Pulmonary deposition of inhaled terbutaline: comparison of scanning gamma camera and urinary excretion methods

Deposition from a multidose powder inhaler (Turbuhaler, Astra) delivering 500 micrograms of terbutaline sulfate per metered dose was compared by two techniques in a group of six healthy volunteers. The deposition of the radionuclide 99mTc, which was used to label terbutaline sulfate powder, was quantified by gamma camera (G-method). Simultaneously, the gastrointestinal absorption of swallowed drug was blocked with activated charcoal, and the amount of terbutaline in a urine sample, collected over a period of 48 h and corrected by a pharmacokinetic internal standard of intravenous deuterated terbutaline, was used as a measure of lung deposition (U-method). The mean (standard deviation) depositions in lung were 26.9 (3.8%) of the dose for the G-method and 21.1 (3.2)% of the dose for the U-method. Possible reasons for the differences between the two means are discussed. Both methods are suitable for assessing deposition from medical aerosol inhalers; the U-method requires access to gas chromatography-mass spectrometric equipment, and the G-method requires access to gamma camera facilities.