Modeling targeted mediated drug disposition using Quasi-equilibrium model

Dear users, I am working on monoclonal antibody exhibiting target mediated drug disposition and trying to fit a quasi-equilibrium model described by Mager and Krzyzanski [Pharm Res 22(10):1598]. In the paper, the quasi-equilibrium was fitted to total drug conc. and total target conc. We observed total drug conc., total target conc., and free target conc. I am trying to fit the model to not only total drug conc. and total target conc. but also free target conc. simutaneously. But I did not figure out how I can incorporate free target conc. in the model using KD instead of Kon and Koff. Could anybody please give me some suggestion or help? Thank you. Best, Xiaofeng