Retatrutide

Retatrutide (development code LY3437943) is a synthetic peptide agonist at the GIP, GLP-1 and glucagon receptors. It is built on a GIP-based backbone with modifications conferring activity at the other two, and carries fatty acid acylation for albumin binding and Aib substitution for protease resistance — the same engineering vocabulary as the rest of the class, applied to a harder target.

Balancing three agonist activities in one molecule is substantially harder than balancing two. The relative potency at each receptor is a designed parameter, and much of the preclinical literature concerns what the right ratio is rather than whether the approach works.

Glucagon opposes insulin and raises hepatic glucose output, so agonising its receptor in a metabolic compound appears self-defeating. The argument for it rests on a second set of glucagon actions:

• Energy expenditure. Glucagon receptor agonism increases metabolic rate, adding a mechanism the incretins do not have — they act largely on intake rather than expenditure.

• Hepatic lipid handling. Glucagon signalling promotes fatty acid oxidation in the liver, which is why this class is of particular interest in MASH research.

• Glycaemic offset. The GLP-1 and GIP components suppress glucagon's hyperglycaemic effect, so the combination is designed to retain the expenditure benefit while neutralising the glucose penalty.

Whether that balance holds across doses and over time is precisely what the trial programme is testing, and it is the reason retatrutide is interesting as a research object rather than only as a candidate.

Retatrutide is at an earlier stage than the approved compounds, and its data should be read accordingly:

• Phase 2 obesity, published in the New England Journal of Medicine in 2023 — the result that brought the compound to wide attention.

• Phase 2 type 2 diabetes, reported in parallel.

• TRIUMPH — the phase 3 programme in obesity and related conditions.

• MASLD and MASH endpoints, where the glucagon component gives a specific mechanistic rationale.

• Knee osteoarthritis and obstructive sleep apnoea, studied as obesity-associated conditions.

It is not approved in any jurisdiction. Phase 2 results have historically attenuated in phase 3 across many therapeutic areas, and the class is no exception.

Supplied as a lyophilised powder. Reconstitute gently in bacteriostatic water; acylated peptides aggregate at the air-liquid interface under vigorous agitation. Store reconstituted material refrigerated and protected from light, and record the batch against the certificate of analysis.

Tirzepatide is the dual-agonist predecessor and the obvious comparator. Survodutide and Mazdutide isolate the GLP-1/glucagon pairing without GIP, which makes them the useful controls for attributing retatrutide's effects to a specific receptor.