Radiopharmaceutical preparation of L-6-[123I]-iodo-m-tyrosine a potential SPECT brain imaging agent

Abstract

A method for radiopharmaceutical preparation of L-6-[123I]-iodo-m-tyrosine a potential SPECT brain imaging agent is described. The method is based on direct electrophilic radioiodination of L-m-tyrosine with [123I] NaI/chloramine-T (CAT) and small amount of KI as a carrier at pH 1.0 where L-6-[123I]-iodo-m-tyrosine is the predominant isomer. A high radiochemical yield of L-6-[123I]-iodo-m-tyrosine up to 70% has been achieved by adding small amount of KI (0.001 μg) as a carrier to the reaction mixture. The pure 6-isomer was separated by reverse phase radio high pressure liquid chromatography (HPLC) on RP-18 column using 0.02M sodium acetate/ethanol (90∶10) adjusted to pH 3.9 with glacial acetic acid at a flow rate 2 ml/min. According to the signals of the UV detector (λ=254) the 6-isomer was eluted at a retention time 12.5 minutes,K′=6. The eluted fraction of L-6-[123I]-m-tyrosine pooled together, evaporated under reduced pressure, then dissolved in 5 ml isotonic phosphate buffer and sterilized by passing through 0.22 μm millipore filter. The sterile solution was now ready for nuclear medical applications. The biological distribution of L-6-[123I]-iodo-m-tyrosine in mice was studied. The results showed that 3% of the injected dose is taken up in dopamine rich striatum 30 minutes after injection and not in norepinephrine-rich hypothalamus.