Pharmacokinetic/Pharmacodynamic Relationship And Susceptibility Breakpoint Of Valnemulin Against Clostridium Perfringens In Rabbits

Epizootic rabbit enteropathy?ERE?has become a highly lethal disease of fattening rabbits that caused significant economic loss in the rabbit industry.Clostridium perfringens and?-toxin are considered to be the dominant pathogen and associated agent in many ERE cases.Valnemulin is a well-known semisynthetic pleuromutilin antibiotic which shows potent activity against a range of bacteria including C.perfringens.Currently,valnemulin has been authorized for use in rabbits to reduce disease mortality of ERE.This study aims to assess the pharmacokinetic?PK?of valnemulin in plasma and jejunal fluids of rabbits after intravenous?IV?and oral dosing,and to predict ex vivo activity of valnemulin in jejunal fluids of rabbits against C.perfringens.In addition,valnemulin MIC distribution for rabbit C.perfringens were established based on reported data,and the wild-type cutoff(CO_WT)and PK/PD cutoff(CO_PD)were therefore established.The PKs of valnemulin against C.perfringens were investigated in plasma and jejunal fluids of rabbits following IV and oral administration at 3 mg/kg bodyweight?BW?.The drug concentrations in plasma and jejunal fluids were determined using a high-performance liquid chromatography-tandem mass spectrometry?HPLC-MS/MS?.The PK parameters in plasma after IV dosing are as follows:T1/2??0.22±0.11 h?,T1/2??2.93±0.27 h?,AUC?1.43±0.17?g·h/mL?,V_ss?8.05±1.04 L/kg?,Cl_B?2.19±0.36 L/kg·h?.The PK parameters in plasma after oral dosing are as follows:T_1/2Ka?0.52±0.11 h?,T_1/2Kel?2.78±0.49 h?,T_max?1.67±0.18 h?,C_max?0.03±0.02?g/mL?,AUC?0.18±0.04?g·h/mL?,F?12.3±1.79%?.The PK parameters in jejunal fluids after oral dosing are as follows:T1/2?z?7.79±2.08 h?,AUC_last?48.1±6.67?g·h/mL?,T_max?3.27±0.56 h?,C_max?5.32±1.16?g/mL?.Fairly poor oral bioavailability was observed in rabbits after oral administration.Of note,the terminal half-live of valnemulin in jejunum was significantly longer than in plasma,indicating that valnemulin could provide a persisting antibacterial effect in the intestinal tract.The time-kill curves were established in jejunal fluids to evaluate ex vivo antibacterial activity of valnemulin against C.perfringens.Valnemulin exhibited rapid,time-dependent killing feature,and ex vivo dose-response profile was closely fitted to sigmoid E_max model.The surrogate index of AUC_24h/MIC ratios required to achieve bacteriostatic,bactericidal and virtual bacterial elimination effects were 28.8,57.5 and 90.1 h,respectively.Based on calculated PK data,AUC_24h/MIC target and Monte Carlo simulation,the adequacy of the current dosage of valnemulin?3.0 mg/kg BW?was explored by computing population doses distribution covering different MICs.The predicted daily valnemulin dose for bactericidal elimination of C.perfringens was 2.33 mg/kg BW,which was consistent with the European proposed in-feed dosing regimen for rabbits?3 mg/kg/day?.These results indicated that the current dosage of valnemulin in rabbits would be sufficient to achieve satisfactory efficacy against ERE due to C.perfringens infections.The MIC distribution of valnemulin against rabbit C.perfringens isolates?n=121?were obtained from some previous studies in Europe?France,Spain and Italy from 2006 to2009?.The log₂-transformed MIC distribution was subjected to the goodness-of-fit and non-linear least squares regression tests,thus the CO_WT vase determined to be 0.5?g/mL.A10,000-subject Monte Carlo simulation was used to calculate the PK/PD cut-off(CO_PD)of valnemulin against C.perfringens using Crystal Ball Professional software,based on the current PK parameters,MIC distribution and the PK/PD targets.The probability of target attainment?PTA?reached>97.48%when the MIC?0.25?g/mL.Therefore,the CO_PD for valnemulin against rabbit C.perfringens was defined as a MIC of?0.25 mg/mL.In this study,we investigated ex vivo PK/PD targets and susceptibility breakpoint of valnemulin against C.perfringens.These findings provided a rational approach to optimize dosing regimen of valnemulin in rabbits expected of having C.perfringens infections.In addition,the established susceptibility breakpoint of valnemulin may be useful in resistance surveillance of pleuromutilins and development of clinical breakpoints.