Retatrutide: chemistry

Retatrutide (development code LY3437943) is a synthetic peptide, a short chain of amino acids, built so that one molecule activates three hormone receptors. The phase 2 trial describes it as “an agonist of the glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, and glucagon receptors”.[1] Semaglutide acts at the GLP-1 receptor only and tirzepatide at the GIP and GLP-1 receptors, so the glucagon receptor is what is new.

Three receptors, unequal strengths

A structural study in Cell Discovery compared retatrutide with each receptor's natural hormone and reported that “retatrutide is 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, respectively”.[2] In other words, it is stronger than the body's own GIP at the GIP receptor and weaker than the natural hormones at the other two. The same paper cites earlier animal work in which “retatrutide induces greater body weight losses in obese mice than tirzepatide due to an increased energy expenditure through GCGR activation”, which is the rationale for adding glucagon activity.[2]

Structure

To see how one peptide binds three different receptors, the researchers wrote that “we used cryo-electron microscopy (cryo-EM) to determine the structures of GLP-1R, GIPR and GCGR bound to retatrutide”.[2] Like semaglutide and tirzepatide, retatrutide carries a fatty-acid side chain: the paper notes that these peptides “are acylated with various fatty diacid moieties via a linker connected to the lysine residues”.[2]

Peptide or biologic

How the molecule is classified matters for its regulatory path. BioSpace reports that “the FDA determined that retatrutide is neither a protein nor a biological product”, a decision Lilly has disputed.[3] The legal section covers what follows from that.

What is in a sold vial

Testing of vials sold as retatrutide in Australia, reported by ABC News, found that “the peptide usually only made up about 10 percent of what was in the vials”.[4] A purity certificate saying a vial contains the peptide does not say how much, or what else: buyers who got such tests back learned that “all that they know from this testing is that there is some retatrutide in their vials”, and the researchers warned of “multiple other serious risks including endotoxins such as bacteria and contaminants such as heavy metals”.[4]

For more on testing and its limits, see Retatrutide from a research vial.

References

1. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial (Jastreboff et al., N Engl J Med 2023;389:514-526) — PubMed (New England Journal of Medicine)
2. Structural insights into the triple agonism at GLP-1R, GIPR and GCGR manifested by retatrutide (Li et al., Cell Discovery 10, 77, published 17 July 2024) — Cell Discovery (Springer Nature)
3. Lilly, FDA retatrutide biologic dispute comes to a head as submission nears (August 5, 2026) — BioSpace
4. New testing of black-market peptides reveals dangerous dosing levels (Rachel Carbonell, Caitlyn Gribbin and Andi Yu for ABC News, 28 August 2026) — RNZ