INS and type 2 diabetes mellitus: A recent retrospective study performed for 2 years in T2DM that aimed to determine the hypoglycemic effect of 2 years of sitagliptin administration revealed that the HbA1c levels decreased and C-peptide immunoreactivity (CPR) index increased from baseline to 3, 6, 12, 18, and 24 months after sitagliptin initiation [266], suggesting that sitagliptin improves glycemic control via an improved intrinsic insulin response.