SURPASS-T1D-1 begins: the first properly powered incretin trial in type 1 diabetes

Eli Lilly has begun enrolling a phase 3 trial of tirzepatide in adults who have type 1 diabetes plus obesity or overweight. The study, registered as NCT06914895 and known as SURPASS-T1D-1, plans to randomize 905 participants to once-weekly tirzepatide or placebo, with change in HbA1c at week 40 as the primary endpoint over a total treatment and follow-up period of roughly 49 weeks ^[1]. Primary completion is listed as November 2026 ^[1].

That size matters. The existing human evidence for an incretin drug in type 1 diabetes at this dose class comes largely from a 24-person phase 2 study — small enough that it could suggest a signal but not settle questions about blood sugar control or safety. A trial of 905 people is the first attempt to answer those questions with enough statistical power to support a regulatory decision, should Lilly pursue one ^[1].

What the trial is measuring

The primary outcome is the change in HbA1c from the start of the study to week 40 ^[1]. HbA1c is the standard blood test that estimates average blood sugar over the prior two to three months. Because everyone in the trial has type 1 diabetes, all participants continue to need insulin; the question is whether adding tirzepatide on top of insulin lowers HbA1c more than placebo does, and what happens to weight, insulin doses, and hypoglycemia along the way.

The registry listing available here does not spell out the full set of secondary endpoints, the specific tirzepatide doses being tested, the number of study sites, or the countries involved. Those details are not yet confirmed from the material reviewed for this story.

The safety question is the real test

Incretin drugs have been tried in type 1 diabetes before. Novo Nordisk's ADJUNCT program tested liraglutide, a GLP-1 receptor agonist, as an add-on to insulin in type 1 diabetes. That program raised concerns about hypoglycemia and ketosis, and it never produced an approval for type 1 diabetes. Those findings are the reason clinicians watch adjunctive therapy in type 1 diabetes closely, and they are why the safety readout from SURPASS-T1D-1 will be scrutinized as hard as the HbA1c result. The specific event rates from the ADJUNCT trials are not contained in the source listing for this story.

The theoretical worry is straightforward: drugs that slow stomach emptying, cut appetite and reduce insulin needs can make insulin dosing harder to match to food, and in people who make no insulin of their own, under-dosing can tip into diabetic ketoacidosis. Whether tirzepatide does or does not increase those events in a large randomized population is exactly what this trial is designed to find out.

Why it matters for patients

Tirzepatide is not approved for type 1 diabetes. Some people with type 1 diabetes and obesity already take GLP-1 or dual-agonist drugs off-label, usually with close attention to insulin adjustment, but they are doing so without controlled trial data on hard endpoints. A properly powered trial changes that: within about two years, there should be randomized evidence on how much HbA1c moves, how insulin needs change, and how often severe low blood sugar and ketoacidosis occur.

It also matters for coverage. Insurers generally follow approved indications. If a trial like this succeeds and leads to a label expansion, the path to paid access would be clearer for a group that currently has none. If safety signals emerge instead, that would be useful information too — it would give patients and clinicians a firmer basis for weighing off-label use.

What happens next

Primary completion is listed for November 2026 ^[1]. Full data typically follow a primary completion date by several months to a year, through a conference presentation or journal publication. Lilly has not announced whether or when it would file for a type 1 diabetes indication, and no regulatory timeline has been disclosed in the sources reviewed here. Anyone with type 1 diabetes who is considering enrollment would need to check the trial's eligibility criteria and site list directly on ClinicalTrials.gov ^[1].