This is a 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 in 6-18 year-olds with CF who are not infected with Pseudomonas 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.

Inclusion Criteria:

• Male or female, 6-18 years of age at enrollment
• Confirmed diagnosis of CF
• Written informed consent (and assent when applicable)
• Clinically stable at enrollment as assessed by the site investigator
• FEV1 % predicted > 50%
• Ability to comply with medication use, study visits, and study procedures
• Ability to swallow a 250 mg tablet

Exclusion Criteria:

• Weight less than 18.0 kg
• Respiratory culture positive for P. aeruginosa, NTM, or B. cepacia complex within 1 year or at screening, or AFB positive at screening
• Allergy to macrolide antibiotics
• Use of macrolide antibiotics (e.g., azithromycin, clarithromycin) within 60 days of screening
• Use of systemic corticosteroids or intravenous or oral antibiotics within 14 days of screening
• Initiation of high dose ibuprofen, Pulmozyme®, hypertonic saline or aerosolized antibiotics within 30 days of screening
• Chronic therapy with drugs known to have rare but serious interactions with azithromycin: amiodarone, digoxin, disopyramide, lovastatin, pimozide, rifabutin, and nelfinavir
• Investigational drug use within 30 days of screening
• Laboratory abnormalities (creatinine, liver function or neutropenia) at screening and confirmed at follow-up testing prior to randomization
• History of biliary cirrhosis, portal hypertension, or splenomegaly, or splenomegaly on physical exam
• History of ventricular arrhythmia
• Other major organ dysfunction, excluding pancreatic dysfunction
• History of lung transplantation or currently on lung transplant list
• Relative decrease in FEV1 % predicted ≥ 20% between the screening and enrollment visit
• Positive serum pregnancy test at screening
• Pregnant, breastfeeding, or if post-menarche female, unwilling to practice birth control during participation in the study
• History of alcohol, illicit drug or medication abuse within 1 year of screening in the judgment of the site investigator
• Presence of a condition or abnormality that in the opinion of the site investigator would compromise the safety of the subject or the quality of the data