Mechanism of Action
Leflunomide is an isoxazole immunomodulatory agent which
inhibits dihydroorotate dehydrogenase (an enzyme involved in de novo pyrimidine
synthesis) and has antiproliferative activity. Several in vivo and in
vitro experimental models have demonstrated an antiinflammatory effect.
Pharmacokinetics
Following oral administration, leflunomide is metabolized to
an active metabolite A77 1726 (hereafter referred to as M1) which is
responsible for essentially all of its activity in vivo. Plasma levels of
leflunomide are occasionally seen, at very low levels. Studies of the
pharmacokinetics of leflunomide have primarily examined the plasma concentrations
of this active metabolite.
Absorption
Following oral administration, peak levels of the active
metabolite, M1, occurred between 6 - 12 hours after dosing. Due to the very
long half-life of M1 (~2 weeks), a loading dose of 100 mg for 3 days was used
in clinical studies to facilitate the rapid attainment of steady-state levels
of M1. Without a loading dose, it is estimated that attainment of steady-state
plasma concentrations would require nearly two months of dosing. The resulting
plasma concentrations following both loading doses and continued clinical
dosing indicate that M1 plasma levels are dose proportional.
Table 1 : Pharmacokinetic Parameters for M1 after Administration
of Leflunomide at Doses of 5, 10, and 25 mg/day for 24 Weeks to Patients (n=54)
with Rheumatoid Arthritis (Mean ± SD) (Study YU204)
| Maintenance (Loading) Dose |
| Parameter |
5 mg (50 mg) |
10 mg (100 mg) |
25 mg (100 mg) |
| C24(Day 1) (μg/mL)1 |
4.0 ± 0.6 |
8.4 ± 2.1 |
8.5 ± 2.2 |
| C24(ss) (μg/mL)2 |
8.8 ± 2.9 |
18 ± 9.6 |
63 ± 36 |
| t½(DAYS) |
15 ± 3 |
14 ± 5 |
18 ± 9 |
1 Concentration at 24 hours after loading
dose
2 Concentration at 24 hours after maintenance doses
at steady state |
Relative to an oral solution, ARAVA tablets are 80% bioavailable. Co-administration
of leflunomide tablets with a high fat meal did not have a significant impact
on M1 plasma levels.
Distribution
M1 has a low volume of distribution (Vss = 0.13 L/kg) and is
extensively bound ( > 99.3%) to albumin in healthy subjects. Protein binding
has been shown to be linear at therapeutic concentrations. The free fraction of
M1 is slightly higher in patients with rheumatoid arthritis and approximately
doubled in patients with chronic renal failure; the mechanism and significance
of these increases are unknown.
Metabolism
Leflunomide is metabolized to one primary (M1) and many
minor metabolites. Of these minor metabolites, only 4-trifluoromethylaniline
(TFMA) is quantifiable, occurring at low levels in the plasma of some patients.
The parent compound is rarely detectable in plasma. At the present time the
specific site of leflunomide metabolism is unknown. In vivo and in
vitro studies suggest a role for both the GI wall and the liver in drug
metabolism. No specific enzyme has been identified as the primary route of
metabolism for leflunomide; however, hepatic cytosolic and microsomal cellular
fractions have been identified as sites of drug metabolism.
Elimination
The active metabolite M1 is eliminated by further metabolism and subsequent
renal excretion as well as by direct biliary excretion. In a 28 day study of
drug elimination (n=3) using a single dose of radiolabeled compound, approximately
43% of the total radioactivity was eliminated in the urine and 48% was eliminated
in the feces. Subsequent analysis of the samples revealed the primary urinary
metabolites to be leflunomide glucuronides and an oxanilic acid derivative of
M1. The primary fecal metabolite was M1. Of these two routes of elimination,
renal elimination is more significant over the first 96 hours after which fecal
elimination begins to predominate. In a study involving the intravenous administration
of M1, the clearance was estimated to be 31 mL/hr. In small studies using activated
charcoal (n=1) or cholestyramine (n=3) to facilitate drug elimination, the in
vivo plasma half-life of M1 was reduced from > 1 week to approximately 1
day. (See PRECAUTIONS - General - Need
for Drug Elimination). Similar reductions in plasma half-life were observed
for a series of volunteers (n=96) enrolled in pharmacokinetic trials who were
given cholestyramine. This suggests that biliary recycling is a major contributor
to the long elimination half-life of M1. Studies with both hemodialysis and
CAPD (chronic ambulatory peritoneal dialysis) indicate that M1 is not dialyzable.
Special Populations
Gender. Gender has not been shown to cause a
consistent change in the in vivo pharmacokinetics of M1.
Age.Age has been shown to cause a change in the in vivo pharmacokinetics
of M1 (see CLINICAL PHARMACOLOGY – Special
Populations - Pediatrics).
Smoking. A population based pharmacokinetic
analysis of the phase III data indicates that smokers have a 38% increase in
clearance over non-smokers; however, no difference in clinical efficacy was
seen between smokers and nonsmokers.
Chronic Renal Insufficiency. In single dose
studies in patients (n=6) with chronic renal insufficiency requiring either
chronic ambulatory peritoneal dialysis (CAPD) or hemodialysis, neither had a
significant impact on circulating levels of M1. The free fraction of M1 was
almost doubled, but the mechanism of this increase is not known. In light of
the fact that the kidney plays a role in drug elimination and without adequate
studies of leflunomide use in subjects with renal insufficiency, caution should
be used when ARAVA is administered to these patients.
Hepatic Insufficiency. Studies of the effect
of hepatic insufficiency on M1 pharmacokinetics have not been done. Given the
need to metabolize leflunomide into the active species, the role of the liver
in drug elimination/recycling, and the possible risk of increased hepatic
toxicity, the use of leflunomide in patients with hepatic insufficiency is not
recommended.
Pediatrics
The pharmacokinetics of M1 following oral administration of
leflunomide have been investigated in 73 pediatric patients with polyarticular
course Juvenile Rheumatoid Arthritis (JRA) who ranged in age from 3 to 17
years. The results of a population pharmacokinetic analysis of these trials
have demonstrated that pediatric patients with body weights ≤ 40 kg have a reduced clearance of M1 (see
Table 2) relative to adult rheumatoid arthritis patients.
Table 2: Population Pharmacokinetic Estimate of M1 Clearance
Following Oral Administration of Leflunomide in Pediatric Patients with Polyarticular
Course JRA Mean ± SD [Range]
| N |
Body Weight (kg) |
CL (mL/h) |
| 10 |
< 20 |
18 ± 9.8 [6.8-37] |
| 30 |
20-40 |
18 ± 9.5 [4.2-43] |
| 33 |
> 40 |
26 ± 16 [9.7-93.6] |
Drug Interactions
In vivo drug interaction studies have demonstrated a
lack of a significant drug interaction between leflunomide and tri-phasic oral
contraceptives, and cimetidine.
In vitro studies of protein binding indicated that warfarin did not
affect M1 protein binding. At the same time M1 was shown to cause increases
ranging from 13 - 50% in the free fraction of diclofenac, ibuprofen and tolbutamide
at concentrations in the clinical range. In vitro studies of drug metabolism
indicate that M1 inhibits CYP 450 2C9, which is responsible for the metabolism
of phenytoin, tolbutamide, warfarin and many NSAIDs. M1 has been shown to inhibit
the formation of 4´-hydroxydiclofenac from diclofenac in vitro. The clinical
significance of these findings with regard to phenytoin and tolbutamide is unknown;
however, there was extensive concomitant use of NSAIDs in the clinical studies
and no differential effect was observed. (See PRECAUTIONS – DRUG
INTERACTIONS).
Methotrexate. Coadministration, in 30 patients, of ARAVA (100
mg/day x 2 days followed by 10 - 20 mg/day) with methotrexate (10 - 25 mg/week,
with folate) demonstrated no pharmacokinetic interaction between the two drugs.
However, co-administration increased risk of hepatotoxicity (see PRECAUTIONS
- DRUG INTERACTIONS – Hepatotoxic Drugs).
Rifampin. Following concomitant administration
of a single dose of ARAVA to subjects receiving multiple doses of rifampin, M1
peak levels were increased (~40%) over those seen when ARAVA was given alone.
Because of the potential for ARAVA levels to continue to increase with multiple
dosing, caution should be used if patients are to receive both ARAVA and
rifampin.
Clinical Studies
Adults
The efficacy of ARAVA in the treatment of rheumatoid
arthritis (RA) was demonstrated in three controlled trials showing reduction in
signs and symptoms, and inhibition of structural damage. In two placebo
controlled trials, efficacy was demonstrated for improvement in physical
function.
- Reduction of signs and symptoms
Relief of signs and symptoms was assessed using the American College of Rheumatology
(ACR)20 Responder Index, a composite of clinical, laboratory, and functional
measures in rheumatoid arthritis. An “ACR20 Responder” is a patient who had
20% improvement in both tender and swollen joint counts and in 3 of the following
5 criteria: physician global assessment, patient global assessment, functional
ability measure [Modified Health Assessment Questionnaire (MHAQ)], visual
analog pain scale, and erythrocyte sedimentation rate or C-reactive protein.
An “ACR20 Responder at Endpoint” is a patient who completed the study and
was an ACR20 Responder at the completion of the study.
- Inhibition of structural damage
Inhibition of structural damage compared to control was assessed using the
Sharp Score (Sharp, JT. Scoring Radiographic Abnormalities in Rheumatoid Arthritis,
Radiologic Clinics of North America, 1996; vol. 34, pp. 233-241), a composite
score of X-ray erosions and joint space narrowing in hands/wrists and forefeet.
- Improvement in physical function
Improvement in physical function was assessed using the Health Assessment
Questionnaire (HAQ) and the Medical Outcomes Survey Short Form (SF-36).
In all Arava monotherapy studies, an initial loading dose of
100 mg per day for three days only was used followed by 20 mg per day
thereafter.
US301 Clinical Trial in Adults
Study US301, a 2 year study, randomized 482 patients with
active RA of at least 6 months duration to leflunomide 20 mg/day (n=182),
methotrexate 7.5 mg/week increasing to 15 mg/week (n=182), or placebo (n=118).
All patients received folate 1 mg BID. Primary analysis was at 52 weeks with
blinded treatment to 104 weeks.
Overall, 235 of the 508 randomized treated patients (482 in
primary data analysis and an additional 26 patients), continued into a second
12 months of double-blind treatment (98 leflunomide, 101 methotrexate, 36
placebo). Leflunomide dose continued at 20 mg/day and the methotrexate dose
could be increased to a maximum of 20 mg/week. In total, 190 patients (83
leflunomide, 80 methotrexate, 27 placebo) completed 2 years of double-blind
treatment. The rate and reason for withdrawal is summarized in Table 3.
Table 3: Withdrawals in US301
| |
n(%) patients |
Leflunomide
190 |
Placebo
128 |
Methotrexate
190 |
| Withdrawals in Year-1 |
| Lack of efficacy |
33 (17.4) |
70 (54.7) |
50 (26.3) |
| Safety |
44 (23.2) |
12 (9.4) |
22 (11.6) |
| Other1 |
15 (7.9) |
10 (7.8) |
17 (9.0) |
| Total |
92 (48.4) |
92 (71.9) |
89 (46.8) |
| Patients entering Year 2 |
98 |
36 |
101 |
| Withdrawals in Year-2 |
| Lack of efficacy |
4 (4.1) |
1 (2.8) |
4 (4.0) |
| Safety |
8 (8.2) |
0 (0.0) |
10 (9.9) |
| Other1 |
3 (3.1) |
8 (22.2) |
7 (6.9) |
| Total |
15 (15.3) |
9 (25.0) |
21 (20.8) |
| 1 Includes: lost to follow up, protocol
violation, noncompliance, voluntary withdrawal, investigator discretion.
|
MN301/303/305 Clinical Trial in Adults
Study MN301 randomized 358 patients with active RA to
leflunomide 20 mg/day (n=133), sulfasalazine 2.0 g/day (n=133), or placebo
(n=92). Treatment duration was 24 weeks. An extension of the study was an
optional 6-month blinded continuation of MN301 without the placebo arm, resulting
in a 12-month comparison of leflunomide and sulfasalazine (study MN303).
Of the 168 patients who completed 12 months of treatment in
MN301 and MN303, 146 patients (87%) entered a 1-year extension study of double
blind active treatment (MN305; 60 leflunomide, 60 sulfasalazine, 26 placebo/
sulfasalazine). Patients continued on the same daily dosage of leflunomide or
sulfasalazine that they had been taking at the completion of MN301/303. A total
of 121 patients (53 leflunomide, 47 sulfasalazine, 21 placebo/sulfasalazine)
completed the 2 years of double-blind treatment.
Patient withdrawal data in MN301/303/305 is summarized in
Table 4.
Table 4: Withdrawals in study MN301/303/305
| |
n(%) patients |
Leflunomide
133 |
Placebo
92 |
Sulfasalazine
133 |
| Withdrawals in MN301 (Mo 0-6) |
| Lack of efficacy |
10 (7.5) |
29 (31.5) |
14 (10.5) |
| Safety |
19 (14.3) |
6 (6.5) |
25 (18.8) |
| Other1 |
8 (6.0) |
6 (6.5) |
11 (8.3) |
| Total |
37 (27.8) |
41 (44.6) |
50 (37.6) |
| Patients entering MN303 |
80 |
|
76 |
| Withdrawals in MN303 (Mo 7-12) |
| Lack of efficacy |
4 (5.0) |
|
2 (2.6) |
| Safety |
2 (2.5) |
|
5 (6.6) |
| Other1 |
3 (3.8) |
|
1 (1.3) |
| Total |
9 (11.3) |
|
8 (10.5) |
| Patients entering MN305 |
60 |
|
60 |
| Withdrawals in MN305 (Mo 13-24) |
| Lack of efficacy |
0 (0.0) |
|
3 (5.0) |
| Safety |
6 (10.0) |
|
8 (13.3) |
| Other1 |
1 (1.7) |
|
2 (3.3) |
| Total |
7 (11.7) |
|
13 (21.7) |
| 1 Includes: lost to follow up, protocol violation,
noncompliance, voluntary withdrawal, investigator discretion. |
MN302/304 Clinical Trial in Adults
Study MN302 randomized 999 patients with active RA to
leflunomide 20 mg/day (n=501) or methotrexate at 7.5 mg/week increasing to 15
mg/week (n=498). Folate supplementation was used in 10% of patients. Treatment
duration was 52 weeks.
Of the 736 patients who completed 52 weeks of treatment in
study MN302, 612 (83%) entered the double-blind, 1-year extension study MN304
(292 leflunomide, 320 methotrexate). Patients continued on the same daily
dosage of leflunomide or methotrexate that they had been taking at the
completion of MN302. There were 533 patients (256 leflunomide, 277
methotrexate) who completed 2 years of double-blind treatment.
Patient withdrawal data in MN302/304 is summarized in Table
5.
Table 5: Withdrawals in MN302/304
| |
n(%) patients |
Leflunomide
501 |
Methotrexate
498 |
| Withdrawals in MN302 (Year-1) |
| Lack of efficacy |
37 (7.4) |
15 (3.0) |
| Safety |
98 (19.6) |
79 (15.9) |
| Other1 |
17 (3.4) |
17 (3.4) |
| Total |
152 (30.3) |
111 (22.3) |
| Patients entering MN304 |
292 |
320 |
| Withdrawals in MN304 (Year-2) |
| Lack of efficacy |
13 (4.5) |
9 (2.8) |
| Safety |
11 (3.8) |
22 (6.9) |
| Other1 |
12 (4.1) |
12 (3.8) |
| Total |
36 (12.3) |
43 (13.4) |
| 1 Includes: lost to follow up, protocol
violation, noncompliance, voluntary withdrawal, investigator discretion.
|
Clinical Trial Data
1. Signs and symptoms Rheumatoid Arthritis
The ACR20 Responder at Endpoint rates are shown in Figure 1.
ARAVA was statistically significantly superior to placebo in reducing the signs
and symptoms of RA by the primary efficacy analysis, ACR20 Responder at
Endpoint, in study US301 (at the primary 12 months endpoint) and MN301 (at 6
month endpoint). ACR20 Responder at Endpoint rates with ARAVA treatment were
consistent across the 6 and 12 month studies (41 - 49%). No consistent
differences were demonstrated between leflunomide and methotrexate or between leflunomide
and sulfasalazine. ARAVA treatment effect was evident by 1 month, stabilized by
3 - 6 months, and continued throughout the course of treatment as shown in
Figure 2.
Figure 1
| |
Comparisons |
95% Confidence Interval |
p Value |
| US301 |
Leflunomide vs. Placebo |
(12, 32) |
< 0.0001 |
| Methotrexate vs. Placebo |
(8, 30) |
< 0.0001 |
| Leflunomide vs. Methotrexate |
(-4, 16) |
NS |
| MN301 |
Leflunomide vs. Placebo |
(7, 33) |
0.0026 |
| Sulfasalazine vs. Placebo |
(4, 29) |
0.0121 |
| Leflunomide vs. Sulfasalazine |
(-8, 16) |
NS |
| MN302 |
Leflunomide vs. Methotrexate |
(-19, -7) |
< 0.0001 |
Figure 2
ACR50 and ACR70 Responders are defined in an analogous
manner to the ACR 20 Responder, but use improvements of 50% or 70%,
respectively (Table 6). Mean change for the individual components of the ACR
Responder Index are shown in Table 7.
Table 6. Summary of ACR Response Rates*
| Study and Treatment Group |
ACR20 |
ACR50 |
ACR70 |
| Placebo-Controlled Studies |
| US301 (12 months) |
|
| Leflunomide (n=178)† |
52.2‡ |
34.3‡ |
20.2‡ |
| Placebo (n=118) † |
26.3 |
7.6 |
4.2 |
| Methotrexate (n=180)† |
45.6 |
22.8 |
9.4 |
| MN301(6 months) |
|
| Leflunomide (n=130)† |
54.6‡ |
33.1‡ |
10.0&dect; |
| Placebo (n=91)† |
28.6 |
14.3 |
2.2 |
| Sulfasalazine (n=132)† |
56.8 |
30.3 |
7.6 |
| Non-Placebo Active-Controlled Studies |
| MN302 (12 months) |
|
| Leflunomide (n=495)† |
51.1 |
31.1 |
9.9 |
| Methotrexate (n=489)† |
65.2 |
43.8 |
16.4 |
* Intent to treat (ITT) analysis using last
observation carried forward (LOCF) technique for patients who discontinued
early.
† N is the number of ITT patients for whom adequate data were available
to calculate the indicated rates.
‡ p < 0.001 leflunomide vs placebo
&dect; p < 0.02 leflunomide vs placebo |
Table 7 shows the results of the components of the ACR response criteria for
US301, MN301, and MN302. ARAVA was significantly superior to placebo in all
components of the ACR Response criteria in study US301 and MN301. In addition,
Arava was significantly superior to placebo in improving morning stiffness,
a measure of RA disease activity, not included in the ACR Response criteria.
No consistent differences were demonstrated between ARAVA and the active comparators.
Table 7. Mean Change in the Components of the ACR Responder
Index*
| Components |
Placebo-Controlled Studies |
Non-placebo Controlled Study |
| US 301 (12 months) |
MN 301 Non-US (6 months) |
MN 302 Non-US (12 months) |
| Leflunomide |
Methotrexate |
Placebo |
Leflunomide |
Sulfasalazine |
Placebo |
Leflunomide |
Methotrexate |
| Tender joint count1 |
-7.7 |
-6.6 |
-3.0 |
-9.7 |
-8.1 |
-4.3 |
-8.3 |
-9.7 |
| Swollen joint count1 |
-5.7 |
-5.4 |
-2.9 |
-7.2 |
-6.2 |
-3.4 |
-6.8 |
-9.0 |
| Patient global assessment2 |
-2.1 |
-1.5 |
0.1 |
-2.8 |
-2.6 |
-0.9 |
-2.3 |
-3.0 |
| Physician global assessment2 |
-2.8 |
-2.4 |
-1.0 |
-2.7 |
-2.5 |
-0.8 |
-2.3 |
-3.1 |
| Physical function/disability (MHAQ/HAQ) |
-0.29 |
-0.15 |
0.07 |
-0.50 |
-0.29 |
-0.04 |
-0.37 |
-0.44 |
| Pain intensity2 |
-2.2 |
-1.7 |
-0.5 |
-2.7 |
-2.0 |
-0.9 |
-2.1 |
-2.9 |
| Erythrocyte Sedimentation rate |
-6.26 |
-6.48 |
2.56 |
-7.48 |
-16.56 |
3.44 |
-10.12 |
-22.18 |
| C-reactive protein |
-0.62 |
-0.50 |
0.47 |
-2.26 |
-1.19 |
0.16 |
-1.86 |
-2.45 |
| Not included in the ACR Responder Index |
| Morning Stiffness (min) |
-101.4 |
-88.7 |
14.7 |
-93.0 |
-42.4 |
-6.8 |
-63.7 |
-86.6 |
* Last Observation Carried Forward; Negative Change Indicates
Improvement
1 Based on 28 joint count
2 Visual Analog Scale - 0=Best; 10=Worst |
2. Maintenance of effect
After completing 12 months of treatment, patients continuing
on study treatment were evaluated for an additional 12 months of double-blind
treatment (total treatment period of 2 years) in studies US301, MN305, and
MN304. ACR Responder rates at 12 months were maintained over 2 years in most
patients continuing a second year of treatment. Improvement from baseline in
the individual components of the ACR responder criteria was also sustained in
most patients during the second year of Arava treatment in all three trials.
3. Inhibition of structural damage
The change from baseline to endpoint in progression of
structural disease, as measured by the Sharp X-ray score, is displayed in
Figure 3. ARAVA was statistically significantly superior to placebo in
inhibiting the progression of disease by the Sharp Score. No consistent
differences were demonstrated between leflunomide and methotrexate or between
leflunomide and sulfasalazine.
Figure 3
| |
Comparisons |
95% Confidence Interval |
p Value |
| US301 |
Leflunomide vs. Placebo |
(-4.0, -1.1) |
0.0007 |
| Methotrexate vs. Placebo |
(-2.6, -0.2) |
0.0196 |
| Leflunomide vs. Methotrexate |
(-2.3, 0.0) |
0.0499 |
| MN301 |
Leflunomide vs. Placebo |
(-6.2, -1.8) |
0.0004 |
| Sulfasalazine vs. Placebo |
(-6.9, 0.0) |
0.0484 |
| Leflunomide vs. Sulfasalazine |
(-3.3, 1.2) |
NS |
| MN302 |
Leflunomide vs. Methotrexate |
(-2.2, 7.4) |
NS |
4. Improvement in physical function
The Health Assessment Questionnaire (HAQ) assesses a
patient's physical function and degree of disability. The mean change from
baseline in functional ability as measured by the HAQ Disability Index (HAQ DI)
in the 6 and 12 month placebo and active controlled trials is shown in Figure
4. ARAVA was statistically significantly superior to placebo in improving
physical function. Superiority to placebo was demonstrated consistently across
all eight HAQ DI subscales (dressing, arising, eating, walking, hygiene, reach,
grip and activities) in both placebo controlled studies. The Medical Outcomes
Survey Short Form 36 (SF-36), a generic health-related quality of life
questionnaire, further addresses physical function. In US301, at 12 months,
ARAVA provided statistically significant improvements compared to placebo in
the Physical Component Summary (PCS) Score.
Figure 4
| |
Comparison |
95% Confidence Interval |
p Value |
| US301 |
Leflunomide vs. Placebo |
(-0.58, -0.29) |
0.0001 |
| Leflunomide vs. Methotrexate |
(-0.34, -0.07) |
0.0026 |
| MN301 |
Leflunomide vs. Placebo |
(-0.67, -0.36) |
< 0.0001 |
| Leflunomide vs. Sulfasalazine |
(-0.33, -0.03) |
0.0163 |
| MN302 |
Leflunomide vs. Methotrexate |
(0.01, 0.16) |
0.0221 |
Maintenance of effect
The improvement in physical function demonstrated at 6 and
12 months was maintained over two years. In those patients continuing therapy
for a second year, this improvement in physical function as measured by HAQ and
SF-36 (PCS) was maintained.
Pediatrics
Clinical Trials in Pediatrics
ARAVA was studied in a single multicenter, double-blind, active-controlled
trial in 94 patients (1:1 randomization) with polyarticular course juvenile
rheumatoid arthritis (JRA) as defined by the American College of Rheumatology
(ACR). Approximately 68% of pediatric patients receiving ARAVA, versus 89% of
pediatric patients receiving the active comparator, improved by Week 16 (end-of-study)
employing the JRA Definition of Improvement (DOI) 30 % responder endpoint.
In this trial, the loading dose and maintenance dose of ARAVA was based on three
weight categories: < 20 kg, 20-40kg, and > 40 kg. The response rate to
ARAVA in pediatric patients 40 kg was less robust than in pediatric patients
> 40 kg suggesting suboptimal dosing in smaller weight pediatric patients,
as studied, resulting in less than efficacious plasma concentrations, despite
reduced clearance of M1. (See Pharmacokinetics - Pediatrics).
Last updated on RxList: 6/23/2009