title: Mechanisms of glucose lowering of dipeptidyl peptidase-4 inhibitor sitagliptin when used alone or with metformin in type 2 diabetes: a double-tracer study.
abstract: OBJECTIVE
  To assess glucose-lowering mechanisms of sitagliptin (S), metformin (M), and the two combined (M+S).
  
  
  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 [(14)C]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 (≈ 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 β-cell function, 3) decrease plasma glucagon, and 4) inhibit fasting and postmeal EGP compared with M or S monotherapy.
authors.affiliation: University of Texas Health Science Center, Texas Diabetes Institute, San Antonio, Texas, USA.
authors.forename: Carolina
authors.initials: C
authors.lastname: Solis-Herrera
authors.forename: Curtis
authors.initials: C
authors.lastname: Triplitt
authors.forename: Jose de Jesús
authors.initials: Jde J
authors.lastname: Garduno-Garcia
authors.forename: John
authors.initials: J
authors.lastname: Adams
authors.forename: Ralph A
authors.initials: RA
authors.lastname: DeFronzo
authors.forename: Eugenio
authors.initials: E
authors.lastname: Cersosimo
mesh_terms: D001786:Blood Glucose / Q000378:metabolism; D002096:C-Peptide / Q000097:blood; D003924:Diabetes Mellitus, Type 2 / Q000097:blood* / Q000188:drug therapy*; D054873:Dipeptidyl-Peptidase IV Inhibitors / Q000627:therapeutic use*; D005215:Fasting / Q000097:blood; D005749:Gastric Inhibitory Polypeptide / Q000097:blood; D052216:Glucagon-Like Peptide 1 / Q000097:blood; D006801:Humans; D007004:Hypoglycemic Agents / Q000627:therapeutic use*; D007328:Insulin / Q000097:blood; D008687:Metformin / Q000627:therapeutic use*; D011719:Pyrazines / Q000627:therapeutic use*; D000068900:Sitagliptin Phosphate; D014230:Triazoles / Q000627:therapeutic use*
chemical_list: D001786:Blood Glucose; D002096:C-Peptide; D054873:Dipeptidyl-Peptidase IV Inhibitors; D007004:Hypoglycemic Agents; D007328:Insulin; D011719:Pyrazines; D014230:Triazoles; D005749:Gastric Inhibitory Polypeptide; D052216:Glucagon-Like Peptide 1; D008687:Metformin; D000068900:Sitagliptin Phosphate