COMT inhibitors

COMT inhibitors work by blocking an enzyme called Catechol-O­methyl Transferase (COMT) that breaks down levodopa. As a result they slow the destruction of levodopa in the body. COMT inhibitors are therefore used with levodopa to prolong the duration of action.

These are:

• Entacapone (Comtess)
  
  Taken with each levodopa dose and tolcapone three times a day. Both drugs should be effective from the first dose with noticeable benefits from the first or second day.
  
  
• Tolcapone (Tasmar)
  
  Recently reintroduced in March 2005. It was withdrawn from the UK in 1998 on the recommendation of the European Medical Agency, due to an association with rare but potentially fatal liver failure.
  
  
•  Levodopa/carbidopa/entacapone (Stalevo)
  
  Introduced in November 2003, is a combination drug to treat Parkinson's, and contains levodopa, carbidopa and entacapone in one tablet.

To avoid any potential risks, only physicians experienced in the management of advanced Parkinson's will be able to prescribe the drug and patients must undergo regular medical checks and undergo blood tests to check liver function.

Advantages

When used with levodopa, COMT inhibitors can reduce the daily 'off' time and increase the 'on' time. In many cases, the levodopa dose and dosing frequency can also be reduced.

Disadvantages

Due to how these drugs work, there may be an increase in the side effects caused by levodopa, notably dyskinesias, nausea and vomiting. Where these side effects increase after starting the drug, people should raise the issue with their GP/consultant, as reducing the levodopa dose can often help. Be aware that other drugs (for Parkinson's or other conditions) can affect the action of COMT inhibitors, although the combination of apomorphine and entacapone needs careful supervision.

Other reported side effects of COMT inhibitors include abdominal pain, loose stools and diarrhoea. Also urine can become discoloured; this is simply due to the substances in the drug and is harmless.