Milk and Blood Pharmacokinetics of Tylosin and Tilmicosin following Parenteral Administrations to Cows

The aim of this study is to determine the pharmacokinetics of tylosin and tilmicosin in serum and milk in healthy Holstein breed cows (n = 12) and reevaluate the amount of residue in milk. Following the intramuscular administration of tylosin, the maximum concentrations (C-max) in serum and milk were found to be 1.30 +/- 0.24 and 4.55 +/- 0.23 mu g/mL, the time required to reach the peak concentration (C-max) was found to be 2nd and 4th h, and elimination half-lives (t(1/2 beta)) were found to be 20.46 +/- 2.08 and 26.36 +/- 5.55 h, respectively. Following the subcutaneous administration of tilmicosin, the C-max in serum and milk were found to be 0.86 +/- 0.20 and 20.16 +/- 1.13 mu g/mL, the C-max was found to be 1st and 8th h, and the t(1/2 beta) were found to be 29.94 +/- 6.65 and 43.02 +/- 5.18 h, respectively. AUC(milk) /AUC(serum) and Cmax-milk /Cmax-serum rates, which are indicators for determining the rate of drugs that pass into milk, were, respectively, calculated as 5.01 +/- 0.72 and 3.61 +/- 0.69 for tylosin and 23.91 +/- 6.38 and 20.16 +/- 1.13 for tilmicosin. In conclusion, it may be stated that milk concentration of tylosin after parenteral administration is higher than expected like tilmicosin and needs more withdrawal period for milk than reported.