The aim of this study in subjects with obstructive sleep apnea is to investigate pharmacodynamics, safety and tolerability after a single nasal administration of BAY2253651 and to evaluate first safety and tolerability of multiple dosing over 5 consecutive nights in OSA patients.

Inclusion Criteria:

• Subjects need to be diagnosed with OSA (obstructive sleep apnea) but should be otherwise healthy at the discretion of the investigator.
• Patients must be pretreated with CPAP (continuous positive airway pressure) for OSA for at least 3 months before randomization.
• AHI of 15-50 per hour after 48 hours of CPAP withdrawal documented by baseline PSG (polysomnography, evaluated by the site staff) and at least 4 hours of total sleep time. (One retesting allowed).
• Female subjects must be of non-childbearing potential, i.e. post-menopausal (no menses for at least 1 year prior to randomization) or surgically sterile (tubal ligation, hysterectomy or bilateral oophorectomy)
• Men of reproductive potential together with their female partner(s) must agree to use at least two adequate contraception methods when sexually active. This applies for the time period between signing of the informed consent form and 3 months after the last administration of study drug.

Acceptable methods of contraception include, but are not limited to, (i) condoms (male or female) with or without a spermicidal agent; (ii) diaphragm or cervical cap with spermicide; (iii) intra-uterine device; (iv) hormone-based contraception

• Only Part B: Patients having completed Part A and had valid PSG after dosing in Part A.

Exclusion Criteria:

• Inability to comply with planned study procedures or to comply with study protocol requirements; this includes completing required data collection and attending required follow up study visits.
• Neck circumference above/equal 44 cm.
• Not predominantly obstructive sleep apnea evidenced by baseline PSG.
• Severely impaired breathing within two days prior to randomization (e.g. acute nasal congestion during upper airway infection).
• Subject with known allergies or hypersensitivities to the study drugs (active substances or excipients of the preparations). Known severe respiratory tract allergies e.g. allergic asthma.
• Intake of a nasal decongestant during the intervention time (48 hours before visit 1 until end of visit 2).
• Use of any topical medication containing local anesthetics for nose and throat within 7 days before first investigational medicinal product (IMP) administration.
• Participation in another trial with an investigational drug within 30 days or 5 half-lives of the investigational drug, whichever is longer before first application of study drug or concomitant participation in another clinical study with investigational medicinal product(s).
• Any other condition, which would make the subject unsuitable for this study and will not allow participation for the full planned study period (e.g. active malignancy or other condition limiting life expectancy to less than 12 months)
• Known history of severe heart failure (NYHA 3-4) or severe COPD (GOLD 3-4).
• Heavy smoking, i.e. more than 20 cigarettes per day and/or unable to stop smoking during the stay in the sleep laboratory.
• Suspicion of drug or alcohol abuse.
• Regular daily consumption of more than 1 L of xanthine-containing beverages.