Tellurium is a rare
element, which has been regarded as a toxic, non-essential
trace element; its
biological role, if any, has not been clearly established to date. The investigation of therapeutic activities of
tellurium compounds is rather limited in the literature, despite the relative abundance of
tellurium in the human body. Nevertheless, the varied activities of
tellurium agents in both malignant and normal cells are extremely exciting, though very complex. Not surprisingly, an increased interest in
tellurium among
biological chemists and pharmacists has fuelled the search for more and more diverse
tellurium compounds. The present review will focus on two small inorganic
tellurium complexes,
ammonium trichloro(dioxoethylene-O,O')tellurate (
AS101) and Octa-O-bis-(R,R)-tartarate ditellurane (SAS), thoroughly investigated by us, converging at their anti-
cancer properties, and elucidating their mechanism of action.
AS101 is probably the most extensively studied synthetic
tellurium compound from the standpoint of its
biological activity. It is a potent
immunomodulator (both in vitro and in vivo) with a variety of potential therapeutic applications. It is probably the only
tellurium compound to be tested in phase I/II clinical studies in
cancer patients. The effects of
AS101 and SAS are primarily caused by their specific Te(IV) redox-modulating activities enabling the inactivation of
cysteine proteases such as
cathepsin B, inhibition of specific
tumor survival
proteins like
survivin, or obstruction of
tumor IL-10 production. All of these have profound consequences regarding anti-
tumor activity or sensitization of
tumors to
chemotherapy. These properties, coupled with the excellent safety profile of the compounds, suggest promising anti-
cancer therapeutic potential for
tellurium compounds such as
AS101 or SAS.