A Trial of Pulmozyme Withdrawal on Exercise Tolerance in Cystic Fibrosis Subjects With Severe Lung Disease (TOPIC)

Inclusion Criteria: * Signed Informed Consent Form and, if applicable, Assent Form * Age ≥ 14 years old at screening * Proven diagnosis of CF * Ability to perform acceptable and reproducible spirometry maneuvers at screening * FVC ≤ 45% predicted for race, height, age, and sex at screening * Chronic use of Pulmozyme for at least 3 months in the last year and daily or twice daily use during the 14 days prior to screening * Stable regimen of chest physiotherapy (CPT) for at least 14 days prior to screening * Ability to complete the 6-minute walk test at screening * Ability to complete the 6-minute walk test and spirometry at Visit 2 * If on routine tobramycin solution for inhalation (i.e., TOBI®) or other cycled antibiotic therapy, intention to either start or stop therapy at the time of Visit 2 (based on routine therapy cycle; no other planned change in this antibiotic regimen for 28 days before or 28 days after Visit 2) * Clinically stable with no change in medications during the 14 days prior to screening Exclusion Criteria: * Use of an investigational drug or device within 28 days prior to screening * Previous episode of acute respiratory failure requiring assisted ventilation within 2 years prior to screening * Previous lung transplant * Any cardiac disease that would contraindicate performing the 6-minute walk test * Pregnancy or nursing * Known hypersensitivity or other contraindication to the use of Pulmozyme * Previous completion or premature discontinuation of study drug or withdrawal from this study * More than one prior screening failure for this study at any time or any prior screening failure for this study within the last 28 days