Metaxalone
FDA Label NDC 68151-4668

Metaxalone tablets are available as an 800 mg oval, convex pink tablet.

Chemically, metaxalone is 5-[(3,5- dimethylphenoxy) methyl]-2-oxazolidinone. The empirical formula is C H NO , which corresponds to a molecular weight of 221.25. The structural formula is: ₁₂₁₅₃

Molecular Structure (Metaxalone Tablets 800 mg 1)

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.

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.

The pharmacokinetics of metaxalone have been evaluated in healthy adult volunteers after single dose administration of metaxalone tablets under fasted and fed conditions at doses ranging from 400 mg to 800 mg.

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 metaxalone tablets from 400 mg to 800 mg results in a roughly proportional increase in metaxalone exposure as indicated by peak plasma concentrations (C ) 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. _max

The single-dose pharmacokinetic parameters of metaxalone in two groups of healthy volunteers are shown in Table 1.

A randomized, two-way, crossover study was conducted in 42 healthy volunteers (31 males, 11 females) administered one 400 mg metaxalone 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 C by 177.5% and increased AUC (AUC , AUC ) by 123.5% and 115.4%, respectively. Time-to-peak concentration (T ) was also delayed (4.3 h 3.3 h) and terminal half-life was decreased (2.4 h 9.0 h) under fed conditions compared to fasted. _max0-t∞maxversusversus

In a second food effect study of similar design, two 400 mg metaxalone tablets (800 mg) were administered to healthy volunteers (N=59, 37 males, 22 females), ranging in age from 18 to 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 C by 193.6% and increased AUC (AUC , AUC ) by 146.4% and 142.2%, respectively. Time-to-peak concentration (T ) was also delayed (4.9 h 3.0 h) and terminal half-life was decreased (4.2 h 8.0 h) under fed conditions compared to fasted conditions. Similar food effect results were observed in the above study when one metaxalone 800 mg tablet was administered in place of two metaxalone 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). _max0-t∞maxversusversus

Figure 1 (Mean (SD) Concentrations of Metaxalone following an 800 mg Dose under Fasted and Fed Conditions.)

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.  Hepatic  Cytochrome P450 enzymes play a role in the metabolism of metaxalone.  Specifically, CYP1A2, CYP2D6, CYP2E1, and CYP3A4 and, to a lesser extent, CYP2C8, CYP2C9, and CYP2C19 appear to metabolize metaxalone.

Metaxalone does not significantly inhibit major CYP enzymes such as CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4.  Metaxalone does not significantly induce major CYP enzymes such as CYP1A2, CYP2B6, and CYP3A4 in vitro.

Metaxalone tablets are 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.

Known hypersensitivity to any components of this product.

Known tendency to drug induced, hemolytic, or other anemias.

Significantly impaired renal or hepatic function.

Metaxalone tablets may enhance the effects of alcohol and other CNS depressants.

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 metaxalone tablets with food may enhance general CNS depression; elderly patients may be especially susceptible to this CNS effect.  (See  and  sections).



CLINICAL PHARMACOLOGY: PharmacokineticsPRECAUTIONS: Information for Patients

Metaxalone tablets 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.

The sedative effects of metaxalone tablets and other CNS depressants (e.g., alcohol, benzodiazepines, opiods, trycyclic antidepressants) may be additive. Therefore, caution should be exercised with patients who take more than one of these CNS depressants simultaneoulsy.

The carcinogenic potential of metaxalone has not been determined.

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.

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.

Safety and effectiveness in children 12 years of age and below have not been established.

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.

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 LD in rats and mice, progressive sedation, hypnosis and finally respiratory failure were noted as the dosage increased. In dogs, no LD could be determined as the higher doses produced an emetic action in 15 to 30 minutes. ₅₀₅₀

- Gastric lavage and supportive therapy. Consultation with a regional poison control center is recommended. Treatment

The recommended dose for adults and children over 12 years of age is one 800 mg tablet three to four times a day.

NDC:68151-4668-1 in a PACKAGE of 1 TABLETS