To investigate the clinical pharmacokinetics and pharmacodynamics of intravenous alfaxalone in young Thoroughbred horses, seven Thoroughbred horses were randomly anaesthetised twice with either 1 or 2 mg/kg of intravenous alfaxalone after premedication with medetomidine (6 µg/kg intravenous) and midazolam (20 µg/kg intravenous). Blood samples were collected at predetermined time points up to two hours after administration. Plasma alfaxalone concentrations were quantified by a liquid chromatography tandem-mass spectrometry method and analysed by non-compartmental pharmacokinetic analysis. Induction and recovery qualities were good to excellent for both doses. Recovery time for the 2 mg/kg (median 90 minutes) was significantly longer than that for the 1 mg/kg (median 50 minutes). Respiratory rate for the 2 mg/kg was significantly lower than that for the 1 mg/kg, resulting in hypoxaemia. The median (range) elimination half-life, total clearance and volume of distribution were 58.2 (42.3–70.7) minutes, 11.6 (10.3–14.5) ml/minute/kg and 0.8 (0.7–0.9) l/kg for the 1 mg/kg and 59.8 (47.5–68.0) minutes, 14.7 (12.1–16.0) ml/minute/kg and 0.9 (0.9–1.2) l/kg for the 2 mg/kg, respectively. Alfaxalone is rapidly eliminated from the plasma in young Thoroughbred horses. Respiratory depression should be especially noted when alfaxalone is used in clinical practice.
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Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Ethics approval The study protocol was approved by the Experimental Animal Committee, Equine Research Institute, Japan Racing Association.
Provenance and peer review Not commissioned; externally peer reviewed.
Correction notice This article has been corrected since it was published. Typos in Figure 1 have been amended.
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