Re: [R] Michaelis-menten equation

From: Chun-Ying Lee <>
Date: Wed 20 Jul 2005 - 12:21:22 EST


   We are doing a pharmaockinetic modeling. This model is described as intravenous injection of a certain drug into the body. Then the drug molecule will be eliminated (or decayed) from the body. We here used a MM eq. to describe the elimination process and the changes of the drug conc.. So the diff. eq. would be: dCp/dt = -Vm/Vd * Cp/(Km+Cp). Vd is a volume of distribution. We used lsoda to solve the diff. eq. first and fit the diff. eq. with optim first (Nelder-Mead simplex) and followed by using nls to take over the fitting process of optim. However, we can not obtain the correct value for Km if we used the above model. The correct Km can be obtained only when we modeled the diff eq. with dCp/dt= -Vm/Vd * Cp/(Km/vd + Cp). Now we lost. The data were from simulation with known Vm and Km. Any idea? Thanks.

--- Chun-ying Lee

> it is not clear to me what you are trying to do:
> you seem to have a time-concentration-curve in PKindex and you seem
> to set up a derivative of this time dependency according to some
> model in dCpdt. AFAIKS this scenario is not directly related to the
> situation described by the Michaelis-Menten-Equation which relates
> some "input" concentration with some "product" concentration. If Vm and
> Km are meant to be the canonical symbols,
> what is Vd, a volume of distribution? it is impossible to see (at least
> for me) what exactly you want to achieve.
> (and in any case, I would prefer "nls" for a least squares fit
> instead of 'optim').
> joerg
> > ------------------------------------------------------------------------
> >
> > ______________________________________________
> > mailing list
> >
> > PLEASE do read the posting guide!
guide.html mailing list PLEASE do read the posting guide! Received on Wed Jul 20 12:25:37 2005

This archive was generated by hypermail 2.1.8 : Fri 03 Mar 2006 - 03:33:50 EST