Study to Evaluate the Safety & Tolerability of MRT5005 Administered by Nebulization in Adults With Cystic Fibrosis

Inclusion Criteria:

- Confirmed diagnosis of CF as defined by all of the following:

- A sweat chloride value of ≥60 mmol/L by quantitative pilocarpine iontophoresis
(documented in the subject's medical record).

- Two CF disease-causing CFTR mutations in Class I or II (genotype confirmed at the
screening visit).

- Chronic sinopulmonary disease and/or gastrointestinal/nutritional abnormalities
consistent with CF disease.

- Clinically stable CF disease, as judged by the investigator.

- FEV1 ≥50% and ≤90% of the predicted normal for age, gender, and height at screening.

- Resting oxygen saturation ≥92% on room air (pulse oximetry).

Exclusion Criteria:

- An acute upper or lower respiratory infection, pulmonary exacerbation, clinically
significant episode of hemoptysis or change in chronic respiratory medications
(including antibiotics) for CF lung disease within 28 days prior to dosing with
investigational product on Day 1.

- Receiving treatment with ivacaftor monotherapy (KALYDECO).

- Subjects with a Class III, IV, or V CFTR gene mutation in at least 1 allele.

- Infection with highly virulent bacteria associated with accelerated decline in
pulmonary function and/or decreased survival (e.g., Burkholderia cenocepacia,
Burkholderia dolosa, Mycobacterium abscessus).

Treatment with ORKAMBI is not an exclusion for this study.