99mTcsestamibi (99mTcTc(MIBI)6+; MIBI = 2-methoxybutylisonitrile) is a clinically established myocardial perfusion SPECT tracer. Its one-pot kit-based synthesis from 99mTcpertechnetate (99mTcTcO4−) is complex, involving a 6-oxidation state transition (Tc(VII) to Tc(I)) and complete ligand replacement. We aimed to unravel this complex reaction, to inform rational quality control and identify new technetium synthons for molecular imaging. Generator-produced 99mTcpertechnetate was added to commercial or bespoke clinically used kits, varying the reaction time, temperature, and concentrations of reagents (individually and collectively) and carrier technetium-99. Radioactive products were analysed by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) with optical, radiometric, and mass spectrometric (MS-ESI+) detection. At least 11 radioactive intermediates were detected by radio-HPLC. Technetium(V) and technetium(I) intermediates were identified or imputed by radio-HPLC-MS, including TcVO(cysteinate)2+, TcI(MIBI)4L2+, and TcI(MIBI)5L1+ (L = labile monodentate ligand, e.g., H2O). Tc(III) intermediates TcIII(cysteinate)2(MIBI)+ and TcIII(cysteinate)2(MIBI)2+ were indicated by weak MS-ESI+ ions. We conclude that the reaction proceeds via reduction from TcVIIO4− via unknown intermediates to TcVO(cysteinate)2+, then via Tc(III) intermediates containing both cysteinate and MIBI ligands (e.g., TcIII(cysteinate)2(MIBI)2+), to form Tc(I) without cysteine and with <6 MIBI ligands, followed by further ligand displacement by MIBI to form Tc(MIBI)6+. Once formed, Tc(MIBI)6+ undergoes no further reaction.
Machado et al. (Mon,) studied this question.