Hi Orlando, This is somewhat dependent on how exactly you want the fraction to be coded. Most likely, you want to restrict the volume to be positive and not bounded between 0 and 1 (i.e. it is ≥0). Given that, you can define V3 as:
V3=EXP(THETA(8))*V1 You would then just exponentiate THETA(8) to find the fraction. You can separately estimate the volume using the same ETA as the parent central volume as: V3=THETA(8)*EXP(ETA(1)) That way, it won’t explicitly be a fraction of the parent, and that may lead to a more stable model. Thanks, Bill From: [email protected] [mailto:[email protected]] On Behalf Of Carlos Orlando Jacobo Cabral Sent: Monday, May 21, 2012 2:19 PM To: nonmem users Subject: [NMusers] VD as a fraction of another VD Dear NMusers, Hi, I have data of a parent drug (intravenously administered) and its metabolite, ¿How can I code the volume of distribution of metabolite as a fraction of the central volume of parent? An example of the model that I want to fit is: $PK V1=THETA(1)*EXP(ETA(1)) ; Central Volume of parent V2=THETA(2)*EXP(ETA(2)) ; Peripheral Volume of parent Q=THETA(3) ; intercompartmental clearance V3= ???????????? ; Volume of metabolite as a fraction of V1 with variability CL1=THETA(5)*EXP(ETA(4)) ; Formation clearance of metabolite CL2=THETA(6)*EXP(ETA(5)) ; Elimination clearance of metabolite CL0=THETA(7)*EXP(ETA(6)) ; Parent drug excretion by routes other than formation of metabolite - Thank you in advance. Orlando. PhD student Carlos Orlando Jacobo Cabral Departamento de Farmacología, Lab.34 Centro de Investigación y de Estudios Avanzados del I. P. N. Email: [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>
