a question about unstable baseline for a soluble antibody target

Dear NMusers, I am trying to develop a TMDD model for an antibody against its soluble target in a monkey study. I have observations of the target before a single antibody IV bolus dose. My model has two compartments for antibody and target respectively. The challenging part is that the baseline levels of the target before the antibody dose increased instead of being stable. So I am trying to model the changing target baseline level. In my data file, the antibody dose is on Time=4 (d) instead of Time 0 (d). I have observations of target from day 0 to day 4. My question is that if it is possible to let NONMEM compute only the differential equation for target from day 0 to day 4 and compute both differential equations for antibody and target after day 4. If yes, how should I set up initial values for the two differential equations? Any suggestion and help will be appreciated. Thank you. Best, Xiaofeng