Brain Imaging to Assess the Effects of Dopamine Agonists on Progression of Parkinson Disease

To the Editor: Based on the results of SPECT imaging, Dr Marek and colleagues¹ discussed a possible "neuroprotective" effect of dopamine receptor agonists. Inspection of Figure 3 and the data in Table 2, however, suggest that the rates of decline of DAT binding in levodopa and pramipexole groups from 22 to 46 months have identical slopes. We found that 2-way analysis of variance, using the data from Table 2, demonstrated significant differences between the levodopa and pramipexole treatment groups but no interactions between time and treatment, confirming lack of difference in the rates of decline of DAT binding between the 2 groups. This is consistent with the identical clinical ratings of both groups at 46 months and argues against differential disease-modifying effect of the treatment arms. A true neuroprotective effect should produce diverging rates of decline with the effective treatment exhibiting slower rate of progression.² In the study of Marek et al,¹ only the slopes of DAT binding between initial scans (unmedicated baseline) and the second scan (first onmedication scan) are significantly different. This could be attributable to differential effects of levodopa and pramipexole on DAT binding.