Retatrutide: Triple Receptor Agonism and Phase II Weight Loss Data

Retatrutide (developmental name LY3437943) is a synthetic peptide under development by Eli Lilly and Company that simultaneously activates three 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 makes it the first "triple hormone receptor agonist" to reach large-scale human clinical trials.

As of this writing, retatrutide has not received FDA approval for any indication. It is in Phase III clinical trials. The results described in this article come from Phase II data — a critical distinction for interpreting what is and is not established.

For context on how Phase II data differs from Phase III evidence, see Understanding Clinical Trials for Peptide Drugs.

The Rationale for Adding Glucagon Receptor Activity

To understand why Lilly added glucagon receptor agonism to an already-active GIP/GLP-1 dual agonist like tirzepatide, it is necessary to understand what glucagon does beyond its classical role in raising blood glucose.

Glucagon's metabolic roles beyond glycemia:

• Energy expenditure: Glucagon receptor activation in adipose tissue and the liver may increase fat oxidation and thermogenesis — processes that increase the number of calories the body burns at rest.
• Hepatic fat reduction: Glucagon receptor agonism has been associated with reduced hepatic steatosis (liver fat) in animal models, which is relevant for the treatment of metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD).
• Satiety signaling: Glucagon may contribute to appetite suppression through central nervous system pathways.

These properties suggested that adding glucagon receptor activity to GLP-1R/GIPR agonism might further increase weight loss — particularly through energy expenditure — beyond what dual agonism achieves.

The challenge: Glucagon classically raises blood glucose. In people with diabetes, glucagon receptor activation is typically counterproductive. Retatrutide's Phase II program deliberately enrolled people with obesity without diabetes, and also enrolled people with type 2 diabetes in a separate arm, to characterize the safety and efficacy profile in each population.

Phase II Trial Results (NEJM, 2023)

The Phase II data for retatrutide was published in the New England Journal of Medicine in 2023 by Jastreboff et al.[] This represents the highest-quality published evidence available for this compound at this time.

The trial enrolled 338 adults with obesity (BMI ≥27 with comorbidity or ≥30) without diabetes, randomized to placebo or retatrutide at 1mg, 4mg, 8mg, or 12mg weekly over 48 weeks.

Key efficacy outcomes at 48 weeks:

Dose	Mean Weight Change	% Achieving ≥15% Loss
Placebo	−1.6%	2%
Retatrutide 4mg	−8.7%	26%
Retatrutide 8mg	−17.3%	75%
Retatrutide 12mg	−24.2%	83%

The 24.2% mean weight loss at 12mg over 48 weeks was the largest pharmacological weight loss effect reported in a clinical trial at the time of publication — exceeding the approximately 20.9% observed with tirzepatide 15mg in SURMOUNT-1 (at 72 weeks, a longer duration) and the approximately 14.9% with semaglutide 2.4mg at 68 weeks.

Important caveats for interpreting these numbers:

• Phase II trials are not powered to detect all adverse events
• The trial duration (48 weeks) differs from the semaglutide and tirzepatide trials used for comparison
• Patient populations and entry criteria differ across trials — indirect comparisons are hypothesis-generating, not definitive
• Phase II results frequently change, sometimes substantially, when Phase III trials are conducted in larger, more diverse populations

Molecular Design

The preclinical characterization of LY3437943 was described by Coskun et al. (2022).[]

Retatrutide is a 36-amino acid peptide based on a GIP sequence scaffold, modified to achieve balanced activity across all three receptors. Key features:

• Balanced triple receptor potency: Designed to activate all three receptors at comparable potencies, rather than having one dominant pharmacology
• C20 fatty diacid modification: Enables albumin binding for extended half-life (~6 days), supporting once-weekly dosing
• DPP-4 resistance: Structural modifications protect against the rapid N-terminal cleavage that inactivates native GLP-1

The preclinical Cell Metabolism paper provided proof-of-concept in rodent models, demonstrating that the triple agonist produced greater reductions in body weight and liver fat than GLP-1R/GIPR dual agonism alone.

Liver Disease Research

One distinctive aspect of the retatrutide Phase II program was inclusion of liver fat as a pre-specified secondary endpoint. Participants with elevated liver fat at baseline showed marked reductions in hepatic fat content — with some participants achieving normalization of liver fat content.

This finding has generated interest in potential applications for MASLD, which affects a large proportion of people with obesity and type 2 diabetes and for which approved pharmacological treatments remain limited.

Phase III Status

As of this writing, retatrutide is in Phase III trials. The TRIUMPH program (Retatrutide's Phase III) is a large-scale international program. TRIUMPH-1 (NCT05394519) is evaluating retatrutide in adults with obesity without type 2 diabetes; other trials in the program are examining type 2 diabetes and cardiovascular outcomes.[]

Phase III enrollment and the time required to accumulate enough endpoint events means that regulatory submission and any potential approval are years away from Phase II completion. The Phase II results, however compelling, establish only that the compound has sufficient efficacy signal and manageable safety profile to justify the Phase III investment.

Safety Profile from Phase II

The adverse event profile in Phase II was broadly consistent with the GLP-1 receptor agonist class:

Common (>10%): Nausea, vomiting, diarrhea, constipation — particularly during dose escalation. The 12mg group had higher rates of gastrointestinal side effects than lower doses.

Dose-dependent GI tolerability: Rates of discontinuation due to adverse events were higher in the 12mg group than in the lower-dose groups, suggesting a dose-response relationship for both efficacy and tolerability.

Glycemia: Despite glucagon receptor agonism, clinically meaningful increases in fasting blood glucose were not observed in the non-diabetic population — consistent with the glucose-dependent nature of the other two receptors modulating the glucagon effect. This will be a critical area of monitoring in Phase III, particularly in the diabetes population.

Class warnings: Given the GLP-1R component, the same class warnings applied to semaglutide and tirzepatide (rodent thyroid C-cell tumors) are relevant to retatrutide.

Where Retatrutide Fits in the Incretin Pharmacology Landscape

The three incretin-based weight loss compounds represent successive generations of the same mechanistic approach:

Compound	Receptors	Status	Peak Trial Weight Loss
Semaglutide 2.4mg	GLP-1R	FDA approved (Wegovy)	≈15% (STEP 1, 68 weeks)
Tirzepatide 15mg	GLP-1R + GIPR	FDA approved (Zepbound)	≈21% (SURMOUNT-1, 72 weeks)
Retatrutide 12mg	GLP-1R + GIPR + GCGR	Phase III	≈24% (Phase II, 48 weeks)

Whether the apparent incremental benefit of each generation will be confirmed in Phase III, and whether the safety profile supports the efficacy advantage, remains to be determined. For a framework on how to evaluate Phase II data relative to Phase III requirements, see How to Evaluate Peptide Research Claims.