Dopamine agonists in Parkinson's disease: a look at apomorphine

Summary— Apomorphine is a D1 and D2 dopamine receptor agonist with anti‐parkinsonian properties qualitatively similar to those seen with l‐dopa. It was first used in the treatment of Parkinson's disease by Schwab in the 1950s but owing to its short duration of action, the need for parenteral administration, and adverse reactions including nausea, vomiting, postural hypotension and sedation, it was not widely prescribed. In the early 1970s, Cotzias confirmed its potent anti‐parkinsonian effects and that some of its secondary effects were diametrically opposite to those seen with l‐dopa. The advent of peripheral dopamine receptor antagonist drugs, which counteract the unwanted effects of apomorphine, and the development of new drug delivery systems including insulin pens and ambulatory mini pumps have led to the resurrection of apomorphine for the treatment of Parkinson's disease. Over the last five years in Europe, the drug has proved to be a major advance in the treatment of refractory “on‐off” oscillations in Parkinson's disease. It has also been used as a diagnostic test for dopaminergic responsiveness in Parkinson syndromes and tremors of uncertain aetiology. The drug has also proved particularly useful in dealing with certain “off‐period” disabilities, including pain, bladder dysfunction, dystonia and gastro‐intestinal symptoms. Continuous steady state infusion of apomorphine by mini‐pump may reduce the severity of “on” phase dyskinesias over time. The drug has also proved useful in the clinical pharmacological investigation of the pathophysiology of the motor response to dopaminergic drugs in Parkinsons's disease and the occurrence of involuntary movement sequences. Neuropsychiatry side‐effects are relatively infrequent when compared with ergolene dopamine agonists. Studies with apomorphine raise the hope that it may be possible to develop a long acting, orally active dopamine receptor agonist with therapeutic effect comparable to that observed with l‐dopa and with fewer long term adverse reactions.