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Abstract

OBJECTIVE

To determine the effects of coadministration of naltrexone, a human opioid abuse deterrent, on the pharmacokinetics and pharmacodynamics of a methadone-fluconazole combination administered orally to dogs.

ANIMALS

12 healthy Beagles.

PROCEDURES

Dogs (body weight, 10.7 to 13.9 kg) were randomly allocated to 2 groups in a parallel design study. All dogs received fluconazole (100 mg [7.19 to 9.35 mg/kg], PO). Twelve hours later (time 0), dogs were administered methadone (10 mg [0.72 to 0.93 mg/kg]) plus fluconazole (50 mg [3.62 to 4.22 mg/kg]; methadone-fluconazole) or methadone (10 mg [0.72 to 0.93 mg/kg]) plus fluconazole (50 mg [3.60 to 4.67 mg/kg]) and naltrexone (2.5 mg [0.18 to 0.23 mg/kg]; methadone-fluconazole-naltrexone), PO, in a gelatin capsule. Blood samples were collected for pharmacokinetic analysis, and rectal temperature and sedation were assessed to evaluate opioid effects at predetermined times up to 24 hours after treatment.

RESULTS

Most dogs had slight sedation during the 12 hours after drug administration; 1 dog/group had moderate sedation at 1 time point. Mean rectal temperatures decreased significantly from baseline (immediate pretreatment) values from 2 to ≥ 12 hours and 2 to ≥ 8 hours after methadone-fluconazole and methadone-fluconazole-naltrexone treatment, respectively. Geometric mean maximum observed concentration of methadone in plasma was 35.1 and 33.5 ng/mL and geometric mean terminal half-life was 7.92 and 7.09 hours after methadone-fluconazole and methadone-fluconazole-naltrexone treatment, respectively. Naltrexone was sporadically detected in 1 dog. The active naltrexone metabolite, β-naltrexol, was not detected. The inactive metabolite, naltrexone glucuronide, was detected in all dogs administered methadone-fluconazole-naltrexone.

CONCLUSIONS AND CLINICAL RELEVANCE

Opioid effects were detected after oral administration of methadone-fluconazole or methadone-fluconazole-naltrexone. Further studies assessing additional opioid effects, including antinociception, are needed.

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in American Journal of Veterinary Research