Second pass at: Albumin binding and how it produces a long half-life

Posting this under the heading it deserves: Second pass at: Albumin binding and how it produces a long half-life Everything below is what sits behind that.

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.

I had written a reply contradicting the opening post and deleted it. Here is what survived.

Counterpoint on Albumin binding, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.

Adding the measurement that the opening post says would settle it.

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.

Speaking for myself and not for anyone else who has posted here.

Protein binding reduces the free fraction that is pharmacologically active and extends duration. Both effects come from the same property and only one of them is usually mentioned.

Terminal half-life estimated from a short sampling window underestimates the true value. That is a common source of discrepant figures between sources.

That is consistent with mine, for whatever one more account is worth.

Picking up post #4: that is the part I would want checked first.

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.

Noting that I have skin in this question and have tried to discount for it.

On post #7 — agreed on the reasoning, with one qualification.

Reporting rather than recommending, on Albumin binding. What happened is above. Whether it should have is a different question and not one I am qualified to answer.

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.

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.

Subcutaneous absorption is the rate-limiting step for most of these compounds, which is why the apparent half-life is absorption-limited rather than elimination-limited.

Somebody will have a better source than mine, and I hope they post it.

Adding a data point of agreement rather than a data point.