Follow-up: Glucagon receptor agonism in a weight-loss compound

Glucagon receptor agonism in a weight-loss compound Writing it up because I had to work it out twice and would rather nobody else did.

A follow-up question about Glucagon receptor agonism that I did not know to ask the first time.

The earlier thread answered what I asked. What I should have asked is below, and I think it is the one that matters.

Taking the opening post at face value and following it one step further.

GIP receptor signalling: the glucose-dependent insulinotropic peptide receptor (GIP) is involved in glucose-stimulated insulin secretion. GIP agonism is thought to contribute to tirzepatide's effect but the mechanism is not fully settled.

I have seen it go both ways, which is why I hedge.

Receptor occupancy required for a clinical effect is not the same as full occupancy, and dose-response curves flattening at the top is what you would expect from that.

Receptor distribution explains the side-effect profile better than anything else. GLP-1 receptors in the gastrointestinal tract and the area postrema account for most of what people report.

Pharmacological class effects: all GLP-1 agonists slow gastric emptying and suppress appetite. Those are class effects, not unique to one compound. Effects that differ between compounds are usually about potency or receptor selectivity.

I will take the caveat as seriously as the claim, which is the point of putting it there.

Species differences in receptor pharmacology are substantial in this family, which is one reason rodent data has translated unevenly.

The short answer was in the first line; everything after is the working.

Biased agonism — where different ligands at the same receptor favour different downstream pathways — is a plausible explanation for differences between compounds in this class and is not a demonstrated one for any specific pair.

Central versus peripheral action: GLP-1 agonism works through both central nervous system effects (appetite) and peripheral effects (gastric motility, insulin). The balance is not fully characterised.

Nothing in receptor biology tells you what is in the vial, which is worth remembering when a mechanistic thread starts being used to justify a sourcing decision.