Entacapone may be useful in patients being treated with levodopa-carbidopa combination because it :

Levodopa is converted to dopamine by dopa decarboxylase (that cannot cross the blood-brain barrier). Carbidopa is an inhibitor of dopa decarboxylase in the periphery. It therefore increases the amount of l-dopa that reaches the brain (due to inhibition of peripheral metabolism to dopamine). Due to inhibition of l-dopa metabolism (by dopa decarboxylase), an alternative pathway of metabolism by catechol-O-methyl transferase (COMT) is activated. COMT metabolizes l-dopa to 3-O-methyldopa (3-OMD) that can complete with l-dopa for entry in the brain. Entacapone and tolcapone is useful in Parkinsonism by inhibiting COMT.