Safety and Efficacy Study of Sublingual Immunotherapy (SLIT) to Treat House Dust Mite Allergic Rhinitis

Inclusion Criteria: 1. Male or female outpatients aged 18 to 50 years (inclusive). 2. Patients who have been informed of the nature and aims of the study and have given their written consent 3. Patients must be in general good health as determined by past medical history, physical examination and safety laboratory tests. 4. Female patients of childbearing potential are eligible 5. Negative urine pregnancy test on female patients of childbearing potential. 6. House dust mite-related allergic rhinitis for at least 1 year. 7. Sensitised to D. pteronyssinus or D. farinae (positive SPT with wheal diameter greater than 3 mm and a specific IgE level ≥ 0.7 kU/L). 8. Baseline ARTSS \> 5 (after completion of the 7-day daily record card). 9. Patients who are willing to comply with the protocol. 10. Patients who are able to understand the information given and the text of the consent form, and who are able to complete the daily record card and the RQLQ. Exclusion Criteria: 1. Whatever the considered allergens, co-sensitisation leading to clinically relevant allergic rhinitis, sinusitis, conjunctivitis or asthma likely to significantly change the symptoms of the patient throughout the study (that means patient symptomatic to another allergen than house dust mites). 2. Patients with any nasal condition that could confound the efficacy or safety assessments (for example nasal polyposis). 3. Patients sensitised to cat or dog allergens and living with these animals at home. 4. Pregnant, breast-feeding / lactating or sexually active women of childbearing potential who are not using a medically accepted contraceptive method. 5. Asthma requiring treatment other than beta-2 inhaled agonists. Patients with intermittent asthma not necessitating inhaled or systemic corticoid treatment may be included (corresponding to the Global Initiative for Asthma \[GINA\] Step 1). 6. Patients treated with systemic, nasal or inhaled steroids (whatever the indication) within 4 weeks before Visit 1. 7. Patients treated with long acting systemic steroids (whatever the indication) within 12 weeks before Visit 1 and before Visit 2. 8. FEV1 \< 80% of predicted value at Visit 1. 9. Patients who received allergy specific immunotherapy for house dust mites in the last 10 years. 10. Patients at risk of non-compliance. 11. Participation in any clinical study within the 12 weeks before Visit 1. 12. Investigators, co-investigators, as well as their children or spouses and all the study collaborators should not be enrolled in the study. 13. Any change in environmental measures for allergen avoidance during the study.