This pilot study will be conducted in adolescents with cystic fibrosis (CF) without diabetes but with abnormal glucose tolerance, and will assess the effects of sitagliptin on glucose regulation. An oral glucose tolerance test (OGTT) and a mixed meal tolerance test (MMTT), will be performed at baseline and again ~4 weeks after treatment with study drug. We will also look at blood sugars throughout the day using a continuous glucose monitor (CGM) before each time the MMTT/OGTT are performed. Several hormones that may affect the way the body regulates blood sugars will be measured in blood when the OGTT and MMTT are done. We will assess the effect this medicine has on blood sugars (using CGM) and the effect the medicine has on the hormones measured during the OGTT and MMTT.

Inclusion Criteria:

• Patients with CF between 13 and <21 yrs old
• Known impaired or indeterminate glucose tolerance (based on a prior OGTT)
• No history of CFRD

Exclusion Criteria:

• Insulin use in the last two months
• Acute pulmonary exacerbation / oral corticosteroid use in the last 6 weeks
• History of pancreatitis in the last 12 months
• Skin rashes or conditions that may affect CGM placement and wear
• Pregnancy or intent on becoming pregnant
• Patients on growth hormone therapy
• Renal insufficiency with creatinine clearance <50 ml/min (based on Schwartz formula)