Tirzepatide shows greater improvements than semaglutide in 28-week islet function trial

A 28-week Phase 1 trial published April 22, 2022 in The Lancet Diabetes & Endocrinology reported that tirzepatide 15 mg produced greater improvements in pancreatic islet function and insulin sensitivity than semaglutide 1 mg or placebo in adults with type 2 diabetes ^[1].

The study was led by Heise and colleagues and described as a multicentre, randomised, double-blind, parallel-arm Phase 1 clinical trial ^[1]. It compared three treatment groups: tirzepatide 15 mg, semaglutide 1 mg, and placebo ^[1].

Rather than tracking routine clinical endpoints, the trial used glucose clamps, a research technique that measures how the body responds to controlled changes in blood sugar. The researchers used clamp-derived measurements to look at two things: insulin sensitivity, which reflects how well the body responds to insulin, and insulin secretion, which reflects how much insulin the pancreas releases ^[1]. The study's title states its focus as the effects of subcutaneous tirzepatide versus placebo or semaglutide on pancreatic islet function and insulin sensitivity ^[1].

According to the trial, tirzepatide 15 mg led to greater improvements on those clamp-derived measures of insulin sensitivity and insulin secretion than semaglutide 1 mg or placebo ^[1].

Tirzepatide and semaglutide are different molecules. Semaglutide is sold as Ozempic, Wegovy and Rybelsus; tirzepatide is sold as Mounjaro and Zepbound. The trial compared the two active drugs at specific doses — tirzepatide 15 mg and semaglutide 1 mg — and those doses may not match what any individual patient is prescribed.

Why it matters for patients

The findings address how the two drugs act on the pancreas, not how well they work over time in everyday use. Insulin sensitivity and insulin secretion are laboratory measurements taken under controlled conditions. They are not the same as long-term blood sugar control, weight change, heart or kidney outcomes, or how a person feels day to day, and this trial did not report results on those endpoints ^[1].

That distinction matters for anyone comparing the two medications. A difference measured with a glucose clamp in a Phase 1 study does not by itself establish that one drug will control blood sugar better, produce more weight loss, or cause fewer side effects than the other in routine care. The study population was adults with type 2 diabetes, so the results do not directly describe people taking these drugs for weight management without diabetes.

The trial also does not establish whether the differences it found were large enough to change clinical decisions, and the publication does not report how the differences compare with other measures of diabetes control. It does not report cost, insurance coverage, or a head-to-head comparison of side effects.

What has not been reported

Key details are not available in the published report as described here, including how many participants were enrolled, how the three groups differed on standard diabetes measures such as A1C, and whether the differences in insulin sensitivity and secretion reached statistical significance for every measure tested. Whether the mechanism differences seen at 28 weeks translate into different long-term outcomes for patients has not been established by this trial ^[1].

The comparison at the heart of the study — tirzepatide 15 mg against semaglutide 1 mg — was conducted at doses chosen for a Phase 1 mechanism trial, and results at those doses should not be assumed to apply to other doses or to other groups of patients.