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Levodopa brought about a revolution in Parkinson's disease treatment by reversing the neurochemical abnormality responsible for the symptoms and making dopamine available for the brain. Dopamine itself cannot cross the blood/brain barrier (a tight "net" of blood vessels that protects the brain by preventing many agents from crossing into the brain). Levodopa was the first drug that was formulated in such a way that some dopamine was able to reach the brain.

Sinemet (levodopa combined with carbidopa) is the standard formulation of levodopa for Parkinson's disease patients today. This was a significant modification of levodopa because while carbidopa does not cross the blood brain barrier, it inhibits levodopa from being metabolized into dopamine in the digestive system (outside the central nervous system) and leaves a greater concentration of levodopa available to reach the brain with each dose. Thus, Sinemet reduces the amount of levodopa necessary to achieve desired motor control. Sinemet also minimizes some of the short term side effects of pure levodopa, including nausea and vomiting.

When a Parkinson's disease patient begins treatment with levodopa, there is a dramatic improvement of symptoms. However, as time progresses, the patient begins to find that the Parkinson's symptoms recur and the dose to achieve patient comfort may need to be raised. Combining levodopa, carbidopa and entacapone (a COMT inhibitor) in a drug called Stalevo, has been found to prolong the action of levodopa, thereby, providing relief with lower doses of levodopa.

There is a slow, sustained release formulation of Sinemet available which reduces the uncomfortable side effects but is not as effective as the regular formulation since its absorption into the blood is slower than the normal dose formulation. The controlled release Sinemet is only moderately effective and does not significantly reduce motor complications in most patients. Some doctors prescribe the slow release Sinemet to be taken as the last dose before the patient goes to bed.

Levodopa is considered to be so effective for Parkinson's disease that only approximately 10% of patients with Parkinson's disease do not respond. Indeed, some physicians are of the opinion that if a patient with Parkinsonian symptoms does not respond to levodopa, other causes for the symptoms should be investigated.

Side-Effects of Levodopa

Over time, patients taking increasingly large doses of levodopa find that not only is the effectiveness reduced but major-side effects become evident. This typically takes place in about 50% of the patients after approximately five years. Some of these events occur because levodopa has a short half-life (the time it takes for half the dose of the drug to be eliminated from the body) of approximately 1.5 hours so the effect of the drug wears off quickly and symptoms reappear.

Side-effects of levodopa can have a profound effect on the quality of life of the patient and include:

• Dyskinesia - involuntary movements (e.g., twitching, nodding, or jerking). The movements can be mild, severe, slow, or rapid. The only way to control them is to cut back on the amount of levodopa, but a return of the Parkinson symptoms quickly recurs. At this point, other medications are utilized. This is the most problematic side effect and has a significant influence on quality of life.