SURPASS-T1D-2 adds a 20-month long-term arm in type 1 diabetes

A second, longer trial of tirzepatide in type 1 diabetes has started enrolling. SURPASS-T1D-2, registered on ClinicalTrials.gov as NCT06962280, is randomizing 465 adults who have type 1 diabetes along with obesity or overweight to tirzepatide or placebo, with treatment running about 20 months and the primary endpoint measured at week 40 ^[1].

The trial is the long-term companion to SURPASS-T1D-1. Both use the same primary measure — change in HbA1c at week 40 — but SURPASS-T1D-2 keeps participants on assigned treatment far longer, which allows researchers to look at what happens to blood sugar control, body weight and side effects after the first year ^[1]. Taken together, the two trials are set to enroll 1,370 people, which makes this the largest test to date of a dual GIP/GLP-1 receptor agonist in type 1 diabetes ^[1].

What the trial is testing

Tirzepatide is the molecule sold as Mounjaro for type 2 diabetes and as Zepbound for obesity. It has not been approved for type 1 diabetes, and these trials are the kind of controlled evidence that would be needed before any regulator considered that question.

The design is straightforward: participants are randomly assigned to tirzepatide or matching placebo, on top of their usual insulin therapy, and HbA1c change at week 40 decides the main result ^[1]. Primary completion — the point when the last participant provides data for the main endpoint — is listed as November 2026 ^[1].

Several details that patients and clinicians will want are not spelled out in the registry information available here. That includes the dose levels being tested, the number and location of study sites, the full list of secondary endpoints such as body weight or insulin dose changes, and how the trials will count episodes of hypoglycemia or diabetic ketoacidosis. Those are important safety questions in type 1 diabetes specifically, and the answers are not yet known from the material at hand.

Why it matters for patients

Most people with type 1 diabetes have been left out of the GLP-1 and GIP/GLP-1 evidence base. The large cardiovascular and weight-loss trials that built the case for semaglutide and tirzepatide enrolled people with type 2 diabetes, obesity, or both. That gap matters because a substantial share of adults with type 1 diabetes also carry extra weight and use large daily insulin doses, and they and their doctors have had little randomized data to work from.

In practice, that has meant off-label use decided case by case, without trial-grade numbers on how much HbA1c or weight changes, or how often serious problems occur. A 1,370-person program with a placebo comparator and a 20-month arm is designed to replace guesswork with measured effects ^[1].

It is also worth setting expectations on timing. A primary completion date of November 2026 is not the same as approval ^[1]. Results would then need to be analyzed, presented or published, and — if the sponsor pursues it — submitted to the FDA, which adds more time. Nothing about these trials changes what is approved today.

What happens next

The key dated milestone on the record is November 2026 for primary completion of SURPASS-T1D-2 ^[1]. Because the treatment period runs about 20 months while the primary endpoint is read at week 40, topline HbA1c results could become available before the full long-term dataset is complete, though no such reporting date is listed ^[1]. Enrollment status, site lists and any protocol changes are tracked on the trial's registry record ^[1].