D(1) dopamine receptor agonists are more effective in alleviating advanced than mild parkinsonism in 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine-treated monkeys.

Selective D(1) dopamine receptor agonists exert antiparkinsonian effects in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model of Parkinson's disease and in human Parkinson's disease. Motor impairment in idiopathic Parkinson's disease progresses from mild to severe, but the therapeutic potential of D(1) dopamine receptor agonists in early and advanced stages of parkinsonism is not known. To compare the effectiveness of D(1) agonists at different levels of impairment, we developed a model of mild and advanced parkinsonism in nonhuman primates and a rating scale that differentiated the two models. D(1) dopamine receptor agonists (SKF 81297, dihydrexidine) and D(2) dopamine receptor agonists [quinelorane, (+)-PHNO were administered to monkeys (Macaca fascicularis) displaying either mild parkinsonism (two doses of 0.6 mg/kg i.v. MPTP 1 month apart) or advanced parkinsonism (three doses of 0.6 mg/kg i.v. MPTP within 10 days). In normal monkeys (n = 3), SKF 81297 and dihydrexidine did not promote increased motor activity. In advanced parkinsonism (n = 4), D(1) and D(2) dopamine agonists effectively reversed the motor deficits. In contrast, the therapeutic benefits of D(1) agonists SKF 81297 and dihydrexidine were relatively limited in mild parkinsonism (n = 4). The D(2) agonists quinelorane and (+)-PHNO alleviated some symptoms in mild parkinsonism but also reduced balance and induced more dyskinesias than did D(1) agonists. Mild and advanced parkinsonism in nonhuman primates can be produced with fixed dosing regimens of MPTP. Based on the therapeutic efficacy and side effect profiles derived from these models, D(1) agonists are more promising for the treatment of advanced than of mild Parkinson's disease.