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Hi,I performed a CYP liability study using rCYPs and observed 60%
inhibition. When i performed the experiment in PHLM, i dont see any
inhibition. The protocol is similar for both the experiments. I will
be glad if someone enlists the possible reasons for the observed
phenomenon. thanking you
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The following message was posted to: PharmPK
There is generally much more non-specific binding in HLMs than in rCYPs
because there is a higher concentration of protein and lipid. 60%
inhibition is only just over the IC50, so if you have 10x more
non-specific binding in HLMs, your inhibition will drop to around 10%
which you might not be able to distinguish from control. You can buy
blank recombinant microsomes to add your rCYP to increase the total
protein content, but even that is probably not as effective as HLMs.
You can then wonder about which is more useful. Personally I prefer to
relate the most sensitive system (rCYPs) to the free concentration in
plasma at Cmax, but sometimes there will be a significant time-lag
between the plasma conc and the liver conc. If you use HLMs, you need
to know the free fraction in HLMs and the free fraction in liver, which
is not easy.
Regards.
Ted
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You should rather rely on PHLH data as recommeneded by the FDA
guidance...you don't need to investigate and waste your time on
something that would not help your drug discovery process.
Thanks
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Dear Ksar
It appears that your statement "... you don't need to investigate and
waste your time on something that would not help your drug discovery
process" is in sharp contrast with the findings of the following paper:
Youdim KA, Zayed A, Dickins M, Phipps A, Griffiths M, Darekar A, et
al. Application of CYP3A4 in vitro data to predict clinical drug-drug
interactions; predictions of compounds as objects of interaction.
British journal of clinical pharmacology 2008 May;65(5):680-92.
in which the authors stated:
"Using a generic approach for all test compounds, the findings from
the current study showed the use of recombinant P450s provide a more
robust in vitro measure of P450 contribution (fraction metabolized,
fm) than that achieved when using chemical inhibitors in combination
with human liver microsomes, for the prediction of potential CYP3A4
drug-drug interactions prior to clinical investigation."
Regards
Masoud
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