Posting this because the summary going around does not say what the paper says, and the difference matters for how people here are using it.
The pharmacokinetics explain nearly every practical question asked here. Albumin binding above 99% slows clearance enough to make weekly dosing possible; a terminal half-life near a week means four to five weeks to steady state and therefore a four-week titration interval; subcutaneous bioavailability around 89% means injection site barely matters. Those three facts answer most timing questions before they are asked.
Where I think it is weakest: the follow-up is short relative to how long people actually take these drugs, so durability is an assumption here rather than a finding.
What I am trying to establish is which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working. Practical detail welcome, however dull — the duller the better.
Figures above are from the primary publication rather than the press summary. If a number here disagrees with one you have, post yours and we will work out which of us is reading a secondary source.
Dr.Martinez said:The pharmacokinetics explain nearly every practical question asked here.
Semaglutide structural biology relevant to the pharmacology: semaglutide is a 31-amino acid peptide with 94% homology to native GLP-1(7-36). Three key modifications enable its pharmacokinetic profile:
- Aib8 substitution: DPP-4 resistance (prevents enzymatic degradation)
- Arg34 substitution: improved chemical stability
- C18 fatty diacid at Lys26: albumin binding → long half-life
These three modifications transform a peptide with a 2-minute half-life (native GLP-1) into one with a 168-hour half-life (semaglutide). A masterclass in peptide engineering.
Dr.Martinez said:The pharmacokinetics explain nearly every practical question asked here.
This is where I part company with the consensus forming above. The mechanism is more central than most summaries suggest. Receptor agonism in the arcuate nucleus activates POMC neurons and inhibits AgRP/NPY signalling, and the downstream MC4R pathway is the same one disrupted in monogenic obesity — convergent genetic evidence that the target is the right one. Peripherally there is glucose-dependent insulin secretion, glucagon suppression and delayed gastric emptying, but the gastric component largely adapts over months while the central effect persists, which is why the durable effect is appetite rather than fullness.
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Browse GL BiochemInsuranceTom said:The mechanism is more central than most summaries suggest.
Receptor pharmacology relevant to the pharmacology: semaglutide is a GLP-1R agonist with a C-18 fatty acid chain that enables albumin binding (>99%), creating a depot effect with a ~168-hour half-life enabling weekly dosing[1].
Tirzepatide is a dual GIP/GLP-1R agonist with higher GIP affinity (5:1 GIP:GLP-1 potency ratio). The GIP component may enhance beta-cell function and adipocyte lipid metabolism beyond what GLP-1 alone achieves.
For the pharmacology, the pharmacology explains the clinical differences between these agents.
[1] Lau J, et al. J Med Chem. 2015;58(18):7370-7380.
DataDave said:Semaglutide structural biology relevant to the pharmacology: semaglutide is a 31-amino acid peptide with 94% homology to native GLP-1(7-36).
Same pattern here, and in the same order. Posting only so the count is not one.