Endogenous drug

From:Sophie.Glatt@serono.com Subject:[NMusers] Endogenous drug Date:Fri, 12 Jul 2002 11:01:46 +0100 Dear all, I am modelling, multiple dose, 2-compartments IV and SC pharmacokinetics of a drug X which exhibits endogenous plasma levels. Endogenous production appears constant on the time scales (placebo). In each subject, the baseline concentration of X was measured in one pre-dose sample, and samples were measured for X post-dose, exhibiting a return to the endogenous levels. I know how to write the code for an endogenous baseline drug concentration, and used the following code for the residual error: $ERROR FTOT=F+CBAS, FTOT is the total plasma concentration estimate, comprising the sum of the exogenous (F) and endogenous (CBAS) drug concentrations. W=FTOT Y=FTOT+W*(EPS(1)) IPRED=FTOT IRES=CP*IPRED IWRES=IRES/W The PK model is ADVAN 4 TRANS 4 In other software, one can initialize each compartment to the ratio of Rin/CL, thereby generating a steady-state in the body. In above case, NONMEM code only adds to the amount (once) in the central compartment (CBAS) ignoring the peripheral compartment. Is there some way of coding the model to place a constant amount in all compartments. Thanks in advance. Sophie.