This study evaluates the efficacy and safety and population PK of HMS5552 add-on to Metformin in adult type 2 diabetic subjects. There will be 2 groups in the first 24 weeks, one group will receive HMS5552 plus Metformin, while the other group will receive placebo plus Metformin; after 24 weeks, all subjects will receive HMS5552 plus Metformin for 28 weeks.

This study is a phase III study in subjects with T2DM. As designed, the study will start with a 4-week, single-blind, placebo run-in period based on diet and exercise interventions for screening eligible subjects, meanwhile, subjects are treated with Metformin (Glucophage) at 1500mg/day as basic therapy throughout the treatment period. 3 weeks after run-in, eligibility is confirmed with required laboratory tests at -5±2 days prior to randomization. The eligible subjects are randomly assigned to HMS5552 75mg BID group or placebo BID group with ratio 1:1 to receive a 24-week double-blind treatment. Then all subjects receive 28-week open-label treatment of HMS5552 75mg BID add-on to Metformin. After 52-week treatment, all investigational drugs should be discontinued, followed by 1 week for safety evaluation. The dosage of Metformin (Glucophage) should be maintained at 1500 mg/day from run-in period to treatment completion.

Inclusion Criteria:

1. Male or female, aged 18~75 years old;
2. T2DM and treated with Metformin ≥ 1500mg/day constantly for at least 12 consecutive weeks;
3. 7.5% ≤ HbA1c ≤ 10.0% at screening;
4. 18.5 kg/m2 < BMI < 35.0 kg/m2 at screening;

Exclusion Criteria:

1. Any anti-diabetic therapy other than Metformin within 12 weeks before screening;
2. Received insulin therapy more than 30 days within 1 year before screening;
3. Fasting C-peptide <0.81 ng/ml (0.27 nmol/L) at screening;
4. Medical history of severe hypoglycemia or frequent hypoglycemia, diabetic ketoacidosis, diabetes lactic acidosis or hyperosmotic nonketotic diabetic coma, severe cardio-cerebrovascular, unstable or rapidly progressive kidney disease, active liver diseases, diagnosed mental disease etc.
5. T1DM;