Preliminary study on the pharmacokinetics of phenobarbital in the neonatal foal

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Abstract
Summary: Pharmacokinetic characteristics of the anticonvulsant phenobarbital were studied in seven pony and two Thoroughbred foals aged between four and 10 days. A single, 20 mg/kg body‐weight (bwt) dose of phenobarbital was given intravenously over 25 mins and the serum concentrations of the drug were measured using an EMIT AED assay (coefficient of variation 1.37 per cent at 30 μg/ml, n = 7). Phenobarbital elimination was found to follow first order kinetics. The mean (±sd) peak phenobarbital serum concentration was 18.6 ± 2.1 μg/ml at 1h after initiation of infusion with a mean (± se) half‐life of 12.8 ± 2.1h. The mean (± se) volume of distribution was 0.86 ± 0.026 litres/kg bwt and mean (±se) total body clearance was 0.0564 ± 0.0065 litres/kg bwt/h. Sedation was noticed 15 to 20 mins after the beginning of infusion and lasted for up to 8h. All foals could be aroused and could walk although they were ataxic for the first 1 to 2h. A degree of delayed hyper‐excitability occurred 3 to 8h after infusion. In equine neonatal seizure disorders it is recommended to use a loading dose of 20 mg/kg bwt of phenobarbital, followed by maintenance doses of 9 mg/kg bwt at 8h. With this regimen, average steady state serum phenobarbital concentrations should range between approximately 11.6 and 53 μg/ml. Phenobarbital serum concentrations should be monitored following the loading dose and 24h after initiating the maintenance doses to check that levels remain within the suggested (human) therapeutic range of 15 to 40 μg/ml.