That's the point, under the current system you can't enforce a royalty. You could theoretically use e-medical records to detect infringement, but you can't build a business case on suing individual patients and doctors taking an old drug for a new indication. You could outlaw prescribing drugs off-label but that isn't politically feasible. Pharmacies and doctors are encouraged to automatically prescribe low cost generics where available vs a branded drug, by reference to the INN.