title: Simultaneous determination of paracetamol and methocarbamol in human plasma By HPLC using UV detection with time programming: application to pharmacokinetic study.
abstract: BACKGROUND
  A combination of methocarbamol (MET) and paracetamol (PAR) is a widely used treatment approach. It provides complementary modes of action for treatment of pain associated with muscle spasm. The aim of this work was to develop and validate a new sensitive and reproducible isocratic reversed phase HPLC-UV detection method for simultaneous determination of MET and PAR in human plasma for the routine use in a therapeutic drug monitoring and pharmacokinetic laboratories.
  
  
  METHODS
  A simple HPLC assay was developed and validated for the simultaneous determination of the above-mentioned drugs in small samples of human plasma (0.25 mL). After protein precipitation with methanol, satisfactory separation was achieved on a Hypersil® BDS C18 column (250 mm × 4.6 mm, 5 μm) using a mobile phase comprising 20 mM sodium dihydrogen phosphate buffer (pH=3) and methanol at a ratio of 80:20, v/v; the elution was isocratic at ambient temperature with a flow rate of 1.2 ml/min. The UV detector was programmed at 254 nm for 7.0 min to measure PAR and IS and at 272 nm for the subsequent 3 min to measure MET.
  
  
  RESULTS
  Linearity was demonstrated over the concentration range from 0.02 to 20 µg/ml (mean R(2) = 0.9998, n = 10). The observed within- and between-day assay precision ranged from 1.11 to 9.4 and 2.46 to 10.0% for PAR and MET, respectively; whereas, accuracy varied between 95.2-101% and 93.9-102.2% for PAR and MET, respectively. Mean drug recovery was 99.8 for PAR and 99.0% for MET. PAR and MET were stable in frozen plasma over a period of 3 months at -80 °C.
  
  
  CONCLUSIONS
  The validated method was applied successfully to a bioequivalence study of PAR/MET (500/400 mg) fixed dose combination tablet in healthy volunteers (n=24).
authors.affiliation: Department of Pharmaceutics, Faculty of Pharmacy, Damanhour University, Damanhour, Egypt.
authors.forename: S A
authors.initials: SA
authors.lastname: Helmy
authors.affiliation: Department of Pharmaceutics, Faculty of Pharmacy, Damanhour University, Damanhour, Egypt.
authors.forename: H M
authors.initials: HM
authors.lastname: El-Bedaiwy
mesh_terms: D000082:Acetaminophen / Q000737:chemistry* / Q000493:pharmacokinetics*; D000293:Adolescent; D000328:Adult; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D006801:Humans; D057230:Limit of Detection; D008297:Male; D008721:Methocarbamol / Q000737:chemistry* / Q000493:pharmacokinetics*; D013607:Tablets / Q000737:chemistry / Q000493:pharmacokinetics; D014466:Ultraviolet Rays; D055815:Young Adult
chemical_list: D013607:Tablets; D008721:Methocarbamol; D000082:Acetaminophen