Receptor-binding and pharmacokinetic properties of dopaminergic agonists

This review describes symptoms and pathophysiology of Parkinson's diseases (PD) and restless legs syndrome (RLS), and discusses the relationship between clinical outcome of DA agonists and their receptor-binding and pharmacokinetics. Oral DA agonists are divided into 2 classes; the ergots and the non-ergots. Both classes are in general equally effective against PD motor symptoms. Ergots (apart from bromocriptine) stimulate the DA D(1) subreceptor and increase dyskinesia. Furthermore, valvular heart disease (VHD) and pulmonary and retroperitoneal fibrosis appear to represent a class effect of 8beta-aminoergolines as cabergoline and pergolide The side effects profile therefore seems more beneficial for non-ergots than ergots. The main improvement of motor functions by DA agonists is related to D(2) agonism. However, in monotheraphy, the selective D(2)-receptor DA agonist sumanirole seemed less effective than ropinirole which is selective for D(2)-like DA-receptors (D(2), D(3) and D(4)). Given as adjunctive to L-dopa both drugs had equal efficacy on motor-symptoms, indicating that D(2)-receptor activity must be accompanied with stimulation of other DA receptors for optimizing the efficacy on motor symptoms. Striatal D(3) receptor loss may be more important than D(2) receptor loss for reduced response to dopaminergic treatment. D(3) stimulation may also be beneficial for the non-motor symptom depression/mood in PD and for neuron-protection. This makes D(3)-receptors a potential therapeutic target in PD. 5-HT(1A)-receptor agonism and alpha(2) adrenergic antagonism may contribute to prevention of dyskinesia. However, 5-HT-receptor activity is also associated with side effects. 5-HT(2B) agonism (and possibly 5-HT(1B) agonism) is associated with fibrotic reactions, and valvular heart disease (VHD). By interfering with the CYP450 system DA agonists may contribute to drug-drug interactions. Lack of CYP2D6 activity is also suggested as important for etiology and CNS-symptoms of PD. Based on current knowledge D2-like receptor activities (preferences for the D(3) receptor) seem most beneficial. 5-HT(1A)-receptor agonism (prevention of dyskinesia), 5-HT(2B) antagonism or no 5-HT(2B)-receptor activity also seems beneficial. Development of DA agonists containing these properties, without interfering with CYP2D6 may be beneficial.