- On 3 Aug 2005 at 12:47:38, james evans (jamesrevans.-a-.gmail.com) sent the message

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The following message was posted to: PharmPK

Hi there,

I am working with a drug that appears to be rapidly and completely

metabolised (to glucuronide and/or sulphate forms) after i.v. and oral

administration.

I am trying to work out the volume of distribution for the metabolite

after i.v. bolus admin.

Can I use the simple formula Vd = dose/Cp ?

(where Cp is the estimated concentration at time 0 by extrapolating

back to time 0 on the y-axis of a semi-log plot.)

It seems wrong to use "dose" in the above equation when I am unsure of

the identity of the actual metabolite.

Is there another simple way of calculating the Vd of this metabolite?

Thanks for your help!

James - On 3 Aug 2005 at 14:13:10, "Steve Duffull" (sduffull.-at-.pharmacy.uq.edu.au) sent the message

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The following message was posted to: PharmPK

James

To use V(metab) = Dose(parent) / C(metabolite) would require the

following assumptions:

1) that all parent is converted to metabolite,

2) that the parent is adminstered by iv-bolus,

3) that the metabolite is formed instantaneously.

You indicate that you are happy with the first 2 assumptions in your

email - but the 3rd assumption would be a bit of a leap of faith.

Therefore I suggest the answer is no you should not use this equation

- but instead should model the data.

Steve

Stephen Duffull

School of Pharmacy, University of Queensland, Brisbane 4072, Australia

Tel +61 7 3365 8808, Fax +61 7 3365 1688, Email:

sduffull.at.pharmacy.uq.edu.au

University Provider Number: 00025B

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