Pharmacokinetics of Doxycycline in Plasma and Milk after Intravenous and Intramuscular Administration in Dairy Goats

Doxycycline is a second-generation tetracycline, marketed in different species for treating infections caused by susceptible bacteria. Little information is available on the pharmacokinetics of doxycycline in lactating goats. The objective of this study was to establish the disposition kinetics of doxycycline after parenteral administration (intravenous and intramuscular) in dairy goats and its elimination in milk. A cross-over model was designed (<i>n</i> = 6). Doxycycline was dosed at 5 mg/kg for intravenous administration and 20 mg/kg for extravascular administrations. Noncompartmental pharmacokinetic methods were used to calculate plasma concentration-time data. The V_z value suggests a moderate distribution of this antibiotic in goats, with a value of 0.85 L/kg. A low bioavailability (F = 45.60%) of doxycycline following an intramuscular injection was observed, with all animals exhibiting signs of lameness. Doxycycline rapidly crossed the blood-milk barrier, but exposure to the antimicrobial and the concentrations reached in milk were lower than those obtained in plasma. Although PK/PD ratios may be low with the pharmacokinetic data obtained with this formulation of doxycycline, at this dose and route of administration, doxycycline after IM administration could be useful for infections by moderate or highly susceptible bacteria in the mammary gland of goats. However, it may be necessary to test different doses of doxycycline or other routes of administration to achieve better surrogate markers and to establish repeated dosing regimens and clinical efficacy.