Study to Evaluate the Safety,Tolerability, Pharmacokinetics and Exploratory Efficacy Parameters of AIR DNase™in Patients With Cystic Fibrosis Previously Treated With Pulmozyme®

Main Inclusion Criteria: 1. Age ≥ 12 years of age (inclusive) at the time of screening 2. Weight ≥ 36 kg 3. Prior confirmed diagnosis of CF 4. At least 4 months treatment with Pulmozyme® prior to screening. 5. The subject is medically stable for at least one month prior to the screening visit. 6. Stable inhaled regimen of either: antibiotics, steroids, hypertonic saline at least four months prior to screening visit. 7. FEV1 of \>40% and \<90% and FVC ≥ 40% of predicted normal for age, gender, and height at screening . 8. Female and male subjects whose co-partners are of child bearing potential must agree to use two medically acceptable methods of contraception, not including the rhythm method. 9. Be willing and able to adhere to the study visit schedule and other protocol requirements 10. Be willing and able to provide voluntary written informed consent Main Exclusion Criteria: 1. Has a history of lung transplantation. 2. Female subjects who are pregnant or lactating. 3. History of severe or unexplained adverse reactions during aerosol delivery of any medicinal product. 4. History or presence of hypersensitivity or reaction to inhaled proteins. 5. Participation in another clinical trial within 60 days prior to screening. 6. Presence of any medical, emotional, behavioral or psychological condition that in the judgment of the Investigator would interfere with the subject's safety or compliance with the requirements of the study. 7. Have positive serology for human immunodeficiency virus (HIV) or Hepatitis B or Hepatitis C infection.