Half-life, steady state, and accumulation worked through — a second dataset posts 121–127

Building on post #120 rather than restating 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.

The short version is the first sentence; the rest is why.

Half-life has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.

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.

This is the sort of exchange that makes the archive worth searching.

Post #123 is the version of this I will quote in future. One addition.

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.

Reading it back, the second half matters more than the first.

Where I part company with post #122, and it is a narrow parting.

Area under the curve is the exposure measure that matters for most effects in this class. Peak concentration matters more for tolerability.

I would want to see it done twice before believing it once.

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

A partial answer, offered because a partial answer beats none.