Bioequivalence and Bioavailability Forum

Hi VSP,

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

There is no “absorption/elimination phase”. Both absorption / elimination start almost immediately with dosing. See here (k_a > k_el) and there (k_a = k_el; 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 λ_z (as an estimate of k_el) consider the TTT-method (see this post). At the inflection point of the profile (2×t_max) 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 t_max. Once you have an estimate of elimination you can obtain absorption f.i. by the method of residuals or by Wagner-Nelson’s.