Retatrutide (LY3437943) is an investigational peptide developed by Eli Lilly and Company that acts as an agonist at three receptor systems simultaneously: the glucagon-like peptide-1 (GLP-1) receptor, the glucose-dependent insulinotropic polypeptide (GIP) receptor, and the glucagon receptor. Early-phase clinical data published in late 2023 and early 2024 placed retatrutide at the forefront of incretin-based pharmacotherapy, with the Phase 2 obesity trial reporting the largest mean body weight reductions seen in any anti-obesity medication trial to date (Jastreboff et al. (2024)).
Mechanism of Action
Triple Receptor Agonism
Each of the three receptor pathways contributes a distinct pharmacological effect:
GLP-1 Receptor Activation
- Enhances glucose-dependent insulin secretion
- Suppresses glucagon release from pancreatic alpha cells
- Slows gastric emptying, promoting postprandial satiety
- Modulates central appetite circuits in the hypothalamus and brainstem
GIP Receptor Activation
- Augments insulin secretion in a glucose-dependent manner
- GIP receptor signalling has been associated with enhanced fat oxidation and improved energy expenditure (Coskun et al. (2023))
- May attenuate the nausea commonly reported with selective GLP-1 receptor agonism
Glucagon Receptor Activation
- Increases hepatic glucose output and overall energy expenditure
- Glucagon receptor activation promotes lipolysis and hepatic fat oxidation
- Contributes to reductions in liver fat content, an effect not typically seen with GLP-1-only agents
Rationale for Triple Agonism
Combining all three receptor targets into a single molecule was designed to produce additive or synergistic effects on body weight, glycaemic control, and hepatic metabolism. GLP-1 receptor agonism provides the primary glycaemic and anorectic signal. GIP receptor activation augments insulin secretion while potentially reducing GLP-1-associated gastrointestinal side effects. Glucagon receptor activation adds an energy expenditure component that pure GLP-1 or dual GLP-1/GIP agonists do not provide (Coskun et al. (2023)).
Phase 2 Clinical Trial Data
Obesity Trial (NCT04881706)
In the Phase 2 trial published in the New England Journal of Medicine in February 2024, Jastreboff et al. reported results from a 48-week, randomised, double-blind, placebo-controlled study of retatrutide in 338 adults with obesity or overweight (Jastreboff et al. (2024)):
- Dose-dependent reductions in body weight occurred across all retatrutide treatment groups
- At the highest dose (12 mg weekly), participants achieved a mean body weight reduction of 24.2% at 48 weeks
- Approximately 90% of participants receiving the 12 mg dose lost more than 10% of body weight; over 75% lost more than 15%
- Improvements in glycated haemoglobin, fasting glucose, systolic blood pressure, and triglycerides were recorded alongside weight loss
- The most frequent adverse events were gastrointestinal - nausea, diarrhoea, and constipation - consistent with the GLP-1 receptor agonist class effect profile
These results, initially presented at the American Diabetes Association Scientific Sessions in June 2023, represented the largest mean weight reduction reported in a Phase 2 obesity trial at that time.
Type 2 Diabetes Trial
A separate Phase 2 trial evaluated retatrutide in participants with type 2 diabetes (NCT04882002):
- Statistically significant reductions in HbA1c were observed relative to placebo across all active dose groups
- Body weight reductions paralleled those seen in the obesity cohort
- Glycaemic improvements followed a dose-response relationship consistent with the obesity data
Ongoing Phase 3 Programme
The TRIUMPH programme comprises multiple Phase 3 trials evaluating retatrutide in broader populations:
- NCT05929775 - TRIUMPH-1: Obesity/overweight with weight-related comorbidities
- NCT05930166 - TRIUMPH-2: Obesity/overweight in a broader population
- NCT05946422 - TRIUMPH-3: Type 2 diabetes with obesity
These trials are expected to provide the safety and efficacy data required for regulatory submissions.
Regulatory Status in Australia
Retatrutide is not approved by the Therapeutic Goods Administration (TGA) and is not listed on the Australian Register of Therapeutic Goods (ARTG). As an investigational compound, it cannot be prescribed or supplied as a registered medicine in Australia.
The TGA evaluates new medicines based on clinical trial data submitted by the sponsor. Approval of retatrutide in Australia will depend on the outcomes of the Phase 3 TRIUMPH programme and whether Eli Lilly submits a registration application to the TGA. No timeline for Australian registration has been announced.
Retatrutide is available only through participation in registered clinical trials. Researchers interested in Australian trial sites can search ClinicalTrials.gov using the NCT identifiers listed above.
Why Retatrutide Is Generating Research Interest
Multiple factors have driven scientific attention toward this compound:
- Magnitude of weight reduction: The 24.2% mean body weight loss at the 12 mg dose exceeded results reported for tirzepatide (a dual GLP-1/GIP agonist) at comparable timepoints in its Phase 3 programme
- First-in-class pharmacology: Retatrutide is the first triple agonist (GLP-1/GIP/glucagon) to reach advanced clinical trials, representing a genuine pharmacological novelty
- Broad metabolic effects: Participants in the Phase 2 trial showed improvements across glycaemic, lipid, and blood pressure endpoints, not weight loss alone (Jastreboff et al. (2024))
- Hepatic fat reduction: The glucagon receptor component addresses non-alcoholic fatty liver disease (NAFLD), a condition with limited approved pharmacotherapies
- Energy expenditure: Unlike GLP-1-only agents that work primarily through appetite suppression, the glucagon agonist component may increase total energy expenditure
Research Limitations
Several important caveats apply to the current evidence base:
- Phase 2 findings, while striking, require confirmation in the larger Phase 3 TRIUMPH trials
- No long-term safety data beyond 48 weeks of treatment have been published
- The complete risk-benefit profile in diverse populations remains under investigation
- Individual variability in response to triple agonism has not been fully characterised
- Preclinical receptor pharmacology does not always translate to clinical outcomes at scale
Related Compounds
- GLP-1 Peptides - Overview of GLP-1 receptor agonist research including semaglutide and tirzepatide
- MOTS-c - A mitochondrial-derived peptide studied in metabolic research
- Tesamorelin - A GHRH analogue studied in body composition research
For research literacy and educational purposes only. This content does not constitute medical advice or therapeutic recommendation. Consult a qualified healthcare professional for medical decisions.