This study is designed to explore the efficacy and safety of nasal aztreonam administered using the Pari Sinus Nebulizer combined with oral Cayston aerosol therapy compared to placebo on clinical and laboratory endpoints such as risk of antibiotic-resistant Pseudomonas aeruginosa (PA), time to pulmonary infection exacerbation, nasal quality of life, pulmonary function, nasal and lower airway cultures, and properties of mucus.