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.
The bit I cannot resolve on my own is which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working.
Tell me what I have not thought of.
HealthEcon_DC 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…
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.
Dr.BariatricHTX said:PK/PD modeling for the pharmacology: understanding the pharmacokinetics helps optimize dosing.
That is correct as far as it goes, and here is where it stops going. 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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Browse GL BiochemHealthEcon_DC 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…
This matches mine closely enough to be worth saying so out loud. Posting only so the count is not one.
Adding the clinical framing, because it changes how the question reads.
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.