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The following message was posted to: PharmPK
Hi all,
Does anyone have a first-hand experience with valsartan in CaCO-2 model?
In vivo it is definitely bioavailable (we have not done the classic
IV/PO experiment, but given PO, it gets in), but I see very small CaCO-2
permeability. Published FA I found was 40%. Explanations about
mechanism?
Thank you!
Katya
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The following message was posted to: PharmPK
Dear Katya,
Keep in mind that Caco-2 permeability is not a reliable predictor of
human
permeability. Sometimes it's OK, often it is not. It is also highly
dependent on the lab and the methods used. Caco-2 Papp differences of
100-fold have been observed for the same compound in different labs.
If your compound is absorbed only by passive transcellular diffusion,
and if
your Caco-2 lab follows good procedures, you should see a good
correlation
with most passively diffused drugs.
If your drug undergoes paracellular transport, Caco-2 permeability
can be
low (e.g., atenolol, which has Fa ~ 50%), yet permeability in the small
intestine may be good because the tight junctions in small intestine are
larger than in Caco-2. MDCK permeability is a better indicator for such
compounds.
If your drug undergoes carrier-mediated transport, then it may be
impossible
to get a reliable in vitro permeability measurement if there are no cell
cultures that express the transporter adequately.
Best regards,
Walt
Walt Woltosz
Chairman & CEO
Simulations Plus, Inc. (AMEX: SLP)
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.-at-.simulations-plus.com
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The following message was posted to: PharmPK
Dear Katya
For permeability classification, it is ideal to run standards (Known
low, medium and high permeability class) with your test drug.
The values for Papp vary from lab. to lab. and there can be solvent
effect.
Hope this helps
Regards
Gurpreet Saini
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The following message was posted to: PharmPK
Dear Katya,
I do not have any experience with, or knowledge about, valsartan. I
believe there will be some people who will give you information.
I agree with the opinion that, considering observed Papp values,
caution should be given first to Caco-2 experimental conditions, and
that reference compounds should be used to calibrate the assay.
As W. Woltosz nicely explained, we also found weak Papp for atenolol on
Caco-2 cells, despite reported significant absorption in man.
Besides, attention should be paied to compound ionisation (see the pKa
values) : transport may be dramatically dependent on pH (see Boisset et
al, Eur J Pharm Sci. 10 (2000), 215-224.)
Another possible explanation can be investigated quickly, by running a
simple transport experiment in both directions (A to B and B to A): that
is the possibility that active transport (efflux) would limit
permeability. If so, it should be saturable, and you might find higher
permeability at increased test compound concentration.
Hope this helps. Good luck.
Frederic MASSIERE,
OROXCELL.
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The following message was posted to: PharmPK
Dear Gurpreet and Walt,
We do run 3 controls on every plate (high, low, pgp substrate), and
validated the assay on over 20 commercial controls with known mechanism
of transport. All compounds are dissolved in transport buffers at
appropriate pH.
It was the only control that did not make sense to me. My question was
rather specific about valsartan and not about general in vivo-in vitro
correlations.
-Katya
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The following message was posted to: PharmPK
Dear Katya,
Valsartan is a tetrazole derivative containing carboxylic group with
following physico-chemical and PK properties:
1. pKa :3.9
2. cLogP: 4.8
3. LogP o/w: 3.9
4. Aqueous Solubility: 1.0 mg/ml
5. BA in humans: ~40%
6. BDDCS class: III (High solubility, poor metabolism)
7. BCS class:??
You can also refer the following articles for above information:
1. Predicting Drug Disposition via Application of BCS: Transport/
Absorption/
Elimination Interplay and Development of a Biopharmaceutics Drug
Disposition Classification System. Chi-Yuan Wu1 and Leslie Z. Benet,
Pharmaceutical Research, (2005) 22: 11-23.
2. Absolute bioavailability and pharmacokinetics of valsartan,
an angiotensin II receptor antagonist, in man. G. Flesch . Ph. MuE
ller . P. Lloyd, Eur J Clin Pharmacol (1997) 52: 115-120
3. Molecular Properties of WHO Essential Drugs and
ProvisionalBiopharmaceutical Classification. Nehal A. Kasim, Marc
Whitehouse, Chandrasekharan Ramachandran, MarivalBermejo, Hans
Lennernas, Ajaz S. Hussain, Hans E. Junginger, SalomonA. Stavchansky,
Kamal K. Midha, Vinod P. Shah, and Gordon L. Amidon; Molecular
Pharmaceutics, 2004January/February, Vol 1(1), 85-96.
Based on the above information, I suggest following reasons for poor
Caco-2 permeability of valsartan:
1. pH dependent permeability - if valsartan is a substrate for uptake
transporters such as MCT or OATP, apical pH plays major role.
Structure suggests that there is a strong possibility for valsartan
being a substrate for any of these transporters. To confirm this you
can perform permeability experiments in the presence (AP pH6.0/BL
pH7,4) and absence of pH gradient (AP pH7.4/BL pH7,4). Role of efflux/
uptake transporters can be monitored by looking at the effect of
specific inhibitors and/or performing concentration dependency.
2. Performing permeability experiments using cold compound with trace
amounts of radiolabelled marker - In these cases when you calculate
permeability values using flux data of radiolabelled species only and
if valsartan is substrate for any uptake transporter, would result in
low estimates if the concentration of cold is above Km of the uptake
transporter. Vice versa can be expected with efflux transporters.
Make sure this is not the case. Either use total flux for
permeability calculations or use radiolabelled marker alone.
3. Low solubility and high lipophilicity - lead to loss of drug from
the donor buffer either due to precipitation or adsorption. If that
is the case you would under estimate permeability when you calculate
using initial donor concentration. To make sure this is not the case
monitor donor concentration at the end of the experiment.
4. UWL and Sink effects - For highly lipophilic drugs unstirred water
layer and lack of efficient sink in the receptor greatly limit
permeability. Use of albumin in receptor (up to 4% w/v) would improve
sink (for basic drugs AAG is suggested). UWL, a major issue with cell
monolayer models, can be minimised with increased mixing of the
transport buffer (I am not sure how you can achieve this in Caco-2
expts, may be you can use modified diffusion apparatus - Refer:
Characterization of the unstirred water layer in Caco-2 cell
monolayers using a novel diffusion apparatus. Hidalgo IJ, Hillgren
KM, Grass GM, Borchardt RT., Pharm Res. 1991 Feb;8(2):222-7).
5. Use of excipients - If you are using surfactants such as Tween-80
or Cremophor-EL - make sure you correct your permeability estimates
for the free concentration. Also, surfactants are known to inhibit
transporters (both efflux and uptake).
Therefore, observation of poor Caco-2 permeability for valsartan
could be due to any/combination of these and other factors.
I suggest you also refer the following article which discussed some
of the above issues in terms of monolayer permeability determinations.
In vitro trans-monolayer permeability calculations: often forgotten
assumptions. Youdim KA, Avdeef A, Abbott NJ., Drug Discov Today. 2003
Nov 1;8(21):997-1003.
Hope this helps.
Kasiram.
>
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The following message was posted to: PharmPK
Dear Katya
I have no experience with Valsartan.
In that case, to Know the mechanism of absorption of valsartan,
experiment can be done at 4-5 different concentrations.
If the papp value increases with increasing concentration then its
passive diffusion and if it decreases with increasing concentration
its carrier mediated.
In my experience, some drugs have shown a 3 fold increase in Papp
value when experiment was carried out at 100, 50, 25 and 10 uM
concentration. Maximum at 10 uM.
Hope this helps
Gurpreet Saini
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The following message was posted to: PharmPK
Dear Kasimar and Gurpreet,
Thank you very much for your detailed responses, especially Kasimar.
This is very helpful.
In fact, valsartan is a class III (High solubility, low permeability)
drug and it could be acting as atenolol in this model.
Again, thank you.
-Katya
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