Retatrutide: Triple Receptor Agonism and Phase 3 Clinical Development
A summary of published research on retatrutide (LY3437943), a synthetic peptide currently under Phase 3 investigation. This overview covers its mechanism of action, receptor pharmacology, Phase 2 outcomes, and comparison with dual GIP/GLP-1 agonists. For research purposes only. Not for human therapeutic use.
What Is Retatrutide?
Retatrutide (development code LY3437943) is a synthetic, acylated peptide developed by Eli Lilly. It is an agonist at three distinct G-protein-coupled receptors: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R), and the glucagon receptor (GCGR).
This triple-receptor profile distinguishes retatrutide from currently approved agents in the incretin space. Semaglutide acts at the GLP-1 receptor only (single agonist). Tirzepatide acts at GIP and GLP-1 receptors (dual agonist). Retatrutide adds glucagon receptor agonism as a third mechanism — making it the first triple incretin agonist to reach Phase 3 clinical trials.
The peptide has a C18 fatty diacid moiety attached via a linker, giving it a plasma half-life of approximately one week and enabling once-weekly subcutaneous dosing — consistent with other long-acting GLP-1-class compounds currently in clinical use.
Mechanism of Action: Three Receptor Pathways
GLP-1 Receptor Agonism
GLP-1R agonism is the primary mechanism shared across most agents in this class. Activation enhances glucose-dependent insulin secretion, suppresses glucagon release, slows gastric emptying, and acts centrally to reduce appetite via hypothalamic pathways. These combined effects produce reductions in both caloric intake and postprandial glycaemic excursions.
GIP Receptor Agonism
GIPR agonism enhances the insulinotropic response and has demonstrated favourable effects on adipose tissue metabolism in preclinical models. The GIP component in tirzepatide has been hypothesised to contribute to its improved tolerability profile compared with GLP-1-only agents, though the precise contribution in humans is still under investigation.
Glucagon Receptor Agonism
The glucagon receptor component is the distinguishing pharmacological feature of retatrutide relative to currently approved agents. Glucagon receptor activation increases hepatic glucose output and stimulates thermogenesis and energy expenditure — effects that appear potentially counterintuitive in a metabolic context. However, when glucagon agonism is combined with GLP-1R agonism, the glycaemic effects of glucagon are blunted, while its thermogenic and lipolytic effects on adipose tissue and energy balance are preserved.
This mechanism has been of particular interest in research into non-alcoholic fatty liver disease (NAFLD/MASH), where glucagon receptor signalling in the liver may provide additional hepatic benefits beyond what is achievable through GLP-1 agonism alone.
Phase 2 Trial Outcomes (Published Data)
Results from the Phase 2 trial of retatrutide (NEJM, 2023, Jastreboff et al.) in adults with obesity were published in 2023. The trial was a double-blind, randomised, placebo-controlled study across multiple dose cohorts (1mg, 4mg, 8mg, 12mg once weekly) over 24 weeks, with a 24-week extension period.
Selected Phase 2 Findings (Highest Dose Cohort at 48 Weeks)
- Mean body weight reduction of approximately 24% from baseline in the highest dose cohort (12mg) at 48 weeks
- Reductions in waist circumference, systolic blood pressure, and fasting serum triglycerides observed across multiple dose groups
- Dose-dependent reductions in liver fat fraction, assessed via MRI-PDFF in a subset of participants
- Adverse events were consistent with the GLP-1 class (predominantly gastrointestinal: nausea, vomiting, decreased appetite), with dose-dependent incidence
Source: Jastreboff AM et al. "Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." N Engl J Med. 2023;389(6):514-526. DOI: 10.1056/NEJMoa2301972. All data represents Phase 2 results and does not constitute evidence of safety or efficacy for any therapeutic indication.
Phase 3 Clinical Programme: TRIUMPH Trials
Following Phase 2 results, Eli Lilly initiated the TRIUMPH Phase 3 programme. This programme includes multiple trials evaluating retatrutide across different populations:
- TRIUMPH-1: Adults with obesity (BMI ≥30 or ≥27 with weight-related comorbidity) without type 2 diabetes. Primary endpoint: percentage change in body weight at 72 weeks.
- TRIUMPH-2: Adults with type 2 diabetes and obesity. Evaluating glycaemic and weight outcomes at 52 weeks.
- TRIUMPH-3: Cardiovascular outcomes trial — evaluating major adverse cardiovascular events (MACE) in a high-CV-risk population.
- TRIUMPH-NASH: Adults with biopsy-confirmed non-alcoholic steatohepatitis (NASH/MASH), assessing histological liver improvement at 48 weeks.
Phase 3 results for the obesity and type 2 diabetes cohorts are anticipated from 2025–2027 depending on trial timeline and regulatory submission strategy. The NASH trial represents a particularly distinct area of investigation, as no GLP-1 class agent has yet achieved broad regulatory approval specifically for MASH resolution with fibrosis improvement.
Retatrutide vs Tirzepatide: Dual vs Triple Agonism
Tirzepatide (approved under the Mounjaro brand for type 2 diabetes and as Zepbound for obesity) is the closest comparator to retatrutide in terms of mechanism. Both share GIP and GLP-1 agonism; retatrutide adds glucagon receptor agonism.
| Feature | Tirzepatide | Retatrutide |
|---|---|---|
| GLP-1R Agonism | ✓ | ✓ |
| GIPR Agonism | ✓ | ✓ |
| GCGR Agonism | ✗ | ✓ |
| Dosing Frequency | Once weekly | Once weekly |
| Regulatory Status (AU) | TGA Approved | Phase 3 (not approved) |
| MASH/Liver Research | Under investigation | Dedicated Phase 3 trial |
The primary research question distinguishing retatrutide from tirzepatide is whether the additional glucagon receptor component translates to meaningfully different outcomes in hepatic, cardiovascular, or thermogenic pathways under controlled clinical conditions. Phase 3 data will be the first definitive test of this hypothesis at scale.
Retatrutide for Laboratory Research
Retatrutide is available as a research peptide for qualified laboratory use. As with all research peptides in Australia, purchase and handling are subject to applicable regulatory requirements. Products are not approved for human therapeutic use.
Eternal Peptides Wholesale supplies retatrutide as part of its wholesale research peptide catalogue. All batches are submitted to Janoshik Analytical for third-party HPLC purity verification, sterility testing, endotoxin testing, and heavy metals analysis. Certificates of Analysis are available on each product page.
Frequently Asked Questions
Key References
- Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526.
- Coskun T, et al. 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 Metab. 2022;34(9):1234-1247.
- ClinicalTrials.gov. TRIUMPH-1: A Study of Retatrutide (LY3437943) in Adults With Obesity. NCT05882045.
- Drucker DJ. The Incretin System: Glucagon-Like Peptide-1 Receptor Agonists and Dipeptidyl Peptidase-4 Inhibitors in Type 2 Diabetes. Lancet. 2006;368(9548):1696-1705.