Levodopa (LD), the most effective drug for Parkinson’s disease treatment, has poor oral availability; therefore, it is taken concomitantly with inhibitors of LD-metabolizing enzymes, including dopa deoxycarboxylase (DDC) and catechol-O-methyltransferase (COMT) (Gershanik, 2015). Here, COMT is linked to Parkinson disease.