COMT inhibition: a new treatment strategy for Parkinson's disease.

During the initial stages of Parkinson's disease, treatment with levodopa plus a decarboxylase inhibitor (carbidopa or benserazide) provides adequate control of symptoms. However, as the disease progresses, the clinical response to treatment often begins to fluctuate, becoming increasingly correlated with fluctuations in plasma concentrations of levodopa-the "wearing-off" phenomenon. Many strategies have attempted, with various degrees of success, to increase the availability of levodopa and its active metabolites, thus reducing these fluctuations in response. This review focuses on the role of the new catechol O-methyltransferase (COMT) inhibitors tolcapone and entacapone as adjuncts to levodopa therapy. These agents act effectively and safely to increase the amount of levodopa that is available to enter the brain by extending the half-life of levodopa, resulting in more stable levels in the plasma and prolonging "on" time.