Dopamine Agonists
Dopamine agonists (DAs) stimulate the same brain centers as dopamine does. Because of this, DAs can be used instead of levodopa, or with levodopa to reduce the levodopa dose. Many physicians are now prescribing DAs as the first drug for Parkinson's disease, instead of beginning with levodopa.
There are currently seven DAs available throughout the world:
Apomorphine**
Bromocriptine (Parlodel®)
Cabergoline* (Dostinex®)
Lisuride** (Dopergine®)
Pergolide (Permax®)
Pramipexole (Mirapex®)
Ropinirole (Requip®)
* Not currently approved in the United States for the treatment of PD
**Not currently available in the United States
The various dopamine agonists differ in several respects, including the duration of action of a single dose. Each has slightly different side effects, and both side effects and drug efficacy may vary from person to person. DAs differ in their degree of stimulation of the various subtypes of dopamine receptors (for a more complete discussion of dopamine receptors and the properties of dopamine agonists, click here). Finally, some DAs are ergot-derived, while others are not.
Dopamine agonists are used to treat all the motor symptoms of Parkinson's disease. If a particular DA is not effective at a tolerable dose, another DA may be tried.
Side effects typical of all DAs include drowsiness, nausea, vomiting, dry mouth, dizziness, and orthostatic hypotension, or lowering of blood pressure upon standing. At higher doses, DAs may cause confusion, hallucinations, or psychosis.
Sleepiness and "Sleep Attacks" with Dopamine Agonists
Sleepiness, drowsiness, or sedation may be a significant side effect of some dopamine agonists in some people, and may interfere with driving or other activities. If you are taking a dopamine agonist, you may wish to consult with your physician about the risks of driving or other activities requiring high levels of alertness.
"Ergot" Versus "Non-ergot" Dopamine Agonists
Bromocriptine, cabergoline, lisuride, and pergolide have chemical structures based on ergot, a plant alkaloid produced by the fungus Claviceps purpurea. They are termed "ergot-derived" or "ergoline" DAs. In contrast, pramipexole and ropinirole are not chemically related to ergot, but have structures similar to dopamine, and are called "non-ergoline" DAs.
While side effects occur with all DAs (and indeed, with all medications of any type), ropinirole and pramipexole may lead to fewer of those side effects that are linked specifically to ergot compounds, such as decreased circulation to the extremities (digital vasospasm) and fibrous growth in the thoracic and abdominal cavities (pleuropulmonary and retroperitoneal fibrosis).
Early Montherapy with Dopamine Agonists
Dopamine agonists are increasingly being prescribed as the first drug for patients with Parkinson's disease, instead of levodopa. In many patients, DAs may be effective as monotherapy for several years, depending on the rate of disease progression. Recent clinical trials indicate that delaying levodopa therapy may delay the onset of dyskinesias. These findings suggest that DAs may be the most appropriate initial therapy for most patients, especially those with earlier onset who are expected to require drug therapy for many years.
