The Devel­op­ment History of Retatrutide

Retatrutide did not appear out of nowhere. It grew out of more than a decade of work on gut hormones and on the idea that one engineered peptide could act on several hormone receptors at once. This page follows that path from early concept to today's large clinical programme.

From One Receptor to Three

The first GLP-1-based medicines of the 2000s acted on a single receptor. During the 2010s, several academic and industry groups began designing "co-agonists" that combined GLP-1 activity with GIP or glucagon activity, hoping for larger metabolic effects than either hormone alone. Lilly's dual GIP and GLP-1 agonist tirzepatide was the first of these to reach the market. Retatrutide took the concept a step further by adding the glucagon receptor to the mix.

Laboratory Design and First Human Doses

Lilly scientists described the molecule, then known only as LY3437943, in a 2022 paper in Cell Metabolism whose title summed up its scope: from discovery to clinical proof of concept. The paper reported three things: its receptor profile in cell assays, its effects on body weight and blood sugar in obese mice, and a first single-ascending-dose study in people. After one injection, weight reduction in that study was still visible 43 days later, and the drug's behaviour in the body supported weekly dosing.

A multiple-dose Phase 1b study in 72 adults with type 2 diabetes followed. It ran at four US centres between December 2019 and December 2020 and was published in The Lancet in late 2022. Gastrointestinal complaints were the most common side effects, blood sugar fell in the higher-dose groups, and body weight decreased in a dose-dependent way.

A Name and a Breakthrough Phase 2

By the time its Phase 2 results appeared in 2023, the compound had received its international non-proprietary name, retatrutide. In June 2023 two Phase 2 trials were presented at the American Diabetes Association meeting and published the same day: one in 338 adults with obesity (New England Journal of Medicine) and one in 281 adults with type 2 diabetes (The Lancet). The obesity trial's weight results, around 24% average loss in the top dose group after 48 weeks, drew wide media coverage because they were larger than anything seen in earlier trials of hormone-based drugs.

Follow-up analyses of the same trials looked at liver fat (Nature Medicine, 2024), body composition measured by DXA scans (The Lancet Diabetes & Endocrinology, 2025) and blood lipid and inflammation markers (2026).

The Phase 3 Programmes

Lilly began Phase 3 testing in 2023 under two programme names. TRIUMPH covers obesity and its complications. According to its published design paper, it includes four registrational trials with more than 5,800 participants, looking at weight, obstructive sleep apnoea, knee osteoarthritis and people with established cardiovascular disease. TRANSCEND covers type 2 diabetes. Separate studies examine kidney function (TRANSCEND-CKD) and long-term heart and kidney outcomes (TRIUMPH-Outcomes).

2026: First Phase 3 Results

• May 2026: Lilly announced the first Phase 3 obesity results, reporting an average weight loss of 28% after 80 weeks of treatment.
• June 2026: TRANSCEND-T2D-1, the first Phase 3 diabetes trial, was published in The Lancet.
• July 2026: Lilly reported that two more Phase 3 obesity trials had met their goals and said it intends to file for regulatory approval in 2027.
• August 2026: Lilly opened an early-access programme for a limited group of patients ahead of any approval.

What Comes Next

Regulators will only review retatrutide once Lilly submits a full application, and a review usually takes a year or more. Until a decision is made, retatrutide remains an investigational compound, whatever the results of individual trials. Several trials, including long-term outcome studies, are still running and will shape how regulators judge its benefits and risks.