A 16-week randomised, placebo-controlled, double-blind, double-dummy, parallel-group study comparing the efficacy and safety of tiotropium inhalation solution delivered by the Respimat® inhaler (2 puffs of 2.5 µg once daily) with that of salmeterol from the hydrofluoroalkane metered dose inhaler (2 puffs of 25 µg twice daily) in moderate persistent asthma patients with the B16-Arg/Arg genotype