The limits of mechanism: why receptor data will not tell you who responds

Everything in this department is written on the assumption that mechanism is worth understanding. It is also worth being explicit about what mechanism cannot do, because the gap between what receptor pharmacology explains and what a reader wants explained is wide and rarely acknowledged. Receptor data predicts average effects reasonably well. It predicts almost nothing about which individual will lose twenty-five per cent of their body weight and which will lose four.

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.

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.²

Amylin analogues are a different class

Cagrilintide is not a GLP-1 receptor agonist and it is repeatedly described as one. It is a long-acting analogue of amylin, a 37-residue peptide co-secreted with insulin from the beta cell, acting at calcitonin and amylin receptor complexes. Its effects — slowed gastric emptying, reduced food intake, satiety signalling through the area postrema — overlap substantially with GLP-1 receptor agonism, which is why the confusion persists and why the co-formulation with semaglutide is pharmacologically interesting rather than redundant.

Two mechanisms converging on the same behavioural endpoint through different receptors is the argument for combining them: the ceiling of each is set by its own receptor-mediated adverse effects, and two half-doses at different receptors may sit below both ceilings. Whether that argument survives phase 3 is an empirical question.

Delayed gastric emptying is the mechanism, not the complication

Slowed gastric emptying is frequently described as a side effect. It is more accurately described as a mechanism that becomes an adverse effect at sufficient magnitude. Delayed emptying blunts the post-prandial glucose excursion, which is part of the glycaemic benefit, and it produces early satiety, which is part of the weight effect. Beyond a threshold it produces nausea, vomiting, reflux and the sensation of food sitting undigested.

Two properties of the effect matter clinically. It is dose-dependent, and it exhibits partial tachyphylaxis: the magnitude of delay attenuates over weeks of continued exposure at a fixed dose, which is the physiological basis for the observation that tolerability improves if a dose is held rather than escalated. The residual delay at steady state is real and is the reason pre-procedural fasting guidance for this class exists at all.³

The heart-rate signal

A resting heart-rate increase of roughly two to four beats per minute is one of the most reproducible findings in the class, observed across molecules, doses and populations. The mechanism is probably direct: GLP-1 receptors are expressed in the sinoatrial node region, and receptor activation has chronotropic effects in isolated preparations.

What it means clinically is unresolved. The cardiovascular outcome trials that reported the heart-rate increase also reported reductions in major adverse cardiovascular events, so whatever the chronotropic effect represents it is not overwhelming the benefit in the populations studied. That is a statement about trial populations and event rates, not a mechanistic reassurance, and the Journal reports it as such.

The response distribution nobody can explain

In the large obesity trials, mean weight reduction is reproducible to within a percentage point or two across programmes. The distribution around that mean is wide and consistent: a substantial minority of participants lose more than a quarter of their body weight, and a smaller but non-trivial group lose almost nothing. Reported non-response rates — usually defined as failing to reach 5% reduction — run to roughly one participant in seven to one in ten depending on the molecule and dose.

Nothing measurable at baseline has been shown to predict which group an individual falls into with useful accuracy. Receptor polymorphisms have been examined and explain little. Baseline BMI, sex, diabetes status and age shift the mean modestly and the variance barely at all. The honest summary is that this is the largest unexplained quantity in the field, and that any source claiming to predict individual response is claiming something the literature does not support.⁴

Desensitisation, and what it does and does not explain

Receptor internalisation following agonist binding is well established in vitro, and the popular inference is that "the receptors get used to it", explaining plateaus. The inference outruns the evidence in two ways. First, plateaus in the trials occur at around sixty to seventy weeks and coincide closely with the point at which reduced body mass lowers energy requirement enough to re-establish balance, which is a sufficient explanation without invoking receptor changes. Second, weight regain on withdrawal is rapid and near-complete, which is difficult to reconcile with a model in which the receptor has become unresponsive.

The tolerability tachyphylaxis discussed above — the attenuation of nausea and gastric delay over weeks at a fixed dose — is separately well supported. Two different phenomena share a name, and conflating them produces confident conclusions about plateaus that the data does not license.

Why any of this belongs in a general publication

An argument could be made that receptor pharmacology is a specialist concern and that readers need practical guidance instead. The Journal’s position is the opposite, for a specific reason: almost every piece of bad advice circulating about this drug class is a mechanistic error with a practical conclusion attached.

Escalating on a fixed calendar regardless of symptoms is an error about accumulation kinetics. Splitting a weekly dose into daily fractions to reduce side effects is an error about half-life and steady state. Assuming a molecule with GIP activity is simply a stronger version of one without is an error about selectivity. Expecting weight to keep falling indefinitely is an error about energy balance. In each case the practical advice is wrong because the mechanism was misunderstood, and in each case understanding the mechanism is not much harder than memorising the rule.

A note on sources

Everything above is drawn from the peer-reviewed pharmacology and clinical literature and from regulatory assessment reports, which are more informative than the papers on questions of dose selection and exposure. Where a claim rests on in-vitro work in transfected cells, this piece says so, because the translation of such work to human physiology has failed often enough in this field to deserve a standing caveat.

Where the Journal reports a trial number it states the estimand behind it, because the treatment-policy and trial-product estimands differ by two to three percentage points in the obesity programmes and the difference is routinely lost in secondary coverage. Nothing here is a recommendation, and none of the compounds discussed as research chemicals are approved for human use.

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.

What has actually changed in the last three years

Three things, on the Journal’s assessment. First, the demonstration that a dual agonist could produce weight reduction approaching bariatric-surgical magnitude moved the field’s expectations, and with them the design of every subsequent programme. Second, the cardiovascular and renal outcome results reframed the class from metabolic-cosmetic to cardiometabolic, which changed reimbursement arguments far more than it changed prescribing.

Third, and least remarked, the pharmacology of oral administration became tractable. That is a manufacturing and access story as much as a scientific one: an oral small molecule has a completely different cost structure, cold-chain requirement and supply profile from an injectable peptide, and if it holds up in phase 3 it will do more to change who can get treated than any of the receptor science described above.

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.