Effect of Azithromycin on Lung Function in 6-18 Year-olds With Cystic Fibrosis (CF) Not Infected With P. Aeruginosa

Azithromycin is an antibiotic that has been shown to improve lung function in patients with cystic fibrosis (CF) whose lungs are infected with a bacterium called Pseudomonas aeruginosa. Scientists are not sure how azithromycin works in cystic fibrosis. It does not appear to work by killing the bacteria Pseudomonas aeruginosa, but it may make these bacteria and other bacteria less damaging to the lungs by reducing their ability to attach to the lining of the lung, or by reducing the bacteria's ability to make substances that damage the lungs of patients with cystic fibrosis. Azithromycin may also work directly on the cells in the lungs to improve lung function. This could occur by reducing inflammation (swelling) in the lungs, and/or making the mucus less sticky, or by affecting the salt channel that doesn't function correctly in CF. If azithromycin works in one or more of these ways; it may also be effective in improving lung function in cystic fibrosis patients who are not infected with Pseudomonas aeruginosa. We are conducting this research study to examine the safety, effect on lung function and frequency of symptoms relating to cystic fibrosis during 24 weeks of treatment with the antibiotic azithromycin. This study is designed to determine if patients with cystic fibrosis whose lungs are not infected with the bacteria Pseudomonas aeruginosa will benefit from 24 weeks of treatment with the antibiotic azithromycin. Benefit will be determined as having better pulmonary function tests and getting sick less often compared to a placebo (sugar pill). This study is also designed to determine if azithromycin is safe when administered for 24 weeks to cystic fibrosis patients not infected with Pseudomonas aeruginosa. By doing this study, we hope to learn more about CF and improve the way in which we treat it. Comparison: Three times weekly azithromycin tablets added to standard care, compared to three times weekly placebo tablets added to standard care.