To demonstrate that efficacy of Gosogliptin as Monotherapy and in Combination with Metformin is non-inferior to efficacy of Vildagliptin as Monotherapy and in combination with Metformin in the effects on glycosilated hemoglobin (HbA1c) at Week 12 and Week 36 compared to baseline (Week 0).

• Screening of patients at Week -2
• Training for all complying patients in Diabetes Program School at Week -1 with glucometer and patient diary distribution for SMBG
• Randomization in one of two groups in the ratio 1:1.
• The 1st stage includes 12 weeks of monotherapy with Gosogliptin or Vildagliptin.
• The 2nd stage includes 24 weeks of combination therapy with Gosogliptin and Metformin or Vildagliptin and Metformin.
• Follow-up period of 4 weeks.

Inclusion Criteria:

1. Men and women from 18 through 78 years
2. Confirmed clinical diagnosis of type 2 diabetes mellitus
3. Drug-naive patients or patients without hypoglycemic drug treatment at least till 12 weeks before screening
4. Patient's agreed to use adequate methods of contraception during the study
5. Body mass index 22-40 kg/m2
6. HbA1c 7.5 - 11.0%
7. FPG < 15 mmol/L
8. Signed informed consent
9. Patient's ability to follow all protocol requirements

Exclusion Criteria:

1. Pregnancy and lactation, women who plan to become pregnant during the clinical study, women of child-bearing potential (i.e. non-stylized surgically and in period of post menopause less than 2 years) without adequate methods of contraception.
2. History of type 1 diabetes mellitus and other forms
3. Severe metabolic complications of diabetes as ketoacidosis, hyperosmolar coma during 6 weeks before screening
4. Severe complication of diabetes, for instance proliferative retinopathy in active stage, autonomic neuropathy or gastroparesis
5. A known allergy, hypersensitivity or contraindications to Gosogliptin, Vildagliptin, Metformin or their components
6. Convulsive disorder; system autoimmune diseases or vascular collagenosis demanding the previous or current treatment by systemic corticosteroids, cytostatics; malignancy within the last 5 years (except for a basal-cell carcinoma);
7. Significant cardiovascular diseases within 12 months of screening including: chronic heart failure of a class III or IV (NYHA), severe arrhythmia demanding treatment with antiarrhythmics, unstable angina, myocardial infarction, heart and coronary vessels surgery, heart valves diseases, transitory ischaemic attack or stroke, uncontrolled arterial hypertension with systolic Blood Pressure > 180 mm Hg and diastolic Blood Pressure > 110 mm Hg, pulmonary embolism or deep venous thrombosis
8. A nephrotic syndrome, a chronic renal failure, serum creatinine > 1.5 mg/dL (132 µmol/L) in men and > 1.4 mg/dL (123 µmol/L) in women or GFR <60 ml/min/1.73m2
9. HBV, HCV or a liver cirrhosis; AST or ALT > 3 ULN; total bilirubin > 2 ULN
10. HIV; severe infection that can affect glycemia within 30 days of screening
11. Anemia (Hb ≤10.5 g/dL in women or ≤11.5 g/dL in men); loss of > 1 unit of blood ( 500 mL) or blood transfusion within 12 weeks of screening
12. Drugs or alcohol abuse
13. Administration of any study drug within 30 days of screening
14. Inability to read or write; unwillingness to understand and follow the protocol procedures