I went looking for why the dosing schedule is what it is and found that almost every practical question on this board has a pharmacokinetic answer nobody states.
Numbers worth memorising for this class: Tmax one to three days for the weekly peptides, terminal half-life about a week for semaglutide and about five days for tirzepatide, steady state at four to five half-lives, subcutaneous bioavailability high enough that site choice is irrelevant.
The narrow version of the question is which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working.
I have searched first, so if this is covered somewhere point me at it and I will read it.
Dr.ObesityLA said:I went looking for why the dosing schedule is what it is and found that almost every practical question on this board has a pharmacokinetic answer…
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.
HPLC_Greg said:Semaglutide structural biology relevant to the pharmacology: semaglutide is a 31-amino acid peptide with 94% homology to native GLP-1(7-36).
No disagreement with HPLC_Greg. One condition attached. 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.
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View ResultsDr.ObesityLA said:I went looking for why the dosing schedule is what it is and found that almost every practical question on this board has a pharmacokinetic answer…
Adding a me-too, because a thread of one person's experience is not much use. Nothing to add that would improve it.
Adding the clinical framing, because it changes how the question reads.
Amycretin (AMY/GLP-1 dual agonist) emerging data relevant to the pharmacology: Phase 1 showed -13.1% body weight at only 12 weeks, the fastest trajectory ever seen for an anti-obesity agent[1].
Amylin receptor agonism enhances satiety signaling through the area postrema and reduces glucagon secretion. Combined with GLP-1R agonism, this dual mechanism may produce even greater efficacy than current agents.
Early-stage data — interpret with caution. But the trajectory is extraordinary.
[1] Novo Nordisk investor presentation, September 2023.