Modeling Rates and Effects of Drug X

I just read the question of the Nonmemuser and Lewis answer and want to add the following: You measure concentration of X as response. Since you do not know the volume of distrubution for that substance you can not estimate the total amount of the substance in the body and it's change. Therefor you can not estimate the real production rates. However you can model concentration of X in response to your drug, if you assume an arbitary volume of distribution for X. I usually set it just to 1, because it does not matter and then the "amount" in that compartment has the same scale as the measured concentrations. The estimated production rates will not correspond to the real ones, i.e. they will not predict the real amount produced, but using them you predict concentration of X correctly and that is probably what you are interested in.