I would recommend against mixing mass-based units and mole-based units for species participating in the same reaction. In such cases, it would be unclear which of these units would apply to the reaction rate. I personally would probably use molarity for all the species. If you also want to know the drug concentration in microgram/milliter, you can calculate that separately as a different output from your simulations. One option introduced in R2020a is to add an "observable" to your model for this. If you need a solution that also works in older versions of SimBiology, you could do something similar with a repeated assignment rule.
-Arthur
