Are you talking in vitro or in vivo? In vitro just do a dose-response curve and see what does cells can tolerate. For in vivo, usually re-purposing implies that this drug is already approved for treatment of some disease. If that's the case, phase I clinical trial usually determine the maximum tolerable dose in vivo (the dose with tolerable side effects). 

Thank u Richard

Repurposing to me implies this is a drug that is already in general clinical use for another indication. In those circumstances in vivo the dose would generally be that in routine use unless there was a mechanistic reason to propose a higher dose ie it was known that the concentrations reached with 'standard' dosing would not be sufficient to inhibit the target of interest. 

Article Lead Phytochemicals for Anticancer Drug Development