Radiopharmaceutical Development of 123I-(+)-3-Iodo-MK 801 for Nmda-Receptor Scintigraphy

125I-(+)-3-iodo-MK 801 exhibited high affinity for the NMDA-receptor in-vitro, thus we aimed to develop the 123I-analog as a potential in-vivo tracer. 3-trimethyl-silyl-MK-801 was used as precursor for electrophilic iodo-desilylation but no 123I-MK 801 was formed in ordinary aqueous acid conditions. In non-aqueous trifluoro-acetic acid using chloramine-T as oxidant up to 60% 123I-MK 801 was obtained within 5 min at 25 °C. It was isolated from the reaction mixture by preparative HPLC. Specific activity was ~ 103 Ci/mmol, Radiochemical purity, determined by analytical HPLC and TLC was > 97% and remained stable for > 20 hours. In-vivo distribution in tissues and brain regions of rats 4 hrs p.i. showed highest concentration in frontal cortex at 0.1% dose/g, reflecting NMDA-receptor distribution.