DPP4 and diabetes mellitus: Insulin therapy and several classes of antidiabetic drugs (i.e., biguanides, dipeptidyl peptidase-4 (DPP-4) inhibitors, meglitinides, sodium-glucose cotransporter-2 (SGLT2) inhibitors, sulfonylureas, and thiazolidinediones (TZDs) are currently available to treat diabetes and reduce the incidence of vascular complications [7,8,9].