When Should Levodopa Therapy be Initiated in Patients with Parkinson’s Disease?

Levodopa is available in three forms: immediate-release, orally disintegrating and sustained-release tablets. Levodopa is metabolised in the gastrointestinal tract, kidney and liver by aromatic acid dopa decarboxylase using pyridoxine as a cofactor. Approximately 70–80% of the dose is eliminated in the urine. Central conversion of levodopa to dopamine likely occurs at surviving dopaminergic terminals and at serotonergic and adrenergic nerve terminals that contain decarboxylase. Dopamine is metabolised by catechol-O-methyltransferase and monoamine oxidase. The major metabolites of dopamine are homovanillic acid and dihydroxyphenylacetic acid.