Development of dipeptidyl peptidase- 4 inhibitors. Dipeptidyl peptidase- 4 inhibitor, Dipeptidyl peptidase- 4, Diabetes mellitus type 2, Hypoglycemia, Gastric inhibitory polypeptide, Crystallography, Takeda Pharmaceutical Company, Vildagliptin, Clinical trial, Sitagliptin, Berberine.