An orally bioavailable DPP-4 inhibitor (IC50 = 6.9 nM) that is selective for DPP-4 over DPP-2, DPP-8, DPP-9, prolyl endopeptidase, FAP, and tryptase (IC50s = >100,000 nM); inhibits DPP-4 activity in vivo in rats, dogs, and cynomolgus monkeys (EC50s = 3.4, 4.9, and 5.6 ng/ml, respectively, in plasma); increases plasma GLP-1 and insulin levels and decreases blood glucose levels during an oral glucose challenge in Zucker fa/fa rats (10 mg/kg); decreases plasma DPP-4 activity and increases GLP-1 levels in diabetic ob/ob mice when administered for 29 days (2.8 mg/kg per day)