Effect of Miglustat on the Nasal Potential Difference in Patients With Cystic Fibrosis Homozygous for the F508del Mutation

Inclusion Criteria: Inclusion criteria at screening visit (Visit 1): * Aged 18 years and older * Male or female * Women of childbearing potential must: * have a negative serum pregnancy test at Visit 1 * agree to use from Visit 1 until 3 months after the last study drug intake a reliable method of contraception * Male patients accepting for the duration of the study and for 3 months thereafter to use a condom * Homozygous for the F508del mutation as confirmed by genetic testing * Sweat chloride ≥ 60 mmol/L * Basal nasal potential difference (NPD) ≤ -30.0 mV (equal to or more electrically negative than -30.0 mV) and total chloride secretion (TCS) ≥ - 5.0 mV for at least one nostril. However, if it is possible to analyze both nostrils, the total chloride secretion (TCS) is to be ≥ - 5.0 mV (equal to or more electrically positive than - 5.0 mV) in both nostrils. * FEV1 ≥ 25% of predicted * Able to comply with all protocol requirements * Signed informed consent prior to any study-mandated procedure Inclusion criteria at randomization visit (Visit 2): * Women of child-bearing potential must have a negative urine pregnancy test * Basal nasal potential difference (NPD) ≤ - 30.0 mV (equal to or more electrically negative than - 30.0 mV) and total chloride secretion (TCS) ≥ - 5.0 mV for at least one nostril. However, if it is possible to analyze both nostrils, the total chloride secretion (TCS) is to be ≥ - 5.0 mV (equal to or more electrically positive than - 5.0 mV) in both nostrils. Exclusion Criteria: * Any condition prohibiting the correct measurement of the NPD such as upper respiratory tract infection * Acute upper or lower respiratory tract infection requiring antibiotic intervention within 2 weeks of screening * Lung transplant recipient or patient on a lung transplant waiting list * Any modification in regular treatments (new treatment initiated or discontinued treatment) or modification in dosing within 2 weeks prior to start of Period 1 * Moderate/Severe renal impairment (creatinine clearance \< 70 mL/min as per Cockroft and Gault) * Systemic corticosteroids (\> 10 mg/day prednisone or equivalent) within 14 days prior to screening and up to start of study * Women who are breast-feeding, pregnant, or who plan to become pregnant during the course of the study * History of significant lactose intolerance * Presence of clinically significant diarrhoea (\> 3 liquid stools per day for \> 7 days) without definable cause within one month prior to screening * Any known factor or disease that might interfere with treatment compliance, study conduct or interpretation of the results such as drug or alcohol dependence or psychiatric disease * Active or passive smoking * Hypersensitivity to Miglustat or any excipients * Planned treatment or treatment with another investigational drug or therapy (e.g., gene therapy) within one month prior to randomization * Known concomitant life-threatening disease with a life expectancy \< 12 months * Indication against Isuprel® (Isoproterenol) including heart diseases.