This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Between-person variability in exposure is substantial and is the reason two people on the same dose can have quite different plasma concentrations. That is inherent rather than a formulation defect.
That holds for the case as described. Change the assumptions and it may not.
Worth separating two things that post #58 runs together.
A missed weekly dose perturbs a slowly moving average rather than creating a trough. That is the pharmacokinetic reason the labelling does not recommend doubling.
Washout after stopping takes roughly the same four to five half-lives as reaching steady state. A month after the last dose is not the same as none.
Adding it in case it saves somebody the afternoon it cost me.
A pharmacokinetic model fitted to trial data describes the population studied. Applying it to somebody outside the enrolled range is an extrapolation, and the model will not tell you it is.
I would not lead a decision with this, but I would not ignore it either.
Picking up post #63: that is the part I would want checked first.
Metabolism for peptide drugs is proteolytic rather than hepatic in the usual sense, which is why the cytochrome interaction questions that dominate small-molecule pharmacology mostly do not apply.
Reading back through, this was answered upthread and I missed it. My fault.
Dose proportionality across the studied range means dose arithmetic behaves the way you would naively expect. It is worth checking whether it holds for a given compound rather than assuming.
That is the practical version. The rigorous version is longer and says the same thing.
The time to maximum concentration after a subcutaneous dose in this class is measured in days rather than hours, which surprises people expecting an injection to act quickly.
I would be interested in a counterexample if anyone has one.
Adding the measurement that post #68 says would settle it.
Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number.
I am reporting what happened, not recommending it.
Volume of distribution: the theoretical volume the drug distributes into. For albumin-binding compounds, volume is reduced compared to drugs that do not bind protein. That is relevant to understanding how much free drug is available.
I would want a second opinion before relying on that.
Accumulation at steady state: with a week-long half-life, steady-state concentration is reached around 4 to 5 half-lives (about 4 to 5 weeks). Before that, concentration is rising with each dose. The clinical implication: escalating before 4 weeks means escalating before steady state.
Loading doses are not used in this class and the pharmacokinetic reason is tolerability rather than efficacy. A loading dose would reach steady state faster and would be intolerable.
Posting it because the silence on this was starting to look like agreement.
Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number.
Half-life: semaglutide ≈ 165–184 hours (about a week). Tirzepatide ≈ 5 days. Liraglutide ≈ 13 hours. The half-life determines how much accumulation happens at steady state and how long it takes to clear after stopping.
A guess, clearly labelled as one.
Post #73 answers the question as asked. The question underneath it is different.
Metabolism for peptide drugs is proteolytic rather than hepatic in the usual sense, which is why the cytochrome interaction questions that dominate small-molecule pharmacology mostly do not apply.
Worth checking against a second source before it gets quoted onward.
Renal clearance: these compounds are cleared partly via the kidney. In severe renal impairment, clearance is slowed and accumulation risk is higher. Dose adjustments might be needed.
It is the kind of thing that is obvious once and never again.
Everything in post #78 holds. The case it does not cover is the one I have.
A pharmacokinetic model fitted to trial data describes the population studied. Applying it to somebody outside the enrolled range is an extrapolation, and the model will not tell you it is.
Nothing above should be read as advice about what anyone else should do.
Narrowing post #78, because the general version has more than one answer.
Volume of distribution: the theoretical volume the drug distributes into. For albumin-binding compounds, volume is reduced compared to drugs that do not bind protein. That is relevant to understanding how much free drug is available.
Steady state is approached in roughly four to five half-lives. For a compound with a week-long half-life that is four to five weeks, which is where the escalation interval in the trials comes from.
Adding the measurement that post #82 says would settle it.
Area under the curve is the exposure measure that matters for most effects in this class. Peak concentration matters more for tolerability.
Worth saying I have only my own numbers here, and n is small.
Post #80 describes the usual case. This is about the unusual one.
Metabolism for peptide drugs is proteolytic rather than hepatic in the usual sense, which is why the cytochrome interaction questions that dominate small-molecule pharmacology mostly do not apply.
If it helps: the failure mode here is usually boring rather than dramatic.
Agreed on all of that, and I have nothing to add to it.
Individual variation: people vary in how quickly they absorb, distribute, metabolise, and excrete these compounds. That variation is partly genetic and partly due to individual biology (gut motility, kidney and liver function). It explains why two people on the same dose have different response magnitudes.
Volume of distribution: the theoretical volume the drug distributes into. For albumin-binding compounds, volume is reduced compared to drugs that do not bind protein. That is relevant to understanding how much free drug is available.
Answering the question post #84 raises rather than the one it answers.
Bioavailability: oral semaglutide has low bioavailability (roughly 1%) due to peptide instability. That is why the oral dose (14 mg) is so much larger than the injectable dose. Comparing them by mass is meaningless.
Take it as a starting point and not as a specification.
Taking post #86 at face value and following it one step further.
A pharmacokinetic model fitted to trial data describes the population studied. Applying it to somebody outside the enrolled range is an extrapolation, and the model will not tell you it is.
Written in the hope of being told what I have missed.
Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number.
Two people can read the same figure differently here and both be reasonable.