What it is
Retatrutide is a 39-amino-acid synthetic peptide (CAS 2381089-83-2, approximate molecular formula C₂₁₇H₃₃₄N₅₆O₆₈, MW ~4,600 Da) engineered by Eli Lilly and designated LY3437943. It is the first triple agonist of the GIP, GLP-1, and glucagon receptors — three class B1 GPCRs on the incretin / glucagon axis. Stability and once-weekly subcutaneous pharmacokinetics are achieved through two engineered features: 2-aminoisobutyric acid (Aib) substitutions that confer DPP-4 resistance, and a C20 fatty diacid moiety at Lys17 that drives albumin binding and depot formation. Plasma half-life is approximately 6 days. Human receptor potency (EC₅₀) is reported as 0.064 nM at GIPR, 0.78 nM at GLP-1R, and 5.8 nM at GCGR — a deliberately unbalanced potency ratio intended to extract thermogenic benefit from GCGR without driving hyperglycemia.
In plain English
Retatrutide is a 39-amino-acid synthetic peptide made by Eli Lilly (lab code LY3437943). It is the world's first "triple agonist" — a drug that turns on all three main metabolic receptors at once: GIP, GLP-1, and glucagon. Each receptor controls a different part of how the body handles food and energy. GLP-1 controls insulin release and appetite. GIP boosts insulin secretion and affects fat cells. Glucagon drives the liver and body to burn fat. Turning on all three at once is the key idea. The drug is engineered to last about 6 days in the body, so it can be given as a once-weekly injection. Two engineering tricks make it stable: small chemical substitutions that block the main breakdown enzyme (DPP-4), and a fat-like chain that anchors the drug to a blood protein (albumin) so it releases slowly instead of clearing quickly.
How it works
- 01
Triple agonism of the incretin / glucagon axis
Retatrutide binds and activates three class B1 GPCRs simultaneously: GLP-1R, GIPR, and GCGR. Zhao (2024, Cell Discovery) published cryo-EM structures of retatrutide bound to each of GLP-1R-Gs (2.68 Å), GIPR-Gs (3.26 Å), and GCGR-Gs (2.84 Å), showing that retatrutide adopts a single continuous α-helix whose N-terminal segment penetrates the receptor transmembrane core and whose C-terminal segment engages the extracellular domain. The measured human EC₅₀ values (0.064 nM GIPR, 0.78 nM GLP-1R, 5.8 nM GCGR) represent an engineered potency ratio intended to preserve glucose-dependent insulinotropic benefit while using GCGR agonism for thermogenesis.
In plain English
It turns on three different receptors at once
Retatrutide grabs onto and switches on three different cell receptors at the same time: GLP-1R, GIPR, and GCGR. A 2024 study captured detailed 3D snapshots (cryo-EM) of retatrutide locked into each receptor. The drug folds into a long spiral shape that fits inside each receptor. It is deliberately much stronger at the GIP receptor than at the glucagon receptor — to get the fat-burning benefit without spiking blood sugar.
- 02
GLP-1R contribution — insulin, glucagon suppression, satiety
Per the published clinical-pharmacology record (Rosenstock 2023, Lancet), GLP-1R activation drives glucose-dependent insulin secretion, glucose-dependent glucagon suppression, delayed gastric emptying (Urva 2024, Diabetes Obes Metab, showed clinically relevant gastric-emptying delay at therapeutic doses), and central appetite reduction via hypothalamic and brainstem circuits. This arm is mechanistically identical to semaglutide and to the GLP-1 arm of tirzepatide.
In plain English
The GLP-1 part controls insulin, glucagon suppression, and appetite
Turning on the GLP-1 receptor tells the pancreas to release insulin when blood sugar is high. It also suppresses glucagon (which normally raises blood sugar), slows the stomach so food is digested more slowly, and tells the brain's appetite centers to feel less hungry. This is the same mechanism used by semaglutide (Ozempic/Wegovy) and the GLP-1 half of tirzepatide.
- 03
GIPR contribution — β-cell potentiation and adipocyte handling
GIPR co-agonism potentiates glucose-dependent insulin secretion beyond what GLP-1R alone delivers, and appears to modulate subcutaneous adipocyte nutrient partitioning. The in-vivo contribution of the GIPR arm to retatrutide's weight-loss effect — distinct from the GLP-1R and GCGR arms — has not been isolated in humans, and the relative weight of GIPR signaling versus GIPR antagonism is still contested across the incretin field.
In plain English
The GIP part boosts insulin and affects fat cells
Activating the GIP receptor adds more insulin-releasing power on top of what the GLP-1 receptor already does. It also appears to change how fat cells handle incoming nutrients. Exactly how much the GIP receptor contributes to the total weight loss — separately from GLP-1 and glucagon — has not been measured in humans in isolation. That question is still debated in the field.
- 04
GCGR contribution — energy expenditure and hepatic lipid mobilization
Glucagon-receptor agonism is the feature that distinguishes retatrutide from dual GIP/GLP-1 agonists like tirzepatide. GCGR activation at the liver increases fatty-acid oxidation and suppresses lipogenesis, which is the proposed driver of the 82.4% MRI-PDFF liver-fat reduction reported in the Phase 2a MASLD trial (Sanyal 2024, Nat Med). GCGR activation also raises resting energy expenditure and is thought to account for the observation — still requiring Phase 3 confirmation — that Phase 2 weight loss did not plateau at 48 weeks despite plateau being typical for GLP-1/GIP dual agonists by that timepoint.
In plain English
The glucagon part tells the body to burn fat for energy
Activating the glucagon receptor tells liver cells to burn more fat and make less new fat. This is likely why retatrutide cut liver fat by 82.4% in the Phase 2 fatty liver trial. The glucagon receptor also raises the body's resting energy burn. That may be why the weight-loss curve hadn't leveled off at 48 weeks — unlike most other drugs, which plateau by then. Phase 3 still needs to confirm this.
- 05
What is NOT yet established in humans
The absolute and relative contributions of GIPR and GCGR to the total weight-loss effect cannot be cleanly separated from published human data. Lean-mass preservation has been examined in a T2D body-composition substudy (Jastreboff 2025, Lancet Diabetes Endocrinol — retatrutide 12 mg preserved lean mass to a similar or better degree than placebo-adjusted expectation) but has not been demonstrated across the full obesity population or over Phase 3 durations. The magnitude of GCGR-driven resting-heart-rate increase, chronic effect on hepatic glucose output, and long-term bone, renal, and cardiovascular consequences remain open until TRIUMPH and its CVOT report.
In plain English
What we still don't know
We cannot cleanly separate how much of the weight loss comes from each receptor (GIP, GLP-1, or glucagon) — they all work at the same time. Lean muscle preservation has been measured in a smaller diabetes study, but not across the full obesity population over Phase 3 timescales. The glucagon receptor's heart rate effects, long-term liver effects, and consequences for bones and kidneys are all unknown until Phase 3 and the cardiovascular outcomes trial report.