Metaxalone

Metaxalone (marketed by King Pharmaceuticals under the brand name Skelaxin) is a muscle relaxant used to relax muscles and relieve pain caused by strains, sprains, and other musculoskeletal conditions. Its exact mechanism of action is not known, but it may be due to general central nervous system depression. It is considered to be a moderately strong muscle relaxant, with relatively low incidence of side effects. Skelaxin is available in an 800 mg scored tablet. Possible side effects include nausea, vomiting, drowsiness and CNS side effects, such as dizziness, headache, and irritability.

Metaxalone
Clinical data
Trade namesSkelaxin
AHFS/Drugs.comMonograph
MedlinePlusa682010
Pregnancy
category
  • US: C (Risk not ruled out)
    Routes of
    administration    		Oral
    ATC code
    • none
    Legal status
    Legal status
    Pharmacokinetic data
    BioavailabilityUnknown
    MetabolismHepatic
    Elimination half-life9.2 (± 4.8) hours
    ExcretionRenal
    Identifiers
    CAS Number
    PubChem CID
    IUPHAR/BPS
    DrugBank
    ChemSpider
    UNII
    KEGG
    ChEMBL
    CompTox Dashboard (EPA)
    ECHA InfoCard100.015.253
    Chemical and physical data
    FormulaC12H15NO3
    Molar mass221.252 g/mol g·mol−1
    3D model (JSmol)
      (verify)
    Not to be confused with Metolazone, a diuretic.

    The metabolism of metaxalone involves the liver cytochrome P450 system. Based on the information in the labeling, patients receiving metaxalone therapy and physicians prescribing metaxalone are directed to take precaution when coadministering it with other medications involving the P450 system.[1][2]

    Because of potential for side effects, this drug is considered high risk in the elderly. As of 2015 the cost for a typical month of medication in the United States is 100 to 200 USD.[3]

    Pharmacokinetics

    Metaxalone exhibits increased bioavailability when taken with food.[4] Specifically, in one study, compared to fasted conditions, the presence of food at the time of drug administration increased Cmax by 77.5%, AUC0-t by 23.5%, and AUC0-∞ by 15.4%.[5] Thus, based on the information in the labeling, patients receiving metaxalone therapy are directed to take metaxalone with food, and are informed that taking metaxalone with food results in an increase in the oral bioavailability of metaxalone compared to taking metaxalone without food.[6][7][8]

    Assay

    A literature survey reveals very few methods are reported for the determination of metaxalone to date. Nirogi et al.[5] reported a liquid chromatographic method coupled to tandem mass spectrometry for the quantification of metaxalone in human plasma. A stability-indicating HPLC method was introduced by P.K. Sahu et al.[9] Metaxalone has been used as an internal standard for few analytical methods[10][11]

    References
    1. United States Patent No. 7,122,566, by Jie Du, et al
    2. United States Patent No. 7,378,434, by Jie Du, et al
    3. Hamilton, Richart (2015). Tarascon Pocket Pharmacopoeia 2015 Deluxe Lab-Coat Edition. Jones & Bartlett Learning. p. 2. ISBN 9781284057560.
    4. Skelaxin Package Insert
    5. Nirogi RV, Kandikere VN, Shukla M, Mudigonda K, Shrivastava W, Datla PV (May 2006). "Quantification of Metaxalone in Human Plasma by Liquid Chromatography Coupled to Tandem Mass Spectrometry". J Anal Toxicol. 30 (4): 245–51. doi:10.1093/jat/30.4.245. PMID 16803662.
    6. id.
    7. United States Patent No. 6,407,128
    8. United States Patent No. 6,683,102
    9. Prafulla Kumar Sahu, M. Mathrusri Annapurna and Dillip Kumar Sahoo, Development and Validation of Stability Indicating RP-HPLC Method for the Determination of Metaxalone in Bulk and its Pharmaceutical Formulations; E-Journal of Chemistry, 2011, 8(S1), S439-S447.
    10. Mistri H N, Jangid A G, Pudage A, Gomes N, Sanyal M and Shrivastav P, J Chromatogr B, 2007, 853(1), 320-332.
    11. Mistri H N, Jangid A G, Pudage A and Shrivastav P, J Chromatogr B, 2008, 864(1-2), 137-148.
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