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Non-specific microsomal protein binding could be an issue when
determining TDI KI or IC50. Does anyone include the unbound ( free)
fraction of microsomal protein binding in the static model for DDI
prediction? The static models in the current FDA and EMA DDI guidance
did not mention of the unbound fraction of microsomal protein binding.
Do you know whether inclusion of Fu, mic would improve the prediction?
Thanks a lot.
Ling Li, Ph.D
Lexicon Pharmaceuticals
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Hi Ling
Non-specific protein binding is important in determining any of Km, Ki,
TDI Ki, or CLint and it is recommended to be incorporated in any of the
equations (both static and dynamic). Generally we have seen improvement
in predictions but it should be incorporated irrespective of this
matter.
Please see the following:
Gao H, Yao L, Mathieu HW, Zhang Y, Maurer TS, Troutman MD, Scott DO,
Ruggeri RB, and Lin J (2008) In Silico Modeling of Non-specific Binding
to Human Liver Microsomes. Drug Metab Dispos:dmd.107.020131.
Gao H, Steyn SJ, Chang G, and Lin J (2010) Assessment of in silico
models for fraction of unbound drug in human liver microsomes. Expert
Opin Drug Metab Toxicol.
Nagar S and Korzekwa K (2012) Commentary: Nonspecific Protein Binding
versus Membrane Partitioning: It Is Not Just Semantics. Drug Metabolism
and Disposition 40:1649-1652.
Regards
Masoud
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The following message was posted to: PharmPK
Hi Ling,
May I suggest also suggest this article:
Poulin P and Haddad S (2011). Microsome Composition-Based Model as a
Mechanistic Tool to Predict Nonspecific Binding of Drugs in Liver
Microsomes. J PHARM SCI, 100(10):4501-4517 (DOI 10.1002/jps)
Regards,
Sami Haddad, Ph.D.
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We do use the fumic in the static model for DDI prediction. However this
binding is usually lower the plasma protein binding, therefore it has an
impact for very few compounds. I guess it improves the prediction but I
did not really check as, as I said, I have too few examples of
significantly bound molecules.
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Dear Masoud,
Recently we did IVIVE of in vitro metabolic stability data from rat
liver microsomes and predicted in vivo CL for 10 drugs from different
BCS class. Predictions were within 1 to 4 fold of observed in vivo CL by
all three liver models, however, when we incorporated fu in the equation
to see if there is any improvement in the predictions we dint see
significant difference. Even Scott (Drug Metab Dispos November 1, 1999
27:1350-1359) concluded in his paper that binding data improved the
predictions.
I was wondering what all possible reasons could be there that I dint see
significant improvement in predictions in my study after including fu in
the models.
Regards
Yunus
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Yunus,
There's more to protein binding than just the % free. Our protein
binding experiments tell you the position of the equilibrium. It does
not tell you the rate of establishment or reestablishment of the
equilibrium (on-off rates). If the rate of uptake into the cell or
metabolism or whatever process you are measuring is slower than the rate
of reestablishment of the equilibrium, as far as this process is
concerned, protein binding will have no contribution. It's a general
principle of chemical kinetics that the rate of a multi-step process
will be that of the slowest step, and as long as the off-rate isn't the
slow step, it doesn't matter. Of course there are numerous literature
examples where correcting for Fu doesn't help, and from my perspective
as a chemist, I am more surprised at the examples where it does, since
that seems to be the exception.
Dale
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The following message was posted to: PharmPK
Dear Dale,
In my opinion, your view on proten binding needs some comments.
> If the rate of uptake into the cell or metabolism or whatever
> process you are measuring is slower than the rate of
> reestablishment of the equilibrium, as far as this process is
> concerned, protein binding will have no contribution.
>
No contribution to what? If you say that protein binding does not have
an
influence on the principal pharmacokinetic parameters clearance (CL) and
volume of distribution (V), I don't agree. A change in the fraction
unbound
(fu=Cu/C) results in changes of both CL and V.
In general, the on-off rates of protein binding are high, as reflected
in
the fact that a high extraction ratio in the liver may occur even for
drugs
with a high degree of protein binding. However, this does not imply that
protein binding 'has no contribution': an increase of fu (e.g. by
displacement by an interacting drug) will further increase the
extraction,
and therefore will reduce oral bioavailability (increase of the
first-pass
effect).
> It's a general principle of chemical kinetics that the rate of
>a multi-step process will be that of the slowest step, and
> as long as the off-rate isn't the slow step, it doesn't matter.
>
This is a too simplistic view, and not necessarily true in the case of
reversible processes; as shown above, the fraction unbound affects CL,
even
in the case where the on-off rate is very high.
>Of course there are numerous literature examples where
>correcting for Fu doesn't help,
>
What exactly do you mean by 'correcting for Fu doesn't help'? Could you
please give an example?
best regards,
Johannes H. Proost
Dept. of Pharmacokinetics, Toxicology and Targeting
University Centre for Pharmacy
Antonius Deusinglaan 1
9713 AV Groningen, The Netherlands
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Hi Yunus
Obviously, I don't have the details of your compounds or the in vitro
assay so whatever I say is simply speculation. But there are different
factors that can contribute in this, including the compound types (in
the paper that you referred to they didn't see improvement for acidic
compounds). Also, as Dale mentioned if the binding rate is not fast
enough (in comparison with other rates) then incorporating fumic may not
improve the predictions.
In any case having a good understanding of what is actually going on in
the assay is very important and I assume generally the less protein is
used the better, though this may not be possible for all compounds.
Regards
Masoud
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