Carbidopa

Carbidopa (Lodosyn) is a drug given to people with Parkinson's disease in order to inhibit peripheral metabolism of levodopa.

Pharmacology
Carbidopa inhibits aromatic-L-amino-acid decarboxylase (DOPA Decarboxylase or DDC), an enzyme important in the biosynthesis of L-tryptophan to serotonin and in the biosynthesis of L-DOPA to Dopamine (DA).

Along with carbidopa, other DDC inhibitors are benserazide (Ro-4-4602), difluromethyldopa, and α-methyldopa.

Uses
Used in tandem with L-DOPA (trade name levodopa, a dopamine precursor converted in the body to dopamine), it increases the plasma half-life of levodopa from 50 minutes to 1½ hours. CarbiDOPA cannot cross the blood brain barrier, so it inhibits only peripheral DDC. It thus prevents the conversion of L-DOPA to dopamine peripherally. This reduces the side effects caused by dopamine on the periphery, as well as increasing the concentration of L-DOPA and dopamine in the brain.

The combination of carbidopa/levodopa carries the brand names of Sinemet, Parcopa and Atamet; whilst Stalevo is a combination with entacapone, which enhances the bioavailability of carbidopa and levodopa.

Carbidopa is also used in combination with 5-HTP, a naturally occurring amino acid which is a precursor to the neurotransmitter serotonin and an intermediate in tryptophan metabolism. Carbidopa prevents 5-HTP's metabolism in the liver and the resulting elevated levels of serotonin in the blood. Research shows that co-administration of 5-HTP and carbidopa greatly increases plasma 5-HTP levels. 5-HTP has no reported cases of heart valve disease associated with it as found in the peer reviewed literature. In Europe, 5-HTP is prescribed with carbidopa to prevent the conversion of 5-HTP into serotonin until it reaches the brain. Several cases of scleroderma-like illness have been reported in patients using carbidopa and 5-HTP.