 To evaluate the clinical efficacy, safety, tolerability and dose-response relationship, using oral corticosteroid OCS modulation, of 3 different doses of VR506 using a twice daily regimen from a new dry powder inhaler nDPI for 16 weeks in subjects with severe persistent asthma requiring OCS therapy, i.e. Step 5 treatment as defined by modified Global Initiative for Asthma GINA guidelines 2011. 