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.

I would like to understand what this number means before I repeat it anywhere.

A Janoshik report on a retatrutide lot gives 96.5% purity. The supplier certificate for the same lot states 98%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

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.

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.

Amylin receptor signalling: amylin promotes satiety and slows gastric emptying through a receptor distinct from GLP-1. The hypothesis behind combination therapy is two complementary satiety mechanisms.

Bias and desensitisation: receptors can be biased (preferentially activating some downstream pathways over others) and can desensitise over time (responding less to the same stimulus with repeated exposure). Both might affect long-term response to these compounds.