A Study Comparing the Safety, Tolerance, and Efficacy of Various Doses of SK-0403 Versus Placebo and Sitagliptin 100 mg in Patients Not Well-Controlled on Metformin Therapy

Inclusion Criteria: * Male and female subjects with type 2 diabetes between 18 and 75 years of age, inclusive * Screening HbA1c between 7.0% to 10.0% for metformin stable subjects, 6.5% to 9.0% for subjects on metformin and 1 other antidiabetic agent (excluding thiazolidinediones, insulin, or incretin therapies \[DPP-4 inhibitors and GLP-1 analogues\]), or 7.5% to 11.0% for subjects on no antidiabetic medication or, if taking metformin, not on a stable dose of 1500 mg/day or maximum tolerated dose. * No antidiabetic medication other than metformin for 3 months prior to randomization. * Fasting plasma glucose less than 270 mg/dL (15 mmol/L) at screening. * Body mass index between 20 kg/m2 and 45 kg/m2 inclusive at screening. Exclusion Criteria: * History of type 1 diabetes. * Received treatment with insulin within 30 days of the screening visit or for more than 1 week within 3 months of the screening visit. * Use of 3 or more oral antidiabetic medications at the time of the screening visit.