Pharmacokinetics of norfloxacin in dogs after single intravenous and single and multiple oral administrations of the drug

Abstract

SUMMARY Norfloxacin was given to 6 healthy dogs at a dosage of 5 mg/kg of body weight iv and orally in a complete crossover study, and orally at dosages of 5, 10, and 20 mg/kg to 6 healthy dogs in a 3-way crossover study. For 24 hours, serum concentration was monitored serially after each administration. Another 6 dogs were given 5 mg of norfloxacin/kg orally every 12 hours for 14 days, and serum concentration was determined serially for 12 hours after the first and last administration of the drug. Complete blood count and serum biochemical analysis were performed before and after 14 days of oral norfloxacin administration, and clinical signs of drug toxicosis were monitored twice daily during norfloxacin administration. Urine concentration of norfloxacin was determined periodically during serum acquisition periods. Norfloxacin concentration was determined, using high-performance liquid chromatography with a limit of detection of 25 ng of norfloxacin/ml of serum or urine. Serum norfloxacin pharmacokinetic values after single iv dosing in dogs were best modeled, using a 2-compartment open model, with distribution and elimination half-lives of 0.467 and 3.56 hours (harmonic means), respectively. Area-derived volume of distribution (Vd area) was 1.77 ± 0.69 L/kg (arithmetic mean ± sd), and serum clearance (ClS) was 0.332 ± 0.115 L/h/kg. Mean residence time was 4.32 ± 0.98 hour. Comparison of the area under the curve (AUC; derived, using model-independent calculations) after iv administration (5 mg/kg) with AUC after oral administration (5 mg/kg) in the same dogs indicated bioavailability of 35.0 ± 46.1%, with a mean residence time after oral administration of 5.71 ± 2.24 hours. Urine concentration was 33.8 ± 15.3 μg/ml at 4 hours after a single dose of 5 mg/kg given orally, whereas concentration after 20 mg/kg was given orally was 56.8 ± 18.0 μg/ml at 6 hours after dosing. Twelve hours after drug administration, urine concentration was 47.4 ± 20.6 μg/ml after the 5-mg/kg dose and 80.6 ± 37.7 μg/ml after the 20-mg/kg dose. Absorption lag time after oral administration ranged from 0.186 ± 0.103 hour after multiple doses (5 mg/kg) to 0.385 ± 0.254 hour after a single dose of 10 mg/kg. The AUC increased (P < 0.01) as the dose increased. However, AUC per unit dose decreased linearly with dose (P < 0.05), most probably because of a dose-dependent decrease in absorption from the gastrointestinal tract.