Testing the Effect of Adding Chronic Oral Azithromycin to Inhaled Tobramycin in People With Cystic Fibrosis (CF)

This study is a prospective, randomized, double-blinded, placebo-controlled trial of azithromycin 500mg taken orally thrice weekly vs. placebo in subjects with cystic fibrosis and chronic airway infection with P. aeruginosa who are utilizing chronic inhaled tobramycin therapy. It will include approximately 120 subjects able to complete a primary 6-week study phase. Subjects will be at least 12 years old with a baseline forced expiratory volume at one second (FEV1) between 25-100% predicted. Subjects will continue to use clinically prescribed inhaled tobramycin cycled on/off every 4 weeks. They will be provided over-encapsulated azithromycin 500mg tablets or placebo during the primary study phase. An optional extension phase will be offered to all subjects completing the primary 6-week study. This 8-week extension phase will include an initial 4 weeks without use of inhaled tobramycin or other inhaled antibiotics, followed by a 4-week period with inhaled tobramycin use. All subjects participating in the extension phase of the study will be provided azithromycin 500mg tablets to be taken thrice weekly for the entire 8-week period. This study will investigate how use of chronic oral azithromycin affects some of the previously demonstrated benefits to health when using inhaled tobramycin. The primary measurements will focus on lung function. Additional measurements will focus on disease-related quality of life as reported by subjects in the trial. Exploratory outcomes, including measurements of safety, are also planned.