Dopamine agonists

How they work

Dopamine agonists are a class of drug that mimic the effects of the brain chemical dopamine. These agonists stimulate nerve cells and produce similar effects to the natural dopamine found in our brains.

Examples

Commonly prescribed dopamine agonists are:

• pramipexole dihydrochloride (Mirapexin®)
• ropinirole hydrochloride (Requip®)
• pergolide mesilate (Celance®)
• bromocriptine mesylate (Parlodel®)
• cabergoline (Cabaser®, Dostinex®)
• rotigotine (Neupro®)
• apomorphine hydrochloride (APO-go®)
  

Why they are used

Dopamine agonists may be used alone or in combination with levodopa to relieve the motor symptoms of Parkinson's.

People who are newly diagnosed with Parkinson's may be prescribed a dopamine agonist alone. This occurs frequently when the person is younger than 60 years of age.

Agonists can also be given in combination with levodopa therapy. This often occurs in people with more advanced Parkinson's, whose response to levodopa begins to fluctuate, and whose motor symptoms are not well-controlled.

Taking levodopa for a long time often leads to motor complications that are difficult to manage. Therefore, by commencing treatment with a dopamine agonist instead of levodopa, the onset of levodopa-related motor problems can be postponed.

How they are taken

A variety of dopamine agonists are currently available. These differ in what form they are delivered, how long their effects last, and potential side effects.

Most agonists are taken by mouth as tablets or capsules for up to several times a day. There is also a patch for rotigotine that is applied once daily onto the skin. Apomorphine is available as a single injection or as a continuous infusion under the skin.

Advantages

The effects of dopamine agonists tend to last longer than levodopa. People on agonists are less prone to side effects such as dyskinesias (uncontrolled movements) and 'on/off' fluctuations.

Agonists are delivered in a range of different forms including: tablets/capsules; skin patch; injection and continuous supply under the skin.

Disadvantages

When agonists are used alone, they can be less effective than levodopa at controlling the motor symptoms of Parkinson's.

Dopamine agonists need to be carefully introduced to minimise their side effects. Agonists are therefore started at a low dose and increased gradually until the best benefits are achieved.

Side effects

Common side effects of dopamine agonists include:

• nausea and vomiting
• drowsiness and sudden 'attacks' of sleepiness
• dizziness or fainting due to low blood pressure
• hallucinations and confusion
• existing dyskinesias may become more troublesome

Compulsive behaviour

There is growing evidence that suggests a link between certain dopamine agonists and compulsive behaviour.

Compulsive behaviour is when a person is unable to resist an impulse or temptation such as gambling, shopping, eating and sex. This behaviour performed repetitively, excessively or obsessively may cause harm to the individual or others.

Studies have shown that compulsive behaviour is an uncommon side effect. It is currently estimated that up to 14% of people with Parkinson's taking dopamine agonists experience some form of compulsive behaviour to varying degrees.

There is also the possibility that some people may become addicted to their dopamine agonists. These people take increasing doses despite experiencing side effects.

Read more on compulsive behaviour

Heart valve and lung problems

Recent studies have shown that people taking pergolide (Celance®) or cabergoline (Cabaser®, Dostinex®) have a higher risk of heart valve and lung problems than people taking other dopamine agonists.

Both pergolide and cabergoline are part of a sub-class of drugs called ergolines. These drugs cause a condition called fibrosis in various organs of the body. Fibrosis is the hardening of tissues that can make the organs not work properly.

In 2007, the US Food and Drug Administration (FDA) announced that manufacturers of pergolide would voluntarily withdraw these drugs from the market due to concerns over its effect on heart valves. Pergolide is still available in the UK, however, the use of the drug is restricted and people are carefully monitored for adverse side effects.

Apomorphine

Apomorphine is a dopamine agonist that is administered by (subcutaneous) injection under the skin. When sudden fluctuations in symptoms occur, in spite of all efforts to adjust medication, injections of apomorphine may be used - almost like a 'rescue remedy' - between doses of a person's usual tablets.

Many people self-inject intermittently with a syringe or a ready-loaded disposable APO-go pen. In some cases, a more continuous supply of apomorphine is required which is administered via a pump. This portable, battery-driven syringe pump works by the insertion of a needle under the skin. It can be injected into the outside thigh, or into the stomach (below the navel).

Advantages

It acts very quickly and reliably so that people who need to be active at specific times can continue with their normal activities.

It can bring great relief to people who are experiencing extreme variations in mobility and prolonged 'off' periods.

Disadvantages

Apomorphine can only help those people with Parkinson's who show a response to Sinemet® or Madopar®. It can cause nausea and therefore another drug called domperidone (Motilium®), which acts specifically against nausea and sickness, has to be taken in addition, at least while apomorphine is being introduced.

More information can be found on our Apomorphine information sheet, available to download from our Publications section