Entacapone may be useful in patients being treated with levodopa-carbidopa combination because it :: (A) Activates COMT, (B) Decreases formation of 3–OMD, (C) Inhibits monoamine oxidase type B, (D) Inhibits dopamine uptake
Answer is B. 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.