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On December 22, 2000, I posed the following question about that part of
pharmacokinetic ADME studies that is related to Distribution in drug
development and research. I did not receive a single response. Is my
conclusion then correct: No drug distribution studies are being done that
seek to identify sites of action??
Here is my question again. I would be grateful for any response:
How do you identify sites of in vivo action, i.e. sites of specific
receptor binding?
Information on in vivo sites of action is important. I have never seen
this question addressed and discussed in your exchange of
letters. Apparently, nobody seems to have any problems with that. Is
identification of in vivo sites of receptor binding a part of your ADME -
DISTRIBUTION studies and, if so, how do you do it? How much confidence do
you have in the results obtained with your approach?
Sites of specific tissue binding (not just blood-related or unspecific
tissue deposition) are these included in your reports, required, not
required?
Walter E. Stumpf, Chapel Hill,NC
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Dear Dr. Stumpf:
I am delighted to answer your question, inasmuch as the question you are
raising are central to what I see pharmacokinetics should be all about:
determining the fraction of the drug administered that will be available at
the target/effector site, and how much drug at that site is required for the
drug to be therapeutically effective.
We have postulated that such measurements must be done noninvasively, for at
least three reasons:
1. The system that is being studied must not undergo any perturbation that
would affect such measurements. This means that any invasive sampling at
target sites (e.g., biopsies) are undesirable because they would affect
(perturb) the system.
2. It would neither be ethically allowable nor even possible to generally
obtain multiple sequential biopsies fom patients.
3. Perturbational measurements (biopsies) would not allow measuring the
effect of modulators and other agents that change the PK of a drug at its
target site(s). Such modulating studies are ideally suited to determine
mechanisms of action.
In an editorial I published several years ago (Imaging Can Be Much More Than
Pretty Pictures, Pharm. Res. 12:1821-1822, 1995) I had discussed the various
noninvasive methods that would allow drug studies, and I had stressed that
Functional Imaging would be ideally suited for measuring Pharmacodynamics at
target sites, and that Pharmacokinetic Imaging would be ideally suited for
measuring Pharmacokinetics at target sites.
We published recently a special issue devoted to Noninvasive Drug Monitoring
in the March 15 issue of Advanced Drug Delivery Reviews (41:1:2000), and in
that issue we have articles showing how such studies can be done using
Nuclear Imaging (Planar and PET), how drugs can be measured noninvasively
using Magnetic Resonance Spectroscopy (MRS) and how MRI can be used for
measuring blood flow and perfusion.
I would be delighted in expanding on any of these aspects in more detail.
The key issue here is that Noninvasive Drug Monitoring is a must in the
proper handling of drugs in acute diseases, where the therapeutic range is
narrow, where individual differences are significant, and where time is
critical in determining whether the patient may live or die. The methods for
such studies are available now, and I hope more clinicians will use these
novel pharmacokinetic approaches that will allow true individualization of
chemotherapy.
--
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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Dear Walter,
You raise important questions which, I am sure, many in PK have asked and will
continue to contemplate on. Often, the exact in-vivo site is not known. In
clinical situations, even when the site is known, sampling of the site is not
possible (e.g. lack of access or analytical sensitivity). This problem is now
addressed (to a very, very small extent) by assuming a relationship between
blood/plasma concentrations and the site in question. For example, in the one
compartment model, one assumes instantaneous distribution equilibrium between
blood and all regions of the body. This way, while the exact concentrations at
the site are unknown, it is assumed that there is a constant ratio
between blood
and the site of action. In more complicated models, this relationship is time
dependent. Pharmacokinetic texts (e.g. M. Gibaldi and D.Perrier,
Marcel Dekker)
discuss this in greater detail.
Therefore, two major areas that need focus are: enhancement of our current
abilities to identify the active site(s) in-vivo (in-vitro studies
might provide
some insight in this regard), and development non-invasive methods to measure
minute concentrations of drug/active metabolite at the site of action.
Sri
Srikumaran Melethil, Ph.D.
Professor of Pharmaceutics and Medicine
University of Missouri-KC
203 B Katz Hall, School of Pharmacy
5005 Rockhill Road
Kansas City, MO 64110
816-235-1794 (voice); 816-235-5190 (fax)
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The following message was posted to: PharmPK
Dear Srikumaran:
Thank you for your comments.
You wrote:
>Therefore, two major areas that need focus are: enhancement of our current
>abilities to identify the active site(s) in-vivo (in-vitro studies might
>provide some insight in this regard), and development non-invasive methods
>to measure minute concentrations of drug/active metabolite at the site of
>action.
The message I tried to convey in my answer to Dr. Stumpf was that we now have
non-invasive methods that can measure drug/active metabolite at their sites
of action. Those methods are available now, and can - and should - be used.
One of the problems is that there is too little interdisciplinary
communication. Most people doing PK/PD do not communicate with imagers, and
imagers have generally no real appreciation of PK/PD. Thus, techniques are
available, but they are not used.
Just to summarize very briefly what would be the essence of doing noninvasive
studies to monitor drugs: Nuclear methods have tremendously high sensitivity,
but have no chemical information. All products that contain the radionuclide
will image the same. NMR methods have tremendous chemical information, but
absolutely atrocious sensitivity. The selection of which method of imaging
to be used depends on both theoretical and practical issues, but they exist
and can be used.
The special issue that I edited now nearly a year ago (Advanced Drug Delivery
Reviews Vol. 41, issue of March 15, 2000) illustrates PK/PD studies using
both techniques. I urge people doing PK/PD to become much more knowledgeable
about the power these techniques can bring to this field.
Thank you for your comments.
======================================================================
| Professor Walter Wolf, Ph.D. E-Mail: wwolfw.-at-.hsc.usc.edu |
| Distinguished Professor of Pharmaceutical Sciences |
| Director, Pharmacokinetic Imaging Program |
| Department of Pharmaceutical Sciences, School of Pharmacy |
| University of Southern California Telephone:323-442-1405|
| 1985 Zonal Ave., Los Angeles, CA 90089-9121 Fax: 323-442-9804|
| |
|Center for Noninvasive Pharmacology, Los Angeles Oncologic Institute|
| MRI at St. Vincent Medical Center Telephone: 213-484-7235 |
| 2131 Third St., Los Angeles, CA 90057 Fax: 213-484-7447 |
======================================================================
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The following message was posted to: PharmPK
Dear Dr. Melethil,
=46or your thoughtful comments to my question I thank you very much.
With your conclusions I fully agree. What you stated characterizes the prese=
nt
situation:
Two major areas that need focus are:
enhancement of our current abilities to identify the active site(s) in-vivo
(in-vitro studies might provide some insight in this regard), and
development non-invasive methods to measure minute concentrations of
drug/active
metabolite at the site of action.=94
I concur with your opinion, 'often, the exact in-vivo site is not known'. It=
is
'assumed' that there is a relationship between blood/plasma
concentrations and the
site in question. It is further 'assumed' that there is a constant
ratio between
blood and the site of action, and that this ratio may also be tissue depende=
nt,
related to the =93model=94 used.
These assumptions can be verified if authentic and detailed
information is obtained
from in vivo cell- and tissue-specific (not organ-specific, since
organs or organ
pieces as well as homogenates consist of several different tissues) binding=
-
not from in vitro studies, not from in silico and rapid through-put procedu=
res
that are known to contribute to a high failure rate in subsequent clinical
evaluations.
Blood/plasma concentrations are important. Some relationship to receptor tar=
get
uptake and retention can be demonstrated. But the binding kinetics
may vary greatly
between plasma proteins and tissue target sites, and also among
different target
tissues =96 as we have demonstrated for vitamin D and the OCT analog.
Of course, the situation is complex and can not be dealt with in brief lette=
rs-
maybe somebody is willing to organize a workshop.
Walter E. Stumpf
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The following message was posted to: PharmPK
Dear Dr. Wolf,
Thank you for your response to my inquiry about in vivo drug distribution
studies.
You postulate that 'such measurements must be done non-invasively' in order =
to
avoid perturbations that would affect such measurement and that biopsies fro=
m
patients would be ethically not allowable and would provide perturbational
measurement.
Noninvasive 'pharmacokinetic imaging would be ideally suited for measuring
pharmacokinetics at target sites.'
Indeed, one cannot put enough emphasis on methods with minimal treatment.
We tried, therefore, to study the unmolested tissue and followed this
concept in
the development of a histochemical technique for drug localization,
that is, the
in vivo distribution of non-covalently bound diffusible substances with high
light microscopic resolution The method is based on autoradiography with 4
micrometer thin frozen sections, without fixation, embedding, alcohol or any
other fluid treatment. High sensitivity and high resolution with
preservation of
tissue topography and with cellular and subcellular detail, avoidance of
redistribution and loss of radiolabeled compound, are decisive features of t=
his
method. It stood the test of time since its development over 30 years ago, =
and
its utility and superiority are documented.
Any technique that provides high resolution and high sensitivity
requires special
care and knowledge and may therefore not be expedient by common standards, a
deterrent to some. Many investigators prefer expedient techniques
which, however,
in the end are likely to be none-expedient, since results obtained in the
expedient fashion are often too limited, contain false negatives, and may tu=
rn
out to be useless, even misleading.
If it were possible, ideally all pharmacokinetic studies would be done in vi=
vo,
in humans, without disturbance. Much progress has been made toward
this goal with
several types of non-invasive techniques. Their development must be further
advanced. Resolution and sensitivity =96 despite continuing improvement
=96 are still
low and in most cases insufficient for receptor binding studies. Small targe=
t
cell populations with high specificity but low capacity binding may not be
recognized. Therefore, correlative studies with low resolution non-invasive =
and
high resolution techniques need to be performed to clarify to which degree
localization is related to receptor binding with =93low capacity-high
specificity=94
or to so-called high capacity sites of deposition, unspecific or less specif=
ic
but probably still relevant. Such comparative studies are essential for
validation. There has been talk about correlative imaging studies for
years, but
very little has been done. Presently, some of the claims made with
the attractive
none-invasive procedures can not be supported. You stated that =93pharmacoki=
netic
imaging would be ideally suited for measuring pharmacokinetics at
target sites.=94
It would be so if the achieved resolution did indeed reveal targets. Under
present conditions this seems barely possible, perhaps only in a few
cases under
special conditions. Resolution, high enough, is a key requirement. Target ce=
ll
populations may be small and embedded in non-target tissues and are therefor=
e
often not recognizable even with radio-assay/HPLC of dissected
tissues and whole
body autoradiography. Resolution of small target cell populations and their
precise topographic identification is difficult or impossible to achieve wit=
h
whole body procedures - unless target cells exist in bulk. We have no choice=
at
this time but to resort to pre-clinical animal experiments if we want
to be well
informed about precise target location and related pharmacokinetics, i.e.,
measurements of binding to target cell populations, half life of receptor
binding, recognition of target cell characteristics and functions.
The solution of the problem requires compromises and a combined evaluation o=
f
data from methods with different levels of resolution. Since a single
laboratory
usually struggles to achieve competence with one method, collaboration with =
and
expertise from different laboratories is necessary albeit complicated.
Because of the complexity of the subject matter, its scientific and practica=
l
importance, regulatory impact, and others, a workshop would be a
helpful vehicle
for advancement. Walter E. Stumpf
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