Dear Harish
Please see the attached project I made that can serve as a template for future modeling.
1:You put a dose in the brain compartment which I believe is not appropriate.
2: I assume 4 different patients because I am not sure if it was cross over one patient, 2 patients, etc..
It does not change how to built the structural model.
If it is one patient only, then we need to reset the compartments to their initial values every time we have a new dose which is what I did with the reset function and by uncheck the sort button
3: For both "one individual" and "4 individuals" I built the model using the graphical option (see attached) and I assume a treatment effect on both Cl and Volume
4: I fit both models and the results are not too bad. Of course the model we used limit the ability to get perfect fit but the idea is there.
Look at it and come back to me if you need further clarifications about what I did.
best Regards
Serge