- On 19 Mar 2009 at 16:11:11, "Jack G. Shi" (bourne5321.-at-.gmail.com) sent the message
I feel this topic interesting to me and didn't find it was discussed
on this forum (correct me if wrong). It is commonly stated in PK
books/papers that:
If renal clearance is contributed by passive filtration only, then its
upper limit is defined by GFR * fu and, if the observed value exceeds
that limit, tubular secretion must be involved. The question is: is
this merely a hypothesis or ever supported by experimental data,
especially for relatively high binding drugs?
This statement seems to give no consideration the possibility that
drug plasma protein binding equilibrium can dynamically shift while
being filtered at the glomerulus. If the statement is true, it must
follow that the unbound concentration of a drug entering a glomerulus
must always be higher than when the blood is leaving the glomerulus,
if there is any renal excretion of the parent drug observed (again for
simplicity, let's assume passive filtration is the only mode of drug
elimination in the kidney). Is there any reported data from real
compounds to support that?
Obviously, the hepatic CYP system can't metabolize a drug if it is
bound to blood protein and can't enter the hepatocytes. There is no
doubt high extraction ratio drugs can dissociate from proteins in
blood quickly enough to compensate for the loss in the unbound pool.
Why this cannot happen in kidney?
Your thoughts and comments are appreciated.
Jack
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- On 20 Mar 2009 at 08:08:43, Nick Holford (bourne5321.-a-.gmail.com) sent the message
Jack,
First of all, if a drug is in fact only passively filtered then it
must be a low extraction ratio drug because GFR is about 6 L/h (with
respect to plasma) and renal plasma flow is about 30 L/h. For low
extraction ratio drugs one can ignore plasma protein binding as it
makes no difference to unbound drug clearance.
If you calculate the unbound drug clearance (i.e. CLtotal/fu) then you
can compare this to the GFR directly because this is reflecting the
clearance of unbound molecules passing by diffusion across the
glomerular filter. If unbound clearance is greater than GFR then
indeed something else must be going on apart from passive filtration.
Most textbooks get this idea backwards (as you illustrate) by
multiplying GFR by fu to get an equivalent GFR as if GFR could be
converted to some kind of total (bound+unbound) clearance. There is
clearly no physical reality associated with GFR x fu. This is just
algebraic silliness and not biology. The rational comparison is to
compare drug unbound clearance to GFR.
If tubular secretion is occurring then this can be a very active
process and the blood extraction ratio can approach 1. This is the
basis of using the clearances of substances like PAH as markers of
renal blood flow. Drugs with high extraction ratio will show changes
in unbound clearance if there are changes in drug delivery to the
kidney. The primary determinant of delivery is blood flow but binding
to plasma proteins can also be important. The plasma proteins act as a
delivery mechanism to the kidney that is independent of blood flow. If
binding is decreased then delivery decreases and unbound clearance
will decrease as well.
These protein binding predictions are based on the assumption that
binding equilibrium occurs rapidly in relation to the transit time
through the eliminating organ. Eg. as unbound drug diffuses across the
glomerulus, leaving the non-filterable protein behind, then bound drug
leaves the plasma protein and re-establishes the unbound concentration
in plasma.
The same thing is assumed to happen in the liver or any other
eliminating organ.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.aaa.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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- On 20 Mar 2009 at 03:38:21, Hartmut Derendorf (bourne5321.-at-.gmail.com) sent the message
The following message was posted to: PharmPK
The situation in the kidney and liver are not equivalent. Filtration is
a physical separation based on size. Plasma water is filtered with drug
dissolved in it. There is no significant change in the unbound drug
concentration and hence no re-equilibration (there is actually a very
small change in unbound drug concentration since the plasma protein
concentration goes up a little during filtration).
That's different to the liver where metabolism decreases unbound drug
concentration which triggers re-equilibration.
Hartmut Derendorf
University of Florida
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- On 20 Mar 2009 at 10:26:42, Guangli Ma (bourne5321.-at-.gmail.com) sent the message
The following message was posted to: PharmPK
Hi Jack,
Very interesting questions. I believe that we can not prove these
assumptions from data no matter how much you collected. A possible way
is to
find an evidence to show this is wrong.
There are three important paramters: Renal blood flow (RBF) is 1.2 L/
min,
while GFR is 125 ml/min and Urine flow is 1.5 ml/min. I think as a limit
value CLren=GFR*fu is 'fast' enough, and the next compartment is
'slower'.
:)
Best regards,
Guangli
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- On 20 Mar 2009 at 10:52:22, Hans Proost (bourne5321.-a-.gmail.com) sent the message
The following message was posted to: PharmPK
Dear Jack,
Thanks for your interesting question on 'GFR * Fu = the upper limit of
renal CL for passive filtration, a myth or truth'. In my opinion it is
a truth, but your qestion with respect to support by experimental data
is appropriate.
My theoretical arguments:
1) Assuming that only unbound drug is filtered, GFR * Cu is the amount
of drug removed by filtration. So the upper limit of renal clearance
is GFR * Fu.
2) The question with respect to dynamic equilibrium is not really
relevant here (but interesting, see below). Please note that there is
an essential difference with the situation in the liver, since during
filtration both water and the unbound drug are removed from the blood.
Consider two situations:
a) Equilibrium in plasma protein binding restores very slowly: so the
unbound concentration would not change by filtration, only the plasma
volume changes (e.g. for renal blood flow 1100 ml/min, GFR 120 ml/min,
hematocrit 0.4, the change in plasma volume would be 120 / (1100 *
(1-0.4) * 100% = 18%).
Since the concentration in the filtrate is lower than the total plasma
concentration, the total drug concentration increases, so Fu decreases
temporarily. However, the unbound concentration does not change, and
so the statement #1 remains true.
b) Equilibrium in plasma protein binding restores very fast: Again,
total plasma concentration increases. Also, the plasma protein
concentration increases due to the filtration. As a result Fu
decreases. Both changes are about proportional to the change in plasma
volume, so Cu will not change significantly. Again, statement #1 holds.
Any further thoughts and comments are appreciated!
best regards,
Hans Proost
Johannes H. Proost
Dept. of Pharmacokinetics, Toxicology and Targeting
University Centre for Pharmacy
Antonius Deusinglaan 1
9713 AV Groningen, The Netherlands
Email: j.h.proost.aaa.rug.nl
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- On 20 Mar 2009 at 11:35:09, "Ed O'Connor" (bourne5321.at.gmail.com) sent the message
The following message was posted to: PharmPK
Changes in glomerular-nephron physiology will impact the clearance of
a drug
whether because of filtration, secretion, or reabsorption. These
changes
can be the result of compensation for pH, ion and homeostasis, hormonal
regulation.
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- On 20 Mar 2009 at 09:41:34, Varma MVS (bourne5321.-at-.gmail.com) sent the message
Jack,
Its an interesting topic.
Even if the equilibrium is slow, I was wondering how relavent is it.
About 10% of the total renal blood flow passes through the glamorous
resulting in an avg GFR of about 125mL/min in 70Kg man. I assume this
indicate only 10% of the unbound drug is passively filtered while the
rest of the 90% unbound drug will get in quick equilibrium, along with
the equilibrium between bound and unbound components. Also nephrons
are not connected in series and the transit time at the glamorous is
very short unlike to the tube or any complex models of the hepatic
extraction, where protein binding may not determine the clearance of
the high intrinsic clearance compounds.
Manthena Varma
Pfizer Inc.
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- On 20 Mar 2009 at 19:30:46, Nick Holford (bourne5321.-at-.gmail.com) sent the message
Hi,
Les Benet pointed out a mistake in my last post to this thread. Thanks
Les!
The unbound clearance must increase (not decrease) when binding
decreases.
Mass balance requires:
Rate out = CLt * CLt = CLu*CLu
CLt=CLu*Cu/Ct=CLu*fu
Well stirred model for organ clearance (ignoring cellular partitioning)
CLt=CLi*fu*Q/(Q+CLi*fu)
When binding changes it is accepted that for high extraction ratio
drugs that CLt does not change.
CLt=Q when Q<
If fu decreases then CLu must increase if CLt is to remain constant.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-a-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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- On 23 Mar 2009 at 13:08:23, "Jack G. Shi" (JShi.at.incyte.com) sent the message
I found the notion that both drug and majority of water in plasma are
removed by passive filtration cannot be correct. As some of you have
noted, the GFR is 125 mL/min and urine flow is 1 mL/min in normal
human subjects (I take numbers from Davies and Morris). Assuming
hematocrit is ~0.4, the plasma volume filtered by glomerula is 125*60%
= 75 mL/min, and we know most volume of plasma is made up by water.
So apparently glomerula filtration removes only a fraction of water in
this process. Therefore, a drug must undergo passive diffusion across
the semi-permeable membrane just like in an equilibrium dialysis
process. To state that drug is filtered along with water is akin to
conducting a dialysis experiment without adding buffer to the other
side of the membrane in the dialysis chamber first.
As we know, drug-protein dissociation does not require drug
transportation through any biological membrane and in concept should
be fast. The dissociation rate constant of drug albumin binding
complex has been reported, and citing from one paper, [Anal. Chem.,
2002, 74 (2), 446-452], the kd of L-tryptophan binding to human
albumin is 6.9 sec-1, which means the half-life of dissociation is
only 0.1 second. As you may agree, for drugs of low free fraction
e.g. ~ 1%, it does not take nearly 50% of dissociation to effect a
huge difference in what can be removed by passive filtration.
So what is the average residence time of a drug molecule inside a
glomerulus? I am not aware of data directly reported (haven't done
any thorough literature search either) but it may be estimated if we
know the volume of a glomerulus. This paper estimated it to be ~ 4x
10^6 micro^3 [Nephrol Dial Transplant (2000) 15: 239-243]. Since we
know there are about 1 million nephron/glomerulus in one human kidney,
the total volume of all glomerula is about 8 mL. The mean transit
time of a drug across the glomerulus is then estimated to be ~ 8 mL/
125 mL/min = 3.8 second. And that's a lot longer than the drug-
protein dissociation t1/2, which in turn means a dramatic shift in
protein-drug binding equilibrium can happen in the passive filtration
process in the kidney.
Jack
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- On 23 Mar 2009 at 21:12:12, Nick Holford (n.holford.-at-.auckland.ac.nz) sent the message
Jack,
I dont understand your calculations for the fraction of water removed
by glomerular filtration. GFR is calculated with reference to plasma
(or sometimes serum). You seem to assume that GFR is relative to whole
blood. Renal plasma flow is about 30 L/h/70kg which can be compared
with GFR of about 6 L/h/70kg so about 20% of of plasma (mainly water)
is cleared.
Thanks for your calculations of plasma protein dissociation times and
glomerulus transit times. These seem very reasonable. But it must be
remembered that glomerular filtration is a low extraction process
(~20% extraction ratio) so protein binding has limited influence on
drug clearance by glomerular filtration. In contrast, drugs secreted
by the renal tubules can have very high extraction and here
differences in plasma protein binding could in theory have an effect
on unbound drug clearance. Renal tubular extraction refers to removal
of drug from plasma. As far as I am aware any water shift is negligible.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-a-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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- On 24 Mar 2009 at 10:03:17, "Jack G. Shi" (JShi.aaa.incyte.com) sent the message
Nick,
You are obviously right in that GFR should not be further corrected by
hematocrit; it was my mistake.
You and several others seemed to suggest that if a clearing organ has
a low extraction rate, then drug-protein binding equilibrium won't
shift or won't matter. I don't agree on this point. An extraction
rate that is low basing on total plasma/blood concentration may be
very high basing on unbound drug concentration. I offer here one real
world example. The IV systemic clearance of a compound in the rat is
0.08L/h/kg, which is ~3% of rat's hepatic liver blood flow. Obviously
it must have very low extraction rate in any organ in the rat.
However, the compound has an unbound plasma fraction of 0.2%. So the
unbound systemic clearance (=dose/unbound plasma AUC) is 40 L/h/kg,
which is more than double of the total cardiac output of the rat.
Obviously the drug binding equilibrium had to shift to unbound side
for this to happen.
My main conceptual mix up with the original question posted was that
the natural kidney uses mainly hemofiltration and not hemodialysis as
its mechanism. So in a hemofiltration model, there is no chance that
the kidney can remove a drug population that's bound to plasma
protein. This would be true even if the fraction of filtration is 80%
rather than ~20%. However, I'd venture that in an artificial kidney
employing hemodialysis, the clearance of dialysis could be higher than
fu * Q, i.e., the entire amount of free drug that is presented to the
system per unit time (and normalized by concentration).
Thanks to everyone for contributing to this discussion, and further
comments are always welcomed.
Jack
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- On 2 Apr 2009 at 09:42:47, jeroen.elassaiss.at.spcorp.com sent the message
The following message was posted to: PharmPK
Dear Jack,
You wrote
[.. ]
The IV systemic clearance of a compound in the rat is
0.08L/h/kg, which is ~3% of rat's hepatic liver blood flow. Obviously
it must have very low extraction rate in any organ in the rat.
However, the compound has an unbound plasma fraction of 0.2%. So the
unbound systemic clearance (=dose/unbound plasma AUC) is 40 L/h/kg,
[..]
Interesting view on clearance. You would have to correct your unbound
clearance for the amount of drug that reaches plasma water. This is
after the initial mixing period in the first seconds-minutes, because
that mixing is neglected (for good reasons) in standard PK theory and
models. I would think that the fraction of drug that reaches plasma
water is pretty close to the unbound fraction perhaps minus some minor
losses. All in all the unbound clearance after such a correction would
be close to the total plasma clearance.
Perhaps your line of thought could be further elaborated on by
considering (and measuring?) the recirculation dynamics from a plasma
protein binding point of view. That way the initially unbound drug bolus
can be incorporated. Should be interesting!
Best regards,
Jeroen
Jeroen Elassaiss-Schaap, PhD
Modeling & Simulation Expert
Pharmacokinetics, Pharmacodynamics & Pharmacometrics (P3)
Early Clinical Research and Experimental Medicine
jeroen.elassaiss.at.spcorp.com
Schering-Plough Research Institute
Room KM2515
PO Box 20
5340 BH Oss, Netherlands
www.schering-plough.com
Refs:
Fast pharmacokinetics and plasma protein binding evaluation could
benefit from the analysis by prof Peletier:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WMD-4TK92NH-2&_user=616883&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000032499&_version=1&_urlVersion=0&_userid=616883&md5=e26661d75b675634798cd31b27eec112
The only example of recirculation kinetics that I am aware of
(additional refs would be appreciated!): J. M. van Rossum, J. E.
de Bie, G. van Lingen, and H. W. Teeuwen. Pharmacokinetics from a
dynamical systems point of view. J Pharmacokinet.Biopharm. 17
(3):365-392, 1989.
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