Phase 3 Study to Compare the Efficacy and Safety of Gemigliptin and Vildagliptin as Add-on Therapy to Metformin in People With Type 2 Diabetes Inadequately Controlled With Metformin

Inclusion criteria: * Patients with Type 2 diabetes mellitus. * Patients inadequately controlled with metformin at the maximal effective and tolerated dose of metformin for at least 12 weeks or who are on a stable dose of ≥1500 mg daily for a minimum of ≥4 weeks prior to study entry. * Signed informed consent obtained prior to any study procedures. Exclusion criteria: * Patients with type 1 diabetes mellitus, gestational diabetes, or diabetes secondary to pancreatic disorders, drug or chemical agent intake. * Male and female younger than 18 and older than 75 years old. * Patients with HbA1c ≤7% and ≥9.5%. * Patients with 20 kg/m\^2 \> body mass index (BMI) \>40 kg/m\^2. * Patients with history of hypersensitivity to metformin. * Patients with history of hypersensitivity to vildagliptin. * Lactose intolerance, lactase deficit, glucose-galactose malabsorption. * Patients who have a history of acute metabolic complications such as lactic acidosis, diabetic pre-coma, diabetic ketoacidosis or hyperosmolar hyperglycemic state within 3 months before study entry. * Patients who were treated with insulin or glucagon-like peptide-1 (GLP-1) analogue. * Patients who have been treated by thiazolidinediones (pioglitazone) in the last 6 months. * Use of systemic glucocorticosteroids (excluding topical application or inhaled forms) for 7 consecutive days or within more than 3 months prior to study entry. * Patients with impaired hepatic function (alanine transaminase \[ALT\], aspartate transaminase \[AST\] activity \>1.5 times the upper limit of normal \[ULN\]). * Patients with impaired renal function (serum creatinine concentration \>135ɥmol/l in men and \>110ɥmol/l in female). * Patients who had experienced myocardial infarction, stroke, unstable angina or coronary artery bypass surgery within 6 months prior to screening, or patients with arrhythmia requiring treatment. * Patients with chronic heart failure II-IV functional class according to the classification of NYHA (New York Heart Association). * Patients taking medicines of thyroid hormones, warfarin, dicoumarin or digoxin. * Patients who took sodium channel blockers and SGLT2 inhibitors (or inhibitors sodium glucose transporters 2) in the last 6 weeks prior to study entry. * Known history of alcohol or drug abuse within 6 months prior to screening. * Pregnant or woman during breast feeding period. * Women of childbearing potential not protected by effective contraceptive method of birth control. * Men whose partners are planning pregnancy. * History of hypersensitivity to the study drug or to a drug with a similar chemical structure. * Use of any investigational drug within 3 months prior to study entry. The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.