Pharmacokinetics of bambuterol in healthy subjects

Abstract

Aims To study the pharmacokinetics and bioavailability of the prodrug bambuterol and its bronchodilator moiety terbutaline in healthy subjects. Methods Eight healthy subjects (four women) received intravenous doses of bambuterol and terbutaline. On a third occasion, they, plus another four subjects, ingested oral bambuterol as a single dose followed by repeated doses once daily for 7 days. Plasma concentrations and urinary excretion of bambuterol and terbutaline were measured. Results After intravenous administration, renal clearances of bambuterol and terbutaline were similar (about 140 ml min⁻¹ ), but there was a five-fold difference in total clearance (bambuterol 1.25 l min⁻¹, terbutaline 0.23 l min⁻¹ ). Volume of distribution (V_ss ) was 1.6 l kg⁻¹ b.w. for both substances. A similar renal clearance of bambuterol was found during oral administration but that of terbutaline decreased (to about 120 ml min⁻¹ ). Mean terminal half-life of bambuterol was 2.6 h after intravenous and 12 h after oral administration, implying that uptake was rate-limiting. Mean residence time of terbutaline generated from oral bambuterol was 34 h compared with 8.0 h when terbutaline as such was infused. Generated terbutaline had a bioavailability of 36% (28–46) after intravenous and 10.2% (6.1–13.2) after oral administration of the prodrug. Bambuterol was well tolerated. The mean activity of plasma cholinesterase, an enzyme catalyzing bambuterol metabolism, was inhibited between 30–60% during repeated oral dosing. It virtually regained original activity within 48 h after the last dose. Conclusions The plasma concentration of terbutaline fluctuated little during repeated oral administration (mean peak:trough ratio 1.9), as a result of prolonged absorption of bambuterol and slow formation of terbutaline. Thus, the pharmacokinetic properties of bambuterol make it suitable for oral once-daily dosage.