TTT1: triple therapy for type 1 diabetes

Most people associate semaglutide with type 2 diabetes and weight management. The drug is approved in those settings in the US, EU, and UK. Type 1 diabetes is a different story. People with T1D depend on exogenous insulin from diagnosis; their pancreatic beta cells can no longer produce it. Adding drugs designed for type 2 disease raises practical and safety questions that remain only partially answered. A Phase 3 international clinical trial published in Diabetes, Obesity and Metabolism on 19 July 2026 lays out the design and methods of the Triple Therapy for Type 1 Diabetes (TTT1) study, which tackles exactly this problem^[1].

Who the trial targets

TTT1 recruits overweight and obese adults with type 1 diabetes whose glycaemic control is suboptimal: baseline HbA1c between 7.5% and 11.0%. That population matters. By most estimates, more than half of adults with T1D in high-income countries now carry excess weight, partly because modern intensive insulin therapy itself promotes weight gain. The overlap between T1D and obesity is the clinical gap TTT1 is designed to address^[1].

The two-period structure

The trial runs in two sequential stages. Period 1 (26 weeks) randomises participants 2:1 to semaglutide plus insulin versus standard insulin alone. Semaglutide is uptitrated to 1.0 mg weekly, the standard dose used in type 2 diabetes. Period 2 (a further 26 weeks) applies only to those receiving semaglutide: they undergo a second double-blind randomisation to either dapagliflozin 10 mg daily or placebo, both on top of continued semaglutide. The result is a sequential experiment: Period 1 tests the GLP-1 add-on; Period 2 tests whether an SGLT2 inhibitor on top of that adds further benefit^[1]. Dapagliflozin, marketed as Farxiga, is already in routine use for type 2 diabetes and chronic kidney disease but carries a DKA warning in T1D, which is why the trial monitors ketosis carefully.

Primary and secondary endpoints

The primary comparison is triple therapy (dapagliflozin, semaglutide, insulin) against dual therapy (placebo, semaglutide, insulin) on HbA1c change. Secondary endpoints extend this to triple versus standard insulin alone, and dual versus standard insulin, making the design nested: it can answer three questions with one cohort^[1]. Safety outcomes include hypoglycaemia and ketosis, the two adverse events most likely to limit adjunct therapy in T1D. An interim masked data analysis revised the planned sample from 114 to 82 participants based on observed variance; the design paper describes this as statistically justified.

Why this combination, and why now

Several lines of evidence support the TTT1 combination. A 2026 systematic review in the Journal of the American Association of Nurse Practitioners found that semaglutide and tirzepatide show promise in type 1 and latent autoimmune diabetes, though current evidence is of moderate quality and randomised trials in well-defined T1D populations are still needed^[2]. Separately, a 2026 crossover trial published in Endocrine Practice found that dapagliflozin as an insulin adjunct raised continuous glucose monitor time-in-range from 55.4% to 68.2% in 44 adults with T1D, with no DKA events^[3]. TTT1 asks whether combining both agents produces additive benefits.

The timing also reflects a shift in how researchers and clinicians think about T1D. For decades the disease was managed almost exclusively with insulin optimisation. The idea of layering drugs designed for metabolic syndrome on top of insulin was seen as risky. Growing real-world evidence from semaglutide use in T1D registries, alongside SGLT2 inhibitor data, has shifted that calculus enough to justify a Phase 3 test.

The safety question the trial must answer

SGLT2 inhibitors like dapagliflozin carry an elevated DKA risk in type 1 diabetes because they lower blood glucose while promoting ketone production, particularly when insulin doses are reduced too aggressively. The TTT1 protocol describes ketosis as a primary safety outcome alongside hypoglycaemia^[1]. The trial was registered as NCT03899402 and funded by JDRF (grant 3-SRA-2019-669-M-B); both facts allow independent monitoring of results as they emerge.

For readers tracking the GLP-1 space: the TTT1 design paper is not a results paper. It describes a trial that is still running. The publication clarifies the protocol and justifies the revised sample size. The actual HbA1c and safety outcomes will follow when the trial completes.

What we do not yet know

The design paper does not report efficacy or safety results. It does not address long-term outcomes beyond 52 weeks, it does not include continuous glucose monitoring as a primary endpoint (though devices are likely used for safety monitoring), and it does not compare triple therapy against established T1D adjuncts like the SGLT2 inhibitor used alone, because the design was built around the additive question.

There is also the regulatory context: semaglutide is not approved for type 1 diabetes anywhere. Dapagliflozin has a DKA-related restriction in T1D in multiple jurisdictions. TTT1 results, when available, will be a data point for regulators, but approval requires more than a single Phase 3 trial. For semaglutide's approved uses and current regulatory status see the semaglutide peptide page.

The bottom line

TTT1 is one of the most logistically ambitious trials in current T1D research: it tests a three-drug combination across a 52-week sequential design in a population defined by both autoimmune disease and excess weight. The design paper describes a well-structured protocol, an interim sample-size revision backed by masked data, and clear safety monitoring for the outcomes clinicians care about most. Whether the combination works in this population is a question the results will answer.

This article is for educational and informational purposes only and does not constitute medical advice. Semaglutide and dapagliflozin are prescription medicines. Neither is approved for type 1 diabetes in any jurisdiction we track. Always consult a qualified healthcare professional before starting, stopping, or changing any medication.