The analytical standard Tilmicosin was procured from Sigma Aldrich private limited, India. Tilmovet® 25% oral solution was obtained
gratis from M/s HuvePharma Private Limited, Pune, India. Acetonitrile and Methanol (HPLC grade) were procured from Merck Specialities Private Limited, Mumbai. Ammonium Formate, Trifluoroacetic acid and Perchloric acid were from Himedia Labs Private Limited, Mumbai.
Thirty-seven healthy Vencobb broiler chicken, four to five weeks old, weighing 1.2±0.2 kg were reared in a deep litter system with
ad libitum water and poultry finisher mash (antibiotic and anticoccidial free). After one week of acclimatization period, the study was conducted with overnight fasting of birds before drug administration. The experimental design and bird usage was approved by Institutional Animal Ethical committee, Madras Veterinary College, Chennai, India (Approval Lr.no.2345/21/DFBS/IAEC/2016, dated: 26.10.2016).
Experimental design
The present experiment was conducted at Madras Veterinary College, Chennai, Tamil Nadu during 2017. Initially, a pilot study was performed in six birds to choose the appropriate dose of tilmicosin for administration through drinking water. In the pilot study, three doses of tilmicosin
viz. 25, 30 and 40 mg/kg b. wt (two birds for each dose) were mixed with drinking water for one day. The blood samples were collected at 0, 0.08, 0.25, 0.5, 1, 2, 3, 4, 8, 12, 24, 48 h post dosing. The plasma was separated and the plasma drug concentration was analyzed by HPLC. Based on the higher C
max value, the dose of 40 mg/kg was fixed for the main study.
The main study was conducted in two phases. In phase I, the pharmacokinetic study was conducted in which 16 birds were randomly categorized into ‘in-crop’ and ‘in-water’ groups (8 birds/group). The dose rate of tilmicosin was 25 mg/kg b.wt (
Jordan and Horrocks, 1996;
Kempf et al., 1997) for ‘in-crop’ route and 40 mg/kg b. wt. (based on pilot study) for ‘in-water’ route. The blood samples were collected from median tarso-metatarsal vein at 0, 0.08, 0.25, 0.5, 1, 2, 3, 4, 8, 12, 24 and 48 h after drug administration. The plasma was separated by centrifugation at ~1000 × g for 10 min and stored at -20°C until analysis.
In phase II, residue study was conducted in 15 birds by administering tilmicosin through drinking water at the dose rate of 40 mg/kg b. wt for 5 days (as per therapeutic regimen). The birds were sacrificed on days 10, 14, 18, 22 and 25 post-last dose (three birds per day) and tissues such as lungs, liver, breast muscle, thigh muscle and kidney were collected, homogenized and stored at -20°C until analysis to assess the tissue residues.
Analytical method
The isocratic separation and quantification of tilmicosin was performed using Syncronis C18 column (5 µm, 4.6×250 mm, Thermo-scientific, USA) in the UFLC Shimadzu system (Japan) which consisted of degassing unit, pump (Prominence, LC 20 AD), Photo Diode Array detector (Prominence, SPD-M20A), auto-sampler (Prominence, SIL-20 AC HT) and column oven (CTO-10AS VP). The mobile phase consisting of 0.1M ammonium formate: acetonitrile: methanol (60:30:10 v/v/v, pH adjusted to 5.0 with trifluoroacetic acid) was used at the flow rate of 1.2 mL/min and the detection wavelength was 287 nm
(Keles et al., 2001). The column temperature was maintained at 30°C.
The plasma and homogenized tissue samples were extracted by liquid-liquid extraction. 50 µl of perchloric acid (10%) was added to 450 µl of sample, vortexed for 30 seconds and centrifuged at 1000 × g for 5 min
(Abu-basha et al., 2007). 10 µl of clear supernatant filtered through 0.2 µ HNN membrane was injected in HPLC system for analysis.
Drug-free poultry plasma and tissue homogenates were spiked with tilmicosin standards ranging from 0.01 to 5 µg/ml and the calibration curve was constructed. The HPLC method was validated for tilmicosin quantification in chicken plasma and tissues. The calibration curves were linear over a range of 0.05-5 µg/ml with r
2 value of 0.999. The inter- and intra-day precision were 13.84% and 4.85% respectively. The limit of quantification was 0.06 µg/ml and the analytical recovery was 100.66%. Hence, the HPLC method in this study was highly sensitive and precise with excellent reproducibility.
Pharmacokinetic analysis
The pharmacokinetic parameters such as area under curve (AUC), area under moment curve (AUMC), mean residence time (AUMC/AUC), clearance (Cl
B/F=Dose/AUC), volume of distribution (Vd
(area)/F= Dose/AUC*β) and elimination half life (t
1/2 = 0.693/β) were calculated by non-compartmental model using MS Excel. The absorption rates were slower than the elimination rate. Hence, based on the principle of flip-flop kinetics, slope of absorption line was used to calculate extrapolated value of AUC
0-∞ AUMC
0-∞(
Bialer et al., 1986). The peak plasma concentration (C
max) and the time to reach C
max (T
max) were taken as observed. Absolute bioavailability could not be measured since the intravenous administration of tilmicosin was not possible. Hence relative bioavailability was determined as ratio of AUC of drinking water to the AUC of ‘in-crop’ administration.
Statistics
The effect of different routes (‘in-crop’ and ‘in-water’) on the mean plasma tilmicosin concentrations and pharmacokinetic parameters was compared using Mann Whitney U test (SPSS version 20).