Primary Objectives:

To assess the effects of tofogliflozin on glycemic control in comparison to placebo as an add-on treatment to insulin treatment in terms of glycated hemoglobin (HbA1c) reduction over a period of 16 weeks in patients with type 2 diabetes mellitus.

To assess the safety of tofogliflozin in combination with insulin treatment throughout 52 weeks.

Secondary Objectives:

To assess the effects of tofogliflozin in comparison to placebo on:

• Body weight
• Fasting plasma glucose (FPG)
• Postprandial plasma glucose (PPG) To assess the long term safety and tolerability of tofogliflozin.

The total study duration from screening for a patient can be approximately up to 1 year, including a screening period of 2 weeks, double-blinded placebo controlled treatment period of 16 weeks, an open-labeled extension period of 36 weeks, and a follow-up period of 3 days.

Inclusion criteria:

• Patients aged 20-75 years old with type 2 diabetes mellitus (T2DM).
• Hemoglobin A1c ≥7.5% - ≤10.5% and FPG ≤220 mg/dL.
• Basal bolus (BB), Bolus, Premix, Basal supported oral therapy (BOT) regimens used for more than 12 weeks before screening.
• Stable dose (-/+20%) of insulin and no change in the dose of oral hypoglycemic agents for more than 12 weeks before screening.
• Only Dipeptidyl peptidase-4 inhibitor is allowed for basal supported oral therapy (BOT) regimen.
• Body mass index (BMI) ≥18.5 kg/m^2 and <35.0 kg/m^2.
• No change of antihypertensive, anti-hypercholesterolemia and uric acid lowering drug 4 weeks before screening.

Exclusion criteria:

• Type 1 diabetes mellitus.
• Pregnancy or lactation.
• Severely uncontrolled glycemic situation.
• History of metabolic acidosis, including diabetic ketoacidosis, within 1 year prior to screening.
• History of myocardial infarction, stroke, or heart failure requiring hospitalization or drug or alcohol abuse within the previous 6 months.
• A measured serum creatinine level greater than 2.0 mg/dL for men and 1.5 mg/dL for women.
• Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) ≥2.5 times the upper limit of the reference range at the central laboratory test facility.
• Has previously received treatment with the investigational product.
• Has received treatment with another investigational product or non-approved drug within 3 months before tests for provisional enrolment.
• Corticosteroid therapy for 14 or more days in total within 8 weeks before tests for provisional enrolment (excluding those used for localized effects, such as drugs for topical [skin] application, eye drops and sprays).
• Patients who are frequently experiencing orthostatic hypotension.
• Required a change in the dosing regimen for the following drugs within 4 weeks before tests for provisional enrolment:
• Lipid-lowering drug
• Antihypertensive drug
• Thyroid hormone preparation
• Uric acid lowering drug

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.