COMT is one of the enzymes that catabolise dopamine - both within and outside the nervous system
COMT inhibitors therefore increase the bioavailability of dopamine (both extra and intracerebrally)
COMT inhibitors can reverse wearing-off effects of L-dopa without the need for an increase in daily dose
a common consequence of initiation of COMT inhibitor therapy is an exacerbation of L-dopa induced side effects e.g. dyskinesias; these should settle when the L-dopa dose has been appropriately adjusted
other side effects of COMT inhibitors include:
diarrhoea - often self-limiting - occurs in approximately 10% of patients on standard dose
may be some orange discoloration of patient's urine
NICE guidance states (1):
COMT inhibitors may be used to reduce motor fluctuations in people with later Parkinson's disease (PD)
in view of problems with reduced concordance, people with later PD taking entacapone should be offered a triple combination preparation of levodopa, carbidopa and entacapone
tolcapone should only be used after entacapone has failed in people with later PD due to lack of efficacy or side effects. Liver function tests are required every 2 weeks during the first year of therapy, and thereafter in accordance with the 'Summary of product characteristics'
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