A Study of Inhaled KB407 for the Treatment of Cystic Fibrosis

Inclusion Criteria: 1. The subject or legally appointed and authorized representative must have read, understood and signed an Institutional Review Board/Ethics Committee (IRB/EC) approved Informed Consent Form and must be able to and willing to follow study procedures and instructions. 2. Male or female subject aged 18 years old or older at the time of Informed Consent. 3. A confirmed diagnosis of cystic fibrosis (CF) that is clinically stable, in the opinion of the Investigator. 4. FEV1 ≥50% and ≤100% of the predicted normal for age, gender, and height at Visit 1 (Screening). Exclusion Criteria: 1. Initiation of any new chronic therapy (e.g., ibuprofen, hypertonic saline, azithromycin, Pulmozyme®, Cayston®, TOBI®) or any change in chronic therapy (excluding pancreatic enzyme replacement therapy) within 28 days of Visit 2 (Day 0). 2. Hospitalization, sinopulmonary infection, CF exacerbation, or other clinically significant infection or illness within 14 days of Visit 2 (Day 0) that, in the opinion of the Investigator, may confound study results. 3. A positive culture (saliva or sputum) indicating infection with highly virulent bacteria associated with accelerated decline in pulmonary function and/or decreased survival (e.g., Burkholderia cenocepacia, Burkholderia dolosa, Mycobacterium abscessus) within 6-months of Visit 2 (Day 0). 4. Participation in another clinical study or treatment with an investigational agent within 30 days or 5 half-lives, whichever is longer, of Visit 2 (Day 0). 5. History of lung transplantation. 6. Any condition (including a history or current evidence of substance abuse or dependence) that, in the opinion of the Investigator, would impact a subject's ability to complete all study-related procedures and/or poses an additional risk to the assessment of safety of the Investigational Product (IP). 7. An active oral herpes infection within 30 Days of Visit 2 (Day 0). 8. Women who are pregnant or nursing. 9. Subject who is unwilling to comply with contraception requirements per-protocol. 10. Clinically significant abnormalities of hematology or chemistry testing at Visit 1 (Screening) that the Investigator believes may interfere with the assessment of safety and/or efficacy of the study treatment. 11. Subject is known to be noncompliant or is unlikely to comply with the requirements of the study protocol, in the opinion of the Investigator.