As I understand it, there are two distinct mechanisms which have been used to reduce the peripheral metabolism of levodopa:

- DDC inhibitor (e.g carbidopa). Carbidopa [1] "inhibits aromatic-L-amino-acid decarboxylase (DOPA decarboxylase or DDC), ... an enzyme important in the ... biosynthesis of L-DOPA to dopamine (DA). DDC exists both outside of (body periphery) and within the confines of the blood brain barrier."

- COMT inhibitor (e.g. entacapone). Entacapone [2] "is a selective and reversible inhibitor of the enzyme catechol-O-methyltransferase (COMT)... When taken together with levodopa (L-DOPA) and carbidopa, entacapone stops catechol-O-methyltransferase from breaking down and metabolizing levodopa, resulting in an overall increase of levodopa remaining in the brain and body."

So, there's Sinemet (= levodopa + carbidopa) and Stalevo (= levodopa + carbidopa + entacapone). But, I've never seen levodopa + entacapone (i.e. without carbidopa). Why is this?

References:

[1] Carbidopa - Wikipedia

[2] Entacapone - Wikipedia

John