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Dear everyone,
I have recently completed some pharmacokinetics of 2 forms of the
same protein and have managed to calculate all relevant parameters such as
clearance, elimination and distribution half lives, etc etc.
These experiments were performed with iodinated human proteins and
injected into the rat by iv.
However, when I removed the organs, most of the radioactivity was
accumulated in the stomach (30% of total counts). I do know that my protein
is stable in serum up to 4 hrs (as assessed by in vitro stability assays
and TCA precipitation).
So has anyone else seen anything like this or can they explain this...
I have a couple of references suggesting that iodination can affect
pharmacokinetics but iodination affecting distribution I still havent found.
Thanks for your time,
Joe Marinaro
B.Sc (Hons)
Department of Medicine,
The University of Melbourne
Department of Medicine
Austin Hospital
HEIDELBERG, VICTORIA 3084, AUSTRALIA
email: joe.at.austin.unimelb.edu.au
Ph: (613) 9496 3595
Fax: (613) 9457 5485
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Joe:
Check the stability (In-vitro and in-vivo) of your radiolabel or tag of
the drug before you administer it to the animals. Also check to see what
exactly you are counting in the stomach samples, that is, is it the
radiolabel by itself or the drug with the radiolabel.
These may be trivial issues, but sometimes the most obvious are the most
missed issues.
Alfredo R. Sancho,Ph.D.
Clinical Pharmacologist
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[A few replies - db]
Date: Tue, 15 Sep 1998 13:23:38 -0400
From: Nabil B. Darwazeh
To: PharmPK.aaa.pharm.cpb.uokhsc.edu
Subject: Re: PharmPK Accumulation of Radioactivity in Stomach
Mime-Version: 1.0
Dear Joe
I have seen this with many of my radioidinated polymers and proteins.
Although your proteins is stable in plasma, for sure can not be stable for
long. Dehalogenation metabolism occures in the liver, free iodin share the
same exchane window of chlorin in the stomach, as a results, free Iodin
will accumulate in the stonach. Additionally, if you sample the saliva or
the tip of the nose you will find continuous activity. So the source of
radioactivity in stomach are secretion through the stomach wall and saliva.
Some minor possibilty that the small fragments of your protein (Amino
acids) that carry the label gain access to stomach by saliva or ather
secretion mechanism.
Sincerly
Nabil B. Darwaze, Ph.D.
Wyeth-Ayerst Research
darwazn.-a-.war.wyeth.com
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Mime-Version: 1.0
Date: Wed, 16 Sep 1998 08:38:40 +1100
To: PharmPK.aaa.pharm.cpb.uokhsc.edu
From: Joe Marinaro
Subject: Accumulation of Radioactivity in Stomach
These experiments are performed on anaesthetised rats. Therefore
the possibility of ingestion by the animal is not possible.
I have checked the in vitro stability of my protein and it is
stable >4 hr in serum at 37=B0C (with < 10% reduction, as assessed by 15% TC=
A
precipitation).
It may be possible that what I am measuring is free iodine or fragmented
protein but at the same time I believe it is intact protein because this
localization has also been seen in the same manner with another protein of
the same family, so I know its not an original observation.
We have frozen the tissues and placed slices on flm and noticed
discrete areas specifically binding the protein.
Yes I have tried to run a gel of stomach protein but it is
difficult to remove in high enough amounts to detect on film.
Thanks for the previous suggestions.
Joe Marinaro
B.Sc (Hons)
Department of Medicine,
The University of Melbourne
Department of Medicine
Austin Hospital
HEIDELBERG, VICTORIA 3084, AUSTRALIA
email: joe.aaa.austin.unimelb.edu.au
Ph: (613) 9496 3595
Fax: (613) 9457 5485
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Date: Wed, 16 Sep 1998 05:59:34 -0600
From: Vinay Desai
To: PharmPK.at.pharm.cpb.uokhsc.edu
Subject: PharmPK Accumulation of Radioactivity in Stomach -Reply
Encoding: 83 Text
Joe,
Dehalogenation or in this case deiodination is a very
common phenomenon. The thyroid and stomach are the
major organs that accumulate iodine following deiodination.
There are numerous references in the monoclonal antibody
fied that document this.
A few references to get you started:
Eary, J.F. et al, Radiochemistry of halogenated antibodies
(1989), Anitbodies in Radiodiagnosis & Therapy (M.R.
Zalutsky, Ed), pp 97-98, CRC Press, Boca Raton, FL
Regoeczi, E. (1987), In Vivo Behavior of Catabolized labels
derived from iodo proteins. Iodine Labeled plasma proteins,
Vol II, Part B, pp 43-71, CRC Press, Boca Raton, FL
Engler, D et al, (1984) The deiodination of the Iodothyronines
& of their derivatives in Man. Endocr. Rev., 5, 151-184
Sinsheimer, J.E. et al (1978), Mechanisms for the
biodehalogenation of iodocompounds. Biochem. Biophys.
Res. Commun. 83, 281-286
If you want to look at alternative iodination chemistry, look up
Bioconjugate Chemistry, Vol3 No6, Nov/Dec1992 for a review
by D. Scott Wilbur titled Radiohalogenation of Proteins: An
overview of Radionuclides, Labeling methods and reagents for
Conjugate labeling.
Regards,
Vinay
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