DPP4 and diabetes mellitus: Sitagliptin is a highly selective inhibitor of dipeptidyl peptidase-4 (DPP-4) and is used for the treatment of diabetes by inhibiting the activity of DPP-4 and prolonging the bioactivity of glucagon-like peptide −1 (GLP-1) and gastric inhibitory polypeptide (GIP) [13].