Subjects on a cyclical on/off anti-Pseudomonal antibiotic regimen will enter the Screening period at a time such that the Baseline visit coincides with the end of their anti-Pseudomonas antibiotic cycle. Study drug will thereby be administered during the off-cycle, and subjects can then resume anti-Pseudomonal therapy during the 28-day observation period. Subjects continuing alternating anti-Pseudomonal therapy can continue their treatment during the study drug administration, and observation period.