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I am wondering if someone can comment on the relative value of using
liver microsomes or the high speed hepatic S9 fraction for predicting in
vivo metabolism of drugs. My understanding is that the cytochrome P450 mixed
function oxidation system is located primarily in the microsomal membrane
fraction while soluble cytoplasmic phase II enzymes are concentrated in the
S9 fraction. How complete is this separation of phase I and phase II
activities? Is there a reason for using one over the other, or is the best
approach to use both?
Richard Knapp, Ph.D.
PO Box 6800
Bridgewater, NJ 08807-0800
Voice (908) 231-2248, Fax (908) 231-4760
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We have been using hepatic microsomes quite extensively to guide our
SAR efforts towards an increased chance for metabolic stability in humans.
The premise is that if the compound is not stable (i.e. has a short
half-life) in the in vitro microsome assay, the likelihood that the compound
would survive a first pass effect is decreased. The commercially available
liver microsome preps that we purchase (i.e. via Gentest) are very clean and
contain little/no 'contaminating' S9 enzymes. There is, however, some
microsome enzymes in the commercially available liver S9 preps. In my
opinion, the usefulness of the systems is very valuable as a prioritization
tool when there are large numbers of compounds to evaluate. The value of
the microsome system is in predicting first pass effects on the compounds of
interest....increased stability in the liver microsomes, therefore increased
chance of surviving first pass effect. Both the S9 and microsome systems
(i.e. also 'mixed' systems) are great for predicting metabolism across
species for a large number of molecules. This allows for a more robust look
at which species would be appropriate for evaluating potential toxicities
(i.e. similar metabolism as humans, therefore expectations of similar
toxicity). We have in a number of cases found identical metabolites in vivo
as we had observed in vitro. The caveat is that at times we also observe
metabolites in vivo the were not present in the in vitro system. I have a
hunch, however, that if we were to go back and look more closely at a larger
scale in vitro prep we may begin to see at least some of the metabolites
that we had previously missed. I hope this helps. Good luck!!
Thomas E. Stephan, Ph.D.
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Copyright 1995-2010 David W. A. Bourne (firstname.lastname@example.org)