This study will evaluate safety and tolerability of ascending doses of nebulized KB407 in adults with cystic fibrosis.

Inclusion Criteria:

1. The subject must have read, understood, and signed an Institutional Review Board/Ethics Committee (IRB/IEC) approved Informed Consent Form and must be able to and willing to follow study procedures and instructions

2. Subjects aged 18 years or older at the time of Informed Consent

3. A confirmed diagnosis of CF as defined by clinical signs and symptoms of CF and at least one of the following:

   • A historical sweat chloride value >60 mmol/L
   • Two copies of a disease causing mutation in the CFTR gene

4. Clinically stable in the opinion of the Investigator

5. Percent predicted FEV1 ≥40% and ≤90% of the predicted normal for age, gender, and height at Screening

6. Resting oxygen saturation ≥92% on room air at Screening

Exclusion Criteria:

1. Initiation of any new chronic therapy (eg, CFTR modulator, hypertonic saline, inhaled antibiotic) or any change in chronic therapy (excluding pancreatic enzyme replacement therapy) within 28 days prior to the first dose
2. Hospitalization, sinopulmonary infection, CF exacerbation, or other clinically significant infection or illness 14 days prior to the first dose that, in the opinion of the Investigator, may confound study results
3. Treatment for Mycobacterium abscessus within 3 months prior to the first dose or more than two pulmonary exacerbations with a history of Burkholderia cenocepacia infection within 6 months prior to the first dose
4. Participation in another clinical study or treatment with an investigational agent 30 days or 5 half-lives, whichever is longer, prior to the first dose
5. History of or listed for solid organ transplantation
6. Any condition (including a history or current evidence of substance abuse or dependence, uncontrolled asthma, or is considered to be immunocompromised) 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 KB407
7. An active oral herpes infection 30 days prior to the first dose
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 Screening that the Investigator believes may interfere with the assessment of safety and/or efficacy of the study treatment
11. Subject has a known hypersensitivity to inhaled glycerol
12. Subject is known to be noncompliant or is unlikely to comply with the requirements of the study protocol in the opinion of the Investigator
13. Bronchoscopy participants only: Unable to tolerate bronchoscopy procedure and airway sampling, in the opinion of the Investigator
14. Cohort 4 participants only: Subjects who are ineligible for, do not tolerate, or do not benefit from modulator therapy