Triple-agonist peptides such as retatrutide are defined by the receptors they activate. To understand the molecule, it helps to understand those receptors first. All three belong to the same family: class B G-protein-coupled receptors (GPCRs), which respond to peptide hormones and signal mainly through cyclic AMP.
The incretin idea
In the 1960s researchers noticed that glucose taken by mouth triggers more insulin release than the same amount of glucose given into a vein. Hormones from the gut cause the difference. These gut hormones are called incretins, and the two main ones are GIP and GLP-1.
GIP receptor
- Natural ligand: glucose-dependent insulinotropic polypeptide (GIP), a 42-amino-acid hormone.
- Source: K-cells in the duodenum and upper jejunum, released after a meal.
- Where the receptor is found: pancreatic beta cells, adipose tissue, bone and parts of the brain.
- Main signals studied: glucose-dependent insulin secretion, and effects on fat tissue.
GLP-1 receptor
- Natural ligand: glucagon-like peptide-1 (GLP-1), processed from proglucagon.
- Source: L-cells in the lower small intestine and colon.
- Where the receptor is found: pancreatic beta cells, the stomach, the heart, the kidney and the brain, including areas involved in appetite.
- Main signals studied: glucose-dependent insulin secretion, suppression of glucagon, slower gastric emptying and central effects on appetite.
Native GLP-1 is broken down by the enzyme DPP-4 within minutes. Research molecules use chemical changes to resist this breakdown.
Glucagon receptor
- Natural ligand: glucagon, a 29-amino-acid hormone from pancreatic alpha cells.
- Where the receptor is found: mainly the liver, also the kidney and adipose tissue.
- Main signals studied: glucose release from the liver, fat oxidation, amino-acid metabolism and energy expenditure.
Glucagon raises blood glucose, so activating its receptor on its own would oppose the incretins. Pairing glucagon-receptor activity with incretin-receptor activity in one molecule is the central design idea behind the triple agonists.
Why one molecule instead of three
Combining several activities in one peptide gives a single, fixed ratio of activity at each receptor and one pharmacokinetic profile. Designers tune that ratio by changing the amino-acid sequence. Comparing the activity profile at each receptor is the clearest way to tell the molecules in this class apart — see single, dual and triple agonists compared.