- On 10 Jul 2002 at 10:13:40, uceph hijazi (uceph_hijazi.at.yahoo.fr) sent the message
Dear all,
I am somehow beginner in extrapolation of
pharmacokinetic parameters from in vitro to in vivo
sytems. I would like to predict the intrinsic
clearance of a drug in human from in vitro metabolism
kinetic data (Vmax1, Km1 & Vmax2, Km2). The drug is
metabolised by 2 different CYP isoforms and the enzyme
kinetic parameters were determined by incubating the
drug with recombinant CYP isoforms responsible for the
metabolism. What is the exact equation that describe
the relationship between the intrinsic clearance and
enzyme kinetic parameters.
Thank you for your cooperation.
Youssef Hijazi
Hôpital neurologique et neurochirurgicale
Service Pharmaceutique
Laboratoire de Pharmacocinétique Clinique
B.P. Lyon Montchat 69394 Lyon Cedex 03-FRANCE
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- On 10 Jul 2002 at 15:28:27, "Paulo Paixao" (ppaixao.aaa.hotmail.com) sent the message
I'm not an expert so I will be only thinking loud...
Clintr = vi/Ci and that should be constant for Ci<<If you have 2 paralel systems then you would have
Clintr(total)=f(1)*Clintr(1)+f(2)*Clintr(2) with f(1) and f(2) the fraccions
metabolized by eatch metabolic system, then the Clh (hepatic clearence)
should be equal to Q*((f*Clintr)/(Q+f*Clintr)) with Q the liver blood flux
and f the free plasma fraction of the drug.
For Ci closer or bigger that the Km then you had enter in saturation kinetics
and the calculus would be necessary more complex...
Am I close to the possible answer?
Paulo Paixao
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- On 10 Jul 2002 at 17:30:18, "Kavanagh, Ronald E" (KAVANAGHR.-a-.cder.fda.gov) sent the message
In extrapolating in vitro intrinsic clearance to in vivo intrinsic clearance
you're making several assumptions.
1. The enzyme conformation, folding, etc.. in vitro is identical to in vivo
2. The enzyme behaves similarly in the in vitro milieu, pH buffer,
cofactors, inhibitors etc.. to the in vivo situation.
3. The free concentration at the enzyme site in vitro is the same as the
free concentration at the enzyme site in vivo, i.e. instantaneous passive
diffusion.
You can't assess any of these factors without assuming the other assumptions
are valid. Regarding free concentrations this assumes no active transport
processes resulting in a lack of concentration differences in free
concentrations intracellularly vs. extracellularly.
This is why I use terms such as apparent in vivo (or in vitro) intrinsic
clearance or enzyme kinetic parameters and why I specify the in vitro
conditions, (e.g. fresh hepatocytes, recombinant enzymes, etc..)
Because of these various unknowns it's currently impossible to accurately
predict in vivo apparent parameters from in vitro parameters. Sometimes they
correlate closely and sometimes they don't. Some people try to use a fudge
factor and use total as compared to free drug concentrations. Thus the in
vitro parameters are only a rough starting estimate. This is why regulatory
agencies sometimes require clinical interaction studies even when you
wouldn't predict an interaction from in vitro data. The question is what is
the margin of safety based on the in vitro data that is acceptable, i.e. a
10 fold, 50 fold or 100 fold margin of safety for when a clinical study is
not required.
Ron Kavanagh
The opinions expressed are my own and do not reflect the opinions of the
FDA.
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- On 11 Jul 2002 at 15:07:21, Walt Woltosz (walt.aaa.simulations-plus.com) sent the message
Dear Dr. Hijazi,
Prediction of in vivo metabolism from in vitro measurements is now
possible, but it is not a simple matter of a few equations. Our
GastroPlus software has successfully predicted in vivo
bioavailability and plasma concentration-time profiles from such
data, including the separate determination of both gut and hepatic
metabolism for drugs metabolized by 3A4 and 2D6. It is not sufficient
to use a simple one-compartment absorption model when gut metabolism
is involved, because it is critical to know where and how fast
absorption is taking place at each time step in the simulation. This
means correctly simulating transit, dissolution/precipitation, time-
and concentration-dependent absorption rate, time- and
concentration-dependent gut and hepatic metabolism, and
pharmacokinetics.
A paper describing this process is:
Agoram et al, Predicting the impact of physiological and biochemical
processes on oral drug bioavailability, Advanced Drug Delivery
Reviews (50) 2001 S41-S67.
To do this requires IV pharmacokinetic parameters, and in vitro data
(typically Vmax and Km from microsomes, hepatocytes, or liver slices)
along with the usual physicochemical properties and permeability
(some of which can often be adequately predicted from structure).
Walt Woltosz
Chairman & CEO
Simulations Plus, Inc. (SIMU)
1220 W. Avenue J
Lancaster, CA 93534-2902
U.S.A.
http://www.simulations-plus.com
E-mail: walt.-a-.simulations-plus.com
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- On 14 Jul 2002 at 23:54:01, jbyczkowski.-a-.netscape.net sent the message
Ron Kavanagh wrote:
>> "... it's currently impossible to accurately predict in vivo apparent
>> parameters from in vitro parameters...">>
Indeed, it is difficult but not impossible.
Again, physiologically based pharmacokinetic (PBPK) modeling technique
may help a lot. The following two references may illustrate the point.
Best wishes.
Janusz Z. Byczkowski, Ph.D.,D.Sc.,D.A.B.T.
Consultant
212 N. Central Ave.
Fairborn, OH 45324
e-mail januszb.-at-.AOL.com
homepage: http://members.aol.com/JanuszB/index.html
JZB Consulting web site: http://members.aol.com/JanuszB/consult.htm
References:
Yu,K.O., Tillitt,D.E., Byczkowski,J.Z., Burton Jr., G.A., Channel, S.R.,
Drerup, J.M., Flemming, C.D., & Fisher, J.: In vivo/in vitro comparison
of the pharmacokinetics and pharmacodynamics of
3,3',4,4'-tetrachlorobiphenyl (PCB77). Toxicol. Appl. Pharmacol. 141:
434-438, 1996.
Lipscomb,J.C., Fisher,J.W., Confer,P.D., & Byczkowski,J.Z.: In vitro to
in vivo extrapolation for trichloroethylene metabolism in humans.
Toxicol. Appl. Pharmacol. 152:376-387, 1998.
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- On 15 Jul 2002 at 10:14:49, "Kavanagh, Ronald E" (KAVANAGHR.at.cder.fda.gov) sent the message
My mistake. It's not impossible to predict in all situations. However,
the
gist of my comments still hold. There are a number of factors that may
effect prediction of in vivo parameters from in vitro parameters for
which
we usually don't have reliable estimates.
For lipophilic compounds that passively diffuse across the cell membrane,
such as many inhaled compounds that are studied in toxicology, cell
transporters may not be an important variable and it may be easier to
find
examples where you can predict. In addition, the route of metabolism is a
factor in our ability to predict, with some enzyme systems easier to
extrapolate than others.
However, individual examples do not eliminate the difficulties in
extrapolating with the majority of compounds.
Although I am a proponent of PBPK, like most tools you need to understand
the advantages and limitations. Consequently, I do not view it as a
panacea,
as it still has a number of limitations. In the industrial setting I'm
uncertain to what extent it will ultimately be used, as it will depend
upon
the ease and timing of the generation of data to input into the models
and
whether the effect will ultimately effect the outcomes of drug
development.
My guess is that it will be useful in certain situations where it is the
best way to get at certain information, but it will not be a tool that is
used regularly.
Ron Kavanagh
The comments are my own and do not reflect the views of the FDA
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