This is a Phase 1b/2a study with the primary objective to determine if BX004-A is safe and tolerable. Exploratory objectives include whether BX004-A reduces sputum Pseudomonas aeruginosa (PsA) bacterial load in CF subjects with chronic PsA pulmonary infection.

This is a randomized, double-blind, placebo-controlled, multicenter study to evaluate the safety and tolerability of BX004-A in CF subjects with chronic PsA pulmonary infection. The study is divided into two parts, a single-ascending and multiple-dose phase (Part 1) and a multiple dose phase (Part 2). Subjects in both parts will be included in a 6-month safety follow-up. A Data Safety Monitoring Board will monitor safety in both parts. The purpose of the study is to evaluate safety and tolerability of BX004-A, and whether BX004-A reduces the PsA burden in the sputum of CF subjects with chronic PsA pulmonary infection. Clinically stable CF subjects with a confirmed diagnosis of CF and chronic PsA pulmonary infection will be enrolled.

Key Inclusion Criteria:

• Cystic fibrosis patients with chronic Pseudomonas aeruginosa pulmonary infection receiving standard of care CF medications
• Age ≥ 18 years
• FEV1 ≥ 40% predicted
• Clinically stable lung disease
• Willing and able to provide adequate sputum samples, using any method (spontaneously expectorated, induced, from home or clinic) at designated study visits.

Key Exclusion Criteria:

• Known hypersensitivity to bacteriophages or excipients in the formulation.
• Receipt of prior bacteriophage therapy within the 6 months prior to Screening
• Recovery of Burkholderia species from respiratory tract within 1 year prior to screening
• Currently receiving treatment for allergic bronchopulmonary aspergillosis
• Currently receiving treatment for active infection with non-tuberculous mycobacteria
• History of severe neutropenia
• History of lung transplant
• History of solid organ transplant
• Acquired or primary immunodeficiency syndrome
• Initiation or change in CF modulator therapy less than 3 months prior to screening
• Pregnant or breastfeeding female