Absorption
In a relative bioavailability study in healthy adult volunteers, the C
max (peak plasma concentration) and AUC (extent of absorption) values of metaxalone from Metaxalone Tablets, 640 mg were found to be similar to those from SKELAXIN 800 mg tablets. After a single dose of Metaxalone Tablets, 640 mg, under fasted conditions, mean C
max and AUC values were 2 mcg/mL and 16 mcg*h/mL, respectively. The time-to-peak plasma concentration (T
max) occurred at 3.5 hours (range 1.5-12 hours). The plasma half-life in adult healthy subjects was approximately 5 hours after administration of Metaxalone Tablets, 640 mg.
Effect of Food: Table 1 displays the effect of a high fat meal on the pharmacokinetics of metaxalone.
Table 1: Effect of a High Fat Meal on the Pharmacokinetics of Metaxalone | Metaxalone Product | C
max
1
| AUC
0-t
1
| AUC
0-inf
1
| T
max
2
|
| Metaxalone Tablets, 640 mg | 23% Increase | 7% Increase | 6% Increase | 8 hours
(3.5-24 hours)
|
| SKELAXIN (metaxalone) tablets, 800 mg | 76% Increase | 32% Increase | 32% Increase | 5 hours
(2.5-24 hours)
|
1= Presented as ratio fed/fasted groups. Fed group received a high fat meal.
2= Presented as median (range) under fed state.
Distribution
For Metaxalone Tablets, 640 mg, the metaxalone apparent volume of distribution (V/F) was approximately 800 Liters. Metaxalone plasma protein binding is unknown.
Elimination
For Metaxalone Tablets, 640 mg, the metaxalone plasma half-life was approximately 5 hours.
Metabolism: Metaxalone is metabolized by the liver. CYP1A2, CYP2D6, CYP2E1, and CYP3A4 and, to a lesser extent, CYP2C8, CYP2C9, and CYP2C19 are involved in metaxalone metabolism.
Excretion: Metaxalone is excreted in the urine as unidentified metabolites.
Specific Populations
For Metaxalone Tablets, 640 mg, the effect of age, renal impairment, and hepatic impairment on the pharmacokinetics of metaxalone is unknown.
Males and Females:The metaxalone Cmax increased by 36% and AUC inf by 31%, in females compared to males following administration of 640 mg of Metaxalone Tablets. These data were not adjusted for other factors (e.g., weight, renal function). The clinical significance of these findings is unknown.
Drug Interaction Studies
In Vitro Studies: Metaxalone does not inhibit CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 or induce CYP1A2, CYP2B6, and CYP3A4.