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.
So the question, as narrowly as I can put it: which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working.
I would rather have one careful answer than five confident ones.
mark_tokyo 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…
Pharmacist here. I want to add the drug interaction perspective on the pharmacology.
Key points from a pharmacokinetic standpoint:
- GLP-1 agonists delay gastric emptying, which can affect Tmax of co-administered oral medications
- Monitor patients on warfarin (INR), levothyroxine (TSH), and oral contraceptives during dose titration
- The albumin-binding mechanism of semaglutide (C-18 fatty acid linker) gives it the ~168-hour half-life that enables weekly dosing
- Steady state is reached at approximately 4-5 weeks after dose initiation or adjustment
Re: the pharmacology specifically — the pharmacology here is well-characterized and the clinical implications are straightforward.
DanielChem_CHI said:I want to add the drug interaction perspective on the pharmacology.
Agreeing with DanielChem_CHI, and the qualification matters more than the agreement. 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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View Resultsmark_tokyo 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. The detail I would add is minor and it is already implied above.
Clinical perspective, offered as context rather than as advice.
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.