A Study of GSK256073 in Subjects With Type 2 Diabetes Mellitus Who Are Being Treated With Metformin

Inclusion Criteria: * A diagnosis of T2DM as determined by a responsible physician based on a medical evaluation including medical history, physical examination, and laboratory tests, with onset at least 6 months prior to Screening. Subjects may be entered if they have stable hypertension or dyslipidemia on therapy. Subjects with other conditions except as noted in the Exclusion criteria may be included only if the investigator and GSK medical monitor agree that the condition is unlikely to introduce additional risk factors and will not interfere with study procedures * HbA1c levels greater than or equal to 7.0 % and less than or equal to 9.5% at Screening * On monotherapy with metformin at the time of screening, and at a maximum tolerated dose greater than or equal to 1000 mg for at least 2 months prior to dosing. * Fasting plasma glucose level less than 13.3 mmol/L (240 mg/dL) at Screening * Male or female between 20 and 70 years of age, inclusive, at the time of signing the informed consent * Fasting Triglycerides lower than 4.52 mmol/L (400 mg/dL) * A female subject is able to participate is she if of non-child bearing potential * Male subjects must agree to use one of the contraception methods listed in the protocol. This criterion must be followed from the time of the first dose of study medication until 3 days after last dose of the study medication * Overweight with BMI greater than or equal to 25 kg/m2 for non-Asian Indians and greater than or equal to 24 kg/m2 for Asian-Indian, and less than 40 kg/m2 * The subject is capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form * Subjects in France will be eligible only if they are affiliated to or a beneficiary of a social security category * Subjects in other countries must meet all local and/or country-specific requirements for registration and reimbursement, as applicable Exclusion Criteria: * Requiring insulin therapy or use of combination oral antidiabetic medications or use of monotherapy other than metformin within the 3 months prior to screening * Past or present disease (other than type 2 diabetes mellitus) that in the opinion of the Investigator may affect the outcome of this study * A positive pre-study Hepatitis B surface antigen, or positive Hepatitis C result within 3 months of screening * Renal impairment as defined by a calculated GFR less than 60 mL/min * Any concurrent serious illness (e.g., severe COPD, history of malignancy other than skin cancer within 5 years of initial diagnosis or with evidence of recurrence) that may interfere with a subject completing the study * Laboratory values as defined per protocol * ECG criteria as defined per protocol * Systolic blood pressure greater than 160 mmHg, or diastolic blood pressure greater than 100 mmHg at Screening * History of uric acid kidney stone, and being treated with drugs for hyperuricemia including Allopurinol or Probenecid * History of peptic ulcer disease (PUD) and/or other gastrointestinal bleeding within the 12 months prior to screening * Use of certain blood pressure medications or certain other medications that are renally excreted as defined per protocol * History of myopathy or CPK value greater than 3 times upper limit of normal at screening * The subject has participated in a clinical trial and has received an investigational product within 30 days or 5 half-lives or twice the biological effect (whichever is longer) * Exposure to more than four new chemical entities within 12 months prior to the first dosing day * Any change in diet, exercise habits or smoking status within six weeks prior to screening. Any subject that cannot refrain from smoking while in the unit must be excluded * History of sensitivity to any of the study medications, including sitagliptin or metformin, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK Medical Monitor, contraindicates their participation * The subject has a positive pre-study drug screen * History of regular alcohol consumption within 6 months of the study as defined per protocol * Where participation in the study would result in donation of blood or blood products in excess of 500 mL within a 56 day period * Pregnant females as determined by positive serum and/or urine hCG test at screening and prior to dosing * Lactating females * Subjects who are unwilling or unable to follow the procedures outlined in the protocol * Subject is mentally or legally incapacitated