Entacapone may be useful in patients being treated with levodopacarbidopa combination because it:

(Ref: KDT 6/e p421) Levo-dopa is conveed to dopamine by dopa decarboxylase (that cannot cross blood brain barrier). Carbidopa is an inhibitor of dopa decarboxylase in the periphery. It therefore increases the amount of 1-dopa that reaches the brain (due to inhibition of peripheral metabolism to dopamine). Due to inhibition of 1-dopa metabolism (by dopa decarboxylase), an alternative pathway of metabolism by catechol-o-methyl transferase (COMT) is activated.COMT metabolizes 1-dopa to 3-o-methyl dopa (3-OMD) that can complete with 1-dopa for entry in the brain. Entacapone and tolcapone is useful in Parkinsonism by inhibiting COMT.