Ex Vivo Pharmacokinetic And Pharmacodynamic Analysis Of Gamithromycin Against Haemophilus Parasuis In Porcine Serum

As a high-mortality infective factor,H.parasuis can invade the respiratory tract of swine and cause the Gl?sser’s disease,including fibrinous serositis and arthritis,causing significant economic losses to the swine industry.H.parasuis possessed up to 15 serotypes with significant regional differences,so that the cross-protection rate of H.parasuis vaccine was fairly low.Thus,antibiotic treatment is still an important strategy to effectively control H.parasuis infection.Gamithromycin is a novel macrolide developed for the treatment of livestock respiratory diseases.However,the pharmacokinetic/pharmacodynamic（PK/PD）relationship of gamithromycin against H.parasuis still remains unclear.Based on gamithromycin PK characteristic in pigs,this study aims to investigate the ex vivo antimicrobial activity and PK/PD target values of gamithromycin against H.parasuis in porcine serum,assessing the rationality of recent recommended dose regimen for gamithromycin.The results could provide a theoretical basis for further applications of gamithromycin in veterinary medicine.Six healthy crossbred piglets received gamithromycin at a single dose of 6.0 mg/kg by intravenous（IV）,intramuscular（IM）and subcutaneous（SC）injections.The drug levels in porcine serum were measured using HPLC-MS/MS method.The limit of quantification（LOQ）and detection（LOD）were 1.0 and 0.5ng/m L,respectively.The recoveries of gamithromycin ranged from 90%to 110%,and the coefficient of variation for both interday and intraday were<10%.The concentration-time data were submitted to Win Nonlin non-compartment analysis to generate PK parameters.For IV dosing,the PK parameters are as follow:T_1/2λz（25.7±2.23 h）,AUC_last（5.93±1.07 mg?h/L）,AUC_infinity（6.15±1.13mg?h/L）,Cl（1.01±0.17 L/kg/h）,V_ss（18.8±3.12 L/kg）,MRT（18.7±1.49 h）;For IM dosing,the PK parameters are as follow:T_max（0.63±0.49 h）,C_max（1.07±0.41 h）,T_1/2λz（26.5±4.67h）,MRT（20.3±3.68h）,F（117.5%）;For SC dosing,the PK parameters are as follow:T_max（2.16±1.68 h）,C_max（0.58±0.15 h）,T_1/2λz（28.1±4.08 h）,Cl/F（1.08±0.37 L/kg/h）,MRT（21.6±2.52h）,F（100.9%）.The PK data indicated that gamithromycin was absorbed rapidly,with a long half-life and high bioavailability.The MIC of gamithromycin against H.parasuis LM15 were determined using the microbroth dilution method,and the results showed that MIC of gamithromycin for H.parasuis LM15 were 0.5 and 0.06μg/m L in CAMHB and porcine serum,respectively.Further MIC comparison for a total of 192 clinical H.parasuis isolates indicated that MICs of gamithromycin in serum were markedly lower than those in broth culture,with broth/serum ratios of 8.93.The post-antibiotic effects（PAE）of gamithromycin against H.parasuis were at 1.5 h（1×MIC）and 2.4 h（4×MIC）and the post-antibiotic sub-MIC effect（PA-SME）ranged from 2.7 to 4.3 h.The ex vivo antimicrobial activity analysis showed that gamithromycin exerted effective growth inhibition or bactericidal effects in serum collected up to 36 h.By integration of PK data and ex vivo antimicrobial activity,the Sigmoid E_max-based PK/PD model was established to obtain ex vivo PD targets of gamithromycin against H.parasuis.The calculated AUC_12h/MIC in serum required producing bacteriostatic,bactericidal and elimination effects were 8.5,15.6 and 21.8 h,respectively.Based on the results of Monte Carlo simulation,probability of target attainment（PTA）of the current recommended dose of gamithromycin（6.0 mg/kg）for bactericidal effect were 88.65%.A predicted dose regimen of 6.64 mg/kg gamithromycin was estimated to be effective,achieving for a bactericidal activity against H.parasuis strains having MIC of≤16μg/m L over 3 days period of treatment.Results indicated that the current gamithromycin recommended dose has positive therapeutic outcomes for infection due to H.parasuis,providing the scientific evidence and data support for adjustment and evaluation of the clinical dosing regimen of gamithromycin in China.