Carbidopa is a competitive inhibitor of aromatic L-amino acid decarboxylase. Carbidopa is often used with L-DOPA [
D0600] for the treatment of Parkinson's disease. L-DOPA, the metabolic precursor of dopamine [
A0305], does cross the blood-brain barrier, and presumably is converted to dopamine in the brain. However, when L-DOPA is administered orally, it is rapidly decarboxylated to dopamine in extracerebral tissues. Administration of dopamine alone is ineffective in the treatment of Parkinson's disease apparently because dopamine does not cross the blood-brain barrier. Carbidopa inhibits decarboxylation of L-DOPA. Since its decarboxylase inhibiting activity is limited to extracerebral tissues, administration of carbidopa with levodopa makes more L-DOPA available for transport to the brain. Carbidopa does not cross the blood-brain barrier and does not affect the metabolism of L-DOPA within the central nervous system. (The product is for research purpose only.)