title: Pharmacokinetics of metronidazole in foals: influence of age within the neonatal period.
abstract: Neonatal foals have unique pharmacokinetics, which may lead to accumulation of certain drugs when adult horse dosage regimens are used. Given its lipophilic nature and requirement for hepatic metabolism, metronidazole may be one of these drugs. The purpose of this study was to determine the pharmacokinetic profiles of metronidazole in twelve healthy foals at 1-2.5 days of age when administered as a single intravenous (IV) and intragastric (IG) dose of 15 mg/kg. Foals in the intravenous group were studied a second time at 10-12 days of age to evaluate the influence of age on pharmacokinetics within the neonatal period. Blood samples were collected at serial time points after metronidazole administration. Metronidazole concentration in plasma was measured using LC-MS. Pharmacokinetic parameters were determined using noncompartmental analysis and compared between age groups. At 1-2.5 days of age, the mean peak plasma concentration after IV infusion was 18.79 ± 1.46 μg/mL, elimination half-life was 11.8 ± 1.77 h, clearance was 0.84 ± 0.13 mL/min/kg and the volume of distribution (steady-state) was 0.87 ± 0.07 L/kg. At 10-12 days of age, the mean peak plasma concentration after IV infusion was 18.17 ± 1.42 μg/mL, elimination half-life was 9.07 ± 2.84 h, clearance was 1.14 ± 0.21 mL/min/kg and the volume of distribution (steady-state) was 0.88 ± 0.06 L/kg. Oral approximated bioavailability was 100%. Cmax and Tmax after oral dosing were 14.85 ± 0.54 μg/mL and 1.75 (1-4) h, respectively. The elimination half-life was longer and clearance was reduced in neonatal foals at 1-2.5 days as compared to 10-12 days of age (P = 0.006, P = 0.001, respectively). This study warrants consideration for altered dosing recommendations in foals, especially a longer interval (12 h).
authors.affiliation: William R. Pritchard Veterinary Medical Teaching Hospital, University of California, Davis, CA, USA.
authors.forename: E A
authors.initials: EA
authors.lastname: Swain
authors.forename: K G
authors.initials: KG
authors.lastname: Magdesian
authors.forename: P H
authors.initials: PH
authors.lastname: Kass
authors.forename: J E
authors.initials: JE
authors.lastname: Edman
authors.forename: H K
authors.initials: HK
authors.lastname: Knych
mesh_terms: D000367:Age Factors; D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D000900:Anti-Bacterial Agents / Q000097:blood / Q000493:pharmacokinetics*; D005260:Female; D006207:Half-Life; D006736:Horses / Q000378:metabolism*; D007275:Injections, Intravenous / Q000662:veterinary; D007441:Intubation, Gastrointestinal / Q000662:veterinary; D008297:Male; D008795:Metronidazole / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*
chemical_list: D000900:Anti-Bacterial Agents; D008795:Metronidazole