How many weeks does it actually take to reach steady state?

The receptor pharmacology I can follow; the in vivo translation is where I lose the thread.

Please show the division. I want to check my own against yours.

I would like the general form as well as the specific number, so I can apply it again.

Where is my error, and what is the correct working?

Specifically, start from the receptor and the rest follows: which receptors, in what ratio, with what signalling bias, reached at what concentration.

Comparing a 2.4 mg dose of one agonist to a 15 mg dose of another tells you nothing, because the molar potencies at their respective receptors differ, the receptor profiles differ, and the exposure per milligram differs. The only defensible comparison is between clinical outcomes in trials with comparable populations and durations, which is why SURMOUNT-5 exists and why indirect comparisons should be read sceptically.

Receptor desensitisation as a plateau mechanism is plausible and poorly evidenced. GLP-1R internalises on agonist binding and recycles, and biased agonists that internalise less have been argued to sustain signalling better. Whether any of that operates at the timescale of a four-month clinical plateau — against the much simpler explanation that energy expenditure fell with mass — is not established.

The structural basis of semaglutide’s pharmacokinetics — Aib-8, the Arg34Lys substitution and the C18 diacid–AEEA linker at Lys26 — is described in the original medicinal chemistry publication, and it is worth reading once because it makes the design logic explicit[1].

One qualification: none of the investigational agents discussed here is approved anywhere, and material supplied for research use is not approved for human use.

Structure predicts pharmacokinetics reliably and clinical effect unreliably. Keep the two claims separate.

Stated carefully, dose equivalence across agents with different receptor profiles is not a defensible concept, and the attempt to construct it is the most common analytical error in this area.

The split between delayed gastric emptying and central satiety matters because they have different time courses. Gastric emptying effects show substantial tachyphylaxis over weeks; the central appetite effect does not, or does so much more slowly. That dissociation is the best available explanation for why nausea fades while appetite suppression persists.

Oral bioavailability of a 4 kDa peptide is essentially zero without help. Oral semaglutide is co-formulated with sodium N-(8-[2-hydroxybenzoyl]amino)caprylate, which raises local gastric pH and transiently increases transcellular permeability in a small area of gastric mucosa. It works, and it delivers roughly one per cent of the dose, which is why the oral tablet strengths are an order of magnitude above the injectable and why fasting and water volume are not optional details.

The role of the area postrema and the hypothalamic arcuate nucleus in GLP-1-mediated appetite suppression is supported by both the neuroanatomy of receptor expression and by the effect of lesioning studies in animal models.

Worth noting that receptor pharmacology measured in a transfected cell line is a starting point, not a physiological measurement, and potency ratios do not transfer cleanly in vivo.

The chemistry is the interesting part and it is also the well-documented part. Read the medicinal chemistry papers; they are short and they explain the design.

Specifically, GLP-1R is a class B G-protein-coupled receptor signalling predominantly through Gs and cyclic AMP, and most of the interesting pharmacology in this class is about where that signalling happens rather than how hard it is driven.

The Aib substitution at position 8 replaces alanine with α-aminoisobutyric acid, which is sterically hindered enough that dipeptidyl peptidase-4 cannot cleave the N-terminal dipeptide. That single change takes the half-life from minutes to hours. The C18 diacid on a linker at Lys26 then binds albumin reversibly, which both shields the molecule from renal filtration and creates a depot that releases slowly — taking hours to about a week.

Oral semaglutide’s absorption mechanism via SNAC is described in the pharmacokinetic literature, and the ~1 per cent bioavailability figure with high inter- and intra-individual variability is why administration conditions are specified so tightly[1].

I would separate what is established structurally from what is inferred clinically. The chemistry is settled; the attribution of clinical effect to specific receptor arms mostly is not.

If you want to reason about a new agent, start from its receptor profile and its half-life. Almost everything else follows.

On the detail: the mechanism question has a clean answer for the peripheral effects and a much less clean answer for the central ones, and it is worth being explicit about which of those you are asking about.

Amylin co-agonism adds to a GLP-1 effect rather than duplicating it because the two act through different circuits: amylin signals through the area postrema via calcitonin receptor complexes, GLP-1 through both the area postrema and the arcuate nucleus. Two non-redundant satiety signals summate, which is the design rationale for a co-formulation rather than a higher dose of either.

The limitation here is that almost all of the human mechanistic work is in the licensed agents, so mechanistic claims about the investigational tri-agonists rest on animal and early-phase data.

The mechanism is settled enough to be useful and unsettled enough to be interesting, which is a reasonable place for a field to be.