Study was terminated after completion of Part 1, because Part 2 was no longer appropriate.

Assessment of the safety, tolerability and early signs of efficacy of three times a day orally administered BAY63-2521 in adult delta F508 homozygous Cystic Fibrosis patients not on treatment with Orkambi

The study consists of two parts. The first part is double-blind, randomized and placebo-controlled. The second part has an open-label study design. In part 1 patients on Orkambi or other CFTR-modulators are excluded. In part 2 patients on Orkambi are allowed to be included under certain conditions.

Inclusion Criteria:

• Signed informed consent available before any study specific tests or procedures are performed
• Patients must be at least 18 years of age at time of inclusion (i.e. upon signature of informed consent)
• Patient diagnosed with Cystic Fibrosis according to standard criteria (i.e. either elevated sweat chloride content above 60 mmol/ L and/ or genetic testing)
• Patient is homozygous for the deltaF508 mutation
• Patient has a mild-to-moderate stage of lung disease as determined by FEV1 (FEV1 between 40 and 100% predicted)
• Patient has a stable condition of lung disease (no ongoing or recent pulmonary exacerbation and no change in current treatment) within the last 4 weeks prior to screening
• Ability and willingness to understand and follow study procedures for the entire study
• Patients do not smoke. Patients with a history of smoking can be included, if they have refrained from smoking for the last 3 months. If a patients starts smoking during the study participation, he/ she needs to be excluded and considered to be a drop out
• Body mass index (BMI): ≥ 16 kg/ m² (calculated by dividing the patient's weight by the square of his/ her height [kg/ m2])

Inclusion criterion valid for study part 1 only:

- Women of childbearing potential must agree to use adequate contraception when sexually active. 'Adequate contraception' is defined as one highly effective form of contraception (intrauterine devices [IUD], contraceptive implants or tubal sterilization) or a combination of methods (hormone method with a barrier method ). If a partner's vasectomy is the chosen method of contraception or if a partner has documented azoospermia, a hormone or barrier method must be used in combination. Adequate contraception is required from the signing of the informed consent form up until 4 weeks after the last study drug administration

Inclusion criteria valid for study part 2 only:

• Women of childbearing potential must agree to use adequate contraception when sexually active. 'Adequate contraception' is defined as one highly effective form of contraception (intrauterine devices [IUD], contraceptive implants or tubal sterilization) or a combination of methods (hormone method with a barrier method). For patients on Orkambi hormonal methods (including hormonal oral contraceptives) cannot be accepted in this study. They need to choose non-hormonal methods. If a partner's vasectomy is the chosen method of contraception or if a partner has documented azoospermia, a hormone or barrier method must be used in combination. Adequate contraception is required from the signing of the informed consent form up until 4 weeks after the last study drug administration
• Patients receiving Orkambi (Lumcaftor + Ivacaftor) as part of their standard care need to be on stable Orkambi treatment for at least 3 months prior to screening (patients on Lumacaftor and/or Ivacaftor are excluded in part 1)

Exclusion Criteria:

• Patients with Cystic Fibrosis with any background other than homozygous deltaF508 mutation
• Exclusion criterion only valid for study part 1: Patients receiving treatment with Lumacaftor and/ or Ivacaftor
• Active state of hemoptysis or pulmonary hemorrhage, including those events managed by bronchial artery embolization. Also any history of moderate hemoptysis within the 3 months prior to inclusion
• Any history of pneumothorax, bronchial artery embolization or massive hemoptysis. Massive hemoptysis being defined as acute bleeding >240 mL in a 24-hour period or recurrent bleeding >100 mL/ d over several days
• A positive sputum culture for Burkholderia cenocepacia, Burkholderia dolosa, and/ or Mycobacterium abscessus either currently or within the previous year
• Active allergic broncho-pulmonary aspergillosis
• Current pulmonary exacerbation
• Known history of solid organ transplantation
• Known history of any form of pulmonary hypertension
• Clinically relevant deviations of the screened laboratory parameters from reference ranges outside of expected changes for Cystic Fibrosis patients, especially a hemoglobin value below 110 g/L or a creatinine clearance based on the Cockcroft-Gault formula < 15 ml/ min

PI(s) shall provide to Bayer any proposed publication or oral presentation relating to the Study or the Study Drug or the Results at least sixty (60) days prior to the intended submission or presentation of the Publication in order to allow Bayer to review it.