TTP399-206 is a Phase 2a multicenter double blind cross over randomized study of cadisegliatin in participants with T1D using hybrid closed loop systems to manager their diabetes. Patients using a hybrid closed loop insulin pump to manage their diabetes will be randomized to either receive blinded cadisegliatin 800 mg QD as an adjunctive therapy to their insulin treatment or placebo QD along with their insulin treatment. The trial begins with a screening period of up to 2 weeks, followed by a device training and insulin adjustment period of 1-2 weeks leading into the first double-blind treatment period of 6 weeks. There will then be a washout period of 1-2 weeks followed by the second double-blind treatment period of 6 weeks where the patient will cross-over to the treatment arm that they did not receive in the first treatment period.