Retatrutide

What Retatrutide is

Retatrutide, also known by its development code LY3437943, is an investigational single-molecule peptide agonist developed by Eli Lilly and Company ^[1]. It was designed to simultaneously engage three metabolically relevant G-protein-coupled receptors: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R), and the glucagon receptor (GCGR) ^[2]. The compound's foundational pharmacology was publicly described by Coskun and colleagues in a 2022 paper in Cell Metabolism, which characterized the molecule's receptor binding profile, reported its behaviour in diet-induced obese animal models, and included first-in-human single-ascending-dose pharmacokinetic data ^[1][3]. To extend its circulating half-life and enable weekly administration, retatrutide was engineered with a fatty diacid moiety conjugated to the peptide backbone — a strategy that promotes reversible binding to human serum albumin, analogous to approaches used with semaglutide and tirzepatide ^[4].

The molecule's receptor potency profile is notably asymmetric: based on cell culture studies, retatrutide is more potent than endogenous GIP at the human GIPR by a factor of approximately 8.9, while being less potent than endogenous ligands at the GLP-1R and GCGR ^[4][5]. This deliberate balance was the product of systematic peptide engineering and screening in cell-based receptor activation assays and preclinical animal models before LY3437943 was selected as the development candidate ^[3]. The pharmacokinetics of retatrutide are considered dose-proportional, with an approximate half-life of six days ^[4]. From its preclinical origins, the compound moved through a Phase 1b proof-of-concept study published in The Lancet in 2022 ^[6], two landmark Phase 2 trials published in 2023 ^[2][7], and is now in the Phase 3 TRIUMPH and TRANSCEND registrational programmes ^[8].

What it is being researched for

1. Obesity and body weight reduction

The most extensively studied application of retatrutide is in obesity. A Phase 2, randomised, double-blind, placebo-controlled trial (NCT04881760, n=338, 48 weeks) published in the New England Journal of Medicine in 2023 found a mean body weight reduction of 24.2% in the highest-dose group, compared with 2.1% in the placebo group ^[2]. A systematic review and meta-analysis of randomised controlled trials confirmed that retatrutide produced significant, dose-dependent reductions in body weight, body mass index, and waist circumference versus placebo ^[9]. Phase 3 TRIUMPH trials, designed to enrol over 5,800 participants across four studies, are evaluating long-term efficacy and safety for weight management in adults with and without type 2 diabetes, and in those with established cardiovascular disease ^[8].

2. Type 2 diabetes and glycaemic control

Retatrutide has been studied in people with type 2 diabetes (T2D) beginning with a Phase 1b multiple-ascending-dose trial published in The Lancet (2022), which established acceptable pharmacokinetics, safety, and early evidence of glucose and weight reduction ^[6]. A subsequent Phase 2 trial in 281 participants with T2D, conducted over 36 weeks, showed significant and clinically meaningful improvements in glycaemic control, including reductions in HbA1c of approximately 2.2%, with 82% of participants reaching an HbA1c at or below 6.5% ^[7]. These findings positioned retatrutide as a potentially more efficacious option for concurrent weight and glucose management compared with current approved agents, though longer-term and larger-scale Phase 3 data remain ongoing ^[8].

3. Metabolic liver disease (MASLD/MASH)

A pre-specified substudy of the Phase 2 obesity trial investigated the effect of retatrutide on metabolic dysfunction-associated steatotic liver disease (MASLD). In the 98 participants with MASLD and at least 10% liver fat at baseline, retatrutide produced dose-dependent reductions in liver fat at 24 weeks, ranging from approximately 43% at the lowest studied level to more than 82% at the higher levels, all statistically significant versus placebo ^[4]. Normal liver fat (below 5%) was achieved by 86% of participants in the highest-dose group at 24 weeks, compared with 0% of those receiving placebo ^[4]. These reductions were also accompanied by improvements in visceral adipose tissue, insulin sensitivity, and biomarkers of liver injury and fibrosis ^[4]. The glucagon receptor component of retatrutide is thought to contribute specifically to liver fat reduction, beyond what is achievable with GLP-1 mono-agonists or dual GIP/GLP-1 agonists ^[5].

4. Cardiovascular risk markers and lipid profile

Phase 2 trial data published in the New England Journal of Medicine and subsequent post-hoc analyses have documented improvements in several cardiometabolic risk markers with retatrutide treatment, including reductions in systolic and diastolic blood pressure, triglycerides, total cholesterol, and LDL-C ^[2][10]. A post-hoc analysis of Phase 2 trials further characterised changes in apolipoprotein and lipoprotein subclasses and inflammatory biomarkers, noting meaningful reductions in atherogenic lipid fractions ^[10]. Researchers have proposed that the glucagon receptor agonism component may contribute to lipid improvements partly through effects on PCSK9 degradation ^[2]. A dedicated cardiovascular and renal outcomes trial (TRIUMPH-Outcomes, NCT06383390) is ongoing to determine whether these surrogate improvements translate to hard cardiovascular endpoints ^[11].

5. Obstructive sleep apnea (OSA)

As part of the Phase 3 TRIUMPH programme, retatrutide is being evaluated for its effect on moderate-to-severe obstructive sleep apnea as a complication of obesity ^[8]. The TRIUMPH-1 and TRIUMPH-2 basket trials include nested OSA protocols, with the primary OSA endpoint being change in the Apnea-Hypopnea Index (AHI) ^[8]. Early phase 3 data have suggested reductions in AHI of up to 60.6% in participants treated with retatrutide, and Eli Lilly has highlighted these as part of the compound's broader obesity-complication profile ^[12]. This research area reflects a growing scientific interest in pharmacological weight reduction as a treatment strategy for sleep-disordered breathing ^[8].

6. Knee osteoarthritis

TRIUMPH-4 (NCT05931367) is a Phase 3 randomised, double-blind, placebo-controlled trial specifically evaluating retatrutide in adults with obesity or overweight and osteoarthritis of the knee ^[11]. The primary endpoint for osteoarthritis includes change in the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain subscale score ^[8]. Phase 3 data reported in 2026 indicated significant relief of osteoarthritis knee pain compared with placebo, alongside substantial reductions in body weight, cholesterol, and inflammatory markers ^[12]. This trial exemplifies the broader hypothesis that effective obesity pharmacotherapy may reduce mechanical and inflammatory burdens on weight-bearing joints.

7. Chronic kidney disease

A completed Phase 2b study (TRANSCEND-CKD, NCT05936151) investigated retatrutide's renal mechanism of action in participants with overweight or obesity and chronic kidney disease (CKD), with or without T2D ^[13]. Secondary analyses from Phase 2 metabolic trials had suggested that higher doses of retatrutide were associated with reductions in urinary albumin, a marker of kidney damage, and some improvements in kidney function parameters in participants without T2D ^[12]. The TRANSCEND-CKD trial was designed to more formally characterise these renal effects, and its findings are expected to inform the ongoing cardio-kidney outcome trial TRIUMPH-Outcomes ^[11].

How it is thought to work

Retatrutide works by simultaneously activating three distinct hormone receptors that collectively regulate appetite, blood glucose, energy expenditure, and fat metabolism ^[1][2]. GLP-1 receptor agonism is the best-characterised component: it slows gastric emptying (reducing the rate of nutrient absorption), suppresses appetite through central nervous system pathways, and stimulates glucose-dependent insulin secretion while suppressing glucagon release from the pancreas ^[5][14]. GIP receptor agonism, at which retatrutide is roughly 8.9 times more potent than the endogenous hormone, is thought to enhance insulin secretion, modulate fat tissue metabolism, and contribute independently to appetite suppression and weight reduction — a mechanism that distinguishes it from pure GLP-1 agonists ^[4][5]. The glucagon receptor component, which is less potent than endogenous glucagon, is understood to drive increases in basal energy expenditure through stimulation of hepatic thermogenesis and fatty-acid beta-oxidation, as well as reductions in liver fat — effects absent from GLP-1 mono-agonists ^[1][5].

The three-receptor strategy is considered synergistic rather than additive: preclinical studies in obese animal models showed that weight reduction with the triple agonist exceeded what was achievable with a selective GLP-1R agonist alone, partly attributable to the additional energy expenditure driven by glucagon receptor activation ^[1]. At the molecular level, cryo-electron microscopy structural studies have described how retatrutide simultaneously engages each receptor's extracellular domains, and researchers have confirmed that all three receptor interactions are required for the compound's full metabolic profile ^[5]. Retatrutide also delays gastric emptying in humans, a property documented in a dedicated Phase 1b sub-study, which contributes to its effects on post-meal glucose excursions and satiety ^[14]. The fatty diacid moiety attached to the peptide enables reversible albumin binding, extending the half-life to approximately six days and making once-weekly administration pharmacologically feasible ^[4].

Where the evidence stands

The evidence base for retatrutide spans preclinical animal studies, Phase 1 human pharmacology, two published Phase 2 randomised controlled trials, and an expanding Phase 3 programme. Preclinical work, reported in the founding 2022 Cell Metabolism paper by Coskun and colleagues, characterised receptor pharmacology in vitro and demonstrated superior weight loss in diet-induced obese rodent models compared with a selective GLP-1R agonist, with the additional weight loss attributed to glucagon receptor-mediated increases in energy expenditure ^[1]. A Phase 1b, placebo-controlled, multiple-ascending-dose trial in people with T2D (Urva et al., Lancet, 2022; n=72, 12 weeks) established acceptable safety and pharmacokinetics and provided clinical proof of concept, showing dose-dependent reductions in glucose and body weight ^[6]. The two landmark Phase 2 trials — one in people with obesity without T2D (Jastreboff et al., NEJM, 2023; n=338, 48 weeks) and one in people with T2D (Rosenstock et al., Lancet, 2023; n=281, 36 weeks) — provided the first robust human efficacy data and represent the strongest published evidence to date ^[2][7]. A systematic review and meta-analysis of these trials, encompassing 640 patients, confirmed the weight-reduction and glycaemic findings and characterised the gastrointestinal adverse event profile, which was primarily mild-to-moderate in severity ^[9].

Several important limitations and evidence gaps remain. Phase 2 trials, while well-designed, are not powered for hard clinical endpoints such as cardiovascular events, mortality, or long-term kidney outcomes, and follow-up periods of 36–48 weeks do not capture long-term durability of effect or late-emerging safety signals. The Phase 2 MASLD substudy (n=98) was pre-specified but small, and the liver fat findings, while striking, require confirmation in dedicated larger trials ^[4]. No long-term head-to-head data comparing retatrutide with approved agents (semaglutide or tirzepatide) in a powered randomised trial have been published as of 2026. Phase 3 TRIUMPH trials are ongoing and are expected to provide the definitive efficacy and safety data needed for regulatory review, including dedicated cardiovascular and renal outcome studies ^[8][11]. Retatrutide has no approved regulatory status in any jurisdiction as of mid-2026, and all research findings are investigational ^[3].

Frequently asked questions

What is retatrutide and how is it different from semaglutide or tirzepatide?

Retatrutide is an investigational peptide that activates three hormone receptors simultaneously — GIP, GLP-1, and glucagon — whereas semaglutide targets only the GLP-1 receptor and tirzepatide targets GLP-1 and GIP receptors. The addition of glucagon receptor agonism is believed to increase energy expenditure and provide greater reductions in liver fat, effects not seen with the other two agents. Phase 2 data suggest greater average weight loss with retatrutide than has been reported with semaglutide or tirzepatide in comparable studies, though no direct head-to-head randomised trials have been published. Retatrutide remains investigational and is not approved for any clinical use.

Is retatrutide approved by the FDA or Health Canada?

No. As of mid-2026, retatrutide has not received regulatory approval from the FDA, Health Canada, the EMA, or any other major regulatory authority. It is still in Phase 3 clinical trials under Eli Lilly's TRIUMPH and TRANSCEND programmes. Approval, if granted, would follow successful completion and regulatory review of those trials.

What did the Phase 2 clinical trial show about retatrutide and weight loss?

In a 48-week Phase 2 randomised, double-blind, placebo-controlled trial in 338 adults with obesity, retatrutide produced an average body weight reduction of 24.2% in the highest-dose group, compared with 2.1% in the placebo group. Reductions in blood pressure, fasting glucose, insulin, and lipids were also observed. These Phase 2 results, published in the New England Journal of Medicine in 2023, are considered the strongest published human evidence to date, though Phase 3 confirmation is ongoing.

What conditions is retatrutide being studied for beyond obesity?

Beyond obesity itself, retatrutide is being investigated for type 2 diabetes, metabolic dysfunction-associated steatotic liver disease (MASLD/MASH), obstructive sleep apnea, knee osteoarthritis, chronic kidney disease, cardiovascular outcomes, and chronic low back pain. These studies are part of Eli Lilly's Phase 3 TRIUMPH and TRANSCEND clinical programmes, which together encompass thousands of participants across multiple trial designs.

What are the known side effects of retatrutide reported in trials?

The most commonly reported adverse effects in Phase 2 trials were gastrointestinal in nature, including nausea, diarrhoea, vomiting, and constipation, consistent with the GLP-1 receptor agonist class. These effects were generally described as mild-to-moderate in severity. A dose-dependent increase in heart rate was also observed in the Phase 2 obesity trial, peaking around week 24 before partially easing. The full long-term safety profile is still being characterised in ongoing Phase 3 studies.

What did research show about retatrutide and fatty liver disease?

A pre-specified Phase 2 substudy in participants with MASLD and at least 10% liver fat showed dose-dependent reductions in liver fat at 24 weeks, with reductions exceeding 80% at the higher doses studied, versus no meaningful change with placebo. Normal liver fat levels were achieved by 86% of higher-dose participants. These results, published in Nature Medicine in 2024, were more pronounced than liver fat reductions reported with either GLP-1 mono-agonists or dual GIP/GLP-1 agonists in comparable studies.

How does the glucagon receptor component of retatrutide contribute to its effects?

Glucagon receptor agonism is thought to increase basal energy expenditure by stimulating hepatic thermogenesis and fatty-acid beta-oxidation — a thermogenic mechanism distinct from the appetite-suppression pathways of GLP-1 and GIP. Preclinical studies demonstrated that this component augmented weight loss beyond what GLP-1 or GIP agonism alone could achieve. Glucagon receptor activity is also believed to contribute to the substantial liver fat reductions observed with retatrutide, and to positive effects on LDL cholesterol potentially through PCSK9 pathways.

What is the TRIUMPH clinical trial programme?

TRIUMPH is Eli Lilly's Phase 3 registrational programme for retatrutide, consisting of four multicenter, randomised, double-blind, placebo-controlled studies enrolling over 5,800 participants. The four trials evaluate weight management in adults with obesity with and without type 2 diabetes (TRIUMPH-1 and 2), weight management in adults with established cardiovascular disease (TRIUMPH-3), and a stand-alone knee osteoarthritis trial (TRIUMPH-4). Nested within the basket trials are protocols for obstructive sleep apnea. The programme uses a novel basket trial design that simultaneously evaluates multiple obesity-related complications.

Is retatrutide being studied for kidney disease?

Yes. A completed Phase 2b study (TRANSCEND-CKD, NCT05936151) specifically investigated retatrutide's effects on renal function in participants with overweight or obesity and chronic kidney disease, with or without type 2 diabetes. Phase 2 analyses had suggested reductions in urinary albumin (a marker of kidney damage) with higher doses. Results from TRANSCEND-CKD are expected to inform the design and interpretation of the longer-term cardio-kidney outcome trial TRIUMPH-Outcomes.

Where can researchers find the primary published literature on retatrutide?

The key peer-reviewed publications include Coskun et al. (Cell Metabolism, 2022) for the founding preclinical and Phase 1 pharmacology; Urva et al. (The Lancet, 2022) for the Phase 1b multiple-ascending-dose trial in T2D; Jastreboff et al. (NEJM, 2023) for the Phase 2 obesity trial; Rosenstock et al. (The Lancet, 2023) for the Phase 2 T2D trial; and a Nature Medicine (2024) publication for the Phase 2a MASLD substudy. Active Phase 3 trials can be found on ClinicalTrials.gov under the TRIUMPH programme identifier.

Glossary

Triple agonist

A molecule that activates three distinct receptor types simultaneously — in retatrutide's case, the GIP, GLP-1, and glucagon receptors — combining multiple pharmacological actions in a single compound.

GLP-1 receptor (GLP-1R)

A cell-surface receptor activated by glucagon-like peptide-1, involved in stimulating insulin secretion, suppressing appetite, and slowing gastric emptying.

GIP receptor (GIPR)

A receptor for glucose-dependent insulinotropic polypeptide, an incretin hormone involved in enhancing insulin secretion after meals and regulating fat tissue metabolism.

Glucagon receptor (GCGR)

A receptor for glucagon, a hormone primarily known for raising blood glucose by stimulating hepatic glucose production; GCGR agonism is also associated with increased energy expenditure and liver fat reduction.

MASLD (Metabolic dysfunction-associated steatotic liver disease)

The current clinical term for excess fat accumulation in the liver linked to metabolic conditions such as obesity, insulin resistance, and dyslipidaemia, formerly called non-alcoholic fatty liver disease (NAFLD).

Apnea-Hypopnea Index (AHI)

A measure of sleep apnea severity, representing the number of breathing interruptions or partial obstructions per hour of sleep; used as a primary endpoint in retatrutide's OSA trials.

Incretin

A class of gut-derived hormones, including GLP-1 and GIP, released in response to food intake that amplify insulin secretion in a glucose-dependent manner.

Fatty diacid moiety

A lipid chain chemically attached to a therapeutic peptide to promote albumin binding, which slows the molecule's clearance from the bloodstream and extends its half-life to enable once-weekly administration.

References

1. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept — Cell Metabolism (2022)
2. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — New England Journal of Medicine (2023)
3. Structural insights into the triple agonism at GLP-1R, GIPR and GCGR manifested by retatrutide — PMC / Nature Communications (2024)
4. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial — PMC / Nature Medicine (2024)
5. Triple Agonism Based Therapies for Obesity — PMC / Current Obesity Reports (2025)
6. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial — The Lancet (2022)
7. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial conducted in the USA — The Lancet (2023)
8. Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials — PubMed / Diabetes, Obesity and Metabolism (2026)
9. Effects of once-weekly subcutaneous retatrutide on weight and metabolic markers: A systematic review and meta-analysis of randomized controlled trials — PMC (2024)
10. Retatrutide-Associated Improvements in Cardiovascular Risk Biomarkers in Adults With Obesity With or Without Type 2 Diabetes — Diabetes, Obesity and Metabolism (2025)
11. TRIUMPH-4: A Study of Retatrutide (LY3437943) Once Weekly in Participants Who Have Obesity or Overweight and Osteoarthritis of the Knee — ClinicalTrials.gov — ClinicalTrials.gov
12. Retatrutide—A Game Changer in Obesity Pharmacotherapy — PMC (2025)
13. Rationale, design and baseline characteristics of the TRANSCEND-CKD trial of retatrutide in patients with chronic kidney disease — PMC / Nephrology Dialysis Transplantation (2025)
14. The novel GIP, GLP-1 and glucagon receptor agonist retatrutide delays gastric emptying — Diabetes, Obesity and Metabolism (2023)