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The following message was posted to: PharmPK
Hello,
I'd like to ask your opinion about the following question.
A drug is P-gp substrate and metabolized by CYP-3A4 in the liver. If
P-gp in the
liver was suppressed, would you think BA (AUC) of the drug (parent drug)
increase or decrease?
There is an article about this study and I have hard time understanding
why
supressing P-gp increase metabolite AUC and decrease parent drug AUC.
I greatly appreciate your opinion.
Regards,
Hue.
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The following message was posted to: PharmPK
At 05:36 AM 11/21/2003,kwonh.at.onid.orst.edu wrote:
"A drug is P-gp substrate and metabolized by CYP-3A4 in the liver. If
P-gp in the
liver was suppressed, would you think BA (AUC) of the drug (parent drug)
increase or decrease?
There is an article about this study and I have hard time understanding
why
supressing P-gp increase metabolite AUC and decrease parent drug AUC."
If the drug is a 3A4 substrate, then it will be metabolized in the gut
wall as well as the liver, especially in the proximal small intestine.
For some drugs, gut wall metabolism can equal or exceed liver
metabolism (midazolam, saquinavir). You should try to identify the
amount of metabolism in both regions.
I believe inhibition of P-gp in the gut should increase bioavailability
for drugs that are both 3A4 and P-gp substrates, because 3A4 is
expressed mostly in the proximal small intestine, and P-gp is expressed
increasingly as you move toward distal small intestine and colon, so
inhibiting P-gp would have the greates effect in regions of low 3A4.
Simulations for saquinavir, for example, show increased AUC with
suppressed P-gp Vmax in the gut.
Inhibition of P-gp in the liver is a different story, and I could guess
that because drug molecules would reside longer in the hepatocytes
without P-gp efflux, hepatic metabolism would increase, but it's just a
guess.
Walt Woltosz
Chairman & CEO
Simulations Plus, Inc. (SIMU)
1220 W. Avenue J
Lancaster, CA 93534-2902
U.S.A.
http://www.simulations-plus.com
Phone: (661) 723-7723
FAX: (661) 723-5524
E-mail: walt.-a-.simulations-plus.com
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regarding: "There is an article about this study and I have hard time
understanding
why supressing P-gp increase metabolite AUC and decrease parent drug
AUC.
In the liver the drug see's the metabolizing enzyme first and then it
see's pgp, so inhibiting/suppressing pgp would decrease the transistion
time through the liver cells, thereby exposing the drug (parent drug)
to the enzyme longer, giving the enzyme in your case 3A4 to metabolize
further etc...hope this helps
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The following message was posted to: PharmPK
Dear Hue,
Thats very good question!
Naturally people get confused with that kind of observation- that is
P-gp
inhibition leading to increase in metabolite AUC and decrease in parent
AUC.
But, that observation seems to be true if we look at the sequential
roles of
Pgp and CYP in enterocyte and hepatocyte.
The reason for confusion could be because of people's understanding of
the
P-gp role in intestine where inhibiting P-gp increase parent AUC and
decrease metabolite AUC, a vice versa phenomenon, which is true with
respect
to intestine.
To understand these conflicting observations one should know the
sequential
roles of P-gp in the absorption/elimination process in intestine and
liver.
Drug paths in intestine and liver are:
Intestine (enterocyte) : Lumen-(p-gp-CYP450)-systemic circulation
Liver (hepatocyte) : Systemic circulation-(CYP450-P-gp)-bile
As P-gp in intestine is thought to expose the drug to CYP enzymes again
and
again, due to the influx-efflux cycles, one would expect higher
metabolism
and hence low parent AUC. Inhining P-gp in enterocyte would therfore
lead to
increase in parent AUC and decrease in metabolite formation.
But, in case of liver P-gp efflux leads to elimination of drug into bile
making the drug unavailable for CYP in the hepatocytes. Inhibiting P-gp
would lead to increased residence time for the drug in hepatocytes to be
metabolised. This could be the reason for the increased metabolite AUC.
I
think decrease in parent AUC (along with increased metabolite AUC) would
only be possible when drug is extensively metabolised in the liver,
otherwise inhibiting P-gp in liver would lead to increase in both
metabolite
and parent AUCs.
I hope this helps.
Kasiram.
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The following message was posted to: PharmPK
Dear All,
Thank you so much for your input. All the comments were very helpful.
The study wasn't done by myself. I was referring a paper done by Dr. Wu
and
Dr.bennet. Perhaps I should provide the reference.
It is a short communication in DMD (drug met disposition) (2003)
31:1292-1295.
Nick Holford was right about BA and AUC. I should have said 'AUC'
instead of
'BA'. Sorry for the confusion.
The drug was given intravenously, not orally and therfore, i don't think
upregulation of P-gp in the small intesting by inhibiting P-gp in the
hepatocytes is happening in this case.
I understand a hypothesis about inhibiting P-gp in the hepatocytes
increase
residence time of the drug to CYP3A4, resulting in higher metabolism.
Then, wouldn't AUC of the metabolite increase by inhibiting P-gp?
However, the
AUC of metabolite was also decreased.
I am still not fully understand why AUC of the parent drug, TAC and
metabolite
decreased...
I greatly appreciate your input.
Best regards,
Hue.
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The following message was posted to: PharmPK
Dear all,
It is with interest I follow the discussion on PGP inhibition and
decreased AUC of both the parent drug and metabolite. Would it be
possible for Hue to give some more information on the compounds? What
are the routes of elimination? It is obviously metabolized but is it
renaly excreted as well? Where does the metabolism take place? Any
chance that kidneys are involved in its metabolism? What about the
metabolite and its route of elimination? Excreted renaly or metabolized
further? Then there is the question of how specific the PGP-inhibitor
is. Could you tell us what inhibitor you used? Several of these
compounds are non-specific and might block other pumps or induce
enzymes.
Then there are some general questions. What AUCs are estimated? If
total, what was the ratio of AUC 0-t over AUC 0-infinity for both the
parent compound and the metabolite? What about the amounts of drug and
metabolite in urine? Do you have any information there before and after
PGP-inhibition?
More information will hopefully help everybody to get closer to the
answer to this "mystery".
Toufigh Gordi
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The following message was posted to: PharmPK
Toufigh Gordi wrote:
>
> It is with interest I follow the discussion on PGP inhibition and
> decreased AUC of both the parent drug and metabolite. Would it be
> possible for Hue to give some more information on the compounds? What
> are the routes of elimination? It is obviously metabolized but is it
> renaly excreted as well? Where does the metabolism take place? Any
> chance that kidneys are involved in its metabolism? What about the
> metabolite and its route of elimination? Excreted renaly or metabolized
> further? Then there is the question of how specific the PGP-inhibitor
> is. Could you tell us what inhibitor you used? Several of these
> compounds are non-specific and might block other pumps or induce
> enzymes.
>
> Then there are some general questions. What AUCs are estimated? If
> total, what was the ratio of AUC 0-t over AUC 0-infinity for both the
> parent compound and the metabolite? What about the amounts of drug and
> metabolite in urine? Do you have any information there before and after
> PGP-inhibition?
>
> More information will hopefully help everybody to get closer to the
> answer to this "mystery".
>
The details can be found here:
Wu CY, Benet LZ. Disposition of tacrolimus in isolated perfused rat
liver: influence of troleandomycin, cyclosporine, and gg918. Drug Metab
Dispos 2003;31(11):1292-5.
The work was done using an ex situ perfused rat liver system.
The authors assert "These results support our hypothesis that the
hepatic metabolic clearance of a dual substrate will be increased by
inhibiting the efflux transporter". However, nowhere is clearance
actually reported and I believe their hypothesis about clearance is
unreasonable and not expected by simply inhibiting PGP efflux. I think
they are confused about the difference between clearance and the
fraction eliminated by a particular clearance pathway.
They show that the area under the curve of tacrolimus (AUCt) decreased
by 25% after a bolus injection to an isolated rat liver when
pre-treated with GG918 (a PGP inhibitor assumed not to affect CYP3A
metabolism). Assuming first-order elimination processes this indicates
that the total clearance of tacrolimus is increased by 30%. If we
assume that PGP is an elimination clearance mechanism for tacrolimus
then inhibiting this clearance pathway with GG918 should decrease the
total clearance of tacrolimus but exactly the opposite was observed
(AUCt decreased with GG918).
It is reasonable to suppose that hepatocyte concentrations would
increase if efflux is inhibited and if this occurred then a greater
fraction of tacrolimus might be metabolized by CYP3A but no effect on
clearance by CYP3A should occur. This is analogous to a drug being
eliminated by renal and non-renal mechanisms. A decrease in renal
clearance will increase drug concentration and increase elimination by
the non-renal pathway BUT the non-renal clearance is not affected.
The observed decrease in tacrolimus AUC with GG918 suggests this
compound stimulates clearance mechanisms but this cannot be explained
simply by inhibition of PGP efflux from the hepatocyte.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
email:n.holford.at.auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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hue, i guess we could go back and forth on this topic, but we would
need as much info as possible. is the ratio of parent to metabolite
the same? does the metabolite follwo a different matabolic pathway
(i.e. metabolic switching).
What are the clearance mechanisms? renal, bile etc...please provide
more input so that we can get a good understandin go f your situation.
Thanks
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The following message was posted to: PharmPK
Dear Hue,
> I would like to ask you one more question.
> As you said, drug is going to systemic circulation first->
> CYP3A4->P-gp-> bile.
> so, when you measure AUC of TAC, do measure it at systemic circulation
> step or
> after passing P-gp? The drug was given IV, not orally. Therefore, I
> don't think
> there is much small intestine P-gp or CYP3a4 metabolism involved in
> this case.
> I understand longer residence time of TAC in the hepatocytes due to
> inhibiting
> P-gp (Lowering bile clearance), which leads to increase CYP3A4
> metabolism. But
> when do you measure AUC? If we don't inhibit P-gp, is this drug going
> back out
> to blood from the bile after being pumped by P-gp into the bile?
> If you measure AUC of TAC before going into the bile, why does AUC of
> TAC
> decrease by increasing metabolism?
Dont get confused comparing Benet's obsearvations with the in vivo
situation. Wu and Benet have done the study in an isolated perfused rat
liver model where they have added substrate (TAC) to perfusate (with and
without inhibitors) and monitored that loss of TAC from the perfusate
over
time (60min). They have calculated AUC from this taking 30min data and
copared among various groups.
I could not get full text of their article but from the abstract it
seems
that they have not reported the TAC and its metabolites data in bile.
Also
it seems they have not mentioned metabolite data in perfusate which
would
have been very handy in explaining the results as these data complement
each
other.
To look at the TAC path in their model:
TAC in Perfusate------->hepatocytes------Pgp--->bile
(collected for 60 min)
|
CYP3A4
|
V
metabolites
(into perfusate or bile)
1. Control ---> (AUC=2260±430)
2. GG918 (no P-gp) ---> (AUC=1730±270) - One cannot claim this as
decreased
value over control as SD is huge and overlapping
3. Troleandomycin (no 3A4) ----> (AUC=5200±2470) - increase in AUC due
to
decreased metabolism
4. Cyclosporin (no P-gp or 3A4)----> (AUC=4390±2080) - increase in AUC
due
to decreased efflux and metabolism
I think confusion starts if one considers the AUC in presence of GG918
as
reduced value over control. This observation can be better explained
together with bile excretion data. If there is no significant
difference in
the bile excretion of TAC in control and GG918 groups, the above
obseravtion
can be considered appropriate due to the fact that role of P-gp is
minimal
and all that controlling the AUC is metabolism. Metabolite data should
complement this observation with similar extents of metabolism in both
groups.
But, if there is huge difference in bile excretion of TAC between
control
and GG918 groups, then the situation becomes hard to explain and one
needs
to look at the metabolite data (both in bile and perfusate) and tissue
data
of TAC to explain the results.
Hope this helps.
Kasiram.
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The following message was posted to: PharmPK
Kasiram Katneni wrote:
> I could not get full text of their article but from the abstract it
> seems
> that they have not reported the TAC and its metabolites data in bile.
> Also it seems they have not mentioned metabolite data in perfusate
> which
> would have been very handy in explaining the results as these data
> complement
> each other.
The tacrolimus and metabolite AUCs measured in the perfusate are
reported as well as total biliary excretion of these substances. The
tac excretion increased slightly with the P-gp inhibitor despite a fall
in tac AUC.
Despite a decrease in perfusate tac AUC the perfusate tac metabolite
AUC decreased even more with the P-gp inhibitor which implies either an
decrease in formation of metabolite from tac or an increase in
metabolite elimination clearance.
> To look at the TAC path in their model:
>
> TAC in Perfusate------->hepatocytes------Pgp--->bile
> (collected for 60 min)
> |
> CYP3A4
> |
> V
> metabolites
> (into perfusate or bile)
>
Why do you draw CYP3A4 in the bile? It's more usually found in
hepatocytes. This is the way these authors draw it in their Figure 1 in
Cummins et al. 2002.
Cummins CL, Wu CY, Benet LZ. Sex-related differences in the clearance
of cytochrome P450 3A4 substrates may be caused by P-glycoprotein. Clin
Pharmacol Ther 2002;72(5):474-89.
> 1. Control ---> (AUC=2260±430)
> 2. GG918 (no P-gp) ---> (AUC=1730±270) - One cannot claim this as
> decreased
> value over control as SD is huge and overlapping
They report the decrease in tac AUC afte P-gp was P<0.05. The
variability measures they give in the text are not defined as SD or SE.
It's possible they are SD with 6 rats per group which gives P=0.02 with
a two-tailed t-test.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
email:n.holford.at.auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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