Carbidopa is an inhibitor of amino acid decarboxylase (dopa decarboxylase), which is the enzyme present in both the peripheral and central tissue. Carbidopa only inhibits the peripheral conversion of levodopa. It does not inhibit the decarboxylase enzyme in the brain because it does not cross the blood brain barrier. This is important since inhibition of both central and peripheral amino acid decarboxylase would prevent levodopa from being converted to dopamine.
A double-blind study comparing the effects of carbidopa and levodopa combined in a single tablet with levodopa alone was undertaken in 50 patients with Parkinson's disease. After 6 months, there was a statistically significant improvement over baseline in total score, rigidity, and tremor only in the patients randomized to carbidopa/levodopa. In addition, 40 percent of the patients treated with carbidopa/levodopa showed obvious clinical improvement (a greater than 50 percent reduction in their total score) over treatment with levodopa alone. However, after 2 years, only 20 percent continued to show this improvement. Nausea, vomiting, and anorexia developed in 56 percent of patients on levodopa but in only 27 percent of patients on carbidopa/levodopa. However, abnormal involuntary movements, observed in 48 percent of patients on levodopa, were present in 77 percent of patients on carbidopa/levodopa. Despite the increase in abnormal involuntary movements, carbidopa/levodopa is more effective than levodopa.
Lieberman, A., Goodgold, A., Jonas, S., & Leibowitz, M. (1975). Comparison of dopa decarboxylase inhibitor (carbidopa) combined with levodopa and levodopa alone in Parkinson's disease. Neurology, 25(10), 911-911.
Levodopa is the most efficacious agent for the treatment of motor features of Parkinson's disease but its chronic use is associated with the development of motor complications. Mounting evidence indicates the short half-life of levodopa and resultant pulsatile stimulation of striatal dopamine receptors leads to wearing off, motor fluctuations and dyskinesias. Longer acting dopaminergic agents, such as dopamine agonists, are less likely to cause motor fluctuations and dyskinesias but are not as efficacious for control of motor symptoms. Therefore, there is interest in exploring ways to deliver levodopa in a more continuous fashion, in an effort to maintain benefit through the day and reduce the development of motor fluctuations and dyskinesias. A dopa decarboxylase inhibitor (DDCI), such as carbidopa or benserazide, is administered with levodopa to attenuate its peripheral conversion to dopamine, reduce nausea and increase central bioavailability. When levodopa is administered with a DDCI, its main route of peripheral metabolism is via catechol-O-methyl transferase (COMT).
Seeberger, L. C., & Hauser, R. A. (2009). Levodopa/carbidopa/entacapone in Parkinson’s disease. Expert review of neurotherapeutics, 9(7), 929-940.