fibro-osseous proliferative
lesions occurred in the femurs of female mice treated for 2 years with 0.3,
2.0, or 10 mg/kg/day (0.3, 2.2, and 11 times the MRHD on a mg/m basis). Similar
lesions were not observed in male mice or rats and monkeys of either sex that
were treated chronically with pramipexole. The significance of this lesion to
humans is not known.
Clinical Studies
Parkinson's Disease
The effectiveness of MIRAPEX tablets in the treatment of
Parkinson's disease was evaluated in a multinational drug development program
consisting of seven randomized, controlled trials. Three were conducted in
patients with early Parkinson's disease who were not receiving concomitant
levodopa, and four were conducted in patients with advanced Parkinson's disease
who were receiving concomitant levodopa. Among these seven studies, three
studies provide the most persuasive evidence of pramipexole's effectiveness in the
management of patients with Parkinson's disease who were and were not receiving
concomitant levodopa. Two of these three trials enrolled patients with early
Parkinson's disease (not receiving levodopa), and one enrolled patients with
advanced Parkinson's disease who were receiving maximally tolerated doses of
levodopa.
In all studies, the Unified Parkinson's Disease Rating
Scale (UPDRS), or one or more of its subparts, served as the primary outcome
assessment measure. The UPDRS is a four-part multi-item rating scale intended
to evaluate mentation (part I), Activities of Daily Living (ADL) (part II),
motor performance (part III), and complications of therapy (part IV).
Part II of the UPDRS contains 13 questions relating to
ADL, which are scored from 0 (normal) to 4 (maximal severity) for a maximum
(worst) score of 52. Part III of the UPDRS contains 27 questions (for 14 items)
and is scored as described for part II. It is designed to assess the severity
of the cardinal motor findings in patients with Parkinson's disease (e.g.,
tremor, rigidity, bradykinesia, postural instability, etc.), scored for
different body regions, and has a maximum (worst) score of 108.
Studies In Patients With Early Parkinson's Disease
Patients (N=599) in the two studies of early Parkinson's
disease had a mean disease duration of 2 years, limited or no prior exposure to
levodopa (generally none in the preceding 6 months), and were not experiencing
the “on-off” phenomenon and dyskinesia characteristic of later stages
of the disease.
One of the two early Parkinson's disease studies (N=335)
was a double-blind, placebo-controlled, parallel trial consisting of a 7-week
dose-escalation period and a 6-month maintenance period. Patients could be on
selegiline, anticholinergics, or both, but could not be on levodopa products or
amantadine. Patients were randomized to MIRAPEX tablets or placebo. Patients
treated with MIRAPEX tablets had a starting daily dose of 0.375 mg and were
titrated to a maximally tolerated dose, but no higher than 4.5 mg/day in three
divided doses. At the end of the 6-month maintenance period, the mean
improvement from baseline on the UPDRS part II (ADL) total score was 1.9 in the
group receiving MIRAPEX tablets and -0.4 in the placebo group, a difference
that was statistically significant. The mean improvement from baseline on the
UPDRS part III total score was 5.0 in the group receiving MIRAPEX tablets and -
0.8 in the placebo group, a difference that was also statistically significant.
A statistically significant difference between groups in favor of MIRAPEX
tablets was seen beginning at week 2 of the UPDRS part II (maximum dose 0.75
mg/day) and at week 3 of the UPDRS part III (maximum dose 1.5 mg/day).
The second early Parkinson's disease study (N=264) was a
double-blind, placebo-controlled, parallel trial consisting of a 6-week
dose-escalation period and a 4-week maintenance period. Patients could be on
selegiline, anticholinergics, amantadine, or any combination of these, but
could not be on levodopa products. Patients were randomized to 1 of 4 fixed
doses of MIRAPEX tablets (1.5 mg, 3.0 mg, 4.5 mg, or 6.0 mg per day) or
placebo. At the end of the 4-week maintenance period, the mean improvement from
baseline on the UPDRS part II total score was 1.8 in the patients treated with
MIRAPEX tablets, regardless of assigned dose group, and 0.3 in placebo-treated
patients. The mean improvement from baseline on the UPDRS part III total score
was 4.2 in patients treated with MIRAPEX tablets and 0.6 in placebo-treated
patients. No dose-response relationship was demonstrated. The between-treatment
differences on both parts of the UPDRS were statistically significant in favor
of MIRAPEX tablets for all doses.
No differences in effectiveness based on age or gender
were detected. There were too few non- Caucasian patients to evaluate the
effect of race. Patients receiving selegiline or anticholinergics had responses
similar to patients not receiving these drugs.
Studies In Patients With Advanced Parkinson's Disease
In the advanced Parkinson's disease study, the primary
assessments were the UPDRS and daily diaries that quantified amounts of
“on” and “off” time.
Patients in the advanced Parkinson's disease study
(N=360) had a mean disease duration of 9 years, had been exposed to levodopa
for long periods of time (mean 8 years), used concomitant levodopa during the
trial, and had “on-off” periods.
The advanced Parkinson's disease study was a
double-blind, placebo-controlled, parallel trial consisting of a 7-week
dose-escalation period and a 6-month maintenance period. Patients were all
treated with concomitant levodopa products and could additionally be on
concomitant selegiline, anticholinergics, amantadine, or any combination.
Patients treated with MIRAPEX tablets had a starting dose of 0.375 mg/day and
were titrated to a maximally tolerated dose, but no higher than 4.5 mg/day in
three divided doses. At selected times during the 6-month maintenance period,
patients were asked to record the amount of “off,” “on,” or
“on with dyskinesia” time per day for several sequential days. At the
end of the 6-month maintenance period, the mean improvement from baseline on
the UPDRS part II total score was 2.7 in the group treated with MIRAPEX tablets
and 0.5 in the placebo group, a difference that was statistically significant.
The mean improvement from baseline on the UPDRS part III total score was 5.6 in
the group treated with MIRAPEX tablets and 2.8 in the placebo group, a
difference that was statistically significant. A statistically significant difference
between groups in favor of MIRAPEX tablets was seen at week 3 of the UPDRS part
II (maximum dose 1.5 mg/day) and at week 2 of the UPDRS part III (maximum dose
0.75 mg/day). Dosage reduction of levodopa was allowed during this study if
dyskinesia (or hallucinations) developed; levodopa dosage reduction occurred in
76% of patients treated with MIRAPEX tablets versus 54% of placebo patients. On
average, the levodopa dose was reduced 27%.
The mean number of “off” hours per day during
baseline was 6 hours for both treatment groups. Throughout the trial, patients
treated with MIRAPEX tablets had a mean of 4 “off” hours per day,
while placebo-treated patients continued to experience 6 “off” hours
per day.
No differences in effectiveness based on age or gender
were detected. There were too few non- Caucasian patients to evaluate the
effect of race.
Restless Legs Syndrome
The efficacy of MIRAPEX tablets in the treatment of RLS
was evaluated in a multinational drug development program consisting of 4 randomized,
double-blind, placebo-controlled trials. This program included approximately
1000 patients with moderate to severe RLS; patients with RLS secondary to other
conditions (e.g., pregnancy, renal failure, and anemia) were excluded. All
patients were administered MIRAPEX tablets (0.125 mg, 0.25 mg, 0.5 mg, or 0.75
mg) or placebo once daily 2-3 hours before going to bed. Across the 4 studies,
the mean duration of RLS was 4.6 years (range of 0 to 56 years), mean age was
approximately 55 years (range of 18 to 81 years), and approximately 66.6% were
women.
Key diagnostic criteria for RLS are: an urge to move the
legs usually accompanied or caused by uncomfortable and unpleasant leg
sensations; symptoms begin or worsen during periods of rest or inactivity such
as lying or sitting; symptoms are partially or totally relieved by movement
such as walking or stretching at least as long as the activity continues; and
symptoms are worse or occur only in the evening or night. Difficulty falling
asleep may frequently be associated with symptoms of RLS.
The two outcome measures used to assess the effect of
treatment were the International RLS Rating Scale (IRLS Scale) and a Clinical
Global Impression - Improvement (CGI-I) assessment. The IRLS Scale contains 10
items designed to assess the severity of sensory and motor symptoms, sleep disturbance,
daytime somnolence, and impact on activities of daily living and mood
associated with RLS. The range of scores is 0 to 40, with 0 being absence of
RLS symptoms and 40 the most severe symptoms. The CGI-I is designed to assess
clinical progress (global improvement) on a 7-point scale.
In Study 1, fixed doses of MIRAPEX tablets were compared
to placebo in a study of 12 weeks duration. A total of 344 patients were
randomized equally to the 4 treatment groups. Patients treated with MIRAPEX
tablets (n=254) had a starting dose of 0.125 mg/day and were titrated to one of
the three randomized doses (0.25, 0.5, 0.75 mg/day) in the first three weeks of
the study. The mean improvement from baseline on the IRLS Scale total score and
the percentage of CGI-I responders for each of the MIRAPEX tablets treatment
groups compared to placebo are summarized in Table 8. All treatment groups
reached statistically significant superiority compared to placebo for both
endpoints. There was no clear evidence of a dose-response across the 3
randomized dose groups.
Table 8 : Mean Changes from Bas eline to Week 12 in
IRLS Score and CGI-I (Study 1)
|
MIRAPEX 0.25 mg |
MIRAPEX 0.5 mg |
MIRAPEX 0.75 mg |
MIRAPEX Total |
Placebo |
No. Patients |
88 |
79 |
87 |
254 |
85 |
IRLS score |
-13.1 |
-13.4 |
-14.4 |
-13.6 |
-9.4 |
CGI-I responders* |
74.7% |
67.9% |
72.9% |
72.0% |
51.2% |
* CGI-I responders = “much improved” and “very much
improved” |
Study 2 was a randomized-withdrawal study, designed to
demonstrate the sustained efficacy of pramipexole for treatment of RLS after a
period of six months. RLS patients who responded to MIRAPEX tablets treatment
in a preceding 6-month open-label treatment phase (defined as having a CGI-I
rating of “very much improved” or “much improved” compared to baseline and an
IRLS score of 15 or below) were randomized to receive either continued active
treatment (n=78) or placebo (n=69) for 12 weeks. The primary endpoint of this
study was time to treatment failure, defined as any worsening on the CGI-I
score along with an IRLS Scale total score above 15.
In patients who had responded to 6-month open label
treatment with MIRAPEX tablets, the administration of placebo led to a rapid
decline in their overall conditions and return of their RLS symptoms. At the end
of the 12-week observation period, 85% of patients treated with placebo had
failed treatment, compared to 21% treated with blinded pramipexole, a
difference that was highly statistically significant. The majority of treatment
failures occurred within 10 days of randomization. For the patients randomized,
the distribution of doses was: 7 on 0.125 mg, 44 on 0.25 mg, 47 on 0.5 mg, and
49 on 0.75mg.
Study 3 was a 6-week study, comparing a flexible dose of
MIRAPEX tablets to placebo. In this study, 345 patients were randomized in a
2:1 ratio to MIRAPEX tablets or placebo. The mean improvement from baseline on
the IRLS Scale total score was -12 for MIRAPEX-treated patients and -6 for
placebotreated patients. The percentage of CGI-I responders was 63% for
MIRAPEX-treated patients and 32% for placebo-treated patients. The
between-group differences were statistically significant for both outcome
measures. For the patients randomized to MIRAPEX tablets, the distribution of
achieved doses was: 35 on 0.125 mg, 51 on 0.25 mg, 65 on 0.5 mg, and 69 on 0.75
mg.
Study 4 was a 3-week study, comparing 4 fixed doses of
MIRAPEX tablets, 0.125 mg, 0.25 mg, 0.5 mg, and 0.75 mg, to placebo. Approximately
20 patients were randomized to each of the 5 dose groups. The mean improvement
from baseline on the IRLS Scale total score and the percentage of CGI-I
responders for each of the MIRAPEX tablets treatment groups compared to placebo
are summarized in Table 9. In this study, the 0.125 mg dose group was not
significantly different from placebo. On average, the 0.5 mg dose group
performed better than the 0.25 mg dose group, but there was no difference
between the 0.5 mg and 0.75 mg dose groups.
Table 9 : Mean Changes from Baseline to Week 3 in IRLS
Score and CGI-I (Study 4)
|
MIRAPEX 0.125 mg |
MIRAPEX 0.25 mg |
MIRAPEX 0.5 mg |
MIRAPEX 0.75 mg |
MIRAPEX Total |
Placebo |
No. Patients |
21 |
22 |
22 |
21 |
86 |
21 |
IRLS score |
-11.7 |
-15.3 |
-17.6 |
-15.2 |
-15.0 |
-6.2 |
CGI-I responders* |
61.9% |
68.2% |
86.4% |
85.7% |
75.6% |
42.9% |
* CGI-I responders = “much improved” and “very much
improved” |
No differences in effectiveness based on age or gender
were detected. There were too few non-Caucasian patients to evaluate the
effect of race.