Penetration of danofloxacin into the respiratory tract tissues and secretions in calves.

Pharmacokinetic determinants of danofloxacin (1.25 mg/kg of body weight, IV) and its penetration into the respiratory tract tissues were studied in sixteen 4- to 6-week-old calves. The disposition curve was best described by an open 3-compartment model. Mean elimination half-life was 7.4 hours and the steady-state volume of distribution was 4.3 L/kg. The large volume of distribution was confirmed by a rapid and high penetration of the drug into respiratory tract tissues and secretions. In all structures (lung tissue, bronchial mucosa, bronchial secretions, and nasal secretions), danofloxacin concentration peaked 1 hour after drug administration. The area under the curve ratio for concentrations in tissue or secretions to concentrations in plasma was approximately 5 for lung tissue, 3 for bronchial mucosa, 0.85 for bronchial secretions, and 0.42 for nasal secretions. Protein binding of danofloxacin was 49% in plasma, 31% in bronchial secretions, and 14% in nasal secretions, resulting in consistently higher free danoflaxacin concentrations in bronchial secretions than in plasma. Accumulation of danofloxacin within bronchial mucosa and the high concentration of free drug in bronchial secretions suggested that an active process may be involved in the transport of danofloxacin across the airway epithelium. The dose of danofloxacin administered provided drug concentrations above the minimal inhibitory concentration of common respiratory pathogens for up to 12 hours in bronchial mucosa, up to 8 hours in bronchial secretions, and up to 4 hours in nasal secretions.