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.
If the honest answer is that nobody knows, that is a useful answer and I would rather have it.
JakeBK_lifts 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.
amsterdam_pete said:Semaglutide structural biology relevant to the pharmacology: semaglutide is a 31-amino acid peptide with 94% homology to native GLP-1(7-36).
Agreeing with amsterdam_pete, and the qualification matters more than the agreement. 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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Shop Reference StandardsJakeBK_lifts 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…
Same experience, arrived at from the opposite direction. I had assumed I was the exception until I read this.
From the other side of the consultation, briefly.
PK/PD modeling for the pharmacology: understanding the pharmacokinetics helps optimize dosing. Semaglutide:
- Tmax: 24-72 hours post-injection
- T½: ~168 hours (7 days) — enables weekly dosing
- Steady state: reached at 4-5 weeks
- Bioavailability (SubQ): ~89%
- Volume of distribution: ~12.5L (primarily plasma)
The albumin binding (>99%) is the key pharmacological innovation — creating a sustained-release effect from a single injection. Previous GLP-1 agonists (exenatide) required BID dosing due to rapid clearance.