Concentrations, pharmacokinetics and selected pharmacodynamics of morphine and its active metabolites following oral administration to horses

The metabolism and pharmacokinetics of intravenous (i.v.) morphine in the horse have been described; however, administration of therapeutic doses has also been associated with neuroexcitation and adverse gastrointestinal effects. In this study, we hypothesized that oral administration would lead to comparable concentrations of morphine and its presumed active metabolite, morphine 6-glucuronide (M6G) without the adverse effects associated with i.v. administration. Eight horses were administered a single i.v. dose of 0.2 mg/kg morphine and oral doses of 0.2, 0.6, and 0.8 mg/kg of morphine in a four-way balanced crossover design, with a 2-week washout period between doses. Concentrations of morphine and metabolites were determined, and pharmacokinetic parameters determined. Physiologic and behavioral outcomes including the number of steps taken, changes in heart rate, and gastrointestinal borborygmi were assessed. Oral administration of morphine resulted in higher concentrations of morphine metabolites, including M6G (C-max: 11.6-37.8 ng/mL (0.6 mg/kg); 15.8-42.6 ng/mL (0.8 mg/kg)), compared with i.v. Bioavailability was 36.5%, 27.6% and 28.0% for 0.2, 0.6 and 0.8 mg/kg, respectively. Behavioral and physiologic changes were noted in all groups but were less prominent with oral compared with i.v. administration. Results of the current study are encouraging for further study, specifically anti-nociceptive effects of morphine following oral administration.