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.

A review of the receptor-binding and pharmacokinetic properties of dopamine agonists.

BACKGROUND: Dopamine agonists (DAs), which can be categorized as ergot derived and non-ergot derived, are used in the treatment of Parkinson's disease. OBJECTIVES: This review describes the pharmacologic and pharmacokinetic properties of selected DAs and relates these characteristics to clinical outcomes, with an emphasis on adverse events. METHODS: Relevant articles were identified through a search of MEDLINE (to May 2006) using the terms dopamine agonists (or each individual drug name) and pbarmacokinetics, metabolism, drug-drug interaction, interactions, CYP450, fibrosis, valvular heart disease, tremor, clinical trials, reviews, and meta-analyses. Abstracts from recent sessions of the International Congress of Parkinson's Disease and Movement Disorders were also examined. Clinical studies with <20 patients overall or <10 patients per treatment group in the final analysis were excluded. All DAs that were graded at least possibly useful with respect to at least 3 of 4 items connected to the treatment/prevention of motor symptoms/complications in the most recent evidence-based medical review update were included. This resulted in a focus on the ergot-derived DAs bromocriptine, cabergoline, and pergolide, and the non-ergot-derived DAs pramipexole and ropinirole. RESULTS: Bromocriptine, cabergoline, pergolide, and ropinirole, but not pramipexole, have the potential for drug-drug interactions mediated by the cytochrome P450 (CYP) enzyme system. The occurrence of dyskinesia may be linked to stimulation of the dopamine D(1) receptor, for which cabergoline and pergolide have a similar and relatively high affinity; bromocriptine, pramipexole, and ropinirole have been associated with a lower risk of dyskinesias. The valvular heart disease (VHD) and pulmonary and retroperitoneal fibrosis seen with long-term use appear to represent a class effect of the ergot-derived DAs that may be related to stimulation of serotonin 5-HT(2B) (and possibly 5-HT(2A)) receptors. The incidence of valvular regurgitation was 31% to 47% with ergot-derived DAs, 10% with non-ergot-derived DAs, and 13% with controls. CONCLUSIONS: As reflected in the results of the clinical trials included in this review, dyskinesia associated with DA therapy may be linked to stimulation of the D(1) receptor. Fibrosis (including VHD) seemed to be a class effect of the ergot-derived DAs. Each of the DAs except pramipexole has the potential to interact with other drugs via the CYP enzyme system.