Retatrutide Research Guide

01 What Is Retatrutide?

Retatrutide (LY3437943) is a synthetic acylated peptide developed by Eli Lilly and Company as the first triple receptor agonist targeting GLP-1, GIP, and glucagon receptors simultaneously in a single molecule. It was designed as a once-weekly subcutaneous injection for the treatment of obesity and type 2 diabetes.

The compound emerged from a broader effort to understand why dual GIP/GLP-1 agonism (tirzepatide) produces greater weight loss than GLP-1 agonism alone — and whether adding glucagon receptor activation could further amplify metabolic effects. The Phase II results published in the New England Journal of Medicine in 2023 attracted significant research attention, reporting weight loss figures that exceeded any previously published pharmacological trial data.

02 Triple Agonism Explained

The incretin system — the hormones GLP-1 and GIP released from the gut after eating — coordinates post-meal insulin release and appetite regulation. Glucagon, released from pancreatic alpha cells during fasting, is the counter-regulatory hormone that drives glucose production and fatty acid mobilisation.

These three axes are traditionally viewed as opposing forces (insulin vs glucagon) but Retatrutide's design exploits a nuanced reality: when GLP-1 agonism prevents hyperglycaemia, selective glucagon receptor activation adds thermogenic and hepatic fat-clearing benefits that pure GLP-1 agonists cannot provide. The result is a three-pronged metabolic intervention:

• GLP-1R: Appetite suppression, slowed gastric emptying, glucose-dependent insulin secretion
• GIPR: Enhanced insulin response, adipose lipid metabolism, possible anabolic and satiety effects
• GlucagonR: Increased energy expenditure, hepatic fat oxidation, basal metabolic rate elevation

03 GLP-1 Component

GLP-1 (glucagon-like peptide-1) is a 30-amino acid incretin hormone secreted by L-cells of the distal small intestine and colon in response to nutrient ingestion. Its actions via GLP-1R include:

• Glucose-dependent stimulation of insulin secretion (insulinotropic only when glucose is elevated)
• Suppression of glucagon secretion from alpha cells
• Delayed gastric emptying, reducing postprandial glucose spikes
• Central appetite suppression via hypothalamic and brainstem GLP-1R signalling
• Potential cardioprotective effects (reduced MACE in cardiovascular outcome trials)

Native GLP-1 has a half-life of approximately 2 minutes due to degradation by DPP-4 (dipeptidyl peptidase-4). Retatrutide — like semaglutide and tirzepatide — uses a fatty acid conjugation strategy to extend half-life to approximately 6 days, enabling once-weekly dosing via albumin binding and protection from DPP-4 cleavage.

04 GIP Component

GIP (glucose-dependent insulinotropic polypeptide) was the first incretin hormone identified, secreted by K-cells of the upper small intestine. For decades it was considered less therapeutically relevant than GLP-1 — partly because GIP receptor agonism alone showed limited glycaemic benefit and paradoxical weight gain in some early studies.

Tirzepatide's clinical success re-opened mechanistic investigation of GIP's role. Current evidence suggests GIPR agonism:

• Enhances insulin secretion in a glucose-dependent manner (additive with GLP-1)
• Acts on adipose GIPR to promote lipid storage and mobilisation depending on the metabolic state
• Has direct central effects on appetite and energy expenditure via hypothalamic GIPR
• May counteract GLP-1R-driven nausea, improving tolerability of combined agonists

The combination of GLP-1R + GIPR agonism in tirzepatide produced greater weight loss than GLP-1 alone — suggesting synergistic rather than additive receptor engagement, possibly through distinct CNS and peripheral pathways.

05 Glucagon Component

Adding glucagon receptor agonism is the defining innovation of Retatrutide versus tirzepatide. Glucagon's primary actions via GlucagonR include:

• Hepatic glucose production — stimulates glycogenolysis and gluconeogenesis (counteracted by GLP-1 co-agonism)
• Fatty acid oxidation — activates hepatic β-oxidation pathways, mobilising liver fat (relevant to NAFLD/NASH)
• Thermogenesis — increases basal metabolic rate via brown adipose tissue activation and uncoupled respiration
• Satiety — direct hypothalamic appetite-suppressing effects independent of GLP-1R

The critical design consideration: pure glucagon agonism raises blood glucose (hyperglycaemia). In Retatrutide's triple agonist design, the GLP-1 component provides sufficient insulinotropic activity to offset glucagon's glucose-raising effect, unlocking the thermogenic and lipolytic benefits of glucagon agonism without metabolic compromise.

06 Phase II Trial Data — TRIUMPH-1

The 24.2% mean weight loss at 48 weeks surpassed the best results from semaglutide 2.4 mg (~15% over 68 weeks, STEP 1) and tirzepatide 15 mg (~22.5% over 72 weeks, SURMOUNT-1) in their respective trials — though cross-trial comparisons carry significant caveats regarding population differences, trial duration, and design.

The liver fat reduction data is particularly notable: ~82% relative reduction in MRI-measured hepatic fat content positions Retatrutide as a potential candidate for NASH (now renamed MASH — metabolic dysfunction-associated steatohepatitis), an area with significant unmet clinical need.

07 Retatrutide vs Semaglutide vs Tirzepatide

08 Frequently Asked Questions