Pharmacokinetic-pharmacodynamic integration of danofloxacin in the calf

The pharmacokinetics and pharmacodynamics of danofloxacin were studied in calves after intravenous (IV) and intramuscular (IM) administration, at a dose of 1.25 mg/kg in a two period cross-over study, using tissue cages to monitor aspects of extravascular distribution. Danofloxacin had a high volume of distribution (3.90 L/kg) and relatively rapid clearance (1.02 L/kgh) after IV dosing. Terminal half-life was 2.65 and 4.03 h, respectively, after IV and IM administration. Danofloxacin penetrated slowly into and was cleared slowly from tissue cage fluid (transudate), elimination half-life (10.2 h after IV and 8.9 h after IM dosing) being greater than for serum. The antibacterial actions of danofloxacin against the pathogen Mannheimia haemolytica 3575 were established in vitro in Mueller Hinton Broth, serum and transudate. These data were used together with in vivo pharmacokinetic parameters, C(max) and AUC to determine the surrogate markers of antimicrobial activity, C(max)/MIC, AUC/MIC and T>MIC. The antibacterial actions of danofloxacin were also determined ex vivo in serum and transudate samples harvested at pre-determined times after IM danofloxacin dosing. Ex vivo AUC/MIC data were integrated with ex vivo bacterial count to establish values producing a bacteriostatic action, inhibition of bacterial count by 50%, reduction in bacterial count by 99.9% (bactericidal action) and elimination of bacteria. Mean values were, respectively, 15.9, 16.7, 18.15 and 33.5h for serum and 15.0, 16.34, 17.8 and 30.7 h for transudate. The AUC/MIC-effect relationships for serum may be regarded as representative of a shallow compartment of blood and well perfused tissues, whilst AUC/MIC-effect relationships for transudate may be considered to represent a deep peripheral compartment of poorly perfused tissues. A novel approach to selecting antimicrobial drug dosage for evaluation in clinical trials, using AUC/MIC values producing either bactericidal activity or elimination of bacteria together with MIC(90) values for calf pathogens, is proposed. This approach can be expected to optimise efficacy and minimise the development of resistance.