A randomised controlled trial of the safety, tolerability and effectiveness of Cayston Cayston (Aztreonam Lysine) compared to placebo in participants with bronchiectasis. Bronchiectasis not due to cystic fibrosis is a chronic inflammatory disease characterised by cough, sputum production and frequent respiratory tract infections. There are currently no licensed therapies for bronchiectasis approved by regulators in the United States or Europe. The disease has a high morbidity, particularly in the presence of chronic P. aeruginosa and other chronic Gram-negative infections. This trial will test the hypothesis that 12 months treatment with Aztreonam lysine for inhalation will be safe and well tolerated, and will result in a significant increase in the time to first pulmonary exacerbation in participants with bronchiectasis and a history of frequent exacerbations. This is a multi-centre randomised double-blind placebo controlled parallel group trial with four treatment arms. It will enroll 100 bronchiectasis patients with a history of at least 3 exacerbations in the previous year and the presence of chronic Gram-negative infection in sputum at screening. Patients will be treated following a one month on, one month off treatment regimen for 12 months. The primary objective is to evaluate the safety and tolerability of Aztrenam lysine in these patients by recording adverse events and trial treatment withdrawals.