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Dear all
In a bioavailability assay, could I calculate the time
of performing the last sample collection so that I
obtained 80% of the AUC?
Thank you for replay.
Maria Ines de Toledo
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Maria,
I think that the time of the last sample for which 80% of
the AUC would be included could be calculated after estimating
the Ke. The following calculations were performed for an IV bolus
in a one compartment model:
Fraction 0.8 = AUC(o->t)/AUC(o->inf)
= [(Co/Ke)(1-e-Ket)]/[Co/Ke]
= (1-e-Ket)
Time 0.8 = Ln(0.2)/(-Ke) = 1.60943791/(Ke)
Example: T1/2= 2 hours Ke= .693/2= .3465 hr-1
Time 0.8= 1.60943791/Ke= 4.6448425 hours
Mike Leibold, PharmD, RPh
ML11439.-a-.goodnet.com
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I assume that you are not modeling the PK for bioavailability. The best
option is to obtain some prior data on the product. The data can be
analysed via non-compartmental methods using only data for selected time
points. For example - choose all concentrations <= 8 hours and
determine AUC(0-t) and AUC(0-inf). If AUC(0-t)/AUC(0-inf) < 0.8 then
samples collected over 8 h would be acceptable. Repeat this process
with other selected times. However, I would urge you to use 90 or 95%
rather than 80% for the measured area relative to extrapolated area.
If you do have a drug that has been modeled, you can use fitted
parameters and the appropriate structural model to calculate the time to
80% of AUC or perform simulations.
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Copyright 1995-2010 David W. A. Bourne (david@boomer.org)