Dopamine receptor agonists in current clinical use: comparative dopamine receptor binding profiles defined in the human striatum

The aim of this study was to compare dopamine receptor binding affinities of all currently approved dopamine receptor agonist treatments for Parkinson's disease (PD) in human brain tissue. alpha-Dihydroergoeryptine and lisuride displayed higher comparative affinities (K-i = 35.4 and 56.7 nM, respectively) for D-1 receptors, than the D-1/D-2 dopamine agonist pergolide (Ki = 447 nM). The second generation non-ergot dopamine receptors agonists pramipexole and ropinirole demonstrated no affinity for D, receptors at concentrations up to 10(-4) M. The ergoline dopamine agonists cabergoline and lisuride displayed the highest affinities for the D-2 receptor (K-i = 0.61 and 0.95 nM, respectively). Surprisingly, the second generation non-ergot doparnine receptors agonists pramipexole and ropinirole only weakly inhibited binding to D2 receptors (K-i = 79.5 and 98.7 muM, respectively using [H-3]raclopride). Interestingly we also found that the affinities of cabergoline (K-i = 1.27 nM), lisuride (K-i = 1.08 nM) and pergolide (K-i = 0.86 nM) for the D-3 receptor subtype were comparable to that of pramipexole (K-i = 0.97 nM). The present results thus support the hypothesis that the antiparkinsonian effect of dopamine receptor agonists is mediated by a more complex interactions with dopamine receptor subtypes than currently believed.