DEVELOPMENT OF NOVEL RADIOPHARMACEUTICALS FOR EARLY STAGE DIAGNOSIS OF CEREBRAL NEURODEGENERATION.

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We studied the PET tracer distributions of ligands for dopamine D_1 receptors ([^<11>C]SCH23390) and D_2 receptors ([^<11>C]raclopride) and of the dopamine precursor analog 6-[^<18>F]fluoro-L-3,4-dihydroxyphenylalanine ([^<18>F]FDOPA) in the brain after 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle in rats. The number of methamphetamine-induced rotation was higher at 14 days than that at 3 days following the 6-OHDA lesions. The brains of 6-OHDA treated rats were analyzed by tissue dissection following i.v.bolus of each tracer at 3 days (acute stage) or 3 weeks (chronic stage) postlesion. [^<11>C]Raclopride, but not [^<11>C]SCH23390,showed a higher accumulation in the striatum on the lesion side compared with that on the non-lesioned (intact) side both 3 days and 3 weeks postlesion. On the other hand, a lower accumulation of [^<18>F]FDOPA was found in the striatum on the lesion side 3 days postlesion, and both in the striatum and cerebral cortex on the lesion side 3 … More weeks postlesion. Our studies demonstrate that an increase in [^<11>C]raclopride and a decrease in [^<18>F]FDOPA uptake in the denervated striatum is evident even at 3 days following the 6-OHDA lesions when the degree of methamphetamine-induced rotation is not established. Therefore the combination of a D_2 antagonist and FDOPA may provide a potentially useful method for assessing the effects of dopamine depletion in Parkinson's disease.For SPECT application, radioiodinated derivatives of L-tyrosine, dopamine biosynthesis precursor, were evaluated as diagnosis agents for early detection of neurodegeneration. 3-[^<123>I]iodo-α-methyl-L-tyrosine (I-IMT) and 3-[^<123>I]iodo-α-difluoromethyl-tyrosine methyl ester (I-FTM) were selected as SPECT imaging agents for cerebral amino acid transpoter and dopa decarboxylase (DDC) activity. In the biodistribution study, I-IMT accumulation in the striatum on the lesion side 3 days postlesion increased, then decreased in 2 weeks postlesion, compared with the intact side. On the other hand, I-FTM showed a lower accumulation in the striatum on the lesion side compared with that on the intact side both 3 days and 2 weeks postlesion. It shows that I-FTM, with an affinity to the final enzyme of dopamine biosynthesis, was sensitive for detection of neurodegeneration in acute stage. Thus, both I-IMT and I-FTM may be new candidates of radiopharmaceuticals that can be useful in assessing function of cerebral dopaminergic neuron. Less