Albumin binding and how it produces a long half-life — setting out what I have, and where I think it stops being reliable.
A narrow question about albumin binding, deliberately narrow, because the broad version has been asked here four times and produced four long threads and no answer.
One question, stated units, stated method, and what I have already ruled out.
Adding what did not work for me on albumin binding, since the failures never get written up and they are half the useful information.
No notes. Posting so the count is not one.
Adding the measurement that post #2 says would settle it.
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.
Answering the question post #4 raises rather than the one it answers.
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.
The arithmetic in post #7 is right; the assumption feeding it is the part to check.
The bit of albumin binding that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
Coming back to post #7, because the follow-up matters more than the original answer.
Albumin binding: semaglutide binds albumin through a fatty side chain, which sequesters the free form and extends the half-life. Tirzepatide also has albumin binding (different mechanism) which extends its half-life compared to an unmodified peptide.
I came in to disagree and I am leaving without a disagreement.
Post #8 is right about the mechanism and I think understates the practical bit.
Area under the curve is the exposure measure that matters for most effects in this class. Peak concentration matters more for tolerability.
I am reporting what happened, not recommending it.
Half-life determines how quickly concentration approaches steady state and does not determine what the steady-state concentration is. Dose and clearance determine that.
Someone should write this up properly, and it should probably not be me.
Confirming post #12 from a second method, which matters more than confirming it from a second person.
Small correction to my own earlier position on albumin binding. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
I had written a reply contradicting post #13 and deleted it. Here is what survived.
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.
Speaking for myself and not for anyone else who has posted here.
Terminal half-life estimated from a short sampling window underestimates the true value. That is a common source of discrepant figures between sources.
The variance between people here is larger than the effect being discussed.
Worth separating albumin binding as a question about the compound from albumin binding as a question about the documentation. They get answered by different people and only one of them is answerable here.
Where I part company with post #18, and it is a narrow parting.
Nobody has said the unglamorous part of albumin binding yet, so: most of the variation is explained by things that are boring to write about and easy to check.
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.
The arithmetic in post #19 is right; the assumption feeding it is the part to check.
The accumulation ratio for weekly dosing with a week-long half-life is around two, which is why the concentration after several doses is roughly double the concentration after the first.
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 rather be precise about what I do not know than vague about what I do.
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.
It took me longer than it should have to see that.
I disagree with the framing of albumin binding above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.
The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.
Post #23 put the caveat in the right place and I want to underline it.
Source for the albumin binding figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.
Speaking only to albumin binding as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
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.
One of those cases where knowing the mechanism does not help the decision.