How Was Retatrutide Discovered? The Triple-Agonist Story

The short answer

Retatrutide was discovered by chemically adding a third hormone action — glucagon-receptor agonism — onto the two-receptor (GIP plus GLP-1) peptide backbone that Eli Lilly had already engineered 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, then bolted on a fatty-acid tail so the drug survives in the body long enough for a once-weekly injection. The receptor pharmacology was first disclosed publicly 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 begin 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 was already an unusual achievement. GIP, GLP-1, and glucagon receptors all belong to the same structurally related family of cell-surface receptors (class B G-protein-coupled receptors in the glucagon/secretin group). Because their natural hormones share a common evolutionary shape, a cleverly designed peptide can be tuned to fit more than one of them. Tirzepatide proved that a single sequence could productively engage two of these receptors. Retatrutide extends that same logic to the third — glucagon — turning a dual agonist into a first-in-class triple agonist. If you want the 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

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

The answer is that glucagon does two other things that are extremely useful for metabolic disease:

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

The obvious risk — that the glucagon component would worsen blood sugar — is deliberately offset by the other two receptors. The GLP-1 arm drives insulin release and suppresses appetite, and the GIP arm amplifies the insulin response. In the balanced molecule, those two effects more than counteract glucagon’s glucose-raising tendency. The net result is the counterintuitive prize: blood sugar improves and energy expenditure rises at the same time. We explore 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 not the concept — it was the tuning. A single peptide sequence has to bind and activate three different receptors, and the ratio of activity across them determines whether the drug helps or harms. Push glucagon activity too high relative to GLP-1 and GIP, and blood sugar could deteriorate. Push it too low, and you sacrifice the metabolic-rate and liver-fat benefits that were the entire reason for adding the third receptor.

So the discovery work was essentially a balancing act played out at the level of individual amino acids. By substituting specific residues along the peptide chain, Lilly’s chemists could dial each receptor’s potency up or down until the three-way profile sat in a therapeutically useful window. The title of the 2022 Cell Metabolism paper captures the arc of that effort precisely: it describes the molecule “from discovery to clinical proof of concept” (Coskun et al.). This is also why triple agonism is genuinely harder to build than a single-target drug — three levers, not one, all connected. The understanding triple agonism post covers why that added complexity is worth the trouble.

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 bloodstream 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. Anchored to albumin, the drug is shielded from the enzymes and kidney filtration that would otherwise destroy it, and it is released slowly over days. The payoff is an approximately 6-day half-life, which is what makes once-weekly dosing possible.

From molecule to proof

Designing a molecule on paper is one thing; showing 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 disclosure of retatrutide’s receptor pharmacology — the evidence that the molecule really did engage all three targets in the intended balance. In parallel, the first-in-human Phase 1 program, which began in 2019, was published in The Lancet (Urva et al.). Phase 1 confirmed the pharmacokinetics that underpin weekly dosing and established that the triple agonist was tolerable in early testing, with gastrointestinal effects being the most common issue.

The later trials then confirmed that the glucagon bet paid off exactly 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). And in the liver-focused substudy, it delivered roughly an 82% relative reduction in liver fat at the highest dose (Sanyal et al., Nature Medicine, 2024) — a magnitude that appetite suppression alone cannot explain, and precisely the signature you would expect from the added glucagon-driven fat oxidation. 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? Energy expenditure and direct liver-fat burning — effects the GLP-1 and GIP receptors alone do not provide.

A note on where this stands

Retatrutide is an investigational drug. As of mid-2026 it has not been approved by the FDA or any other regulator, and everything described here reflects published discovery science and clinical-trial findings rather than a finished, licensed medicine. Nothing in this article is medical advice or a recommendation to seek 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.