Mechanisms of Glucose Lowering of Dipeptidyl Peptidase-4 Inhibitor Sitagliptin When Used Alone or With Metformin in Type 2 Diabetes: A double-tracer study

OBJECTIVE To assess glucose-lowering mechanisms of sitagliptin (S), metformin (M), and the two combined (M+S).RESEARCH DESIGN AND METHODS We randomized 16 patients with type 2 diabetes mellitus (T2DM) to four 6-week treatments with placebo (P), M, S, and M+S. After each period, subjects received a 6-h meal tolerance test (MTT) with [C-14]glucose to calculate glucose kinetics. Fasting plasma glucose (FPG), fasting plasma insulin, C-peptide (insulin secretory rate [ISR]), fasting plasma glucagon, and bioactive glucagon-like peptide (GLP-1) and gastrointestinal insulinotropic peptide (GIP) were measured.RESULTS FPG decreased from P, 160 +/- 4 to M, 150 +/- 4; S, 154 +/- 4; and M+S, 125 +/- 3 mg/dL. Mean post-MTT plasma glucose decreased from P, 207 +/- 5 to M, 191 +/- 4; S, 195 +/- 4; and M+S, 161 +/- 3 mg/dL (P < 0.01). The increase in mean post-MTT plasma insulin and in ISR was similar in P, M, and S and slightly greater in M+S. Fasting plasma glucagon was equal (approximate to 65-75 pg/mL) with all treatments, but there was a significant drop during the initial 120 min with S 24% and M+S 34% (both P < 0.05) vs. P 17% and M 16%. Fasting and mean post-MTT plasma bioactive GLP-1 were higher (P < 0.01) after S and M+S vs. M and P. Basal endogenous glucose production (EGP) fell from P 2.0 +/- 0.1 to S 1.8 +/- 0.1 mg/kg min, M 1.8 +/- 0.2 mg/kg min (both P < 0.05 vs. P), and M+S 1.5 +/- 0.1 mg/kg min (P < 0.01 vs. P). Although the EGP slope of decline was faster in M and M+S vs. S, all had comparable greater post-MTT EGP inhibition vs. P (P < 0.05).CONCLUSIONS M+S combined produce additive effects to 1) reduce FPG and postmeal plasma glucose, 2) augment GLP-1 secretion and -cell function, 3) decrease plasma glucagon, and 4) inhibit fasting and postmeal EGP compared with M or S monotherapy.