Modeling with insufficient data

From: "Hyeong-Seok Lim" Subject: [NMusers] Modeling with insufficient data Date: Tue, October 26, 2004 6:28 am Dear all, I am trying to model a drug and its 3 metabolites using NONMEM. After oral administration, the drug is converted to 2 first stage metabolites, which are subsequently converted to a second stage metabolite. However, I don't have a concentration data of one out of the 2 first stage metabolites. Could anyone advise on how to model all the 4 compounds (the parent drug and its 3 metabolites) with concentration data of only 3 compounds? The only relevant information is that the amount ratio of the 2 first stage metabolites in urine is 4:3. Thanks in advance. HS Lim National Cancer Center, Republic of Korea E-mail: mdlhs@ncc.re.kr