RE: [EXTERNAL] Re: MU referencing and time-varying covariates

-----Original Message----- From: [email protected] <[email protected]> On Behalf Of Leonid Gibiansky Sent: Friday, January 10, 2025 5:21 PM To: Sébastien Bihorel <[email protected]>; [email protected] Subject: [EXTERNAL] Re: [NMusers] MU referencing and time-varying covariates Hi Sébastien, As you did these experiments, can you share the results: have you seen any differences in the fit, parameter estimates, precision, convergence speed (number of iteration), and evaluation time for SAEM/IMP (I think, FOCEI does not have this restriction of time-independence even if you use Mu referencing, so results should be identical or very close). As code is encrypted, only Bob can answer the question but my understanding is that some kind of averaging is used to get time independent value of WT that is then used by the SAEM/IMP algorithm for parameter update procedure. As WT changes slowly and not very significantly, it could be hard to see the differences. A more stringent test would be to use time-dependent and strongly influential ADA (0/1): how bad is the incorrect version 1 in this case? Thank you Leonid On 1/10/2025 2:56 PM, Sébastien Bihorel wrote: > > Happy New Year, > > I hope everybody is ready for a great 2025 ! > > I'll start my message/question by defining 2 different ways of coding > a simple power relationship between body weigh on clearance. > > * > Coding 1 > > MU_1 = THETA(1) + THETA(2) * LOG(WGT/70) CL = EXP( MU_1 + ETA(1) ) > > * > Coding 2 > > MU_1 = THETA(1) > CL = EXP( MU_1 + ETA(1) ) * ( WGT/70 )**THETA(2) > > The reference and training materials for NONMEM clearly indicate that > MU variables should be time invariant within occasions and recommend > using coding 2 when body weight is time varying. Nevertheless, it is > possible for an analyst to use coding 1. As far as I can tell from > some limited testing, this is not a "fatal" error. Either with FOCE(I) > or SAEM/IMP, NONMEM reports a warning but performs the model > optimization. The table outputs also report CL as a time varying > variable changing as body weight changes. > > So my questions are the following: when coding 1 is used and body > weight is time varying, what is NONMEM actually doing during model > optimization? Does NONMEM internally create occasions to break the > records by interval of constant body weight and constant MU1? > Alternatively, does NONMEM internally calculate an average of MU1? > Something entirely different? What's the risk taken by an analyst when > using coding 1 versus coding 2? > > Thank you in advance for you input > > > __ > Sébastien Bihorel > Director, Quantitative Pharmacology > +1 914-648-9581 > [email protected] > > > Regeneron - Internal > > ******************************************************************** > This e-mail and any attachment hereto, is intended only for use by the > addressee(s) named above and may contain legally privileged and/or > confidential information. If you are not the intended recipient of > this e-mail, any dissemination, distribution or copying of this email, > or any attachment hereto, is strictly prohibited. If you receive this > email in error please immediately notify me by return electronic mail > and permanently delete this email and any attachment hereto, any copy > of this e-mail and of any such attachment, and any printout thereof. > Finally, please note that only authorized representatives of Regeneron > Pharmaceuticals, Inc. have the power and authority to enter into > business dealings with any third party. > ********************************************************************