Sitagliptin is a dipeptidyl peptidase-4 (DPP4) inhibitor [1].

DPP4 is an antigenic enzyme expressed on the surface of most cell types associated with immune regulation, signal transduction and apoptosis. DPP4 is an intrinsic membrane glycoprotein and a serine exopeptidase that cleaves X-proline dipeptides from the N-terminus of polypeptides [2].

In vitro: Sitagliptin was a potent inhibitor for DPP-4 with an IC50 of 18 nM [1]. Sitagliptin inhibited DPP-8 with an IC50 of 48 μM. Sitagliptin showed no effect on several related peptidases, including DPP-9, DPP-II, and amino peptidase P [1].

Clinical trials: In patients with type 2 diabetes, once-daily sitagliptin monotherapy improved glycemic control in the fasting and postprandial states, improved measures of β-cell function. Sitagliptin was well tolerated [3]. Sitagliptin did not increase the risk of major adverse cardiovascular events, hospitalization for heart failure, or other adverse events [4].

References:
[1] Biftu T, Feng D, Qian X, et al.  (3R)-4-[(3R)-3-Amino-4-(2, 4, 5-trifluorophenyl) butanoyl]-3-(2, 2, 2-trifluoroethyl)-1, 4-diazepan-2-one, a selective dipeptidyl peptidase IV inhibitor for the treatment of type 2 diabetes[J]. Bioorganic & medicinal chemistry letters, 2007, 17(1): 49-52.
[2] Fleischer B.  CD26: a surface protease involved in T-cell activation[J]. Immunology today, 1994, 15(4): 180-184.
[3] Aschner P, Kipnes M S, Lunceford J K, et al.  Effect of the dipeptidyl peptidase-4 inhibitor sitagliptin as monotherapy on glycemic control in patients with type 2 diabetes[J]. Diabetes care, 2006, 29(12): 2632-2637.
[4] Green J B, Bethel M A, Armstrong P W, et al.  Effect of sitagliptin on cardiovascular outcomes in type 2 diabetes[J]. New England Journal of Medicine, 2015, 373(3): 232-242.