two compartment with nonlinear elimination

Dear NMusers, I'm a NM beginner modeling for a monoclonal antibody. Below is 12 subjects's individual predicted plot , it seems that nonlinear elimination (Michaelis-Menten) model can fit the data well. However, when running the control stream (two compartment models with linear and non-linear elimination), the resulting Q THETA SE% and OMEGA SE% are quite large even thought successful minimization.From the Correlation Matrix , I find that THETA (1) (VMAX) and THETA(2) (KM)are highly correlated (9.67E-01). Another problem is the .fit file can't be produced. I have tried various error models and initial estimations, however, none had better improvement and often got error message (error 134 or parameter estimate is near its boundary). Does re-parameterization helpful ? or need to change to other models? Any suggestions for solving the problems are highly appreciated. (control stream will be send in another e-mail)