This is a randomized, double-blind, placebo-controlled, multicenter study to evaluate BX004 in CF subjects with chronic PsA pulmonary infection. The main purpose of the study is to evaluate whether BX004 reduces the PsA burden in the sputum of CF subjects with chronic PsA pulmonary infection. Secondary endpoints are to see how well BX004 works in improving lung function and quality of life, reducing the amount of PsA in the sputum, getting negative sputum cultures for PsA, and safety and tolerability. Clinically stable CF subjects with a confirmed diagnosis of CF and chronic PsA pulmonary infection will be enrolled. Subjects will be included in a 6-month post-dose safety follow-up. A Data Safety Monitoring Board of the CF Foundation will monitor safety.