- On 10 Apr 1997 at 13:35:12, mike_graham.-a-.Merck.Com (Mike Graham) sent the message
I have carried out PK evaluation of a large number of compounds in rats,
usually within a few months of first synthesis. This is part of the
early biological evaluation of new compounds for some of our drug
discovery projects. Some of these compounds have shown very high plasma
clearance - well above liver blood flow in rats. In our own group
discussions of these results we usually conclude that the high clearance
was a consequence of distribution into red blood cells and/or
substantial extra-hepatic metabolism, but we rarely have the opportunity
to explore this in detail.
The red blood cell argument opens up the question of whether we should
analyse plasma or whole blood (i.e. should we calculate plasma or blood
clearance?). I am sure both parameters can be misleading under certain
circumstances. The dynamics of drug transfer between the plasma and red
blood cell compartments must also be an integral part of interpreting
whole blood clearance data. I would be interested to know how or if
other list members have addressed these issues.
Being a new member of this list, I am not sure if this topic has been
discussed recently; apologies if it has.
Mike Graham,
Drug Metabolism Section,
Merck Research Laboratories,
Terlings Park, Harlow, England.
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- On 11 Apr 1997 at 12:45:02, SWANG (SWANG.aaa.AM.HERL.EPA.GOV) sent the message
whether the tissue clearance is greater or smaller than tissue blood
flow rate depends on which blood concentration you use to calculate
the clearance, as well as to which tissue.
if you do a mass balance on a specific tissue,
VdC/dt=QCin-Qout-rate of excretion
integrat the above equation from zero to infinit, and with the initial
condition that at time=0, C=0, plus at time->infinite, C->0,
the following results obtained,
the amount of excreted=Q(AUCin-AUCout)
based on the definition of mean clearance,
CL=the amount of excreted/AUCin
therefore,
CL=Q(1-AUVout/AUCin)
if the tissue is non conservative, Cin>Cout. Therefore, AUCin>AUCout.
that's why CLusing venous blood to calcaulte CL, it is equivelent that you use
AUCout to calculate CL. form the equation, CL'=Q(AUCin/AUCout-1). If the
metabolism is very large, AUCin/AUCout>2, you can get CL'>Q.
For lung, the situation is opposite. IF you use venous blood sample to
calculate clearance, CLCL can be large than blood flow rate.
Dr.Bischoff and I developed a general relationship between the total body
clearance and tissue clearance with/without lung clearance. We presented
the abstract at SOT'97 conference. We will submitte the paper soon. If
you are interested, please contact me at xwang.aaa.MGA.com and I will send the a
abstract and the poster to you.
xiaofeng Wang, PhD
MGA software, concord, MA 01742
ph# (508)369-5115 ext 236
email xwang.-at-.MGA.com
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- On 11 Apr 1997 at 13:20:43, Bob Phair (rphair.at.ix.netcom.com) sent the message
This is potentially a very useful general question. Perhaps you could
post the time course data for one of your (very) high plasma clearance
compounds, as well as the data for one whose clearance you find within
the expected range. If you post the two tables of time and concentration
as well as the injected dose, those who want to try their hands at
obtaining your clearances could do so using a variety of methods and
software.
An exercise like this might also serve to consolidate the many responses
to the recent thread on AUC(t) and AUC(infinity).
Regards,
Bob
--
Robert D. Phair, Ph.D. rphair.-a-.ix.netcom.com
BioInformatics Services http://www.webcom.com/rphair
Partnering and Outsourcing for Computational Biology
[db - interesting suggestion but please only one data set for the list]
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- On 11 Apr 1997 at 13:24:22, DuncanMFE.-a-.aol.com sent the message
Some thoughts prompted by Mike Graham's e-mail.
Distribution into blood cells is often assumed rapid, but I suspect you may
be right that there are cases where this approximation doesn't apply. In
particular, it is easy to forget that it is not just combustion gases that
have a distribution which is potentially sensitive to oxygen tension or pH
(consider for example hypoxanthine and purine trafficking) and whether the
distribution is at equilibrium during a passage through the lungs and how
relevant in vitro distribution measurements are is worthy of a few seconds'
pause for thought. I certainly don't have all the answers here.
These things apart, for certain classes of compounds an appreciable pulmonary
extraction can appear as a superhigh clearance when the sampling site is a
peripheral venous one and input on the other side of the lungs (the apparent
pulmonary clearance is magnified since the concentration is sampled
downstream of the first pass): and rapid pulmonary elimination is quite
common (apart from volatile drugs) for a number of metabolic mechanisms but
particularly in cases of hydrolytic cleavage (high perfusion, high membrane
surface area for enzyme exposure, also potential for rapid receptor mediated
endocytosis followed by metabolism). Or a pulmonary clearance + some other
clearance...? Of course hydrolytic enzymes are circulating in blood itself as
well.
It is my impression from past experience in pharmaceutical companies that
high clearance compounds can be encountered rather frequently, although their
appearance on the market may be less frequent outside critical care medicine.
What sort of compounds are you looking at?
Regards
Duncan Edwards
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- On 15 Apr 1997 at 16:58:31, "Vladimir Piotrovskij" (VPIOTROV.at.JANBELC1.SSW.JNJ.COM) sent the message
Mike,
I suppose you mean plasma clearance after IV administration. Have you
discussed possibilities of artificially low plasma clearance? It might
be some part of a dose you injected did not reach the systemic
circulation of animals (due to the dead volume of a device you use,
adsorption, etc.). If the assay method you use is not enough sensitive
you may significantly underestimate AUC(0-inf) because of the
overestimated terminal half-life.
If everything is OK with the dose and the assay method, you could
assume a degradation of your compound(s) in blood (esterases, etc.).
Then, the lung might be responsible for the rapid elimination.
Generally, there may be a lot of explanations for really low plasma
clearance!
Best regards,
Vladimir
--------
Vladimir Piotrovsky, Ph.D. Fax: +32-14-605834
Janssen Research Foundation Email: vpiotrov.aaa.janbe.jnj.com
Clinical Pharmacokinetics vpiotrov.-at-.janbelc1.ssw.jnj.com
B-2340 Beerse
Belgium
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- On 18 Apr 1997 at 10:46:45, "Alfredo R. Sancho" (sancho.aaa.hsc.usc.edu) sent the message
Mike:
I gather you have looked at other tissues, e.g. fat, muscle, urine, etc? If
so can you account for the total -or close to the total- amount of drug
injected into the animal?
Also, the route of administration will of course dictate how much is the
liver aacounted for the plasma clearance. First pass effect "kills" our 5FU
kinetics... this is just an example.
You may want to do radiolabelling of your drugs and do biodistribution
studies -inlcuding plasma, urine and most of your tissues. You can follow
this noninvasively using an imaging setup, hence using single rat for several
time points. Or in a worse case, use radiolabels for the drugs, and do the
studies invasively.
Good luck,
Alfredo R. Sancho,
USC PK-Imaging Ctr.
Los Angeles, CA
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- On 18 Apr 1997 at 16:07:13, JNCOGB.aaa.monsanto.com sent the message
Alfredo,
Could you please follow up with more description of the noninvasive procedures
for quantitative determination of tissue distribution? What kinds of test
material would be amenable to this kind of imaging?
Thanks,
Nita Cogburn
jncogb.-at-.ccmail.monsanto.com
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- On 28 Apr 1997 at 11:17:48, mike_graham.at.Merck.Com (Mike Graham) sent the message
Thanks for all the comments and suggestions in response to my posting to
the list a couple of weeks ago.
To recap and with a little more detail:
We have calculated very high plasma clearance values from the plasma
concentration data following intravenous administration of several
compounds to male Sprague-Dawley rats. In response to Bob Phair's
suggestion I have provided data from two of the studies ([db - see below]
attached as an
Excel document - I hope this is transmitted with the message and is
legible when you try to open it). We are confident that each animal
received the correct dose. Blood samples were arterial. The analytical
method was sufficiently sensitive. The dose was 3mg/kg. We calculated
the plasma clearances to be a little over 100ml/min/kg for one of the
compounds and about three times higher for the other (i.e. approximating
to cardiac output!). Both compounds are lipophilic bases, molecular
weight in the 350-450 range. We have not done any further work with
these compounds, so we have no information on blood distribution, tissue
concentrations or susceptibility to plasma esterases.
One respondent suspected that there are many research compounds which
show such high plasma clearances but they rarely get onto the market. I
am inclined to agree - I have seen several tens of compounds with plasma
clearances substantially higher than 100ml/min/kg. Do others have
similar experience?
Mike Graham,
Drug Metabolism Section,
Merck Research Laboratories,
Harlow, UK.
[db - Rather than send the excel spreadsheet to everyone on the list I have
placed it on the WWW server at
http://www.cpb.uokhsc.edu/pkin/xcel/lowauc.xls If you set your browser
helpers with the MIME application/excel for the extension .XLS it should
open into Excel 'automatically' - at least it did for me with Excel 5 Mac
version. After clearing my cache and restarting Netscape]
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- On 1 May 1997 at 17:58:22, Bob Phair (rphair.aaa.ix.netcom.com) sent the message
Since I suggested that he post the data, it seems only fair that I
provide an analysis of Mike Graham's low-AUC data sets.
I labeled the first group A, and the second group of three rats, B.
Taking everything at face value (especially assuming that if you add a
known amount of the drug to rat blood in vitro, wait one minute, and
measure the drug, you get back what you added), and assuming rat plasma
volume of 45 ml/kg, I was able to fit all eight data sets (6 individuals
and two means) with a standard two-compartment model and obtain
parameter estimates with coefficients of variation all less than 19% and
most less than 10%.
The plasma clearances were indeed high. For Group A, I obtained
clearances of 86.1, 93.9, and 74.9 ml/min/kg for the individual rats,
and 81.9 ml/min/kg for the mean data. For Group B, I obtained plasma
clearances of 161, 161, and 149 ml/min/kg for the individual rats, and
158 ml/min/kg for the mean data. Coefficients of variation for all these
clearance estimates were less than 6%.
Since cardiac output in Sprague-Dawley rats is 200 ml/min/kg as measured
by electromagnetic flow probe 5-30 days after chronic implantation (A.
Shoukas, personal communication), all of these clearances are at least
possible. Since only 110 ml/min/kg of that CO is plasma flow, Group B
rats must either be delivering the drug partially in erythrocytes or the
rapid decline in plasma concentration before the first datum is partly
due to sequestration in a compartment that is not turned over on the
time scale of the experiment (6 hours).
The possibility of a very slow compartment could be explored for both
drugs in a number of ways, but perhaps the most important would be to
carry the experiment out further to check for the emergence of an even
slower exponential. If one appears, you could check tissue samples to
help identify the tissue source of the slow component. If that tissue is
the drug's target, you might decide to push forward with development
despite the large clearance; if that tissue is the site of a potential
troublesome side effect, you might decide to pull the drug from further
testing.
I can post details of the parameter estimates, but I wonder if these
results are in accord with your own thinking and with the analysis
performed by other readers of the list.
Regards,
Bob
--
Robert D. Phair, Ph.D. rphair.at.ix.netcom.com
BioInformatics Services http://www.webcom.com/rphair
Partnering and Outsourcing for Computational Biology
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