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Keywords:

  • dog;
  • Greyhound;
  • methadone;
  • opioid;
  • pharmacokinetics;
  • volume of distribution

Abstract

Objective  To determine the behavioral effects and pharmacokinetics of methadone in healthy Greyhounds.

Study design  Prospective experimental study.

Animals  Three male and three female healthy Greyhounds.

Methods  Methadone hydrochloride, 0.5 mg kg−1 IV (equivalent to 0.45 mg kg−1 methadone base), was administered as an IV bolus. Trained observers subjectively assessed the behavioral effects of methadone. Blood samples were obtained at predetermined time points and plasma methadone concentrations were measured by liquid chromatography with tandem mass spectrometry. Pharmacokinetic variables were estimated with computer software.

Results  Methadone was well tolerated by the dogs with panting and defecation observed as adverse effects. Mild sedation was present, but no vomiting, excitement, or dysphoria was observed. The elimination half-life, volume of distribution, and plasma clearance were 1.53 ± 0.18 hours, 7.79 ± 1.87 L kg−1, and 56.04 ± 9.36 mL minute−1 kg−1, respectively.

Conclusions and clinical relevance  Methadone was well tolerated by Greyhounds. The volume of distribution was larger than expected, with resultant lower plasma concentrations than expected. Higher doses may need to be administered to Greyhounds in comparison with non-Greyhound dogs in order to achieve similar plasma drug concentrations. A dosage of 1–1.5 mg kg−1 every 3–4 hours is suggested for future studies of analgesic efficacy of methadone in Greyhounds.