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Understanding the PK of a drug [PK / PD]

posted by Helmut Homepage - Vienna, Austria, 2017-03-20 14:40  - Posting: # 17170
Views: 1,171

¡Hola Javier!

» I was wondering about how to calculate the steady state of a drug when you have two different values of half life

See this post (especially the last sentence) and the warfarin example (slides 21–22). ;-)
If you extrapolate from SD to MD you have to rely on assumptions (linear PK, no saturation/induction/inhibition/capacity-limited excretion).

» (If I understand correctly the steady state of a drug takes 3-5 half lives)

It depends on how one defines sufficiently ‘close’ to true steady state (at t = ∞!). For a one-compartment model, IV administration

i  % SS
1  50
2  75
3  87.5
4  93.75
5  96.875

where i = number of half lives and % SS = 100(1 – ½i ). For EV (and more important: other models) this approach is only approximate.
Coming back to your OP: I would trust the results of the MD study’s PopPK-model more than the SD study’s results derived by NCA.

[image]Regards,
Helmut Schütz 
[image]

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