 The purpose of this study is to assess the safety and tolerability of multiple oral doses of CTX-4430 when administered to cystic fibrosis CF patients once-daily QD for 15 days.This study will include two dose levels. For each dose level, blood samples will be collected for exploratory PK and PD assay validation. In addition, sputum will be collected for exploratory biomarker analysis. Following multiple dose administration, pulmonary function and exploratory lung clearance index LCI measurements will be taken. 