Unveiling the mechanism of activation of the Te(IV) prodrug AS101. New chemical insights towards a better understanding of its medicinal properties

AS101 (Ammonium trichloro (dioxoethylene-O,O') tellurate) is an important hypervalent Te-based prodrug. Recently, we started a systematic investigation on AS101 with the aim to correlate its promising biological effects as a potent immunomodulator drug with multiple medicinal applications and its specific chemical properties. To date, a substantial agreement on the rapid conversion of the initial AS101 species into the corresponding TeOCl₃^- anion does exist, and this latter species is reputed as the pharmacologically active one. However, we realized that TeOCl₃^- could quickly undergo further steps of conversion in an aqueous medium, eventually producing the TeO₂ species. Using a mixed experimental and theoretical investigation approach, we characterized the conversion process leading to TeO₂ occurring both in pure water and in reference buffers at physiological-like pH. Our findings may offer a valuable "chemical tool" for a better description, interpretation -and optimization- of the mechanism of action of AS101 and Te-based compounds. This might be a starting point for improved AS101-based medicinal application.