Triple agonist (revision 9)

A triple agonist is a single engineered peptide that activates three receptors of the glucagon-secretin family — conventionally the receptors for GIP, GLP-1 and glucagon. Retatrutide is the most advanced example and is investigational.^[1]

The design extends the logic of the dual agonists by one receptor, and multiplies the constraints accordingly: the sequence must retain activity at three targets in a ratio fixed by chemistry, and that ratio must remain appropriate across the whole dose range.^[1]

Adding glucagon-receptor agonism introduces an effect that opposes the others glycaemically, in exchange for increased energy expenditure and reduced hepatic fat. The balance is the central difficulty. See Glucagon.^[2]

Each added receptor is intended to contribute a mechanism the others do not. GLP-1 agonism supplies appetite suppression and glucose-dependent insulin secretion; GIP agonism appears to improve tolerability and contributes adipose-tissue effects; glucagon agonism raises energy expenditure and mobilises hepatic fat.^[1]

Because the receptors share a common architecture and their ligands a common precursor family, a peptide can be engineered to engage all three — see Proglucagon — but the sequence space in which all three activities coexist at usable ratios is narrow.^[2]

Reported potency ratios are assay-dependent and are not comparable between publications, a caution that applies with more force here because three numbers are being compared rather than two.^[1]

Two observations in the phase 2 data are attributable to glucagon-receptor agonism: a dose-dependent heart-rate increase larger than that seen with GLP-1 monotherapy, and transient rises in fasting glucose at low doses in participants with diabetes.^[2]

The second is the design's predicted failure mode. At low total dose the glucagon component is not yet offset by sufficient GLP-1 agonism, so glycaemia worsens before it improves — which is why escalation schedules for these compounds are long.^[1]