title: Free and bound enantiomers of methadone and its metabolite, EDDP in methadone maintenance treatment: relationship to dosage?
abstract: OBJECTIVE
  To determine the effects of metabolism and protein binding on the relationship between administered dose, blood levels of R methadone and biological response by measuring the free and protein-bound forms of the R and S enantiomers of methadone and EDDP, its metabolite.
  
  
  METHODS
  To measure free and total drug, trough levels were collected from 45 methadone clients. To measure free methadone, samples were filtered using ultrafiltration with a MW weight cut-off of 10,000 and extracted using liquid-liquid extraction. The solvent was evaporated and samples reconstituted in mobile phase for analysis by LC/MS/MS. Total analyte was determined by extracting unfiltered samples. Enantiomeric separation of methadone and EDDP was by chiral chromatography.
  
  
  RESULTS
  The presence of unmetabolized methadone suggested that none of the patients were very fast metabolizers. R and S forms were metabolized at the same rate at all administered doses. Free R methadone levels correlated both with methadone dose and with the total amount of R methadone. The free fraction of R methadone (%free R) was higher at lower doses than at high doses, varied from 5 to 25% and was inversely proportional to the total dose of administered drug in a relationship that was logarithmic and non-linear.
  
  
  CONCLUSIONS
  By measuring the free, biologically active form of the drug, we were unable to account for the large variations in dose required between different patients to prevent the onset of withdrawal symptoms. The reason for the large range in dosage may be multifactorial.
authors.affiliation: Provincial Laboratory, 3211 Albert Street, Regina, SK, St. Michael's Hospital, University of Toronto, Canada. dlehotay@health.gov.sk.ca
authors.forename: D C
authors.initials: DC
authors.lastname: Lehotay
authors.forename: S
authors.initials: S
authors.lastname: George
authors.forename: M L
authors.initials: ML
authors.lastname: Etter
authors.forename: K
authors.initials: K
authors.lastname: Graybiel
authors.forename: J C
authors.initials: JC
authors.lastname: Eichhorst
authors.forename: B
authors.initials: B
authors.lastname: Fern
authors.forename: W
authors.initials: W
authors.lastname: Wildenboer
authors.forename: P
authors.initials: P
authors.lastname: Selby
authors.forename: B
authors.initials: B
authors.lastname: Kapur
mesh_terms: D001798:Blood Proteins / Q000032:analysis / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D008691:Methadone / Q000008:administration & dosage / Q000097:blood* / Q000378:metabolism*; D008970:Molecular Weight; D011485:Protein Binding; D011759:Pyrrolidines / Q000097:blood*; D013237:Stereoisomerism; D013375:Substance Withdrawal Syndrome / Q000517:prevention & control; D019966:Substance-Related Disorders / Q000097:blood* / Q000188:drug therapy
chemical_list: D001798:Blood Proteins; D011759:Pyrrolidines; D008691:Methadone; C010724:2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine