You may have found a local minima with the microconstant parameterization. There generally isn’t any rhyme or reason for why one chooses macro or micro constants. People will argue that the macro model with Q and Vp is more interpretable than k12 and k21. That, and scaling reasons, are why one might choose one parameterization over another.
Pete Bonate On Feb 3, 2019, at 2:54 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
