The objective of this study is to confirm efficacy of ASP1941 based on the changes in HbA1c and to assess its safety in subjects with type 1 diabetes mellitus receiving ASP1941 once daily in combination with insulin for 24 weeks. This study will also assess the safety/efficacy of long-term treatment (52 weeks).

This study consists of two parts. In Part 1, ASP1941 or placebo will be administered orally in a blind manner. In Part 2, the long-term safety and efficacy of ASP1941 will be evaluated in patients who have participated in the study and completed the Part 1.

Inclusion Criteria:

• The subject has been diagnosed with type 1 diabetes mellitus
• The subject has been receiving insulin therapy for the treatment of diabetes mellitus.
• The subject has not switched from an insulin product to another insulin product or switched between continuous subcutaneous insulin infusion (CSII) and multiple daily injections (MDI).
• The subject has an HbA1c value between 7.5% and 11.0% and the difference of HbA1c value is within ± 2.0%.
• The subject has a fasting blood C-peptide level < 0.6 ng/mL.
• The subject has a body mass index (BMI) between 20.0 kg/m2 and 35.0 kg/m2.

Exclusion Criteria:

• The subject has type 2 diabetes mellitus.
• The subject has participated in a clinical study or post marketing study of another drug or medical equipment within 12 weeks (84 days) before providing written informed consent, or is currently participating in such a study.
• The subject has received treatment with ASP1941 (ipragliflozin) or participated in a clinical study of ASP1941 (excluding subjects who discontinued before the investigational period).
• The subject participated in this study previously.
• The subject has received a hypoglycemic agent other than insulin or an α-glucosidase inhibitor.
• The subject has proliferative retinopathy (except for those who have undergone photocoagulation etc. and whose symptoms are stable).
• The subject has experienced severe hypoglycemia.
• The subject has experienced diabetic ketoacidosis.
• The subject has chronic disease that requires the continuous use of corticosteroids, immunosuppressants, etc.
• The subject has symptomatic urinary tract infection or symptomatic genital infection.
• The subject has a history of recurrent urinary tract infection or recurrent genital infection.
• The subject has a history of cerebral vascular attack, unstable angina, myocardial infarction, vascular intervention, or another serious heart disease.
• The subject has a concomitant malignant tumor or a history of malignant tumor
• The subject has a history of an allergy to ASP1941 (ipragliflozin) and/or similar drugs (drugs possessing SGLT2 inhibitory action).
• The subject has psychiatric disorder that is inappropriate for participation in the study.
• The subject has drug addiction or alcohol abuse.
• The subject has severe infection or serious trauma, or is perioperative.
• The subject has a history of medically significant renal disease such as renovascular occlusive disease, nephrectomy and/or renal transplant.
• The subject has any symptoms of dysuria, anuria, oliguria, or urinary retention.
• The subject has severe renal impairment or end-stage renal failure requiring dialysis.
• The subject has an Aspartate Aminotransferase and/or Alanine Aminotransferase value that exceeds 2 times, or a total bilirubin value that exceeds 1.5 times the upper limit of the reference range.
• The subject has uncontrolled severe hypertension.
• The subject has serious gastrointestinal disease or a history of operation for serious gastrointestinal disease.

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.