Understanding the radiolabelling mechanism of Tc–antimony sulphide colloid

The chemistry of antimony trisulphide colloid (ATC) was examined to elucidate the radiolabelling mechanism with ^99mTcO₄⁻. Ion exchange chromatography and atomic absorption spectrophotometry techniques determined ATC to be resistant to hydrolysis in 0.1 M hydrochloric acid (HCl) at 25°C or 100°C (>97% recovery, Sb³⁺ absent). Hydrogen sulphide gas detected did not participate in the mechanism, where antimony trisulphide and ^99mTcO₄⁻ in HCl/100°C yielded 96% ^99mTc-product from a K₂S-free formulation (versus 98% when K₂S was present). ^99mTcO₄⁻ was reduced >90% by DMSA or dithiothreitol under the same conditions, identifying involvement of thiol groups. Infrared analysis of Re-ATC showed S=O bonds, indicating excess thiol groups at the colloid surface were oxidised at the expense of ^99mTcO₄⁻ reduction.