Tolerance to the bronchoprotective effect of β2-agonists:: Comparison of the enantiomers of salbutamol with racemic salbutamol and placebo

Background: Regular use of racemic salbutamol results in the partial loss of its bronchoprotective effect. The 2 enantiomers of salbutamol, the bronchodilator R-salbutamol and nonbronchodilator S-salbutamol, are now available. Objective: We sought to compare the effect of regular use of S-salbutamol, R-salbutamol, racemic salbutamol, and placebo on the bronchoprotective effect of a single dose of racemic salbutamol against methacholine-induced bronchoconstriction. Methods: Eleven of 13 well-controlled beta(2)-agonist-free asthmatic subjects completed a double-blind, randomized study comparing racemic salbutamol 2.5 mg, S-salbutamoI 1.25 mg, R-salbutamol 1.25 mg, and dilueut placebo nebulized and inhaled 3 times daily for 6 days (greater than or equal to 6-day washout period). Ten to 12 hours after the last dose, the subjects performed measurement of FEV1, methacholine PC20, and a repeat methacholine PC20 done 1 hour after the first methacholine test and 10 minutes after 2 puffs (200 mu g) of racemic salbutamol administered from a metered-dose inhaler. The primary endpoint was the methacholine PC20 dose shift (Delta log PC20 divided by log 2) from before to after administration of 200 mu g of racemic salbutamol. Results: The methacholine dose shift was 3.2 doubling doses (9-fold increase in methacholine PC20 after 200 mu g of racemic salbutamol) during the placebo treatment and was unaltered (3.2) after administration of S-salbutamoI. The dose shift was significantly lower after both the R-salbutamol and racemic salbutamol treatments (2.2 and 2.6 doubling doses, respectively); there was no significant difference between R-salbutamol and racemic salbutamol. There was no treatment effect on baseline FEV1, baseline methacholine PC20, or bronchodilation. Conclusion: Regular treatment with racemic salbutamol or R-salbutamol, but not S-salbutamol, results in a partial loss of bronchoprotection, without loss of bronchodilation, compared with placebo.