This is a double-blind, randomized, parallel arm, controlled, multicenter, and interventional clinical trial. Potential subjects will sign the informed consent form (ICF) and be assessed for eligibility. After satisfying all inclusion & exclusion criteria, subjects will be given a mannitol tolerance test (MTT). Those subjects that pass the MTT will be randomized to receive inhaled mannitol (400 mg b.i.d.) or control b.i.d. for a period of 26-weeks.