Re: Why should we avoid using micro rate constants?

On Feb 5, 2019, at 7:52 AM, Saeheum Song <[email protected]<mailto:[email protected]>> wrote: The input data are composed of amount as dosing and concentration in plasma. Concentration is expressed as amount divided by volume of distribution. The rate constant is movement of either amount or concentration pending on modelers intention. Whence, in the modeling fitting, dosing amount needs to be converted into concentration by introduction of volume of distribution. With introduction of the volume of distribution (K21), K12= Q/V1, and K21=Q/V2 Therefore by introduction of Volume (V1), system can define all other parameters. Simply, by model fitting using micro-constant only means we are treating amount and concentration as same unit. Hence, it should be avoided. You can still fit model with micro-constant but requires defining volume and conversion of amount into concentration to do proper modeling, Regards, SaeHeum Song, Independent Consultant, On Tue, Feb 5, 2019 at 2:05 AM <[email protected]<mailto:[email protected]>> wrote: Hi All, It could also be the statistical model. If you are estimating 4 parameters then different parameterisations should be fairly equivalent if a BLOCK(4) structure is used for both parameterisations. If only the diagonal option is used, then this could be why different results/minimisations are obtained for different parameterisations. Kind regards, Janet Janet R Wade, PhD Occams Senior Consultant From: [email protected]<mailto:[email protected]> <[email protected]<mailto:[email protected]>> On Behalf Of Leonid Gibiansky Sent: 04 February 2019 07:30 To: Singla, Sumeet K <[email protected]<mailto:[email protected]>> Cc: [email protected]<mailto:[email protected]> Subject: Re: [NMusers] Why should we avoid using micro rate constants? It could be just coding error, could you show the control stream? Thanks Leonid On Feb 3, 2019, at 12:44 PM, Singla, Sumeet K <[email protected]<mailto:[email protected]>> wrote: Hello everyone! I have a question. I was trying to build a 2-compartment PK model for marijuana use in occasional and chronic smokers. Initially, I was using providing rate constants K12 and K21 ­in PK block and it resulted in poor fitting. Then, I later changed to CL,V1, V2 , Q and it resulted in proper fitting. I was perplexed as to why I couldn’t get a proper fit by providing rate constants? I tried to look online but couldn’t find any proper explanation about when (or not) we should use micro constants in PK block to define our model in NONMEM? Does anyone has any useful insights into this? Regards, Sumeet Singla Graduate Student Dpt. of Pharmaceutics & Translational Therapeutics College of Pharmacy- University of Iowa