Clearing Lungs With Epithelial Sodium Channel (ENaC) Inhibition in Cystic Fibrosis (CF)

Inclusion Criteria: * Male or female, 12 years of age or older. * Diagnosis of cystic fibrosis as determined by the 1997 CF Consensus criteria (NIH Consensus Statement, 1997) * Non-smoker * FEV1 at Screening Visit 1 between 40% and 90% * Stable regimen of CF medications and chest physiotherapy for the 28 days prior to Screening. Must be willing to discontinue use of hypertonic saline for the duration of the study. * Clinically stable for at least 2 weeks * All females of child-bearing potential must have a negative serum pregnancy test and if sexually active must agree to practice a highly effective form of contraception throughout the study and for 28 days after the last dose of study medication. Exclusion Criteria: * History of any organ transplantation or any significant disease or disorder * Use of diuretics (including amiloride) or renin-angiotensin antihypertensive drugs or trimethoprim in the 28 days prior to Screening * History of significant intolerance to inhaled hypertonic saline, as determined by the Investigator * Known hypersensitivity to the study drug or amiloride * Any clinically significant laboratory abnormalities at Screening Visit 1 as judged by the investigator, or any of the following: * Potassium ≥ 5 milliequivalent per Liter (mEq/L) * Abnormal renal function * Abnormal liver function, defined as ≥ 3 x upper limit of normal (ULN) * Hemoglobin level \< 10.0 g/dL * Female who is pregnant or lactating * History of sputum or throat swab culture yielding Burkholderia species or Mycobacterium abscessus within 2 years of screening * Has previously participated in an investigational trial involving administration of any investigational compound or use of an investigational device with 28 days prior to Screening * Currently being treated with any ivacaftor containing regimen