RADIOPHARMACEUTICAL PREPARATION OF L-6-[I-123]-IODO-M-TYROSINE, A POTENTIAL SPECT BRAIN IMAGING AGENT

A method for radiopharmaceutical preparation of L-6-[I-123]-iodo-m-tyr osine a potential SPECT brain imaging agent is described. The method i s based on direct electrophilic radioiodination of L-m-tyrosine with [ I-123] NaI/chloramine-T (CAT) and small amount of KI as a carrier at p H 1.0 where L-6-[I-123]-iodo-m-tyrosine is the predominant isomer. A h igh radiochemical yield of L-6-[I-123]-iodo-m-tyrosine up to 70% has b een achieved by adding small amount of KI (0.001 mu g) as a carrier to the reaction mixture, The pure 6-isomer was separated by reverse phas e radio high pressure liquid chromatography (HPLC) on RP-18 column usi ng 0.02M sodium acetate/ethanol (90:10) adjusted to pH 3.9 with glacia l acetic acid at a flow rate 2 ml/min. According to the signals of the UV detector (lambda=254) the 6-isomer was eluted at a retention lime 12.5 minutes, K'=6. The eluted fraction of L-6-[I-123]-m-tyrosine pool ed together, evaporated under reduced pressure, then dissolved in 5 ml isotonic phosphate buffer and sterilized by passing through 0.22 mu m millipore filter. The sterile solution was now ready for nuclear medi cal applications. The biological distribution of L-6-[I-123]-iodo-m-ty rosine 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.