This study will assess the efficacy and safety of ACT-129968 in subjects with partly controlled asthma on reliever therapy only.

Inclusion Criteria:

• Signed informed consent prior to any study-mandated procedure
• Males and females age 18 to 65 years
• Women of childbearing potential must use adequate contraception
• Presenting with a diagnosis of asthma according to GINA Guidelines
• Pre-bronchodilator forced expiratory volume in one second (FEV1) < / = 85% of patient's predicted normal value
• Reversibility of airway obstruction of > / = 12% and > / = 200mL from pre-bronchodilator FEV1
• ACQ score > / = 1.5

Exclusion Criteria:

• History of life-threatening asthma
• Any asthma exacerbation requiring treatment with systemic corticosteroids within the last 3 months
• Ongoing or recent treatment with medication for allergic airway disease
• Smoking within the last year, or life-time consumption > / = 10 pack-years (e.g., 20 cigarettes/day for 10 years)
• History of chronic pulmonary disease (other than asthma), such as chronic obstructive pulmonary disease (COPD), fibrosis, tuberculosis or sarcoidosis
• Pregnant or lactating women
• Diagnosis of aspirin or non-steroidal anti-inflammatory drug (NSAID)-induced asthma
• Any hospital admission for asthma within the last 6 months
• Anti-IgE therapy at any time check/update interventions by explicitely writing the experimental drug dosage in the interventions description
• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol