Pharmacokinetics and pharmacodynamics of terbutaline in healthy horses

OBJECTIVE: To determine pharmacokinetics of terbutaline in healthy horses and to relate serum terbutaline concentrations with the drug's pharmacodynamic effects. ANIMALS: 6 healthy horses. PROCEDURE: Horses were given terbutaline i.v. (10 microg/kg of body weight) and, 1 week later, p.o. (100 microg/kg). Responses to drug administration (eg, heart rate and serum lactate concentration) were measured. Serum terbutaline concentration was measured by means of gas chromatography with mass spectrometry. Protein binding was determined in vitro. RESULTS: Following i.v. administration, median maximum serum terbutaline concentration and mean residence time were 9.3 ng/ml and 30 minutes, respectively. Bioavailability following oral administration was < 1%. All horses developed sweating, trembling, excitement, and tachycardia during i.v. infusion. The 2 horses with the highest serum terbutaline concentrations developed severe tachycardia and CNS stimulation; 30 minutes after the i.v. infusion was completed, they were hyperventilating and lethargic. Heart rate and serum lactate concentration increased as serum terbutaline concentration increased. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that terbutaline is rapidly cleared from the bloodstream following i.v. administration to horses, suggesting that continuous i.v. infusion would be needed to maintain therapeutic serum concentrations. Oral administration of terbutaline to horses is not practical because of the low bioavailability.