A Study to Evaluate the Effect of ASP1941 in Combination With Metformin in Adult Patients With Type 2 Diabetes Mellitus

Inclusion Criteria: * Subject has been diagnosed with Diabetes Mellitus type 2 (T2DM) for at least 6 months * Subject has inadequate glycemic control indicated by an HbA1c level between 7.0% and 9.5% at start of the run-in period at Visit 1 AND does not meet any of the FPG discontinuation criteria * Subject has been on a stable dose of at least 1500 mg/day metformin monotherapy for at least 6 weeks prior to Visit 1 * Subject is on a stable diet and exercise program (for at least 6 weeks prior to Visit 1) and is willing to remain on this program for the duration of the study * Subject has a body mass index (BMI) 20 - 45 kg/m2 at Visit 1 * Female subject of childbearing potential has a negative serum pregnancy test (human chorionic gonadotropin \[hCG\]) at Visit 1 and agrees to use an acceptable form of contraception throughout the duration of the study OR is at least 1 year post-menopausal (defined as amenorrhea for at least 1 year) or surgically sterile. Male study subjects should be advised to use a male condom in addition to having their partner use another acceptable method during the study and for 3 months after the last dose Exclusion Criteria: * Subject has any known complication of T2DM indicating a late disease state that in the investigator's opinion should preclude the subject from participation * Subject has type 1 diabetes mellitus * Subject is in need of insulin therapy or has received insulin within 3 months prior to Visit 1, with the exception of acute use of \<7 days * Subject has a serum creatinine higher than upper limit of normal range at Visit 1 * Subject has an alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) higher than 3 times upper limit of normal range or has a total bilirubin more than 2 times upper limit of normal at Visit 1 * Subject has a urinary microalbumin/creatinine ratio above or equal to 300 mg/g at Visit 1 * Subject has a symptomatic urinary tract infection (UTI) or symptomatic genital infection at Visit 1 or during the placebo run-in period, including just prior to randomization at Visit 2 * Subject has persistent, uncontrolled severe hypertension as indicated by a systolic blood pressure \>180 mmHg or a diastolic blood pressure of \>110 mmHg taken in a sitting position after 5 minutes of rest on at least 2 measurements (within 30 minutes of each other) at Visit 1 * Subject has a significant cardiovascular disease, such as myocardial infarction or a vascular intervention (e.g. angioplasty or stent) within 3 months prior to Visit 1, or history of heart failure (New York Heart Association \[NYHA\] Class III IV) * Subject is known to have hepatitis or be a carrier of hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody (enzyme linked immunosorbent assay \[ELISA\] plus confirmatory test), or is known positive for human immunodeficiency virus (HIV) HIV-1 and/or HIV 2 * Subject is currently receiving an excluded medication (loop-diuretics or systemic corticosteroids) or has received any other oral anti-diabetic drug except for metformin within 3 months prior to Visit 1 * Subject has history of lactic acidosis * Subject has a history of drug or alcohol abuse/dependency within 12 months prior to Visit 1 as defined in the Diagnostic and Statistical Manual-IV (DSM-IV) * Subject has had a malignancy in the last 5 years, except for adequately treated basal or squamous cell carcinoma of the skin or carcinoma in situ of the cervix * Female subject who is pregnant, lactating or pre-menopausal with positive serum pregnancy test (hCG) at Visit 1 or has an intention of becoming pregnant * Subject has an unstable medical or psychiatric illness * Subject has known or suspected hypersensitivity to ASP1941 or any components of the formulations used * Subject has previously received ASP1941 * Subject is concurrently participating in another drug study or has received an investigational drug within 30 days (or the limit set by national law, whichever is longer) prior to Visit 1 * Subject has any concurrent illness which, in the opinion of the investigator, may interfere with treatment or evaluation of safety or completion of this study * In the investigator's judgment, the subject is unable to adhere to the treatment regimen, protocol procedures or study requirements