Comparison of the relative airways and systemic potencies of inhaled fenoterol and salbutamol in asthmatic patients

Abstract

Background - There is controversy as to the relative safety of fenoterol and salbutamol. No differences have been found in the relative cardiac β1/β2 receptor activity of inhaled fenoterol and salbutamol in normal subjects. These initial findings have been extended by comparing the respective potencies of equivalent doses by weight of fenoterol and salbutamol in asthmatic subjects, in terms of airways and systemic responses. Methods - Eighteen asthmatic patients of mean (SD) age 40 (14) years and a forced expiratory volume in one second (FEV1)% predicted of 56 (14)% (1.97 (0.66)1) were randomised to inhale fenoterol (100 μg/puff or 200 μg/puff), salbutamol, or placebo (100 μg/puff or 200 μg/puff) on three separate days. Dose-response curves were constructed using cumulative doses of 100 μg, 200 μg, 400 μg, 1000 μg, 2000 μg, and 4000 μg, and airways and systemic responses were measured 20 minutes after each dose with 40 minute increments. Dose ratios for the relative potency of fenoterol versus salbutamol were calculated from the dose-response curves using regression analysis of parallel slopes. Results - There was no difference in bronchodilator potency between fenoterol and Methods salbutamol (as median dose ratio): FEV1 1.1 (95% CI 0.4 to 4.6). In contrast, dose ratios for systemic responses showed that fenoterol was more potent than salbutamol: serum potassium 3.7 (95% CI 2.0 to 6.0), tremor 5.7 (95% CI 1.4 to 10.2), heart rate 1.6 (95% CI 1.0 to 2.3). At a conventional dose of 200 μg the only difference in response between the two drugs was observed for tremor (as mean difference): 0.23 log units (95% CI 0.06 to 0.41 log units). Conclusions - There was no difference in the bronchodilator potency between fenoterol and salbutamol on a microgram equivalent basis. In contrast, systemic potency was greater with fenoterol, although this difference was not clinically relevant at conventional dosages up to 200 μg.