This study will consist of a prospective, open-label, randomized, controlled trial conducted in 24 patients with cystic fibrosis. Twenty subjects will be stratified in a 1:2 ratio based on baseline FEV1 into mild (\> 70%) or moderate (40-70%) pulmonary disease in order to control for disease severity within each dose level. The subjects will be randomized in blocks of four to receive no drug, 40mg, 100mg, or 200mg daily for 28 days.
Sputum samples will be obtained in all groups by induction with hypertonic saline at baseline, 8, 24, and 48 hours following the first dose and then weekly for 4 weeks. Sputum will also be collected at two follow up visits after the treatment period at weeks 5 and 6.
In the doxycycline group, serial blood samples (5 mL) for determination of doxycycline concentrations will be obtained before and at 0, 0.5, 1, 2, 4, 12, 24, and 48 hours following the 1-hr infusion of a single IV dose. Once daily dosing of doxycycline will resume immediately following the 48-hour blood sample and will continue until day 28. Additional levels will be obtained pre-dose, and 1, 2, and 3 hours after doses administered on days 14 and 28. A sample of blood will be obtained at baseline, and at days 28 for inflammatory marker analyses.
