Potent CYP3A4 Inhibitors Following inhibition of CYP3A4 by co-administration of ketoconazole 200 mg twice daily, Cmax and AUC of the active metabolite of fesoterodine increased 2.0 and 2.3-fold in CYP2D6 extensive metabolisers and 2.1 and 2.5-fold in CYP2D6 poor metabolisers, respectively.