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

In order to improve tracers for amino acid transport studies with SPET we have radioiodinated 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-gen 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-gen in homogeneous TFA solution containing about 10% of water. (author)