What is the function of carbidopa in the treatment with carbidopa-levodopa?

Carbidopa plays a crucial role in the treatment regimen of carbidopa-levodopa by preventing the peripheral conversion of levodopa into dopamine before it can reach the central nervous system (CNS). Levodopa is a precursor to dopamine and, when administered alone, can be metabolized outside the CNS into dopamine, particularly in the gut and peripheral tissues. This peripheral conversion can lead to unwanted side effects and reduced effectiveness of the treatment intended to alleviate neurological symptoms.

By inhibiting the enzyme aromatic L-amino acid decarboxylase, which facilitates that peripheral conversion of levodopa to dopamine, carbidopa ensures that more levodopa is available to cross the blood-brain barrier. Once in the CNS, levodopa can be converted into dopamine, thereby addressing the dopamine deficiency seen in conditions like Parkinson's disease. This mechanism enhances the therapeutic efficacy of levodopa while minimizing side effects associated with peripheral dopamine synthesis, thus making the combination of carbidopa and levodopa more effective for managing symptoms of Parkinson’s disease.