Reading about receptor desensitisation and trying to work out which of the effects adapt over time and which do not, because people clearly experience both.
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 question I want answered is which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working.
Numbers rather than impressions, if you have them.
BenResearch_OR said:Reading about receptor desensitisation and trying to work out which of the effects adapt over time and which do not, because people clearly experience…
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.
kate.chem said:Receptor pharmacology relevant to the pharmacology: semaglutide is a GLP-1R agonist with a C-18 fatty acid chain that enables albumin binding (>99%),…
kate.chem has the substance of this right. The condition it depends on is worth stating. 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 StandardsBenResearch_OR said:Reading about receptor desensitisation and trying to work out which of the effects adapt over time and which do not, because people clearly experience…
This matches mine closely enough to be worth saying so out loud. Nothing to add that would improve it.
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.