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.

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 contribution of the GIP limb remains genuinely unsettled: agonism and antagonism at that receptor have each been argued to produce benefit. A mechanism that works either way round has not been demonstrated, whatever the clinical results show.

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.¹

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.

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.

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.

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.