Direct n.c.a. electrophilic radioiodination of tyrosine analogues; their in vivo stability and brain-uptake in mice

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Abstract

In order to improve tracers for amino acid transport studies with SPET we have radioionated methylated tyrosines and compared their brain uptake and in vivo deiodination in mice. O-methylation not only leads to a higher lipophilicity and hence significantly higher brain uptake with a maximum of 5% dose/g for 3-[123I]iodo-O-methyl-l-α-methyltyrosine (OMIMT) but also significantly prevents in vivo deiodination.

High n.c.a. radioiodination yields (⩾80%) are obtained for the activated aromatic compounds l-tyrosine and l-α-methyltyrosine using Iodo-gentm in a heterogeneous aqueous system. Direct n.c.a. radioiodination of the less-activated O-methyl analogues has been achieved in reasonable yields (60%) with Iodo-gentm in homogeneous TFA solutions containing about 10% of water.

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    After extensive experimental trials with different radioiodination methods, we successfully radiolabeled rotenone with 125I and 123I using the direct non-carrier-added electrophilic radioiodination approach with iodogen in trifluoroacetic acid (TFA) solvent. The method was developed for radioiodination of less activated anisole (methoxy benzene) and O-methyl-tyrosine derivatives.31,32 The use of a homogeneous iodogen/TFA/water system allows the direct electrophilic radioiodination of anisole derivatives, which is almost impossible by the classical chloramine-T or iodogen method in aqueous solution.

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