Primary Objective:

To compare the clinical efficacy of gemigliptin and vildagliptin as add-on therapy to metformin in terms of change in Hemoglobin A1c (HbA1c) reduction.

Secondary Objectives:

To compare the safety and tolerability of gemigliptin and vildagliptin:

• Number of patients who experience at least one episode of hypoglycemia.
• Number of patients experiencing adverse event (AE), serious adverse event (SAE).
• Assessment of patients compliance defined as number tablets returned by patients.

The study consists of 2 weeks screening and 24 weeks of treatment (a total of 26 weeks).

The protocol includes a screening period of two weeks, after which they will be randomized to receive gemigliptin or vildagliptin.

Patients will be evaluated through 4 mandatory visits during the study. Visit 1: Week -2 (screening) Visit 2: Week 0 (randomization) Visit 3: Week 12 (efficacy and safety evaluation) Visit 4: Week 24 (efficacy and safety evaluation)

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.