At least 6 cystic fibrosis patients with the S1251N mutation will be treated for 4 weeks, consisting of two consecutive treatment periods of two weeks evaluating one dose of GLPG1837 each. After the treatment period, there is a 7-10 days follow-up period.

During the course of the study, subjects will be examined for any side effects that may occur (safety and tolerability).

Changes in sweat chloride will be assessed as biomarker from baseline onwards, and changes in pulmonary function (efficacy) will be explored throughout the study. The amount of GLPG1837 present in the blood (pharmacokinetics) will also be determined.

Inclusion Criteria:

• Male or female subjects ≥ 18 years of age, with a confirmed diagnosis of cystic fibrosis
• Subjects with gating S1251N CFTR mutation on at least one allele in the CFTR gene
• Subjects currently receiving treatment with ivacaftor on a stable regimen or not on a treatment regimen with ivacaftor, for at least 2 weeks prior to screening
• Weight ≥ 40.0 kg
• Subjects on stable concomitant treatment regimen for at least 4 weeks prior to baseline (excluding ivacaftor)
• Pre- or post-bronchodilator FEV1 ≥ 40% of predicted normal
• Subject will have to use highly effective contraceptive methods

Exclusion Criteria:

• On an ivacaftor-containing treatment regimen and unable or unwilling to discontinue ivacaftor for the washout and treatment periods of the study
• Concomitant use of antifungal drugs within 4 weeks of baseline
• A history of a clinically meaningful unstable or uncontrolled chronic disease
• Liver cirrhosis and portal hypertension
• Any significant change in the medical regimen for pulmonary health within 4 weeks of baseline
• Unstable pulmonary status or respiratory tract infection or changes in therapy for pulmonary disease within 4 weeks of baseline
• Abnormal liver function
• Clinically significant abnormalities on ECG
• History of malignancy, solid organ/haematological transplantation
• Abnormal renal function
• Participation in another experimental therapy study within 30 days or 5 times half-life