Study of Aztreonam for Inhalation in Children With Cystic Fibrosis and New Infection of the Airways by Pseudomonas Aeruginosa Bacteria

Key Inclusion Criteria: * Diagnosis of cystic fibrosis (CF) as determined by the 2008 CF Consensus Conference criteria: Sweat chloride level ≥ 60 milliequivalents per liter (mEq/L) by quantitative pilocarpine iontophoresis; or a genotype with 2 identifiable mutations consistent with CF; or an abnormal nasal transepithelial potential difference (NPD), and 1 or more clinical features consistent with CF * Documented new onset of positive respiratory tract culture for PA within 30 days of screening defined as either first lifetime documented PA-positive culture, or PA recovered after at least a 2-year history of PA-negative respiratory cultures (at least 2 cultures per year) * Forced expiratory volume in one second (FEV1) ≥ 80% predicted (for subjects ≥ 6 years of age who can reliably perform spirometry assessments) * Clinically stable with no evidence of acute significant respiratory symptoms that would require administration of intravenous (IV) antipseudomonal antibiotics, oxygen supplementation, or hospitalization Key Exclusion Criteria: * Use of IV or inhaled antipseudomonal antibiotics within 2 years of screening * Use of oral antipseudomonal antibiotics for a respiratory event within 30 days of study entry (screening visit) * History of intolerance to inhaled short acting β2 agonists * History of lung transplantation * Current requirement for daily continuous oxygen supplementation or requirement of more than 2 L/minute at night * Hospitalization for a respiratory event within 30 days prior to screening * Changes in bronchodilator, corticosteroid, dornase alfa, or hypertonic saline medications within 7 days prior to screening. * Significant changes (per investigators discretion) in physiotherapy technique or schedule within 7 days prior to screening * Abnormal renal or hepatic function results at most recent test within the previous 12 months, defined as Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \> 5 times upper limit of normal (ULN), or Serum creatinine \> 2 times ULN for age * Presence of a condition or abnormality that would compromise the subject's safety or the quality of the study data, in the opinion of the Investigator * Known hypersensitivity to aztreonam, its metabolites, or formulation excipients in AZLI * Respiratory cultures performed within 24 months prior to screening that are positive for ANY Burkholderia spp. or Non-tuberculous mycobacteria (NTM) Note: Other protocol defined Inclusion/Exclusion criteria may apply