How Was Retatrutide Discovered? The Triple-Agonist Story

The short answer

Retatrutide was discovered by adding a third hormone action, glucagon-receptor agonism, to the two-receptor (GIP plus GLP-1) peptide backbone that Eli Lilly had already built into tirzepatide. Working roughly between 2016 and 2018, Lilly’s peptide chemists designed a single 39-amino-acid molecule that switches on all three receptors at once. They then attached a fatty-acid tail so the drug lasts in the body long enough for a once-weekly injection. The receptor pharmacology was first made public in Cell Metabolism in 2022 (Coskun et al.), and the first human results appeared in The Lancet the same year (Urva et al.).

This post is about the design science: why the molecule looks the way it does, and how three competing targets were balanced in one peptide. For the year-by-year account of the program itself, see when was retatrutide created.

Starting from a proven backbone

Retatrutide did not start from a blank page. Its starting point was the same molecular framework that became tirzepatide (Mounjaro/Zepbound), a dual agonist that activates two gut-hormone receptors, GIP and GLP-1, with a single peptide.

That alone was unusual. GIP, GLP-1 and glucagon receptors all belong to the same family of related cell-surface receptors (class B G-protein-coupled receptors in the glucagon/secretin group). Because their natural hormones share a common evolutionary shape, a well-designed peptide can be tuned to fit more than one of them. Tirzepatide showed that one sequence could usefully engage two of these receptors. Retatrutide applies the same logic to the third, glucagon, turning a dual agonist into a first-in-class triple agonist. For a receptor-by-receptor breakdown of what each pathway contributes, the mechanism of action page walks through it.

The counterintuitive part: adding a hormone that raises blood sugar

This is the design choice that surprises most people. Glucagon is the body’s “raise blood sugar” hormone, the opposite of insulin. It tells the liver to release stored glucose. So why would anyone add glucagon activity to a drug meant for people with obesity and type 2 diabetes?

Because glucagon does two other things that are very useful in metabolic disease:

• It increases energy expenditure. Glucagon-receptor activation nudges the body to burn more energy at rest. That is the “calories out” side of the balance sheet, which appetite-suppressing drugs alone never touch.
• It burns liver fat directly. Glucagon-receptor agonism stimulates hepatic (liver) fatty-acid oxidation. The liver actively burns its own fat stores instead of simply losing fat as a side effect of weight loss.

The obvious risk, that the glucagon component would worsen blood sugar, is offset on purpose by the other two receptors. The GLP-1 arm drives insulin release and suppresses appetite, and the GIP arm boosts the insulin response. In the balanced molecule, those two effects more than cancel out glucagon’s push on glucose. The net result is the unexpected prize: blood sugar improves and energy expenditure rises at the same time. We look at that trade-off in more depth in why the glucagon receptor matters.

The engineering problem: balancing three receptors at once

The hardest part of the discovery was the tuning, not the concept. One peptide sequence has to bind and activate three different receptors, and the ratio of activity across them decides whether the drug helps or harms. Push glucagon activity too high relative to GLP-1 and GIP, and blood sugar could get worse. Push it too low, and you lose the metabolic-rate and liver-fat benefits that were the whole reason for adding the third receptor.

So the discovery work came down to a balancing act at the level of single amino acids. By swapping specific residues along the peptide chain, Lilly’s chemists could turn each receptor’s potency up or down until the three-way profile sat in a useful therapeutic window. The title of the 2022 Cell Metabolism paper sums up that effort: it describes the molecule “from discovery to clinical proof of concept” (Coskun et al.). This is also why a triple agonist is harder to build than a single-target drug: there are three connected levers instead of one. The understanding triple agonism post covers why the extra complexity is worth it.

Making it last a week

A balanced triple agonist is only useful if patients do not have to inject it every day. The natural hormones these peptides mimic are cleared from the blood within minutes. To stretch that to a weekly schedule, retatrutide uses a chemistry trick shared across the modern incretin class: lipidation.

Attached to the peptide is a C20 fatty diacid moiety, a long fatty-acid tail. That tail binds tightly but reversibly to albumin, the most abundant protein in blood. While bound to albumin, the drug is shielded from the enzymes and kidney filtration that would otherwise break it down, and it is released slowly over days. The result is a half-life of about 6 days, which is what makes once-weekly dosing possible.

From molecule to proof

Designing a molecule on paper is one thing. Showing that it behaves as intended in people is another. Two 2022 publications closed that gap.

The Cell Metabolism paper (Coskun et al.) was the first public account of retatrutide’s receptor pharmacology, showing that the molecule really did engage all three targets in the intended balance. Alongside it, the first-in-human Phase 1 program, which began in 2019, was published in The Lancet (Urva et al.). Phase 1 confirmed the pharmacokinetics behind weekly dosing and showed that the triple agonist was tolerable in early testing, with gastrointestinal effects the most common problem.

Later trials showed that the glucagon bet paid off as the design predicted. In the Phase 2 obesity trial, retatrutide produced a mean weight reduction of 24.2% at the 12 mg dose over 48 weeks (Jastreboff et al., New England Journal of Medicine, 2023). In the liver-focused substudy, it produced roughly an 82% relative reduction in liver fat at the highest dose (Sanyal et al., Nature Medicine, 2024). Appetite suppression alone cannot explain a drop that large; it is the signature you would expect from the added glucagon-driven fat burning. All of these figures are group averages measured in clinical trials, not individual results.

A few quick questions

Who discovered retatrutide? Eli Lilly, which designed and developed the molecule (internal development code LY3437943).

Is it related to Mounjaro? Yes. Retatrutide is built on the same GIP/GLP-1 peptide framework as tirzepatide, with glucagon-receptor activity added as a third target.

What does the third receptor actually add? Higher energy expenditure and direct liver-fat burning, which the GLP-1 and GIP receptors alone do not provide.

A note on where this stands

Retatrutide is an investigational drug. As of October 2026 it has not been approved by the FDA or any other regulator. Everything described here reflects published discovery science and clinical-trial findings, not a finished, licensed medicine. Nothing here is medical advice or a recommendation to get the drug from any source. If you are weighing options for obesity, diabetes or liver disease, the right next step is a conversation with a qualified clinician who knows your history, not a peptide bought outside a regulated trial or prescription.