Levodopa is a colorless to white odorless solid that is readily soluble in diluted hydrochloric and formic acids and practically insoluble in ethanol, benzene, chloroform and ethyl acetate. It is the prodrug of dopamine and is used in patients with Parkinson's disease (PD). The chemical name of levodopa is (-)-3-(3,4-dihydroxyphenyl)-L-alanine and its structure is shown in fig. 1.
PD is the second most common neurodegenerative disease after Alzheimer's disease with an incidence in the United States of 13.5-13.9 per 100000 person-years. It is a multisystem disorder and the main motor features (tremor, rigidity, and bradykinesia) of PD appear when substantia nigra dopamine neurons have been lost and striatal dopamine has been reduced by 60 to 80%. Other detailed information about PD has already been described in the "Benzhexol Hydrochloride" section and will not be repeated here. Levodopa is generally acknowledged as the gold standard drug that treats the motor features of PD, which has improved the life quality of life expectancy in PD patients.
Research has shown that symptoms of PD are related to the depletion of dopamine in the central nervous system. Administering dopamine to treat PD is ineffective because it does not cross the blood-brain barrier. While levodopa can cross the blood-brain barrier and is converted to dopamine in the central nervous system and periphery by aromatic-amino acid decarboxylase. However, the transformation of levodopa in the periphery is ineffective for the improvement of PD. To increase the bioavailability of levodopa, it is often administered in combination with peripheral decarboxylase inhibitors (such as carbidopa and benserazide). Dopamine decarboxylase inhibitors prevent the conversion of levodopa to dopamine in the periphery, allowing for more levodopa to cross the blood-brain barrier.
Common side effects of levodopa mainly include:
Abnormal hepatic function
Pain in oropharynx
Discoloration of sweat
Upper respiratory infection
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