Synthesis and evaluation of radioiodinated 6-iodo-L-DOPA as a cerebral L-amino acid transport marker.

Regioselective radioiodination of N-trifluoroacetyl 3,4-dimethoxy-6-trifluoroacetoxymercurio-L-phenylalanine ethyl ester 1 under no-carrier-added condition gave 6-[125I]iodo protected L-DOPA with a labeling efficiency of more than 85%, and no-carrier-added 6-[125I]I-L-DOPA was obtained with a radio-chemical purity of over 95% after hydrolysis and chromatography. A nonradioactive standard of 6-iodo protected L-DOPA was synthesized by the iododemercuration of 6-mercuric trifluoroacetate protected L-DOPA 1 using I2 in chloroform. 6-[125I]I-L-DOPA showed high brain accumulation and rapid blood clearance in mice. The rat brain slice studies indicated high affinity of 6-[125I]I-L-DOPA for carrier-mediated and stereoselective active transport systems. The tissue homogenate analysis revealed that most of the accumulated radioactivity was intact 6-[125I]I-L-DOPA. Thus, 6-[123I]I-L-DOPA appears to be a suitable single photon emission computed tomography (SPECT) tracer for the selective measurement of cerebral L-amino acid transport, having no affinity for dopamine metabolism.