18F-Dopa vs dopamine transporter ligands in positron emission tomographic and single-photon emission computed tomographic scans for Parkinson disease.

W e read with interest the article by Ribeiro et al. Although the F-dopa positron emission tomographic (PET) scan is the current imaging gold standard for the diagnosis of early Parkinson disease (PD) and for measuring disease progression in clinical trials, the authors concluded that the ligand, Br-FE-CBT, that binds to the dopamine transporter (DAT) may be more suitable for detecting cases of early PD and assessing disease progression than F-dopa. This is because F-dopa uptake depends not only on the density of dopaminergic terminals in the striatum but also on dopamine turnover, which may be compensatorily increased in early PD. Thus, F-dopa uptake might overestimate the number of striatal dopaminergic nerve terminals in these patients. In support of this theory, we recently described 2 patients who had typical early PD, both with unilateral rest tremor, bradykinesia, and rigidity. They had a positive response to dopamine agonists and underwent F-dopa PET and [I]-altropane single-photon emission computed tomographic (SPECT) scans within 3 months. Like -CIT and Br-FE-CBT, altropane when radiolabeled becomes a ligand that specifically binds to the DAT. In these 2 patients, both PET scans were nondiagnostic, but their SPECT scans showed clear, unilateral striatal reduction of tracer uptake contralateral to their parkinsonian side. Recently, a double-blind F-dopa PET study was conducted that compared disease progression among patients with early, untreated PD who were given ropinirole hydrochloride or levodopa. Of 186 subjects, 11% were excluded from the primary analysis because of “normal” striatal F-dopa uptake on their baseline PET scans and were therefore thought not to have PD. Would a ligand with high affinity to the DAT have prevented their exclusion?