Pharmacokinetics of detomidine and its metabolites following intravenous and intramuscular administration in horses

Authors

  • K. N. GRIMSRUD,

    1. K.L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616, USA
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  • K. R. MAMA,

    1. Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523, USA
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  • S. M. THOMASY,

    1. K.L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616, USA
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  • S. D. STANLEY

    Corresponding author
    1. K.L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616, USA
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K.L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616, USA

Summary

Reasons for performing study: Detomidine is commonly used i.v. for sedation and analgesia in horses, but the pharmacokinetics and metabolism of this drug have not been well described.

Objectives: To describe the pharmacokinetics of detomidine and its metabolites, 3-hydroxy-detomidine (OH-detomidine) and detomidine 3-carboxylic acid (COOH-detomidine), after i.v. and i.m. administration of a single dose to horses.

Methods: Eight horses were used in a balanced crossover design study. In Phase 1, 4 horses received a single dose of i.v. detomidine, administered 30 μg/kg bwt and 4 a single dose i.m. 30 üg/kg bwt. In Phase 2, treatments were reversed. Plasma detomidine, OH-detomidine and COOH-detomidine were measured at predetermined time points using liquid chromatography-mass spectrometry.

Results: Following i.v. administration, detomidine was distributed rapidly and eliminated with a half-life (t1/2(el)) of approximately 30 min. Following i.m. administration, detomidine was distributed and eliminated with t1/2(el) of approximately one hour. Following, i.v. administration, detomidine clearance had a mean, median and range of 12.41, 11.66 and 10.10–18.37 ml/min/kg bwt, respectively. Detomidine had a volume of distribution with the mean, median and range for i.v. administration of 470, 478 and 215–687 ml/kg bwt, respectively. OH-detomidine was detected sooner than COOH-detomidine; however, COOH-detomidine had a much greater area under the curve.

Conclusions and potential relevance: These pharmacokinetic parameters provide information necessary for determination of peak plasma concentrations and clearance of detomidine in mature horses. The results suggest that, when a longer duration of plasma concentration is warranted, the i.m. route should be considered.

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