This randomised, double-blind, placebo-controlled study will evaluate the efficacy and safety of ADC3680 administered once daily as an add-on therapy to inhaled corticosteroids and when co-administered with montelukast in patients with inadequately-controlled asthma. Patients will be randomised to 3 Arms to receive ADC3680, placebo or montelukast.

This is a multi-centre, randomised, placebo-controlled, double blind, parallel group 3-arm study (including montelukast as an active comparator) designed to compare the efficacy and safety of a once daily dose of ADC3680 with placebo in subjects with inadequately-controlled asthma despite receiving a low to moderate dose of an ICS controller therapy, over a 10 week treatment period. At the end of the 10 week treatment period open label montelukast (10 mg) will be added to ADC3680 and placebo arms for a two week extension period to assess whether efficacy of ADC3680 is enhanced by the addition of montelukast compared with montelukast alone. The montelukast arm will continue with 10 mg montelukast alone.

Inclusion Criteria:

• Men and women aged 18 years to 50 years (inclusive)
• Diagnosis of asthma (GINA 2011) including a demonstration of reversible airway obstruction
• Pre-bronchodilator FEV1 value ≥ 40% and ≤ 85% of the predicted normal value at baseline
• A score of 1.5 or greater on the Asthma Control Questionnaire at baseline
• Daily use of low to moderate dose of ICS (equivalent to budesonide ≤ 800 µg per day)
• Prescribed a short-acting inhaled bronchodilator as reliever therapy for relief of symptoms
• A peripheral blood eosinophil count ≥ 0.25 x 109/L
• Non-smoker or former smoker who has not smoked in the last six months
• Body mass index (BMI) ≥ 17 and ≤ 35 kg/m2
• Able to comply with the protocol requirements, instructions and restrictions
• Able to provide signed and dated written informed consent

Exclusion Criteria:

• Subjects with severe asthma exacerbation in the 4 weeks prior to consent
• Subjects with respiratory tract infection in the 4 weeks prior to consent
• Subjects with COPD or other relevant lung diseases
• Subjects with clinically significant condition which may compromise subject safety or interfere with study evaluation

Other protocol-defined inclusion/exclusion criteria may apply.