Residue study of doxycycline and 4-epidoxycycline in pigs medicated via drinking water

A study was performed to determine the residues in edible tissues of health y pigs after continuous administration of doxycycline with drinking water f or five consecutive days at a dose rate of 10.5 mg doxycycline kg(-1) body weight (BW) per day. Quantitation was performed using a validated HPLC meth od with fluorescence detection. The method was able to separate doxycycline and its 4-epimer, 4-epidoxycycline. This epimer was found in kidney, liver , skin, fat and muscle tissue. The method was validated at the maximum resi due limit (MRL), at half the MRL and at double the MRL for both doxycycline and 4-epidoxycycline. Linear calibration curves were obtained in spiked ti ssues (r > 0.99). The accuracy of the calibrators of the calibration curves was within -20% to +10%. The accuracy and precision (expressed as the with in-run repeatability) were found to be within the required ranges for the s pecific concentration. The limits of detection and limits of quantification were below one-half of the MRL. The quantification limits were 50 mu g kg( -1) for doxycycline and 100 mu g kg(-1) for 4-epidoxycycline in kidney and liver, 20 mu g kg(-1) for doxycycline and 50 mu g kg(-1) for 4-epidoxycycli ne in skin and fat and 10 mu g kg(-1) for doxycycline and 50 mu g kg(-1) fo r 4-epidoxycycline in muscle tissue. The withdrawal time was calculated acc ording to the recommendations of the European Agency for the Evaluation of Medicinal Products (EMEA/CVMP/036/95) and was set at 3 days. The plasma con centration of doxycycline and the stability of doxycycline in drinking wate r were also determined during the treatment period. The mean plasma concent ration of doxycycline during the treatment period ranged from 0.83 to 0.96 mu g ml(-1). Thirty-six hours after the withdrawal from medicated drinking water, no plasma levels could be detected. Samples of medicated water were taken at time zero and at 24 h after addition of doxycycline to the drinkin g water. No statistically significant difference in the mean drinking water concentration was seen at time zero and at time 24 h (Student's t-test, al pha = 0.05).