The 28-week, prospective, multicenter, open-label OPTIMIZE study, conducted in Belgium and Canada, evaluated treatment optimization with once-daily (OD) Gla-300 in combination with a prandial rapid-acting insulin analog in adults with T1DM previously uncontrolled (HbA1c 8-10%) on twice-daily (BID) basal insulin as part of basal-bolus therapy. The intent-to-treat (ITT) and safety populations of the Belgian cohort comprised 48 patients (52.1% women); mean (SD) age 51.7 (12.8) years and mean (SD) body mass index 28.2 (4.9) kg/m². Prior to the study, 37.5% were on insulin glargine 100 U/mL and 62.5% were on insulin detemir. There was a statistically significant reduction in HbA1c from baseline to month 6 (primary endpoint, p<0.0001). No significant changes were observed in basal or prandial insulin dose, or in total or nocturnal hypoglycemic event rates. Statistically significant improvements were seen in the Diabetes Treatment Satisfaction Questionnaire (DTSQ) total and perceived hyperglycemia scores (p<0.05) (Table), and in the patient satisfaction score for number of injections (p=0.004). Switching from a BID basal insulin analog to OD Gla-300 in this challenging T1DM population was associated with a statistically significant improvement in HbA1c levels and better overall treatment satisfaction. Disclosure C. Mathieu: Research Support; Self; Novo Nordisk A/S. Advisory Panel; Self; Novo Nordisk A/S. Speaker's Bureau; Self; Novo Nordisk A/S. Research Support; Self; Sanofi. Speaker's Bureau; Self; Sanofi. Advisory Panel; Self; Sanofi. Research Support; Self; Merck Sharp & Dohme Corp.. Speaker's Bureau; Self; Merck Sharp & Dohme Corp.. Advisory Panel; Self; Merck Sharp & Dohme Corp.. Research Support; Self; Eli Lilly and Company. Speaker's Bureau; Self; Eli Lilly and Company. Advisory Panel; Self; Eli Lilly and Company. Research Support; Self; Novartis AG. Speaker's Bureau; Self; Novartis AG. Advisory Panel; Self; Novartis AG, Bristol-Myers Squibb Company. Speaker's Bureau; Self; AstraZeneca. Advisory Panel; Self; AstraZeneca, Pfizer Inc., Janssen Pharmaceuticals, Inc., Boehringer Ingelheim GmbH. Speaker's Bureau; Self; Boehringer Ingelheim GmbH. Advisory Panel; Self; Hanmi Pharmaceutical. Research Support; Self; Roche Diagnostics Corporation. Advisory Panel; Self; Roche Diagnostics Corporation. Research Support; Self; Medtronic. Advisory Panel; Self; Medtronic, MannKind Corporation. Research Support; Self; Intrexon. Advisory Panel; Self; Intrexon, Dianax, UCB, Inc.. Research Support; Self; Abbott. P. Stella: Employee; Self; Sanofi. Stock/Shareholder; Self; Sanofi. J. Bruhwyler: Other Relationship; Self; Sanofi. K. Alexandre: Employee; Self; Sanofi. Stock/Shareholder; Self; Sanofi.