Methadone: Radioimmunoassay and pharmacokinetics in the rat

G. S.F. Ling, J. G. Umans, C. E. Inturrisi

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52 Scopus citations


A radioimmunoassay for the quantitation of methadone in biofluids is described. The antiserum was prepared by using an albumin conjugate of N-methyl-N-(1-methyl-3,3-diphenyl-4-oxohexyl)aminoethanol succinate. By employing tritium-labeled dl-methadone as the radioligand and a sample volume of 0.05 ml, the method has a lower limit of sensitivity of 3 ng/ml. The ability of the antiserum to detect methadone was not influenced by the presence of the metabolites of methadone or members of the methadol series. Morphine, codeine, levorphanol, meperidine, l, α-acetylmethadol and d-propoxyphene do not cross-react. After the i.v. administration of 0.90 or 1.5 mg/kg of methadone to male rats, plasma methadone levels decline biexponentially and elimination is independent of dose. The mean volume of distribution is 7.58 ± 0.87 liters/kg; the mean elimination T 1/2 is 88.6 ± 1.9 min and the plasma clearance is 59.3 ± 1.4 ml/min/kg. These results demonstrate that methadone is eliminated by the rat much more rapidly than previously suspected.

Original languageEnglish (US)
Pages (from-to)147-151
Number of pages5
JournalJournal of Pharmacology and Experimental Therapeutics
Issue number1
StatePublished - 1981

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology


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