Pharmacokinetic/pharmacodynamic Relationship And Susceptibility Breakpoint For Danofloxacin Against Haemophilus Parasuis

Haemophilus parasuis is an important pathogen of swine Glasser’s disease that causes polyserositis and arthritis in piglets.Haemophilus parasuis has the characteristics of high infection and mortality rates,thus causing huge economic loss to the pig industry.Danofloxacin is an animal-specific fluoroquinolone antibiotic that approved for the treatment of livestock respiratory infections.The aim of this study was to determine the pharmacokinetic characteristics of danofloxacin in porcine serum following a single intravenous or intramuscular administration,and to establish an ex vivo pharmacokinetic/pharmacodynamic（PK/PD）model of danofloxacin against H.parasuis to obtain the PK/PD targets associated with effective treatment in piglets.In this study,we further determine the danofloxacin wild-type(CO_WT)and PK/PD cutoffs(CO_PD)for H.parasuis based on in vitro MIC distribution,PK parameters and PK/PD relationship.A single intravenous or intramuscular dose of danofloxacin at 2.5 mg/kg.bw was given to 6 healthy crossbred pigs in a two-period randomized crossover study.The concentrations of danofloxacin in serum were determined by HPLC/MS/MS methods.The PK parameters and the relevant bioavailability were calculated by Win Nonlin version 6.1 software.The concentration-time data of danofloxacin after intravenous dosing was best fitted to two-compartment model.The main PK parameters were as follows:AUC=8.63±2.59μg/m L·h;V_ss=1.41±0.48 L/kg;Cl=0.32±0.09 L/kg·h;MRT=5.10±1.73 h.The correspond-ing PK parameters after intramuscular dosing as follows:V/F=2.26±0.56 L/kg;Cl/F=0.33±0.09 L/kg·h;T_max=1.35±0.72 h;C_max=1.01±0.21μg/m L;AUC=8.30±2.29（μg/m L）·h;F%=88.42%±24.45%.The results indicated that danofloxacin absorbed rapidly with a short time to peak concentration,widely distributed and has a high bioavailability.The post-antibiotic effect（PAE）and post-antibiotic sub-inhibitory concentration effect（PA-SME）of danofloxacin against H.parasuis were determined using the spectroph Otome-try combined with bacterial count method.Results indicated that the PAE of danofloxacin for H.parasuis ranged from 5 to 8 hrs and the PA-SME varied from 9 to 11.5 hrs.It suggested that sub-MIC danofloxacin has a significant inhibition effect on the regrowth of H.parasuis.The ex vivo PK/PD relationship of danofloxacin against H.parasuis was established using the Sigmoid E_maxmodel based on the data from time-kill curve and PK study.The AUC/MIC targets that needed to achieve the bacteriostatic（E=0）,bactericidal（E=-3）and eradication（E=-4）were 4.48,11.21 and 16.54,respectively.The CO_WTof danofloxacin against H.parasuis was determined using the in vitro MIC distribution from 164 clinical strains,normal distribution test and nonlinear regression analysis.The CO_PDof danofloxacin against H.parasuis was determined using PK/PD modeling and Monte Carlo simulation.Finally,the CO_WTand CO_PDof danofloxacin for H.parasuis was determined to be 8 and 0.25μg/m L,respectively.Our finding will be helpful for the interpretation of susceptibility testing results,monitoring of H.parasuis resistance,and guiding the rational use of danofloxacin in veterinary clinical settings.