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]; [email protected]