i don't think carbidopa and entacapone pass the BBB. if carbidopa entered the brain very little l-dopa would be converted to dopamine in the brain. this feature made carbidopa so attractive and reduced the l-dopa dosage from over 10grams and impractical to use to treat pd for the masses to what it is today.


entacapone also doesn't cross the BBB which makes it less potent than Tolcapone (tasmar) which i think is no longer used in the U.S. over liver toxicity risks and the need to monitor that.

Medscape: Medscape Access
Entacapone is highly protein bound, approximately 98%, with a limited body distribution.[42,44] It has poor brain penetration and predominately peripheral effects, although it may exhibit central effects if given in high doses.[