Development of the radiosynthesis of high-specific-activity I-123-NKJ64

Introduction: I-123-NKJ64, a reboxetine analogue, is currently under development as a potential novel single photon emission computed tomography radiotracer for imaging the noradrenaline transporter in brain. This study describes the development of the radiosynthesis of I-123-NKJ64, highlighting the advantages and disadvantages, pitfalls and solutions encountered while developing the final radiolabelling methodology.

Methods: The synthesis of I-123-NKJ64 was evaluated using an clectrophilic iododestannylation method, where a Boc-protected trimethylstannyl precursor was radioiodinated using peracetic acid as an oxidant and deprotection was investigated using either trifluoroacetic acid (TFA) or 2 M hydrochloric acid (HCl).

Results: Radioiodination of the Boc-protected trimethylstannyl precursor was achieved with an incorporation yield of 92+/-6%. Deprotection with 2 M HCl produced I-123-NKJ64 with the highest radiochemical yield of 98.05+/-1.63% compared with 83.95+/-13.24% with TFA. However, the specific activity of the obtained I-123-NKJ64 was lower when measured after using 2 M HCl (0.15+/-0.23 Ci/mu mol) as the deprotecting agent in comparison to TFA (1.76+/-0.60 Ci/mu mol). Further investigation of the 2 M HCl methodology found a by-product, identified as the deprotected proto-destannylated precursor, which co-eluted with I-123-NKJ64 during the high-performance liquid chromatography (H PLC) purification.

Conclusions: The radiosynthesis of I-123-NKJ64 was achieved with good isolated radiochemical yield of 68% and a high specific activity of 1.8 Ci/mu mol. TFA was found to be the most suitable deprotecting agent, since 2 M HCl generated a by-product that could not be fully separated from I-123-NKJ64 using the HPLC methodology investigated. This study highlights the importance of HPLC purification and accurate measurement of specific activity while developing new radiosynthesis methodologies. (C) 2011 Elsevier Inc. All rights reserved.