Primary Outcome Measures:
- Change from baseline in insulin secretion capacity at 16 weeks [ Time Frame: baseline and 16 weeks ]
To compare insulin secretion and maximal insulin secretory capacity prior to initiation of ivacaftor and after 16 weeks of ivacaftor treatment in subjects with CF and at least one G551D CFTR mutation, or other CFTR gating mutation, and to explore the impact of ivacaftor upon incretin secretion, incretin regulation of insulin secretion, and glucose excursion during a mixed meal tolerance test in CF.
Secondary Outcome Measures:
Cystic Fibrosis Related Diabetes (CFRD) is associated with worse nutritional status, greater pulmonary function decline, and increased mortality, highlighting its relevance in Cystic Fibrosis (CF). CFRD arises primarily from compromised insulin secretion - traditionally considered a by-product of pancreatic exocrine tissue damage and fibrosis. Recent developments in the field of diabetes are propelling a re-examination of this basic explanation. The impact of the cystic fibrosis transmembrane conductance regulator (CFTR) potentiator, ivacaftor, upon insulin secretion and glucose regulation has not been examined, but improved glucose tolerance has been appreciated anecdotally. This study aims to understand the impact of ivacaftor therapy upon blood glucose and insulin and incretin secretion.