3-O-Methyltolcapone-d 4 is the deuterium labeled 3-O-Methyltolcapone. 3-O-Methyltolcapone (Ro 40-7591) is a metabolite of Tolcapone. Tolcapone is an orally active, reversible, selective and potent COMT inhibitor. Tolcapone crosses the blood-brain barrier, and can be used for treatment of Parkinson's disease.