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.
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.
- 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.
- 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.
- 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.
- 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.