Retatrutide (GLP-3 RT): A Research Guide to the Published Studies

What retatrutide is

Retatrutide was developed by Eli Lilly and Company. The 2022 discovery paper in Cell Metabolism describes a 39-amino-acid peptide engineered from a GIP backbone. Three positions carry non-coded amino acids: Aib at positions 2 and 20, and alpha-methyl-L-leucine at position 13. The authors report that Aib at position 2 protects the peptide against cleavage by the enzyme DPP-4.

A C20 fatty diacid is attached through a linker to the lysine at position 17. The paper describes this as a way to bind albumin and extend the molecule's half-life.

The number of receptors is what sets it apart. Semaglutide acts on the GLP-1 receptor alone. Tirzepatide acts on the GIP and GLP-1 receptors. Retatrutide adds the glucagon receptor as a third target. In the discovery paper, glucagon receptor activity was linked to higher energy expenditure in mice, on top of the lower calorie intake associated with GIP and GLP-1 receptor activity.

Receptor pharmacology and first human data

Coskun and colleagues measured potency as cAMP accumulation in HEK-293 cell lines that express each human receptor at low density, about 1,300 to 1,700 receptors per cell. The authors chose low receptor numbers to limit signal amplification. Retatrutide acted as a full agonist at all three receptors.

The authors sum this up as more activity at the GIP receptor and balanced activity at the glucagon and GLP-1 receptors. At the GLP-1 receptor, retatrutide showed a partial agonist profile for beta-arrestin recruitment.

In diet-induced obese mice, the paper reported dose-dependent reductions in body weight and calorie intake. The authors report that glucagon receptor activity accounted for 30% to 35% of the body-weight reduction in mice, through increased energy expenditure.

The same paper reported a single-dose phase 1 study in which 45 healthy participants in Singapore received retatrutide or placebo. Mean half-life was about 6 days, and exposure rose nearly in proportion to dose. A 12-week phase 1b study in 72 adults with type 2 diabetes (The Lancet, 2022) also reported a half-life of about 6 days. Its highest-dose group showed a placebo-adjusted body-weight change of up to -8.96 kg.

The phase 2 obesity trial (NEJM, 2023)

Jastreboff and colleagues ran a 48-week, double-blind, placebo-controlled phase 2 trial in 338 adults, 51.8% of them men. Participants had a BMI of 30 or higher, or 27 to under 30 with at least one weight-related condition. The primary endpoint was the percent change in body weight at 24 weeks, with 48 weeks as a secondary endpoint.

At 48 weeks, a reduction of 15% or more had occurred in 60%, 75% and 83% of participants in the 4 mg, 8 mg and 12 mg groups, compared with 2% with placebo.

A substudy of the same trial (Sanyal and colleagues, Nature Medicine, 2024) included 98 participants with metabolic dysfunction-associated steatotic liver disease (MASLD) and at least 10% liver fat. Mean relative change in liver fat at 24 weeks was -42.9%, -57.0%, -81.4% and -82.4% in the 1, 4, 8 and 12 mg groups, versus +0.3% with placebo. Liver fat under 5% was reached by 27%, 52%, 79% and 86% of those groups, and by none of the placebo group.

The type 2 diabetes trials

Rosenstock and colleagues (The Lancet, 2023) randomized 281 adults with type 2 diabetes at 42 US sites to retatrutide, placebo or dulaglutide 1.5 mg, a GLP-1 receptor agonist used as the active comparator. The primary endpoint was the change in glycated hemoglobin (HbA1c) at 24 weeks, with body weight at 36 weeks as a secondary endpoint.

The paper reported HbA1c reductions significantly greater than placebo in every retatrutide group except 0.5 mg. At 4 mg and above, body-weight reductions were significantly greater than with placebo and with dulaglutide.

The first peer-reviewed phase 3 paper appeared in 2026. TRANSCEND-T2D-1 (Bajaj and colleagues, The Lancet) randomized 537 adults whose type 2 diabetes was not controlled by diet and exercise alone, at 48 sites in the USA, Mexico and India. At 40 weeks, using the paper's treatment-regimen estimand, mean HbA1c change was -1.69, -1.86 and -1.94 percentage points in the 4 mg, 9 mg and 12 mg groups, versus -0.81 with placebo. Mean body-weight change was -11.5%, -13.9% and -15.3%, versus -2.6% with placebo.

Adverse events as the papers report them

Across the published trials, the most common adverse events were gastrointestinal. The NEJM obesity paper describes them as dose-related and mostly mild to moderate. In the phase 2 diabetes trial, mild-to-moderate gastrointestinal events such as nausea, diarrhea, vomiting and constipation were reported in 35% of participants in the retatrutide groups (from 13% at 0.5 mg to 50% in one 8 mg group), 13% on placebo and 35% on dulaglutide.

Heart rate is the other recurring finding. The NEJM paper reports dose-dependent increases in heart rate that peaked at 24 weeks and declined afterward. In the single-dose phase 1 study, pulse rate rose in every group including placebo, with numerically larger increases on retatrutide. That study reported no deaths or serious adverse events. It did record transient rises in pancreatic enzymes in the 3 mg and higher groups, back to baseline within 4 to 6 weeks, and anti-drug antibodies in one participant.

In TRANSCEND-T2D-1, discontinuation of the study drug because of adverse events was 2% to 5% with retatrutide, versus 0% with placebo. Neither diabetes trial reported severe hypoglycemia. The phase 2 diabetes trial reported no deaths. TRANSCEND-T2D-1 reported two deaths, both in the 4 mg group, which the paper states were unrelated to the study drug.

Open questions and what is not yet known

Phase 3 obesity results are not yet in the peer-reviewed record. The program's design paper describes four registrational phase 3 trials (TRIUMPH-1 to TRIUMPH-4) in more than 5,800 participants, covering obesity, obstructive sleep apnea and knee osteoarthritis, with one trial in people with cardiovascular disease. ClinicalTrials.gov listed all four as completed on September 28, 2026. The sponsor announced topline TRIUMPH-1 results by press release on May 21, 2026, and said detailed results would go to peer-reviewed journals. A PubMed search on September 28, 2026 found no peer-reviewed TRIUMPH results paper, so this page reports none of those numbers.

No head-to-head results against tirzepatide or semaglutide had been published as of the same date. ClinicalTrials.gov listed TRIUMPH-5, a comparison with tirzepatide, and TRANSCEND-T2D-2, a comparison with semaglutide, as active and not recruiting.

Long-term outcomes are open. TRIUMPH-Outcomes, a cardiovascular and kidney outcomes trial with an estimated 10,000 participants, lists an estimated primary completion date of February 2029.

Most of the evidence comes from one source. The human studies summarized here were sponsored by the developer, and the receptor pharmacology comes from its own discovery paper. Independent replication is still needed. The discovery paper's authors also call for clinical work on resting energy expenditure and appetite to show how the glucagon component contributes in people.

Laboratory handling

Retatrutide reference material is usually supplied as a lyophilized (freeze-dried) powder. Standard practice for lipidated peptides applies.

• Store sealed vials cold, dry and protected from light. Long-term storage is usually frozen.
• Let a cold vial reach room temperature before opening, so moisture does not condense on the powder.
• Once in solution, divide the material into single-use aliquots so no portion goes through repeated freeze-thaw cycles.
• Record the lot number, solvent, concentration and preparation date for every stock.
• Note the protein content of buffers and media. The fatty diacid is built to bind albumin, which is why the discovery paper ran its cAMP assays with and without added human serum albumin.

This page gives no dosing, administration route or reconstitution volumes, by design.

How to read a lot report

A lot report (certificate of analysis) is useful only if it answers specific questions. A complete report for this material covers the points below.

• Traceability: the lot number matches the vial label, and the report names the testing lab, the test date and the methods used.
• Identity: mass spectrometry confirms the expected mass. KEGG lists an average molecular weight of 4731.34. Published LC-HRMS methods, such as a 2026 Journal of Chromatography A method covering nine GLP-1 receptor agonists including retatrutide, pair accurate mass with fragment ions for structural confirmation.
• Purity by HPLC: the main peak as a share of total peak area, ideally with the chromatogram attached. Purity says how much of the peptide material is the target molecule. It does not say how much peptide is in the vial.
• Content (assay): the measured amount of peptide per vial compared with the label claim. Net peptide content accounts for water and counter-ions.
• Endotoxin: usually measured with a LAL-based test and reported in endotoxin units (EU). It matters for cell-based work.
• Sterility: whether microorganisms grew from the samples tested. If the test was not run, the report should say so.
• Heavy metals: elemental impurities, usually measured by ICP-MS. ICH Q3D is the elemental impurity guideline used for pharmaceutical products.