3-O-Methyldopa-d3 (hydrate) is the deuterium labeled 3-O-Methyldopa. 3-O-Methyldopa (3-Methoxy-L-tyrosine) is a metabolite of L-DOPA which is formed by catechol-O-methyltransferase (COMT). 3-O-Methyldopa competitively inhibits the pharmacodynamics of l-DOPA and dopamine[1].