Tolcapone-related liver dysfunction: implications for use in Parkinson's disease therapy

Levodopa is the cornerstone of idiopathic Parkinson's disease (PD) treatment. However, after long-term use of levodopa, a significant percentage of patients experience motor fluctuations, which worsen their quality of life. Catechol-O-methyltransferase (COMT) inhibitors reduce levodopa metabolism and enhance the respective plasma levels, resulting in improvements in symptoms and overall quality of life. Tolcapone was the first drug of this class to be marketed, but was withdrawn in the European Union due to its implication in the deaths of three PD patients due to hepatic failure. Three deaths from fulminant hepatic failure in 40000 patient-years is a number that is 10-100 times higher than the expected incidence in the general population and, according to the manufacturer's own information, the number is probably underestimated due to under-reporting of cases. In the US, tolcapone was not withdrawn, but restrictive liver enzyme monitoring measures were issued by authorities, which severely limited its use. No further deaths from hepatic failure were reported since these measures were implemented. The mechanisms by which tolcapone may induce liver toxicity are still under debate. It was thought that mitochondrial uncoupling of oxidative phosphorylation by tolcapone, and consequent impairment of energy production by hepatocytes, could be responsible for the observed effects. Some experts consider that the restrictive guidelines issued in the US regarding tolcapone use may be loosened with no consequential reductions in safety. It was suggested that ongoing clinical information about safety should be considered and periodical revisions of the restrictions made accordingly. The identification of the molecular and biochemical basis of tolcapone hepatotoxicity, when completed, should also provide important indications for the clinical use of this drug. In conclusion, appropriate monitoring of liver function can ensure adequate safety in PD patients receiving tolcapone, who can therefore benefit from the symptomatic improvements obtained with this drug.