Managing blood sugar in type 1 diabetes is not simply a matter of taking insulin. Insulin itself can cause weight gain, and weight gain can make blood sugar harder to control, creating a cycle that is difficult to break. Researchers have long wondered whether compounds originally developed for type 2 diabetes might help people with type 1 diabetes break that cycle.
A newly published trial record describes the design of the Triple Therapy for Type 1 Diabetes study, known as TTT1. It is the first phase 3 clinical trial to test what happens when a GLP-1 receptor agonist and an SGLT2 inhibitor are both added to insulin therapy in overweight or obese adults with type 1 diabetes whose blood sugar is running above the recommended target. The abstract, published in a diabetes and metabolism journal, lays out the methods researchers are using and the questions they are trying to answer.
Background on the two compound classes
To understand what the trial is testing, it helps to know what each class of compound does at a mechanistic level.
GLP-1 receptor agonists mimic a hormone called glucagon-like peptide 1. In research settings, this class of compound has been shown to stimulate insulin release in response to meals, suppress glucagon, slow gastric emptying, and reduce appetite. The cumulative effect observed across published trials in type 2 diabetes is lower average blood sugar and, typically, lower body weight.
SGLT2 inhibitors work through a completely different pathway. They block a protein in the kidneys called the sodium-glucose co-transporter 2, which is responsible for reabsorbing glucose that would otherwise be filtered into urine. By inhibiting that transporter, more glucose leaves the body through urine rather than staying in the bloodstream.
Both classes are well established in type 2 diabetes research. The open question is whether they add meaningful benefit when layered onto insulin therapy in type 1 diabetes, and whether the combination is safe enough given the real risks of low blood sugar and a potentially dangerous condition called ketosis, in which the body produces excess acid-forming ketone bodies.
Who the trial enrolled and why
The trial specifically enrolled overweight and obese adults with type 1 diabetes whose glycated hemoglobin, a measure of average blood sugar over roughly three months, fell between 7.5 percent and 11.0 percent inclusive. That range was chosen to capture people whose blood sugar is meaningfully above the commonly recommended target but not so elevated as to suggest acute crisis.
Researchers focused on overweight and obese participants because weight is a key variable in the cycle they are trying to interrupt. Excess weight increases insulin resistance, which pushes insulin doses higher, which can cause further weight gain. A compound that reduces appetite or removes glucose through the kidneys could, in theory, address multiple parts of that loop at once.
The original recruitment target was set at 114 participants. During the trial, researchers conducted a masked data analysis and recalculated the sample size, revising the target down to 82 participants. The trial record notes this adjustment transparently as part of the published methods.
Two-period trial design
The TTT1 study uses a sequential, two-period design that is somewhat unusual and worth understanding clearly.
In the first 26-week period, participants are randomized in a two-to-one ratio. Two thirds receive the GLP-1 receptor agonist plus insulin, with the GLP-1 compound uptitrated to a weekly dose of 1.0 milligrams. The remaining third continues on standard insulin therapy only. This part of the study is open-label, meaning both researchers and participants know which group is which.
In the second 26-week period, the design changes. Only those who received the GLP-1 receptor agonist plus insulin in the first period continue. They are then randomly assigned to receive either an SGLT2 inhibitor at 10 milligrams daily or a placebo, in addition to continuing the GLP-1 receptor agonist and insulin. This part is double-blind, meaning neither participants nor researchers know who is receiving the active compound.
The result is three comparison groups by the end of the trial: people on all three agents together, people on the GLP-1 receptor agonist plus insulin only, and people on standard insulin therapy alone.
Primary and secondary objectives
The primary objective of TTT1 is to measure the difference in glycated hemoglobin change between the triple-therapy group, taking all three agents, and the dual-therapy group, taking only the GLP-1 receptor agonist plus insulin and a placebo instead of the SGLT2 inhibitor. This comparison is designed to isolate the specific contribution the SGLT2 inhibitor makes when added on top of GLP-1 receptor agonist and insulin.
Secondary objectives expand the picture. Researchers will also compare the triple-therapy group against the standard insulin group, and the dual-therapy group against the standard insulin group. Together, these comparisons allow the trial to assess whether adding one adjunct compound helps, whether adding two helps more, and by how much at each step.
Safety monitoring priorities
The trial record places particular emphasis on two safety outcomes: hypoglycemia and ketosis.
Hypoglycemia, or dangerously low blood sugar, is a persistent risk whenever insulin doses are adjusted or when additional glucose-lowering compounds are introduced. Adding a GLP-1 receptor agonist or an SGLT2 inhibitor on top of insulin creates a situation where the usual calibration of insulin dose may no longer be correct, potentially pushing blood sugar too low.
Ketosis is a more specific concern with SGLT2 inhibitors in people with type 1 diabetes. Because these compounds cause glucose to leave the body through urine, the body may shift toward burning fat for fuel and producing ketones. In type 1 diabetes, where insulin production is absent, this can escalate toward a serious condition called diabetic ketoacidosis. Tracking the rate of ketosis events carefully is therefore central to the safety analysis of the TTT1 trial.
The published trial record notes that these risks were a primary motivation for the careful, staged design of the study. Researchers built in the GLP-1 receptor agonist as a first step before introducing the SGLT2 inhibitor, partly to allow insulin doses to be adjusted before a second glucose-lowering compound was added.
What the results may add to the field
Published trials in type 2 diabetes have consistently shown that both GLP-1 receptor agonists and SGLT2 inhibitors can lower average blood sugar and body weight as add-on therapies. The evidence base in type 1 diabetes is much thinner, and no phase 3 trial had previously tested both classes together alongside insulin in this population.
The TTT1 trial is designed to fill that gap. If the results show that triple therapy meaningfully reduces glycated hemoglobin compared with insulin alone or insulin plus one adjunct compound, and that the safety profile is acceptable, the literature will have a much stronger foundation for understanding whether this approach warrants broader investigation.
Early data from smaller studies have pointed at possible benefits, but phase 3 evidence carries a different weight because it is larger, more rigorously controlled, and more directly informative about real-world safety at scale. The trial record describes this study as providing clinically useful information on combination adjunct therapy in type 1 diabetes, which is a carefully measured claim that reflects where the evidence currently stands and what this trial is designed to contribute.
