One receptor, several signals: the case for reading retatrutide as a biased agonist

There is a version of incretin pharmacology in which the receptor is a switch and the drug is a key, and the only variable worth discussing is how much key you use. It is a convenient model and it fails almost immediately. The GLP-1 receptor couples to more than one intracellular pathway; different agonists stabilise different receptor conformations and therefore weight those pathways differently; the receptor is expressed in the pancreas, the stomach, the heart, the kidney and several nuclei of the brainstem and hypothalamus, and its density and coupling differ in each. A drug is not a dose. It is a pattern of signalling across tissues.

A class B receptor, and why that matters

The GLP-1 receptor belongs to class B of the G-protein-coupled receptor superfamily — the secretin-like receptors — which is a structural classification with practical consequences. Class B receptors have a large extracellular domain that captures the C-terminal portion of a peptide ligand first, in what is usually described as a two-domain binding model: the extracellular domain provides affinity, and the N-terminal residues of the peptide then insert into the transmembrane bundle to provide activation.

That architecture is why these receptors are difficult small-molecule targets and why, for two decades, every marketed agonist was a peptide. It is also why the orally available non-peptide agonists now in late-stage development are genuinely notable pharmacology rather than a formulation trick: they bind a site that a peptide does not occupy in the same way, and they activate the receptor through a partially distinct mechanism.¹

The consequence for a reader trying to compare molecules is that structural class predicts a great deal about route, durability and formulation, and rather less about efficacy.

Biased agonism, stated carefully

When the GLP-1 receptor is activated it can couple to Gα_s, raising cyclic AMP, and it can recruit beta-arrestin, which contributes to receptor internalisation and desensitisation. An agonist that favours the first over the second is described as G-protein-biased. The therapeutic argument for bias is that sustained cAMP signalling without proportionate internalisation should produce a more durable effect at the same occupancy.

The evidence for that argument is real but narrower than its popularity suggests. Bias is measured in transfected cell systems at receptor densities that bear no relationship to a beta cell or a vagal afferent, and the translation from a bias factor in vitro to a clinical difference in vivo has been demonstrated convincingly for very few ligands.² The Journal’s position is that bias is a legitimate and probably important variable, that it is one of several plausible explanations for the differences observed between molecules, and that anybody presenting it as the explanation is ahead of the data.

Selectivity, potency and efficacy are three measurements

Three quantities are routinely conflated in discussions of this class. Affinity is how tightly a ligand binds, usually reported as a dissociation constant. Potency is the concentration producing half-maximal response, reported as an EC₅₀. Efficacy is the maximal response achievable, reported relative to a reference agonist. A molecule can be more potent and less efficacious than another, and a molecule can bind a second receptor with high affinity and produce almost no response there.

Selectivity is the ratio of activities across receptors, and it is where the current pipeline diverges most sharply. Reported GIP-to-GLP-1 activity ratios for dual agonists vary by more than an order of magnitude between molecules; glucagon receptor arms in triple agonists vary similarly. Those ratios are properties of the sequence and they are not adjustable by dose. Two molecules with different ratios are different drugs at every dose, which is the reason head-to-head trials cannot be replaced by cross-trial comparison.³

The oral non-peptide agonists

An orally bioavailable small molecule that activates a class B GPCR was, for a long time, considered close to impossible. The current crop of non-peptide GLP-1 receptor agonists achieves it by binding a site that overlaps only partially with the peptide binding pocket, stabilising an active conformation without the two-domain capture mechanism.

Pharmacologically this matters for three reasons. Absorption does not depend on a permeation enhancer, so bioavailability is far less variable and far less dependent on fasting state than oral semaglutide’s. Elimination is hepatic rather than largely renal and proteolytic, which changes the interaction profile. And potency at the receptor is achieved without a fatty-acid albumin depot, so the concentration-time profile looks like a conventional small molecule rather than a peptide. None of this predicts efficacy; all of it predicts a different practical drug.

A short glossary, because the words are used loosely

Agonist: a ligand that binds a receptor and produces a response. Full agonist: one producing the maximal response the system permits. Partial agonist: one producing less than maximal response even at full occupancy. Analogue: a molecule structurally derived from a natural ligand. Mimetic: a molecule reproducing a natural ligand’s effect without structural derivation.

Orthosteric site: the binding site the natural ligand occupies. Allosteric site: a distinct site whose occupancy modulates activity at the orthosteric one. Biased agonism: preferential activation of one downstream pathway over another. Tachyphylaxis: diminishing response to repeated administration. Steady state: the condition in which the rate of drug entering the body equals the rate leaving it.

Precision here is not pedantry. Several of the arguments this publication receives by post turn out, on inspection, to be disagreements about which of these words the writer meant.

The next instalment in this department takes up the question this one deliberately set aside: not what the receptor does, but what happens when the molecule reaching it is not quite the molecule on the label. That is an analytical question, and it is answered in a different department.