Pharmacokinetic And Ex Vivo Pharmacodynamic Of Tulathromycin Against Streptococcus Suis Serotype 2 In Piglets

Streptococcus suis is a zoonotic agent causing severe infection in both swine and humans,in which S.suis serotype?is the most virulent and dominant pathogenic serotype capable of causing severe infections.Tulathromycin is a new generation of macrolide antibiotics solely for veterinary use.Howe ver,there is no data about the ex vivo activity and PK/PD profiles of tulathromycin against S.suis?were reported.Therefore,the aim of this study was to investigate the PK/PD relationship of S.suis?using ex vivo models.Moreover,we studied the effect of varying testing matrix on susceptibility,the postantib iotic effect?PAE?and postantibiotic sub-MIC effect?PA-SME?of tulathromycin against S.suis.These results could provide a framework for further design and optimization of tulathromycin dose regimens in piglets expected of having S.suis infectionsIn this work,seven healthy piglets were randomly divided into two groups.The pharmacokinetics?PK?of tulathromycin performed by following intramuscular?IV?and intravenous?IM?injection at a single dose of 2.5 mg/kg b.w.The drug concentrations in serum were determined by liquid chromatography tandem mass spectrometry?LC-MS/MS?,and the time-concentration data were analyzed by WinNonlin 5.2.1 program submitted to a non-compartmental model.After IV dosing,the main PK parameters were as follows:T_1/2?z?69.4±5.71 h?,C_max?4.78±0.54?g/mL?,AUC_last?21.8±3.54?g·h/mL?,V_ss?11.3±0.99 L/kg?,CL?115±19.6 mL/h/kg?and MRT?89.3±10.9 h?.After IM injection,the mean values of PK parameters were as follows:T1/2?z?74.1±10.6 h?,T_max?0.23±0.14 h?,C_max?0.82±0.25?g/mL?,AUC_last?17.1±4.18?g·h/mL?,AUC_infinity?17.9±4.46?g·h/mL?,MRT?97.6±11.3 h?and F?78.3±19.2%?.The PK data,indicated that tulathromycin absorbed rapidly,with short T_max.The elimination of tulathromycin was fairly long and the distribution appeared to be extensive.The protein binding rate of tulathromycin in porcine serum was determined using ultrafiltration method.The degree of binding rate ranged from 32.3%to 39.1%at spiked concentrations of 0.05,0.5 and 5?g/mL,with a mean binding rate of 36.3%,indicating a character of nonspecific concentration independent.The results transformed to the free/unbound fraction of tulathromycin in serum would be approximately 63.7%.The MIC_s and of tulathromycin against S.suis?in swine serum and MHB were 0.03 and1?g/mL,respectively.The MIC values determined in serum were significantly lower than those determined in MHB,with the MHB/serum ratios of more than 30.In vitro time-killing curves showed that either bactericidal or virtual elimination activity of tulathromycin could achieve against S.suis when the drug concentrations were more than 8to 16 multiples of MIC?8-16?g/mL?.The ex vivo time-kill curve for tulathromycin in serum indicated that tulathromycin exerted either effective growth inhibition or bactericidal effect after 12 h of incubation in serums collected up to 200 h.The post-antibiotic effect?PAE?and post-antibiotic sub-MIC effect?PA-SME?of tulathromycin against S.suis were investigated in vitro using a spectrophotometric technique and classic viable count methods.The mean PAEs were 1.27 h?1×MIC?and 2.03 h?4×MIC?,and the mean PA-SMEs were2.47 h?0.1×MIC?,2.63 h?0.2×MIC?and 3.10 h?0.3×MIC?.The results indicated that PAE and PA-SME could suppress bacterial regrowth while permitting drug levels to fall below MIC without losing effectiveness.Based on in vitro MIC data and in vivo PK parameters,the surrogate markers of antibacterial efficacy,namely the ratio of AUC over 12 h to the MIC(AUC_12h/MIC)was calculated for each tulathromycin concentration using the ex vivo killing curves.The AUC_12h/MIC in serum and the antimicrobial effect(?Log₁₀CFU/mL).This model were fitted by sigmoid E_max model that derived from the Hill equation.The calculated AUC/MIC ratios in serum required to achieve bacteriostatic and bactericidal activity were 9.62 and58.3,respectively.