Potent, orally active dipeptidyl peptidase (DPP)-IV inhibitor (Ki = 11 nM, IC50 = 14 nM) that displays > 15 000-fold selectivity over DPP-II and a range of proline-cleaving proteases. Exhibits antidiabetic activity in vivo; improves glucose tolerance, increases glucagon-like peptide 1 (GLP-1) and insulin levels, augments insulin secretion and GLUT-2 levels, and reduces (preserves) islet size.