administrations in

Aim: This work was performed to investigate the pharmacokinetics of the triamilide antibiotic, tulathromycin in healthy rabbits. 
 
Materials and Methods: Ten rabbits in each group were given a single dose of 2.5 mg/kg body weight (bw) of tulathromycin via intravenous (IV), intramuscular (IM) and subcutaneous (SC) administrations. The concentration of tulathromycin in plasma was determined by microbiological assay Bacillus subtilis ATCC 6633 as the test organism. 
 
Results: Following IV administration, the total body clearance (Cltot) was 321.70 ml/kg.h, the volume of distribution at steady-state (Vdss) was 13.26 L/kg and the value of the elimination half-life (t1/2β) was 29.29 h. After SC administration, the elimination half-life (t1/2el), mean residence time (MRT) and maximum plasma concentration (Cmax) were significantly higher (36.22 h, 52.54 h and 882.19 ng/ml) than after IM route (31.69 h, 45.89 h and 714.72 ng/ml), respectively. Tulathromycin was bound to the extent of 36% to plasma protein of healthy rabbits. The absolute bioavailabilities were 88.07 and 94.25% after IM and SC injections. 
 
Conclusion: Thus a single dose of tulathromycin is promising treatment for most respiratory disease in rabbits.


Introduction
Repeated administration of these products over several days is typically required to achieve a therapeutic Snuffles is considered the most common effect.Prolonged exposure to antibiotics is important infectious respiratory disease observed in pet rabbits for the treatment and/ or prevention of several diseases [1].Common clinical signs include nasal discharge, in rabbits, however single administration therapy is sneezing and conjunctivitis [2,3].The most commonly desirable for producers that wish to minimize animal agent implicated with these symptoms is Pasteurella handling.Tulathromycin is a novel triamilide antibiotic multocida [4,5].However, other pathogens are cited, approved for the treatment of respiratory diseases in such as Staphylococcus spp.and Bordetella animals.It may be retained in the lung for many days bronchiseptica, although apparently this is by no after administration of a single dose [8][9][10].The drug means an exhaustive list [1].Macrolide antibiotics are has a long plasma elimination half-life in cattle of 90 h antibacterial agents that are widely used in the [7,11], and therapeutic concentrations have been treatment of infections of the respiratory system, soft detected in lung tissue for 10 to 15 days after a single tissue and skin in humans and domestic animals [6].dose [11,12].Therefore, tulathromycin possibly will Tulathromycin, like other macrolides, binds to the 50S be recommended for use in rabbits for several subunit of bacterial ribosomes and thereby inhibits respiratory diseases including pneumonia.To our protein synthesis, leading to inhibition of cell division knowledge there have not been any reports published and cell death.Although macrolides are generally on the pharmacokinetics of tulathromycin in rabbits.regarded as bacteriostatic, tulathromycin actually exhibits Consequently, the aim of the present study was to mixed bacteriostatic and bactericidal activity [7].
investigate the pharmacokinetic profile of tulathro-mycin in rabbits following a single IV, IM and SC limit of quantitation by this method was 20 ng/ml in injections to estimate an appropriate dosage regimen plasma.The response of tulathromycin was linear over of tulathromycin in rabbits.the range of concentration between 20 and 2000 ng/ml 2 with a correlation coefficient (r ) of 0.998.The intra-

Materials and Methods
assay coefficient of variation rate of elimination (CV) Drug: Tulathromycin was used as 10% injectable was 8%.Negative control samples (non-treated) ® solution (Draxxin , Pfizer Animal Health, New York, showed no bacterial inhibition, indicating no intrinsic ® NY), Each ml of Draxxin contains 100 mg of antibacterial activity of the samples.tulathromycin as the free base in a 50% propylene In vitro plasma protein binding: The extent of glycol vehicle, monothioglycerol (5 mg/ml), with protein-binding was determined in vitro using the citric and hydrochloric acids added to adjust pH.method of Craig & Suh [14] which is based on the Animals and Husbandry: Thirty healthy New diffusion of the free antibiotic into the agar medium.Zealand white rabbits of both sexes, 10-12 months old The drug was dissolved in phosphate buffer (pH 6.2) and weighing 2.7-3.5 kg were used.Rabbits were and antibiotic-free rabbit's plasma at concentrations of housed separately in individual cages under a 12-h 200, 400, 600, 800 and 1000 ng/ml.The differences in light/dark cycle and fed good quality hay (alfalfa) the diameter of the inhibition zone between the and/or a pelleted feed concentrate (fiber 18%, protein solutions of the drugs in the buffer and plasma were 14%, calcium >1 and fat 2%) with free access to water.calculated.The percentage of protein bound fraction The room temperature and relative humidity were was calculated according to the following equation: 0 maintained at 20 and 22 C, and between 30 and 60%, Protein binding % = Zone of inhibition in bufferrespectively.The animals were allowed to acclimatize Zone of inhibition in Plasma × 100 / Zone of inhibition and did not receive any drug treatment for at least 15 in buffer days preceding the study.The experiments were Pharmacokinetic analysis: Plasma concentrations carried out according to the National regulations on of tulathromycin after IV, IM and SC administrations animal welfare and institutional animal Ethical were subjected to a compartmental analysis using a Committee (IAEC).
nonlinear least-squares regression analysis with the Experimental design: The animals were allocated to help of a computerized curve-stripping software three groups of 10 rabbits each.Rabbits were package (R Strip; Version 5.0; Micromath Scientific individually weighed before drug administration and Software, Salt Lake City, UT, USA).Data were doses were calculated precisely.Rabbits of all groups examined by sequential weighted nonlinear regression. of groups were injected a at a dose of 2.5 mg/kg bw of Monoexponential, biexponential and triexponential 10% solution of tulathromycin intravenously into the equations were fitted to individual plasma concenleft ear vein, intramuscularly into the left semitration-time data.Akaike's Information Criterion membranous muscles and subcutaneously in the scruff (AIC) [15], residual sum of squares (Rs) and analysis using graduated 1 ml syringes, respectively. of residual's plots were used to discriminate between Blood samples from the all groups (0.5 ml each) models [16].The distribution and elimination halfwere taken via indwelling catheter into heparinized lives (t and t ), the volume of distribution at steady (MRT) and bioavailability (F), where: Assay for Tulathromycin: Tulathromycin concen-F = [mean AUC / mean AUC ] X 100.

IM or SC IV
trations in plasma were determined by a micro-Statistical analysis: The statistical analysis was biological agar plate assay [13] using Bacillus subtilis ® performed using the SPSS 10.0 software package ATCC 6633 as the test organism.Standard curves of (SAS, Cary, NC, USA).Results are presented as tulathromycin (Pfizer Animal Health, New York, NY) arithmetic mean ± standard errors (SE).The were prepared in pooled antibacterial-free plasma.All nonparametric Wilcoxon test was used to compare the samples were directly added to the culture plate.The parameters obtained in healthy and diseased rabbits two compartment open model.The total body .following each route of administration.Means were clearance (Cl ) was 321.70 ml/kg h, the volume of tot considered significantly different at p< 0.05 and P<0.01.distribution at steady-state (V ) was 13.26 L/kg and dss the value of the elimination half-life (t ) was 29.29 h.

Results
After SC administration, the elimination half-life All animals remained in good health throughout (t ), mean residence time (MRT) and maximum 1/2el the acclimatization and study periods.No adverse plasma concentration (C ) were significantly higher max effects were observed in the present studies when the (36.22 h, 52.54 h and 882.19 ng/ml) than after IM route tulathromycin formulation was administered to (31.69 h, 45.89 h and 714.72 ng/ml), respectively.rabbits at 2.5 mg/kg bw following IV, IM and SC Tulathromycin was bound to the extent of 36% administrations.Mean (±SE) tulathromycin plasma to plasma protein of healthy rabbits.The absolute concentration-time curves in healthy rabbits bioavailabilities were 88.07 and 94.25% after IM and following IV, IM and SC injections are plotted on a SC injections, respectively.The slow elimination and semi-logarithmic graph in Figures 1&2.Values of extensive distribution following a single dose of pharmacokinetic parameters are presented in Table 1.tulathromycin are fortunate pharmacokinetic Following a single IV route the plasma concentration characteristics and promising for the treatment of time curve of tulathromycin was best fitted to follow a respiratory disease in rabbits.
Dickinson vacutainer Systems, state (V ) were calculated according to standard equations dss Rutherford, NJ, USA), from the right ear vein at 0 [17].The total body clearance was calculated as Cl = tot (blank sample), 0.25, 0.33, 0.5, 0.75, 1, 2, 4, 6, 8, 10, Dose/ AUC.Statistical moments were used to 12, 18, 24, 48 h and everyday for 10 days after all compute the non-compartmental model of area under routes of injections.Plasma was separated by 0 the concentration-time curves (AUC), area under the centrifugation at 2000g for 10 min and stored at -20 C first moment curve (AUMC); mean residence time until assayed.

Table - 1: Mean ± SE plasma pharmacokinetic parameters of tulathromycin in normal rabbits (n=10) following IV, IM and SC administrations at a dose rate of 2.5 mg/kg bw.
t : distribution half-life; t : absorption half-life; t (t ): elimination half-life; V : volume of distribution; Cl : total body clearance; AUC: area under the curve by the trapezoidal integral; AUMC: area under moment curve by the trapezoidal integral; MRT: mean residence time; C : maximum plasma concentration; T : time to peak concentration; F%: bioavailability.Values alpha