The blood serum pharmacokinetics of a Poloxamer-407 based new pharmaceutical preparation of 39% tilmicosin was determined in cows.  Two dose levels injected SC, were assessed: 10 mg/kg (Til-LA₁₀) and 26 mg/kg (Til-LA₂₆). Tilmicosin was determined using HPLC, also electrocardiographic (EKG) monitoring in all animals before and at 1, 2 and 4 h after the injection of the drug was performed to measure key EKG parameters, including heart rate. Maximum serum concentration values were 2.6 µg/mL and 1.23 µg/mL, occurring 4.9 and 5.1 h after the injection of Til-LA₂₆ and Til-LA₁₀, respectively.  Mean residence time was statistically larger for Til-LA₂₆ (50.4±5.8 h) than Til-LA₁₀ (37.4±4.7 h) (P<0.05), with T½_el of 39.8 h for Til-LA₂₆ and 33.2 h for Til-LA₁₀. There were no differences in relative bioavailability as adjusted for dose (AUMC of 8663 µg/mLžh² for Til-LA₂₆ and 2858 µg/mLžh² for Til-LA₁₀). This new formulation as dosed for Til-LA₂₆ possesses a long T½_el of tilmicosin and resulted in a lack of changes in the EKG and heart rate. Based on PK/PD ratios tilmicosin is regarded as a time-dependent antibacterial drug. Considering a theoretical minimum therapeutic serum concentration (MTC) of  0.1 µg/mL useful concentrations can be achieved for up to 192 h with Til-LA₂₆ and an AUC/MTC ratio of 1514 without cardiac toxicity. Further studies are necessary to correlate PK/PD parameters obtained with clinical efficacy and a more thorough analysis of cardiac toxicity is required to determine the suitability of this preparation in bovine medicine.

Key words: Cow, Long-acting, New-preparation, Pharmacokinetics, Tilmicosin