Retatrutide (LY3437943) Up Close: Structure, Synthesis and Detection of a Triple Agonist

Retatrutide, coded LY3437943 by Eli Lilly, is one peptide engineered to activate three receptors: the glucagon receptor (GCGR), the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R) (Coskun 2022). Rather than recap trial headlines, this Vial Post explainer looks at the molecule itself: how structural biologists pictured it bound to all three receptors, how chemists are making it, how anti-doping labs detect it, and how a current kidney study is designed.

The Vial Post covers retatrutide only as a research subject. Nothing here is guidance on use in people or animals.

Key Points

• Developer code: LY3437943 (Coskun 2022).
• Receptor profile in vitro: balanced GCGR and GLP-1R activity, with more GIPR activity (Coskun 2022).
• Relative potency: compared with the native hormones, a 2024 structural paper reports retatrutide as more potent at GIPR by a factor of 8.9 and less potent at GCGR and GLP-1R by factors of 0.3 and 0.4 (Li 2024).
• Unusual residues: the same paper's sequence figure marks Aib (alpha-aminoisobutyric acid) and alpha-methyl-L-leucine, and notes a fatty diacid attached through a linker to the lysine at position 17 (Li 2024).
• Clinical stage: the 2024 structural paper notes that phase 3 trials were underway (Li 2024).

Three Receptors, Three Structures

Li and colleagues at the Shanghai Institute of Materia Medica used single-particle cryo-electron microscopy to solve retatrutide bound to each receptor in complex with the Gs protein. Overall resolutions were 2.68 Å for GLP-1R, 3.26 Å for GIPR and 2.84 Å for GCGR (Li 2024).

What they saw:

• Shared contacts. Interactions common to all three receptors include two salt bridges formed through the N-terminal nitrogen of the first residue, a tyrosine.
• Receptor-specific shapes. Differences appeared particularly in extracellular loop 1 (ECL1), which the authors say lets the peptide act at multiple receptors.
• Mutation check. Alanine substitutions at receptor position E6.53 lowered retatrutide's cAMP signaling potency by 3.6-fold at GLP-1R, 1.6-fold at GIPR and 8.3-fold at GCGR.
• Design lesson. Comparing dual and triple agonists, the authors conclude that the peptide's N- and C-terminal regions set receptor selectivity, while the middle region leaves room for sequence tuning (Li 2024).

The original discovery paper adds the animal pharmacology: in obese mice, body weight decreased and glycemic control improved, with GCGR-driven increases in energy expenditure adding to GIPR- and GLP-1R-driven reductions in calorie intake (Coskun 2022). For how this compares with a two-receptor design, see our tirzepatide explainer.

Making It: SPPS and a Liquid-Phase Alternative

A 2026 Organic Letters paper from Sichuan University notes that retatrutide synthesis currently relies mainly on solid-phase peptide synthesis (SPPS), which the authors say has limits in efficiency, scalability and structural flexibility. They report a hydrophobic tag-assisted liquid-phase peptide synthesis (LPPS) route as a practical alternative (Mao 2026). For buyers of research material, the takeaway is that route and scale affect the impurity profile, which is why batch-level analysis matters.

Detecting It: An Anti-Doping Method

A Rio de Janeiro group built a liquid chromatography high-resolution mass spectrometry (LC-HRMS) method for retatrutide in plasma and urine, validated against WADA criteria (Carneiro 2026):

• Detection approach: the [M + 3H]3+ precursor ion by targeted selected-ion monitoring, confirmed with parallel reaction monitoring of diagnostic fragment ions.
• Limits of detection: 6.14 ng/mL in plasma and 2.44 ng/mL in urine.
• Samples: collected from four people who were already self-administering the compound.
• Matrix finding: neither intact retatrutide nor metabolite signals appeared in urine, while plasma showed a detection window of about 56 days after first administration. The authors conclude plasma is the appropriate matrix for testing.

How a Current Trial Is Built

TRANSCEND-CKD, described in Nephrology Dialysis Transplantation, is a double-blind, placebo-controlled Phase 2b mechanistic study in adults with overweight or obesity and chronic kidney disease, with and without type 2 diabetes (Heerspink 2026). Design points:

• Randomization: 1:1 to once-weekly retatrutide or matched placebo; 146 of 367 screened participants were randomized.
• Primary measure: change in measured glomerular filtration rate by iohexol clearance from baseline to Week 24.
• Imaging: MRI of kidney hemodynamics and volume, including perirenal and renal sinus fat.
• Registration: NCT05936151, with a separate cardio-kidney outcome trial, TRIUMPH-Outcomes (NCT06383390).

The paper reports design and baseline data only, and The Vial Post does not report trial doses or outcomes.

Reader Questions

Is retatrutide still being studied in trials? Yes. The 2024 structural paper notes phase 3 trials underway (Li 2024), and TRANSCEND-CKD is a Phase 2b mechanistic study (Heerspink 2026).

Which receptor does retatrutide favor? In vitro it shows more GIPR activity than GCGR or GLP-1R activity (Coskun 2022; Li 2024).

Can retatrutide be detected in testing? Yes. A validated LC-HRMS method detects it in plasma; it was not detected in urine in that study (Carneiro 2026).

Lab Note

Retatrutide is a long, lipidated peptide with non-standard residues, so identity is best confirmed by high-resolution mass spectrometry and purity by HPLC. Our guides on HPLC versus mass spec and what a COA should show explain what to look for.

For research and educational use only. This article is not medical advice and does not describe dosing or human use.