Pharmacokinetics and lung tissue concentrations of tulathromycin in swine

The absolute bioavailability and lung tissue distribution of the triamilide antimicrobial, tulathromycin, were investigated in swine. Fifty‐six pigs received 2.5 mg/kg of tulathromycin 10% formulation by either intramuscular (i.m.) or intravenous (i.v.) route in two studies: study A (10 pigs, i.m. and 10 pigs, i.v.) and study B (36 pigs, i.m.). After i.m. administration the mean maximum plasma concentration ( C max ) was 616 ng/mL, which was reached by 0.25 h postinjection ( t max ). The mean apparent elimination half‐life ( t 1/2 ) in plasma was 75.6 h. After i.v. injection plasma clearance ( Cl ) was 181 mL/kg·h, the volume of distribution at steady‐state ( V ss ) was 13.2 L/kg and the elimination t 1/2 was 67.5 h. The systemic bioavailability following i.m. administration was >87% and the ratio of lung drug concentration for i.m. vs. i.v. injection was ≥0.96. Following i.m. administration, a mean tulathromycin concentration of 2840 ng/g was detected in lung tissue at 12 h postdosing. The mean lung C max of 3470 ng/g was reached by 24 h postdose ( t max ). Mean lung drug concentrations after 6 and 10 days were 1700 and 1240 ng/g, respectively. The AUC inf was 61.4 times greater for the lung than for plasma. The apparent elimination t 1/2 for tulathromycin in the lung was 142 h (6 days). Following i.m. administration to pigs at 2.5 mg/kg body weight, tulathromycin was rapidly absorbed and highly bioavailable. The high distribution to lung and slow elimination following a single dose of tulathromycin, are desirable pharmacokinetic attributes for an antimicrobial drug indicated for the treatment of respiratory disease in swine.