Disulfide and multisulfide antitumor agents and their modes of action

A variety of natural products that contain disulfide or multisulfide bonds were found to display potent biological activities, including antitumor activities. At the center of these biological activities are disulfide or multisulfide moieties. The importance of disulfide or multisulfide groups in the areas of chemistry, biology, and pharmacology has been well recognized. Among these agents, especially noteworthy are mitomycin disulfides, leinamycin, thiarubrines, varacins, calicheamicins, and esperamicins. Their general features, including their biological profiles, peculiar structures, and related chemistries, were summarized and more importantly, their working mechanisms were elucidated in detail in this review. Mechanistic studies of these compounds have provided evidence of the key role of disulfide or multisulfide groups. In general, the cleavage of disulfide or multisulfide bonds produces thiol (or thiolate), which triggers an activation cascade leading to the generation of highly reactive electrophile(s) or cytotoxic species that may cause DNA strand scission. The main concerns with the mode of action are the reactivity and stability of disulfide and multisulfide bonds, their cleavage conditions, and the generation of toxic species. A range of studies for each agent was executed to gather important information on their activation, and the obtained information was gradually integrated to give some clues to the agents' working mechanisms. Such information may be further used to generate biomechanistically designed and more potent derivatives.