CatecholOMethyl Transferase Inhibitors

A recently introduced class of drugs for the treatment of parkinsonism is the catechol-O-methyl transferase (COMT) inhibitors. COMT metabolizes catechol compounds, including dopamine and levodopa (see Chapter

9), producing the inactive compound 3-O-methyl DOPA. The rationale for the use of COMT inhibitors is analogous to that for carbidopa; that is, since COMT is present in the periphery as well as in the CNS, inhibition of peripheral COMT results in an increase in the plasma half-life of levodopa, thereby making more drug available for transfer to the brain. Additionally, compounds that block COMT in the CNS also prolong the brain concentration of levodopa.

The two COMT inhibitors in clinical use are tol-capone (Tasmar) and entacapone (Comtan). They are used in combination with levodopa-carbidopa. In patients with motor fluctuations, they increase the "on" time. Adverse effects are similar to those observed with levodopa-carbidopa alone. Tolcapone therapy can cause fatal hepatotoxicity and so should be used only in patients who do not respond to other therapies. Patients taking tolcapone require close monitoring of liver enzymes for signs of hepatic changes.

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