Coming back to: Time to steady state after a dose increase

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.

On reflection I would soften that slightly.

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.

The evidence for this is thinner than the way I have phrased it suggests.

This follows post #14 rather than contradicting it.

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.

Saving this. It is the version I will quote when the question comes round again.

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.

The interesting part of this is the exception, and I do not understand the exception.

Everything in post #16 holds. The case it does not cover is the one I have.

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.

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.

I checked the source rather than the summary, and they differ.

Half-life determines how quickly concentration approaches steady state and does not determine what the steady-state concentration is. Dose and clearance determine that.

That is the honest state of it as of this week.

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.

The reasoning is more useful than the number, which is why I have shown it.

Picking up post #21: that is the part I would want checked first.

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 had written a reply contradicting post #21 and deleted it. Here is what survived.

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.

Understood, and I withdraw the assumption I opened with.

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.

A single observation, in a thread that deserves better than single observations.

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.

I would be glad to be shown a cleaner way of putting this.

Where I part company with post #25, and it is a narrow parting.

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.

Post #29 is the version of this I will quote in future. One addition.

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.