skelaxin
Generic Name: (
Metaxalone)
Dosage Type: tablet Organization: King Pharmaceuticals, Inc.
Description
SKELAXIN® (metaxalone) is available
as an 800 mg oval, scored pink tablet.
Chemically,
metaxalone is 5-[(3,5- dimethylphenoxy) methyl]-2-oxazolidinone. The
empirical formula is C12H15NO3, which
corresponds to a molecular weight of 221.25. The structural formula
is:
Metaxalone is a white to almost white, odorless crystalline
powder freely soluble in chloroform, soluble in methanol and in 96%
ethanol, but practically insoluble in ether or water.
Each tablet contains 800 mg metaxalone and the following
inactive ingredients: alginic acid, ammonium calcium alginate, B-Rose
Liquid, corn starch and magnesium stearate.
Clinical Pharmacology
Mechanism of Action
The mechanism of action of metaxalone in humans has
not been established, but may be due to general central nervous system
depression. Metaxalone has no direct action on the contractile mechanism
of striated muscle, the motor end plate or the nerve fiber.
Pharmacokinetics
The pharmacokinetics of metaxalone have been evaluated
in healthy adult volunteers after single dose administration of SKELAXIN
under fasted and fed conditions at doses ranging from 400 mg to 800
mg.
Absorption
Peak plasma concentrations of metaxalone occur approximately
3 hours after a 400 mg oral dose under fasted conditions. Thereafter,
metaxalone concentrations decline log-linearly with a terminal half-life
of 9.0 ± 4.8 hours. Doubling the dose of SKELAXIN from 400 mg
to 800 mg results in a roughly proportional increase in metaxalone
exposure as indicated by peak plasma concentrations (Cmax) and area under the curve (AUC). Dose proportionality at doses
above 800 mg has not been studied. The absolute bioavailability of
metaxalone is not known.
The single-dose pharmacokinetic
parameters of metaxalone in two groups of healthy volunteers are shown
in Table 1.
Table 1: Mean (%CV) Metaxalone Pharmacokinetic Parameters
| Dose (mg) |
Cmax (ng/mL) |
Tmax (h) |
AUC8 (ng•h/mL) |
t˝(h) |
CL/F (L/h) |
| 4001 |
983 (53) |
3.3 (35) |
7479 (51) |
9.0 (53) |
68 (50) |
| 8002 |
1816 (43) |
3.0 (39) |
15044 (46) |
8.0 (58) |
66 (51) |
| 1Subjects received 1x400 mg
tablet under fasted conditions (N=42) 2Subjects
received 2x400 mg tablets under fasted conditions (N=59)
|
Food Effects
A randomized, two-way, crossover study was conducted
in 42 healthy volunteers (31 males, 11 females) administered one 400
mg SKELAXIN tablet under fasted conditions and following a standard
high-fat breakfast. Subjects ranged in age from 18 to 48 years (mean
age = 23.5 ± 5.7 years). Compared to fasted conditions, the
presence of a high fat meal at the time of drug administration increased
Cmax by 177.5% and increased AUC (AUC0-t, AUC8) by 123.5% and 115.4%, respectively. Time-to-peak
concentration (Tmax) was also delayed (4.3 h versus 3.3 h) and terminal half-life
was decreased (2.4 h versus 9.0 h) under fed conditions compared to fasted.
In a second food effect study of similar design, two 400 mg SKELAXIN
tablets (800 mg) were administered to healthy volunteers (N=59, 37
males, 22 females), ranging in age from 18-50 years (mean age = 25.6± 8.7 years). Compared to fasted conditions, the presence of
a high fat meal at the time of drug administration increased Cmax by 193.6% and increased AUC (AUC0-t, AUC8) by 146.4% and 142.2%, respectively. Time-to-peak
concentration (Tmax) was also delayed (4.9 h versus 3.0 h) and terminal half-life
was decreased (4.2 h versus 8.0 h) under fed conditions compared to fasted conditions. Similar
food effect results were observed in the above study when one SKELAXIN
800 mg tablet was administered in place of two SKELAXIN 400 mg tablets.
The increase in metaxalone exposure coinciding with a reduction in
half-life may be attributed to more complete absorption of metaxalone
in the presence of a high fat meal (Figure 1).
Distribution, Metabolism, and Excretion
Although plasma protein binding and absolute bioavailability
of metaxalone are not known, the apparent volume of distribution (V/F
~ 800 L) and lipophilicity (log P = 2.42) of metaxalone suggest that
the drug is extensively distributed in the tissues. Metaxalone is
metabolized by the liver and excreted in the urine as unidentified
metabolites.
Pharmacokinetics in Special Populations
Age:
The effects of age on the pharmacokinetics of metaxalone
were determined following single administration of two 400 mg tablets
(800 mg) under fasted and fed conditions. The results were analyzed
separately, as well as in combination with the results from three
other studies. Using the combined data, the results indicate that
the pharmacokinetics of metaxalone are significantly more affected
by age under fasted conditions than under fed conditions, with bioavailability
under fasted conditions increasing with age.
The bioavailability of metaxalone under fasted and fed conditions
in three groups of healthy volunteers of varying age is shown in Table
2.
Table 2: Mean (%CV) Pharmacokinetic Parameters Following
Single Administration of Two 400 mg SKELAXIN Tablets (800 mg) under
Fasted and Fed Conditions
|
|
Younger Volunteers |
Older Volunteers |
| Age (years) |
25.6 ± 8.7 |
39.3 ± 10.8 |
71.5 ± 5.0 |
| N |
59 |
21 |
23 |
| Food |
Fasted |
Fed |
Fasted |
Fed |
Fasted |
Fed |
| Cmax (ng/mL) |
1816 (43)
|
3510 (41)
|
2719 (46)
|
2915 (55)
|
3168 (43)
|
3680 (59)
|
| Tmax (h) |
3.0 (39)
|
4.9 (48)
|
3.0 (40)
|
8.7 (91)
|
2.6 (30)
|
6.5 (67)
|
| AUC0-t (ng·h/mL) |
14531 (47)
|
20683 (41)
|
19836 (40)
|
20482 (37)
|
23797 (45)
|
24340 (48)
|
| AUC8 (ng·h/mL) |
15045 (46)
|
20833 (41)
|
20490 (39)
|
20815 (37)
|
24194 (44)
|
24704 (47)
|
Gender:
The effect of gender on the pharmacokinetics of metaxalone
was assessed in an open label study, in which 48 healthy adult volunteers
(24 males, 24 females) were administered two SKELAXIN 400 mg tablets
(800 mg) under fasted conditions. The bioavailability of metaxalone
was significantly higher in females compared to males as evidenced
by Cmax (2115 ng/mL versus 1335 ng/mL) and AUC8 (17884 ng·h/mL versus 10328 ng·h/mL). The mean
half-life was 11.1 hours in females and 7.6 hours in males. The apparent
volume of distribution of metaxalone was approximately 22% higher
in males than in females, but not significantly different when adjusted
for body weight. Similar findings were also seen when the previously
described combined dataset was used in the analysis.
Hepatic/Renal Insufficiency:
The impact of hepatic and renal disease on the pharmacokinetics
of metaxalone has not been determined. In the absence of such information,
SKELAXIN should be used with caution in patients with hepatic and/or
renal impairment.
Indications and Usage
SKELAXIN (metaxalone) is indicated as an adjunct
to rest, physical therapy, and other measures for the relief of discomforts
associated with acute, painful musculoskeletal conditions. The mode
of action of this drug has not been clearly identified, but may be
related to its sedative properties. Metaxalone does not directly relax
tense skeletal muscles in man.
Contraindications
Known hypersensitivity to any components of this
product.
Known tendency to drug induced, hemolytic,
or other anemias.
Significantly impaired renal
or hepatic function.
Warnings
SKELAXIN may enhance the effects of alcohol and other
CNS depressants.
Precautions
Metaxalone should be administered with great care
to patients with pre-existing liver damage. Serial liver function
studies should be performed in these patients.
False-positive Benedict’s tests, due to an unknown reducing
substance, have been noted. A glucose-specific test will differentiate
findings.
Taking SKELAXIN with food may enhance
general CNS depression; elderly patients may be especially susceptible
to this CNS effect. (See CLINICAL PHARMACOLOGY: Pharmacokinetics and
PRECAUTIONS: Information for Patients).
Information for Patients
SKELAXIN may impair mental and/or physical abilities
required for performance of hazardous tasks, such as operating machinery
or driving a motor vehicle, especially when used with alcohol or other
CNS depressants.
Drug Interactions
SKELAXIN may enhance the effects of alcohol, barbiturates
and other CNS depressants.
Carcinogenesis, Mutagenesis, Impairment of Fertility
The carcinogenic potential of metaxalone has not
been determined.
Pregnancy
Reproduction studies in rats have not revealed evidence
of impaired fertility or harm to the fetus due to metaxalone. Post
marketing experience has not revealed evidence of fetal injury, but
such experience cannot exclude the possibility of infrequent or subtle
damage to the human fetus. Safe use of metaxalone has not been established
with regard to possible adverse effects upon fetal development. Therefore,
metaxalone tablets should not be used in women who are or may become
pregnant and particularly during early pregnancy unless in the judgement
of the physician the potential benefits outweigh the possible hazards.
Nursing Mothers
It is not known whether this drug is secreted in
human milk. As a general rule, nursing should not be undertaken while
a patient is on a drug since many drugs are excreted in human milk.
Pediatric Use
Safety and effectiveness in children 12 years of
age and below have not been established.
Adverse Reactions
The most frequent reactions to metaxalone include:
CNS: drowsiness, dizziness, headache, and nervousness
or “irritability”;
Digestive:
nausea, vomiting, gastrointestinal upset.
Other
adverse reactions are:
Immune System: hypersensitivity
reaction, rash with or without pruritus;
Hematologic:
leukopenia; hemolytic anemia;
Hepatobiliary:
jaundice.
Though rare, anaphylactoid reactions
have been reported with metaxalone.
Overdosage
Deaths by deliberate or accidental overdose have
occurred with metaxalone, particularly in combination with antidepressants,
and have been reported with this class of drug in combination with
alcohol.
When determining the LD50 in rats and mice, progressive sedation, hypnosis and finally respiratory
failure were noted as the dosage increased. In dogs, no LD50 could be determined as the higher doses produced an emetic action
in 15 to 30 minutes.
Treatment - Gastric lavage
and supportive therapy. Consultation with a regional poison control
center is recommended.
Dosage and Administration
The recommended dose for adults and children over
12 years of age is one 800 mg tablet three to four times a day.
How Supplied
SKELAXIN (metaxalone) is available as an 800 mg oval,
scored pink tablet inscribed with 8667 on the scored side and “S”
on the other. Available in bottles of 100 (NDC 60793-136-01) and in
bottles of 500 (NDC 60793-136-05).
Store at
Controlled Room Temperature, between 15°C and 30°C (59°F
and 86°F).
Rx Only
Distributed by:
King Pharmaceuticals, Inc., Bristol, TN 37620
Manufactured by:
Mallinckrodt Inc., Hobart, NY 13788
Prescribing Information as of April 2007.
Revised: 06/2007King Pharmaceuticals, Inc.