- Initiation of cycled, inhaled tobramycin (TOBI) less than 4 months prior to Screening (Visit 1). Note: Chronic TOBI users are allowed to participate in this study, but subjects who have recently initiated chronic TOBI should have at least 2 cycles of TOBI in the preceding 4 months before being enrolled in this study. Chronic TOBI subjects should be starting an 'off- TOBI' period at Day 1 (Visit 2) so there will be no concomitant dosing of TOBI and assigned study medication.