Clear all variables in GUI without clearing GUI handles?

The question is pretty straightforward as is but I will proceed by giving a little background on its application.

I have a GUI in which I upload a bunch of cases from text files. I then, or at least I would like to, run the simulations for each of the cases upon clicking a "RUN" button. Within the callback function of the "RUN" pushbutton I have the for loop to sweep through my DoE (Design of Experiments a.k.a my cases). Currently I have the following:

    for r = 1 : number_cases

         clearvars -global
         clearvars -except r number_cases files case_list

         set(handles.case_list,'Value',r)
         progress = [num2str(r) ' of ' num2str(number_cases)];
         set(handles.progress_text,'String',progress)
         pause(1)

         ...

The problem is when I run this I receive the following error:

??? Reference to a cleared variable handles.

There in lies my dilemma. How do I also avoid clearing all variables except for the GUI handles???

[Walter Roberson]

Why not add "handles" to the -except list ?