The incretin receptors, ranked by how much we actually know about them

The most useful thing an editor can do with a drug class this heavily covered is insist on the distinction between a mechanism and a metaphor. Appetite suppression is a metaphor. Delayed gastric emptying mediated by vagal afferent signalling and central integration in the area postrema is a mechanism. The two are related, they are not interchangeable, and the difference determines which side effects are expected, which are dose-limiting, and which resolve.

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.

Four to five half-lives to approach steady state is the single most useful calculation in this area. For a molecule with a half-life of about a week it means more than a month, and every impression of a dose formed before then was formed on incomplete exposure.

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 long half-life of these agents is engineering rather than accident. Acylation and albumin binding were designed in, they explain the dosing interval, and they also explain a good deal about distribution that is otherwise puzzling.

How you get seven days out of a two-minute peptide

Three engineering strategies account for essentially every long-acting agonist on the market. The first is substitution at the DPP-4 cleavage site: replacing the alanine at position 8 with a residue the enzyme cannot process removes the fastest route of degradation. The second is acylation with a fatty-acid chain, which promotes reversible binding to serum albumin; albumin-bound drug is protected from renal filtration and enzymatic attack, and dissociates slowly to provide a circulating depot. The third is fusion to a large carrier — an immunoglobulin Fc fragment, for instance — which raises the hydrodynamic radius above the glomerular filtration threshold.

Semaglutide uses the first two, with a C18 diacid linked through a spacer. Liraglutide uses a shorter C16 chain and achieves roughly thirteen hours rather than seven days, which is a useful demonstration of how much the chain contributes. Dulaglutide takes the fusion route. The strategies are not interchangeable and they produce different distribution and clearance behaviour, not merely different durations.⁴

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.

The Journal will keep reporting this department from the primary literature and the regulatory assessment reports, and will keep stating when a claim rests on transfected cells rather than on people. Readers who think a paragraph here has outrun its evidence should write in; the standards desk reads every such letter and the correction log records what came of it.