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Group,
We have finished a PK study on radiolabeled glycosaminoglycans in
rats. We measured the radioactivity in the plasma and different
organs. I will appreciate receiving the information on how to
translate DPM to concentration.
Many thanks in advance.
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[A few replies - db]
Sender: PharmPK.-a-.boomer.org
Reply-To: ospiegel.at.unmc.edu
MIME-Version: 1.0
=46rom: ospiegel.-at-.unmc.edu
Date: Wed, 20 Sep 2000 18:19:38 -0500
To: david.-at-.boomer.org
Subject: Re: PharmPK DPM to concentration
What you need to do is:
1. Subtract the background DPM from experimental DPM.
2. Convert the corrected DPM values to Ci units using the conversion factor
of 1Ci=3D2.22 * 10 EXP 12 DPM.
3.Using the specific activity (e.g. Ci/mmol) of your radioactive compound
calculate the amount of radioactive compound in your sample.
4.Divide the amount by the volume of the sample and you should have the
concentration.
Good luck
Ofer Spiegelstein, Ph.D.
Center for Human Molecular Genetics
University of Nebraska Medical Center
Omaha, NE 68198-5455
Tel: 402-559-8187 (Office)
402-559-2863 (Lab)
402-559-4001 (Fax)
402-496-9442 (Home)
E-mail: ospiegel.aaa.unmc.edu
---
I am assuming that you have run the experiment as a biodistribution study,
following administration of radiolabeled glycosaminoglycans to rats. If so,
you should know the specific activity of your compound, given in DPM/g. If y=
ou
determine your dose in DPM, rather than weight units, and the concentration
values you are seeing are in DPM/mL, then you can easily substitute the
specific activity of you radioactive dose into the equation:
Conc. value (in DPM/mL) * Specific activity of the dose (in g/DPM) =3D Conc.
Value (in g/mL).
Since you have not mentioned the specific isotope, I am assuming that its
half-life is long enough so that you do not need to decay-correct your data.
Also, keep in mind that radioactivity counting is nonspecific, and just beca=
use
you see a count it does not mean that it is coming from the intact molecule =
-
50% of your primary metabolites will also be radioactive and will contribute=
to
the overall tracer pk profile.
Sincerely,
Daria Stypinski, Ph.D.
Senior Pharmacokineticist
MDS Pharma Services
Lincoln, NE
---
Sender: PharmPK.at.boomer.org
Reply-To: "David S. Farrier"
Mime-Version: 1.0
=46rom: "David S. Farrier"
Date: Thu, 21 Sep 2000 00:03:00 -0400
To: david.-a-.boomer.org
Subject: PharmPK DPM to concentration
Dear Qing,
Here are some useful equations:
The key to your question involves knowing how to use the specific activity
of a dose form to convert radioactivity to equivalent weights. Assume that
you measured the specific activity of your dose form in units of dpm/=B5g.
=46urther assume that you measured your plasma or other tissue samples in
replicate and computed an average activity concentration in DPM/g
Then,
(CASE 1) for contiguous tissues (those which can be excised in whole like
liver, brain, kidney, etc).:
Average Total Activity (DPM) =3D
Average Activity Concentration (DPM/g) x Total Sample (g)
(CASE 2) however, for non-contiguous samples (those which usually can only
be sampled in part, such as blood, body fat, skin, etc.):
Average Total Activity (DPM) =3D
Ave. Activity Conc. (DPM/g) x ( %BW Factor/100) x Terminal BW (g)
Where Terminal BW =3D Body Weight of the carcass at termination.
Some values often quoted for the Percent Body Weight Factor (%BW Factor)
for the rat are:
Blood 4.04
Plasma 2.35
=46at 10.60
Muscle 45.40
Plasma 2.35
You can then proceed to calculate the average amount of dose form in the
total sample as follows:
Average Total Amount (=B5g) =3D
Average Total Activity (DPM) / Specific Activity (=B5g)
Alternately, you can calculate the concentration of dose form in the sample
as follows:
Average Amount Concentration (=B5g/g) =3D
Average Activity Conc. (DPM/g) / Specific Activity (DPM/=B5g)
=46inally, you can calculate the percent of dose in the sample as follows:
Percent of Dose =3D
100 x Average Total Activity (DPM) / Total Dose Applied (DPM)
-----------
Now for the advertisement. METABASE, our low cost radioanalytical LIMS
collects weight and DPM values and makes all these calculations for you.
Then PK SOLUTIONS takes your plasma level data and computes 75
pharmacokinetic parameters with the ease of a few mouse clicks. Both have
demos and order information on our web site at http://www.SummitPK.com
While there, get a free DOSE CALCULATOR which figures out the specific
activity you need for a dose form based on how sensitive you want your
analysis to be. It will also compute how much to order based on the number
of animals and the dose amount.
=46aithfully,
David
David S. Farrier, Ph.D.
Summit Research Services
Pharmacokinetics and Metabolism Software
Web: http://www.SummitPK.com
Email: DFarrier.at.SummitPK.com
---
Sender: PharmPK.aaa.boomer.org
Reply-To: hmehler.-a-.mehler.com
MIME-Version: 1.0
=46rom: hmehler.aaa.mehler.com
Date: Mon, 20 Nov 2000 21:24:26 -0800
To: david.aaa.boomer.org
Subject: Re: PharmPK DPM to concentration
The following message was posted to: PharmPK
Qing Ma:
You cannot derive reliable parameter estimates in a multicompartmental
model solely by measuring radioactivity flux as a function of time. This
is because of sampling and processing error; ie sampled plasma or other
organ weights or volumes will vary when collected from individual animals or
experimental subjects. The further processing or workup of each sample will
introduce cumulative random error in terms of non uniform losses of
radioactivity across samples. Unless you use an internal standard there is
no way to quantitate this random error.
If you are not concerned with endogenous glycosaminoglycan (lets say
that free glycosaminoglycan does not circulate) you may want to use an
amount (ie preserve proportionality between dose and transfer rate) of
"cold" or unlabeled glycosaminoglycan to monitor these random errors.
Otherwise, you can normalize the radioactivity measurement by measuring the
mass of endogenous glycosaminoglycan in each tissue of interest and
calculating a specific activity ratio (Ci/mole).
If endogenous glycosaminoglycan can be ignored then your "concentration=
"
is simply Ci/ml plasma or Ci/g tissue. Otherwise, with endogenous
glycosaminoglycan or added glycosaminoglycan your concentration is mole
endogenous glycosaminoglycan per ml or gram, since the mass of the tracer is
puportedly negligible.
The important concern that I have from all of this is that the paramete=
r
estimates that you obtained in the first run are imprecise. In order to
improve the fit of your model to the data consider the following protocol:
step 1: purify the metabolite of interest from each plasma and organ sample
and for each time point.
step 2: develop an assay to measure the mass of the metabolite in each
sample and obtain a mass measurement for each sample.
step 3: obtain a radioactivity count of the purified metabolite in each
sample for which a mass measurement is obtained.
step 4: derive a "specific activity" value for each sample as the quotient
of the radioactivity and mass (curie/mole)
step 5: measure the poolsizes (the total amounts) of glycosaminoglycan in
each compartment or tissue sampled.
step 6: construct a database consisting of specific activity (Ci/mole)
ordered by tissue and time of sampling.
step 7: note that your differential equations should be solved for
radioactivity fluxes [(Ci/mole) x (poolsize)] and the computer predicted
values will be evaluated as Ci/poolsize.
step 8: without blood flow measurements, transfer rates within or between
compartments may be expressed as the product of the parameter estimate
(kmin-1)and and the "concentration" of the measured species (mole/ml).
For an example of a multicompartmental model based on a radioactivity
study see Mehler, H. , Lactic Acid Metabolism ISBN 0-9621181-0-9.
Howard S Mehler PhD JD
--
David Bourne, Ph.D., OU HSC College of Pharmacy, Oklahoma City, OK 73117
Voice: (405) 271-6481 x 47213 FAX: (405) 271-7505
Email: david.aaa.boomer.org or david-bourne.-at-.ouhsc.edu
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All the previous comments assume that there is no metabolism, which may or
may not be true. Data calculated by these methods should be reported as
microgram equivalents/g or mL. Also if you have a tritium or iodide label,
the total radiaoctivity data will also include any isotope exchanged off.
In the case of very unstable labels one can end up measuring the kinetics of
tritiated water or iodide, both of which are already known.
Dale Sharp
PharmPK Discussion List Archive Index page
Copyright 1995-2010 David W. A. Bourne (david@boomer.org)