(R)-3-O-Methyldopa-d 3 (hydrochloride) is a deuterium labeled (R)-3-O-Methyldopa, and (R)-3-O-Methyldopa is an R-enantiomer of 3-O-Methyldopa. 3-O-Methyldopa 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.