A potent, selective, reversible, and slow binding dipeptidyl peptidase IV inhibitor (IC₅₀ = 14 nM). Displays >15,000 fold selectivity over DPP-II and a range of Proline-cleaving proteases. Displays anti-diabetic activity in vivo, improves glucose tolerance, increases glucagon-like peptide 1 (GLP-1) and insulin levels, and augments insulin secretion and GLUT-2 levels.