This is a randomized, double-blind, active-controlled, 2 period cross-over clinical trial in subjects with type 1 diabetes mellitus using a Multiple Daily Injection (MDI) regimen.

After a screening visit, eligible subjects will enter a run-in period. Subjects will receive a Continuous Glucose Monitoring (CGM) system for glucose monitoring and control at the beginning of the run-in and for the whole duration of the study. Each eligible subject will then be randomly allocated to a sequence of the 2 treatments, i.e. multiple daily injections of ADO09 and insulin aspart during 2 dosing periods. At Day 1 a mixed meal test (MMT) will be conducted at breakfast and subjects will remain at the clinical site until day 3. At Day 3 subjects will leave the clinical site and continue the treatment with IMP for the next 3 weeks. On Day 23 subjects will come back for a MMT on Day 24.

This study is constituted of 2 parts. In the first part (Part A), only subjects with daily prandial insulin dose ≤ 40 U/day will be enrolled.

Following the completion of the part A, an extension part (Part B) will be conducted to particularly assess the safety and tolerability of higher daily doses of ADO09 in patients with insulin requirements ≥ 40 U/day.

The clinical conduct and procedures will not change for the Extension Part of the study.

Inclusion Criteria:

• Signed and dated informed consent obtained before any trial-related activities.
• Type 1 diabetes mellitus (as diagnosed clinically) ≥ 12 months.
• Treated with insulin ≥ 12 months.
• Using a multiple dosing insulin therapy (MDI) with basal and bolus insulin.
• HbA1c ≤ 9.0%.
• Fasting negative C-peptide (≤ 0.30 nmol/L).
• Total daily prandial dose: ≤ 40U in the Part A and ≥ 40 U in the Part B

Exclusion Criteria:

• Known or suspected hypersensitivity to products used in the clinical trial
• Type 2 diabetes mellitus
• Previous participation in this trial. Participation is defined as randomized.
• Receipt of any medicinal product in clinical development within 3 months before randomization in this trial.
• Known slowing of gastric emptying, including gastroparesis, and or gastrointestinal surgery that in the opinion of the investigator might change gastrointestinal motility and food absorption.
• Presence of clinically significant acute gastrointestinal symptoms (e.g. nausea, vomiting, heartburn or diarrhoea), as judged by the Investigator.
• Intake of medication known to affect gastrointestinal motility, including but not limited to erythromycin, metoclopramide, cisapride, cholestyramine or colestipol within 4 weeks before screening.