Pharmacokinetics and PK-PD modelling of danofloxacin in camel serum and tissue cage fluids

The pharmacokinetics and pharmacodynamics of danofloxacin were studied in the camel in a two period cross-over study. After intravenous (i.v.) administration at a dose rate of 1.25 mg/kg, the pharmacokinetics of danofloxacin indicated a high volume of distribution (V_darea=3.43 L/kg), relatively rapid clearance (0.44 L/kg/h) and half-life of 5.37 h. After intramuscular (i.m.) dosing absorption was complete (F=114.5) and rapid (T_(1/2)abs=0.12 h) and terminal half-life was 5.71 h. Danofloxacin penetrated fairly slowly into both inflamed (exudate) and non-inflamed (transudate) tissue cage fluids and was cleared slowly from these fluids, elimination half-life being at least twice that for serum for both exudate and transudate after both i.v. and i.m. dosing. The antibacterial actions of danofloxacin against the camel pathogen Escherichia coli 0157-H7 were determined by measurement of minimum inhibitory concentration (MIC) in vitro (single measurement) and ex vivo measurements of bacterial count at nine times between one and 48 h after i.m. dosing in each of the fluids, serum, exudate, and transudate. Using in vitro MIC data and in vivo pharmacokinetic parameters, the surrogate markers of antimicrobial activity, C_max/MIC, AUC/MIC and T>MIC, were determined for all three fluids. The ex vivo serum AUC_24h/MIC data were integrated with reduction in bacterial count to provide values producing a bacteriostatic action (no change in bacterial count), inhibition of bacterial count by 50%, reduction in bacterial count by 99.9% (bactericidal action) and elimination of bacteria. Mean AUC_24h/MIC values were 17.20, 20.07, 21.24, and 68.37 h, respectively. To describe the latter, the introduction of a new term to supplement MIC and minimum bactericidal concentration (MBC) is proposed, namely minimum elimination concentration (MEC). A novel means of designing antimicrobial drug dosage schedules for evaluation in clinical trials is proposed, using ex vivo AUC_24h/MIC values for bactericidal activity and elimination of bacteria together with MIC₉₀ data for camel pathogens.