Back to the Top
Dear Colleagues,
What does circulation half-life exactly means? In
terms of one-compartment model (e.g., mono-phasic
profile) we have one half-life and it may be referred
to as circulation half-life but what about
multi-compartment model (e.g., bi-phasic profile)?
Referes to alpha or beta? I tend to think alpha,
please provide your input.
Rostam
Back to the Top
Hi there
The alpha half life indicates the rate of distribution whereas the beta
half life indicates the rate of elimination of drug from the body, which
is used for calculating the half life of the drug.
Thanks and regards
Ashish Jain, M.Pharm.
Group Leader-Bioanalytics
Back to the Top
Hello everyone,
Does the half-life obtained from one-compartmental modeling of plasma/blood
concentration-time data, correspond to the elimination half-life (beta
half-life) obtained in two-compartment models? Or is it as Rostam has referred
to it, a sort of circulation half-life?
Thanks
Back to the Top
You should not expect having beta half-life from 2 compt model
corresponding to the alfa-half life of the one compartment model.
However if I am not wrong, reparametrization of the 2 compt model in
terms of Clearance (what NONMEM is using) would rise to a quantity
(clearance) that is the same as the clearance used in the one
compartment model.
Serge Guzy, Ph.D.
Head of Pharmacometrics
Xoma
Back to the Top
"pkhalili (by way of David Bourne)" wrote:
>
> Does the half-life obtained from one-compartmental modeling of plasma/blood
> concentration-time data, correspond to the elimination half-life (beta
> half-life) obtained in two-compartment models? Or is it as Rostam
>has referred
> to it, a sort of circulation half-life?
Are you familiar with the famous quote of George Box "All models are
wrong but some models are useful"?
In suitable circumstances, it may be useful to consider that the
half-life estimated using a one compartment model is similar to the
terminal half-life estimated from a 2 compartment model but it would
be wrong to think they were identical e.g. the terminal half-life of
a 2 cpt model for digoxin after an IV dose is quite similar to the
half-life obtained using a one cpt model after an oral dose.
The circulation half-life is a harder concept to compare. If I was to
propose a model for circulation times then I would say a parameter of
the model would be the average time for a molecule to do one cycle
around the circulation. This cycle time does not involve a
first-order or exponential model and thus does not have a half-life
as a parameter. One could stretch the idea by proposing that the
cycle time is similar to the mean residence time of a molecule and
thus ln(2)/cycle time might be thought of as the corresponding
"circulation half-life".
But I still find it hard to compare the distribution half-life, which
reflects the process of drugs exiting the circulation, with a
"circulation half-life", which reflects the process of the drug
remaining in the circulation.
Nick
--
Nick Holford, Divn Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
email:n.holford.-at-.auckland.ac.nz
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
PharmPK Discussion List Archive Index page
Copyright 1995-2010 David W. A. Bourne (david@boomer.org)