VSP
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India,
2017-04-25 18:28

Posting: # 17265
Views: 2,186
 

 absorption phase and elimination phase [PK / PD]

Hello,

Please guide me on following question.

Can we define the absorption phase and elimination phase with concentration time data/Plot?

Thanks in advance.

VSP


Edit: Category changed; see also this post #1. [Helmut]
Helmut
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Vienna, Austria,
2017-04-26 13:26

@ VSP
Posting: # 17266
Views: 1,782
 

 absorption / elimination

Hi VSP,

» Can we define the absorption phase and elimination phase with concentration time data/Plot?

[image]There is no “absorption/elimination phase”. Both absorption / elimination start almost immediately with dosing. See here (ka > kel) and there (ka = kel; flip-flop PK).

As long as drug is released from the formulation and dissolved, absorption continues until
  • the formulation leaves the GIT (hence it does not make sense to sample more than 72 hours for an IR formulation) or
  • the formulation proceeded in the GIT beyond the absorption window of the drug.
If you are interested in λz (as an estimate of kel) consider the TTT-method (see this post). At the inflection point of the profile (2×tmax) absorption is practically (!) complete (i.e., 93.75%).

 i    % abs.
0.5   50.00
1     75.00
1.5   87.50
2     93.75
2.5   96.875
3     98.4375
3.5   99.21875
4     99.609375

where i is the multiplier of tmax. Once you have an estimate of elimination you can obtain absorption f.i. by the method of residuals or by Wagner-Nelson’s.

Cheers,
Helmut Schütz
[image]

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