Levodopa

Levodopa (L-DOPA) is the amino-acid precursor of the neurotransmitter dopamine, used in the treatment of Parkinson's disease. Administration of levodopa aims to restore dopamine levels which are significantly reduced in Parkinson's patients. Levodopa has the advantage over dopamine that it can cross the blood-brain barrier, whereas dopamine cannot. Many of the adverse side effects (such as motor complications) seen with use of levodopa on its own arise from peripheral conversion to dopamine. To circumvent this, co-administration of peripheral DOPA decarboxylase inhibitors (DDCI) such as carbidopa (co-careldopa) or benserazide (co-beneldopa) is beneficial, and allow effective brain-dopamine concentration to be achieved with lower doses of levodopa (see image below). End-of-dose deterioration can be somewhat mitigated by using modified-release preparations, which can help reduce nocturnal immobility and rigidity resulting from drug effects wearing off.