LYCOS RETRIEVER 
Levodopa
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During the first few years of Levodopa clinical use, large amounts of Levodopa were needed to be effective. These large doses were associated with side-effects such as nausea and vomiting and were due to the rapid breakdown of Levodopa in the body by enzymes. One of the main enzymes involved in this breakdown of Levodopa is peripheral dopa-decarboxylase (DDC). It was quickly realised that by inhibiting this enzyme using a DDC inhibitor, it was possible to reduce the Levodopa dose by 70%, thereby reducing the severity of side-effects. This combination is so effective that, today, Levodopa is almost always given in the same tablet with a DDC inhibitor, either carbidopa or benserazide. However, when DDC is inhibited another enzyme, catechol-O-methyl transferase (COMT), takes over the breakdown of Levodopa.
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Levodopa, which is nowadays always combined with a decarboxylase inhibitor, is the most efficient drug for the treatment of Parkinson's disease. It is irrelevant which of the two decarboxylase inhibitors is used. Levodopa treatment is normally started when anticholinergic agents and amantadin do not suffice any longer, or when the patients are severely hindered in their daily activities or social contacts. It is still not clear if an early levodopa treatment has a negative impact on the later development of the disease. At the beginning over 75% of the patients respond to levodopa treatment. The combination with other Parkinsonian drugs (especially with bromocriptine or selegilin) makes it possible to reduce the dose and therefore ... the side-effects.
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Levodopa-carbidopa distributes throughout the body, but due to peripheral metabolism, less than 1% of levodopa reaches the CNS; carbidopa does not penetrate the CNS. Approximately 95% of a dose of levodopa is metabolized to dopamine by L-aromatic amino acid decarboxylase in the stomach, intestines, and liver. Carbidopa is not appreciably metabolized. The onset of action of levodopa-carbidopa can be observed 2—3 weeks after therapy is initiated, but some patients require up to 6 months of therapy before beneficial effects are seen. The plasma half-life of both levodopa and carbidopa is roughly 1—2 hours, and the duration of action of a dose is 5 hours. Levodopa-carbidopa is eliminated renally as dopamine metabolites and small amounts as unchanged drug.
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Levodopa and carbidopa are used for the treatment of Parkinson's disease. Levodopa can be used alone, but adding carbidopa lowers the amount of levodopa that is required and may reduce some of the levodopa side effects such as nausea and vomiting. Carbidopa is a medication (called a decarboxylase inhibitor) that, when taken with levodopa, helps prevent the levodopa from converting to dopamine outside the brain. The combination of carbidopa and levodopa allows more levodopa to get to the brain. The levodopa carbidopa combination decreases side effects caused by increased dopamine levels outside the brain by reducing the supply of “free” dopamine outside the brain. Most people take levodopa and carbidopa together, rather than levodopa by itself.
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Levodopa-induced dyskinesia (LID) is characterized by a variety of hyperkinetic movements. Although chorea and dystonia are the most common manifestations of LID, stereotypies, tics, myoclonus, or ballism may occur. LID commonly starts in the lower extremity ipsilateral to the side first affected by PD, usually the most affected side. Early in LID, patients may not notice subtle hyperkinetic movements; ... as LID worsens it may interfere with activities of daily living, resulting in functional impairment, disability, and poor quality of life.2-5
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Levodopa is required by the brain to produce dopamine, an important neurotransmitter. People with Parkinson’s disease have depleted levels of dopamine, leading to debilitating symptoms. Levodopa is given to increase production of dopamine, which in turn reduces the symptoms of Parkinson’s disease. When taken by mouth, most levodopa is broken down by the body before it reaches the brain. Sinemet® combines levodopa with carbidopa, a drug that prevents the breakdown, allowing levodopa to reach the brain to increase dopamine levels.
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