RE: Antwort: Re: change from mass to molar units
Theoretically, the parameter estimates should be the same for the parent (if
the input, e.g., oral, is exactly in the form of the parent), no matter you use
the mass unit or the molar unit. However, for the metabolite, the parameter
estimates should be different when different concentration units are used
UNLESS the molecular weight of the metabolite happens to be the same as or very
close to MW for the parent.
Alan
Quoted reply history
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED]
Sent: Thursday, March 08, 2007 9:32 AM
To: Nick Holford
Cc: [email protected]
Subject: Antwort: Re: [NMusers] change from mass to molar units
Nick,
oh, sorry about that, that was a typo. I have mg and µg/L and µmol and
nmol/L, so I'm afraid that's not the reason. I have a combined error
model. However, although I needed the additive error to get the model to
run, I fixed it to a very small value (0.0001). But to make sure I just
adjusted this value too, with the same result.
Just to get this clear for me...in theory the parameters should be the
same, shouldn't they? So if this keeps on happening, what else can I do? I
was thinking about recalculating the metabolite concentrations by dividing
them with their molar mass and then multiplying with the molar mass of
the parent. Would this be a reasonable approach, just to pretend that the
metabolite is parent by using this calculation? Would the metabolite
parameters still be correct then?
Thanks for your help.
Nele
_________________________
Nele Plock
Bayer Schering Pharma AG
GPD/Pharmacokinetics
Metabolism & Bioanalysis
D- 13342 Berlin
Phone : +49-30-468 15146
Fax: +49-30-468 95146
[EMAIL PROTECTED]
Nick Holford <[EMAIL PROTECTED]>
08.03.2007 15:10
An
[EMAIL PROTECTED]
Kopie
[email protected]
Thema
Re: [NMusers] change from mass to molar units
Nele,
Two suggestions:
1. You have mass units of mg and mg/L but molar units of nmol/1000 and
nmol/L. So the ratio of AMT to DV is different by a factor of 1000
2. If you have an additive error model then the scale of the residual
error model parameter will need to change.
Best wishes,
Nick
[EMAIL PROTECTED] wrote:
>
> Dear all,
>
> I have fitted a simple two-compartment model with linear elimination to
> some compound data (ADVAN4 TRANS4). Everything worked well. Because I
want
> to extend this model to account for metabolite data as well, I now
wanted
> to change all data to molar units, i.e. I changed the AMT and DV column
in
> the same way. To my understanding, if I change both these columns, the
> obtained parameters should end up being the same. However, when I use mg
> and µg/mL the model runs just fine, whereas when I use µmol and nmol/L
the
> parameter estimates change completely and always run into some boundary.
> Can anyone shed some light on why this happens? Is there any way to get
> around this? Could I keep the mass units and do some changes in the
> control stream when incorporating the metabolite data? Any help would be
> highly appreciated.
>
> Best regards
> Nele
>
> _________________________
> Nele Plock
> Bayer Schering Pharma AG
> GPD/Pharmacokinetics
> Metabolism & Bioanalysis
> D- 13342 Berlin
>
> Phone : +49-30-468 15146
> Fax: +49-30-468 95146
> [EMAIL PROTECTED]
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
email:[EMAIL PROTECTED] tel:+64(9)373-7599x86730 fax:373-7556
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/