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Dear collegues,
in case of analysing serial blood samples after
injection of drugs marked with radionuclides or radiopharmaceuticals,
is it better, in your opinion, correct the decay for the time of
sampling or for the time of injection? It can be a difference in the
interpretation of biokinetic data, following one approach rather than
the other?
Thank you very much in advance for your reply and your interest in our
case.
Your sincerely,
Sandro Ridone
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It seems that the calculation should be done beginning at the time of
injection. Then, you can calculate how much should still be there at
time of sampling.
Steven C. Bubnick, Environmental Scientist
Region 8, Ecosystems Protection and Remediation
8EPR-EP
bubnick.steven.aaa.epa.gov
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Dear Sandro,
Your analysis would depend on the radioactive isotope that you are
using. For radioisotope that has a long half life (in years), like for
tritium, I would not expect a significant decay with time between your
injection and sampling. I would presume this calculation would be of
interest in case of gamma emitters.
Would like to know what others think of it though!
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The following message was posted to: PharmPK
Hello Sandro:
In standard tabulations, data are corrected for decay with a time
interval equal to that from injection to sampling. Thus, 3 days after iv
injection, the user would correct for 3 days of decay. These results are
then in %injected dose (ID) and are standard in nuclear medicine
literature. For blood, it is also common to divide by the ml of the
sample to obtain %ID/ml or %ID/g. The latter is standard notation in
other tissues that may be sampled such as liver or spleen.
I would recommend, however, that you do not correct for decay if
modeling of the data is done. Simply include radiodecay as an additional
exit route out of each compartment. This has at least two advantages. 1)
the statistics are correct in that the data are not artifically inflated
by the decay correction term. 2) In non-linear models, the correct
solution can only be obtained if the decay is explicitly kept in the
differential equations. After all, that is the nature of the reality -
material leaves a compartment by both physiological and physical means.
See L.Williams et al Medical Physics 22 1619-1626, 1995 for more about
radiodecay correction and modeling.
There is another caveat. The label may have come off the pharmaceutical.
Iodine is notorious for this behavior. Following the label does not
prove that you are following the entity of interest. Test for free label
in the plasma.
Best wishes.
Larry Williams
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The following message was posted to: PharmPK
Dear Larry,
In reply to Sandro you wrote:
> 2) In non-linear models, the correct
> solution can only be obtained if the decay is explicitly kept in the
> differential equations. After all, that is the nature of the reality
> material leaves a compartment by both physiological and physical
> means.
I agree. However, this is true only if the label represents the total
amount of material, e.g. in nuclear medicine.
In pharmacokinetic studies, however, a radiolabelled compound is often
mixed with cold material, or a tracer dose is given additional to a
multiple dosing regimen. In such cases, PK modeling should take into
account the total concentration or amount of the compound, not the
radiolabel only.
As an example, see our paper:
Proost JH, Beljaars L, Olinga P, Swart PJ, Kuipers ME, Reker-Smit C,
Groothuis GMM, Meijer DKF. Prediction of the pharmacokinetics of
succinylated human serum albumin in man from in vivo disposition data
in animals and in vitro liver slice incubations. Eur J Pharm Sci 2006;
27(2-3): 123-132.
best regards,
Hans Proost
Johannes H. Proost
Dept. of Pharmacokinetics and Drug Delivery
University Centre for Pharmacy
Antonius Deusinglaan 1
9713 AV Groningen, The Netherlands
Email: j.h.proost.-at-.rug.nl
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