The purpose of this trial is to investigate if treatment with house dust mite allergen immunotherapy tablet can reduce the risk of asthma exacerbation in subjects with house dust mite induced asthma.

Key Inclusion Criteria:

• A clinical relevant history consistent with house dust mite induced asthma of at least 1 year prior to trial entry.
• Use of an appropriate amount of inhaled corticosteroid for the control of asthma symptoms.
• Documented reversible airway obstruction.
• Suitable level of asthma control.
• FEV1 ≥ 70% of predicted value.
• Positive Skin Prick Test response to Der pte and/or Der far.
• Positive specific IgE against Der pte and/or Der far (≥ IgE Class 2; ≥ 0.70 KU/L).

Key Exclusion Criteria:

• A clinical history of persistent allergic asthma or rhinitis caused by an allergen to which the subject is regularly exposed and sensitised (except house dust mites).
• A clinical history of intermittent allergic asthma or rhinitis if the seasonal allergen is causing symptoms in the period from October to March.
• Any clinically relevant chronic disease.