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The following message was posted to: PharmPK
On December 22, 2000 I sent an e-mail to all of you, posing the
following questions:
in your ADME studies, how do you identify in vivo sites of action?
I did not mean any sites of deposition, but specific sites of receptor
binding to specific tissues and cells.
Do you conduct such studies and if so, how do you do it?
In the past I have argued that radioassay/HPLC with dissected organs or
chunks of them and whole body autoradiography, tissue homogenates, cell
cultures, PET-scans and other useful non-invasive methods, can not (with
perhaps certain exceptions) inform about in vivo target tissue and
target cell specific receptor binding. All these procedures are lacking
resolution or topographic in vivo context.
My question was, in other words, do you and how do you identify LOW
CAPACITY =96 HIGH SPECIFICITY in vivo sites of drug action?
I received four responses, two from academia and two from pharmaceutical
companies.
>From the results of this inquiry and the silence in this matter one
could conclude: Identification of (tissue and cell) specific in vivo
receptor sites is not commonly done in routine ADME studies at
pharmaceutical companies. In many or most cases, pharmaceutical
companies have no or only inadequate knowledge of in vivo(!)
distribution of sites of specific drug binding and action.
If my impressions are correct, it would follow that this state of
affairs is deplorable. Lack of that knowledge is not, should not be,
acceptable scientifically and administratively. From a public health
viewpoint, is it not irresponsible, if one considers the available
scientific potential? Techniques have been developed and can be applied.
I thank all who responded to my inquiry. I would happily accept any
corrections and further clarification as well as suggestions for
improvement of the situation.
Walter E. Stumpf
PS:
Is it not the purpose of ADME studies to obtain information relevant to
the understanding of drug action (including potential side effects and
toxicity) ? Accordingly, identification of target sites of action would
be an essential part of ADME, perhaps the most important part of it. If
this is so, I am wondering why there is apparently little concern about
the clarification of that subject matter. =96 We know a lot about P450,
about other enzymatic reactions, metabolism, blood/plasma protein
binding, in vitro protein binding =96 all of which is important. But none
of that answers the most important question, namely, where precisely are
the in vivo sites of receptor binding and action relevant for most
compounds.
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Walter:
You wrote:
>I received four responses, two from academia and two from
>pharmaceutical companies.
Could you please summarize what were the answers you got?
And would there be interest by the members of the list-server
of making this a formal discussion topic to try to get more
views? As I stated in my answer (and I guess I am one of
the two academics) the time is now here where we have the
means of going well beyond blood-based measurements.
Professor Walter Wolf, Ph.D.
Distinguished Professor of Pharmaceutical Sciences
Director, Pharmacokinetic Imaging Program
Department of Pharmaceutical Sciences, School of Pharmacy
University of Southern California
1985 Zonal Ave., Los Angeles, CA 90089-9121
E-Mail: wwolfw.aaa.hsc.usc.edu
Telephone: 323-442-1405
Fax: 323-442-9804
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The following message was posted to: PharmPK
Dear Dr. Wolf:
Thank you for your continued interest. I hope, we will be able to arrive
at some clarification and - if appropriate - exposure of the problems at
hand.
Below are copies of the responses I received (except yours) and I included
my individual answers.
[Also available from the PharmPK archive - db]
I am looking forward to your assessment and recommendations.
Best regards, Walter E. Stumpf
--
Date: Mon, 15 Jan 2001 13:04:06 -0600
From:
"Melethil, Srikumaran K."(by way of
David_Bourne)
To:
Multiple recipients of PharmPK - Sent by
Subject:
PharmPK Re: In vivo drug distribution studies
Dear Walter,
Yy and related brain target identification is mostly not
possible (with, perhaps, very few exceptions of large target areas). The
non-invasive methods of PET scan and other nuclear imagings are highly
desirable and need to be developed further. However, their present
resolution
is limited. At the present stage of development, results from PET studies
do
not inform, in most cases, about hormone or drug targets and their nuclear
topography in the brain.
"Pre-clinically they have used autoradiography to show reduced binding in
presence of antagonists/competing agonists." The question is, what kind
of
autoradiography? How is the resolution? Whole body autoradiography with
14C-labeled compounds is frequently used. If that is the case, in vivo
sites of
specific receptor-ligand binding, in most cases, can not be identified
with
this method. Sensitivity and resolution are too low.
Unless you can contradict some of the above, my conclusion would be: at
your
company, as probably at many others, in routine ADME studies in vivo sites
of
specific drug binding are not or insufficiently determined.
My emphasis in this discussion is on "in vivo" and on "specific sites of
receptor binding".
If ADME studies are aimed at understanding mechanisms of drug action,
identification of sites of action is probably the most important part of
it. In
drug development and research this seems inadequately dealt with, perhaps
even
neglected and evaded.
Of course, all the other pharmacokinetic parameters obtained during
routine
ADME studies are relevant. But they inform about sites of deposition or
binding
that usually pertain to what is called 'high capacity-low affinity sites'
("sites of loss"). They are mostly not 'low capacity-high specificity
sites'
of action. E.g., half lives determined with radio-labeled compound in
body
fluids and in dissected tissues may be - and probably usually are -
considerably different from half lives of specific target tissue binding.
Walter E. Stumpf
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The following message was posted to: PharmPK
Dear Dr. Stumpf,
In reaction to your messages:
> in your ADME studies, how do you identify in vivo sites of action?
> Is it not the purpose of ADME studies to obtain information relevant to
> the understanding of drug action (including potential side effects and
> toxicity) ? Accordingly, identification of target sites of action would b=
e
> an essential part of ADME, perhaps the most important part of it. If this
> is so, I am wondering why there is apparently little concern about the
> clarification of that subject matter. =F1 We know a lot about P450, about
> other enzymatic reactions, metabolism, blood/plasma protein binding, in
> vitro protein binding =F1 all of which is important. But none of that
> answers the most important question, namely, where precisely are the in
> vivo sites of receptor binding and action relevant for most compounds.---
I agree with your point that identification of the site of action is
interesting and important. However, I don't not agree that this is the
most important point in pharmacokinetics (PK), or ADME.
The important questions in PK are: is the drug absorbed, and to
which extent; how is the drug eliminated, and which metabolites
are formed, etc. These questions are essential for, among others,
the evaluation of toxicity, the development of dosage forms, dosing
regimens, and dose adjustment in special patient groups.
The most important question 'does the drug work?' is the main
topic of other research areas.
Of course, PK/PD has become a major topic in pharmacokinetics,
and therefore your questions become more and more important,
and deserve more attention. Therefore your question is highly
relevant. However, this does not imply that the more traditional
questions in PK are of less importance. I do not see how that will
change in the future.
Sincerely,
Johannes H. Proost
Dept. of Pharmacokinetics and Drug Delivery
University Centre for Pharmacy
Antonius Deusinglaan 1
9713 AV Groningen, The Netherlands
tel. 31-50 363 3292
fax 31-50 363 3247
Email: j.h.proost.-a-.farm.rug.nl
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The following message was posted to: PharmPK
Dear All,
ADME gives us the knowledge of how much compound that we can
delivery, by which route and for how long. As for biological
responses, this is another matter. It depends on the intrinsic
properties of the organisms(individual) and its relationship to
the host. Cardio-vessecular reponses to drug administration can
be evaluated and antineoplastic(tumor) responses are much more
difficult. Recent use of fluorinated nucleoside for the evluation
of tumor thymydilate synthase(a target) in tumor has been used.
But this is expensive and only few centers has such
facility(cost, cost, cost!). There are many classes of drugs for
different diseases and ADME is only part of solution and for
specific target(bacteria, fungi, etc), there is no one size fit
all solution.
Bill Tong
Pharmacology Analytical Lab
Memorial Sloan Kettering Cancer Center
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The following message was posted to: PharmPK
Dear Dr. Tong:
I wish to raise some issues concerning the message you posted to the
discussion group, because you make a general statement that while it is
generally accepted in the PK community, it is a statement is wrong. You
wrote:
"ADME gives us the knowledge of how much compound that we
"can deliver[y], by which route and for how long."
The above statement is not correct. Data from blood measurements can
only DOCUMENT what we measure - the levels of the drug in blood.
ANYTHING else are estimates based on assumptions that, while they are
probably valid much of the time, are assumptions nevertheless.
However, ADME does NOT tell us how much of a given drug has been
delivered to the target and/or toxic site(s). This can ONLY be
determined by direct measurements at such site(s).
There are two basic modes in which drug concentrations can be measured
at organ/tissue sites:
1. Obtaining a sample by a biopsy, a method that is invasive.
2. Measuring the drug using imaging methods, techniques that are not
invasive.
While an analysis of a sample obtained following a biopsy is probably
more precise by measuring drug concentration at a given time point,
biopsies can only be obtained for very limited samples, only from
certain organs and tissues, and can generally NOT be obtained in a
repetitive manner for kinetic estimates, in addition to many other
limitations.
Noninvasive (imaging) methods are probably less sensitive, and while
they have limitations in spatial localization and sensitivity, they are
however able to obtain repetitive kinetic information that can not be
measured by any other means, at present.
I guess I have put my finger in the ventilator, and I expect some
interesting feedback from many members of this list-server.
But the time has come where we need to recognize that:
"yes, Virginia, there are tissues/organs other than blood", and
"yes, Virginia, noninvasive imaging, when used as ONE of the methods to
generate PK data, will provide unique and novel insights into what
happens to drugs and to their pharmacodynamics in patients".
--
Professor Walter Wolf, Ph.D.
Distinguished Professor of Pharmaceutical Sciences
Director, Pharmacokinetic Imaging Program
Department of Pharmaceutical Sciences, School of Pharmacy
University of Southern California
1985 Zonal Ave., Los Angeles, CA 90089-9121
E-Mail: wwolfw.at.hsc.usc.edu
Telephone: 323-442-1405
Fax: 323-442-9804
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The following message was posted to: PharmPK
Dear Dr. Wolf,
In addition to obtaining a sample by biopsy and by measuring the drug using
imaging methods, we can also use microdialysis technique to obtain tissue
drug concentrations in a repetitive manner for kinetic estimates. This has
not only been used in animal studies but also in humans (especially in some
European countries). We have developed an animal model for simultaneous
sampling of brain extracellular fluid cocaine and metabolites as well as
dopamine concentrations in the rat nucleus accumbens region using
microdialysis technique. Details of this animal model are shown in the
following publication.
Pan, W. J. and Hedaya, M. A.: An animal model for simultaneous
pharmacokinetic /pharmacodynamic investigations: application to cocaine.
Journal of Pharmacological and Toxicological Methods. 39:1-8, 1998.
Regards!
Wei-Jian
Wei-Jian Pan, PhD
Center of Clinical Pharmacology and Pharmacokinetics
Department of Clinical Pharmacokinetics
Abbott Laboratories
100 Abbott Park Road
Abbott Park, IL 60064-6104
weijian.pan.-at-.abbott.com
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The following message was posted to: PharmPK
Dear Dr Wolf,
I have been following your arguments with interest, and agree with your
general conjecture that pharmacokineticists should be more critical about
what information can (and can't be) gathered from measuring systemic blood
concentrations alone.
I would like to add to your list of methods for measuring organ drug
concentrations.
I also think there are more people actively pursuing these areas than the
response
from the list would suggest.
>There are two basic modes in which drug concentrations can be measured
>at organ/tissue sites:
>1. Obtaining a sample by a biopsy, a method that is invasive.
>2. Measuring the drug using imaging methods, techniques that are not
invasive.
3. Measurements of organ venous drug concentrations in vivo.
This requires careful placement of a blood sampling catheter - for example
in the
dorsal sagittal sinus for brain effluent in a sheep. This is something my
laboratory (and others)
have been doing for some time.
With appropriate analysis and comparison to simultaneous arterial
concentrations,
these data can be used to infer a great deal about the kinetics of the drug
in the organ.
It is less invasive than biopsy, and presumably cheaper than imaging.
Animals can be studied repeatedly.
Dynamic effects can be measured simultaneously and correlated
with global organ drugs concentrations. We have shown that this is
appropriate for some
drug/organ combinations.
If you want to follow up this work, the following reference is a good
starting point.
Upton RN, Ludbrook GL. A model of the kinetics and dynamics of the
induction of anesthesia in sheep.
Parameter estimation for thiopentone and comparison with propofol. Br J
Anaesth 82: 890-9 (1999)
Regards,
Richard Upton
Dr Richard Upton
Principal Medical Scientist/Senior Lecturer
Department of Anesthesia and Intensive Care
Royal Adelaide Hospital/University of Adelaide
North Tce, SA 5000, Australia
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The following message was posted to: PharmPK
Dear Dr. Proost:
Thank you for your comments.
Yes, 'is the drug absorbed, and to which extent' and to where?
Pharmacokineticists have data for blood and other compartments,
metabolizing and
excretory organs. All have different half lives of deposition and turnover.
How is the absorption to TARGET tissues? Is this not important - perhaps most
important - in pharmacokinetics? If you determine target tissue absorption, you
will find that these data are different from blood/plasma etc and
different also
among different target tissues.
That's my point. Target tissue in vivo pharmacokinetics need to be included.
Best regards, Walter E. Stumpf, MD, Ph.D.
Professor of Cell Biology and Pharmacology em
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The following message was posted to: PharmPK
Dear All
I seem to have come a little tae into the discussion, but it seems to me we
have done fairly well so far at predicting drug response without exact
knowledge of the drug concentration at the target site. Those of us involved
with TDM have to make this assumption each time we give a consultation.
However I do have a reference to tissue kinetics which we are applying to a
new drug.
In vivo tissue pharmacokinetics by fluorine magnetic reonance spectroscopy:
A study of liver and muscle disposition of fleroxacin in humans.
Clin Pharmac Ther. 1990 Vol 48 (5) 481-489.
Graham Mould
Graham Mould,
Guildford Clinical Pharmacology Unit,
Royal Surrey County Hospital,
Egerton Road,
Guildford, Surrey, UK
GU2 7XX.
Tel:44 (0)1483 406886.
Fax: 44 (0)1483 455375.
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The following message was posted to: PharmPK
Graham:
I am afraid that, at least in the field I work, which is cancer, blood levels
generally do not predict drug targeting, and hence, do not correlate with
response.
You cite a 1990 reference. This field has progressed tremendously since, and I
suggest you have a good look at the theme issue on "Noninvasive Drug
Monitoring", which appeared as the March 15, 2001 issue of Advanced Drug
Delivery Research. It contains articles which illustrate the use of PET, NMR
spectroscopy (MRS) and imaging (MRI) and planar Nuclear Medicine to monitor
drugs pharmacokinetics and pharmacodynamics.
--
Professor Walter Wolf, Ph.D.
Distinguished Professor of Pharmaceutical Sciences
Director, Pharmacokinetic Imaging Program
Department of Pharmaceutical Sciences, School of Pharmacy
University of Southern California
1985 Zonal Ave., Los Angeles, CA 90089-9121
E-Mail: wwolfw.-a-.hsc.usc.edu
Telephone: 323-442-1405
Fax: 323-442-9804
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