Detoxication of sulfur half-mustards by nucleophilic scavengers: Robust activity of thiopurines

Sulfur mustard (bis-(2-chloroethyl)sulfide) has been used in chemical warfare since World War I and is well known as an acutely toxic vesicant. It has been implicated as a carcinogen after chronic low-level exposure and is known to form interstrand cross-links in DNA. Sulfur and nitrogen mustards are currently of interest as potential chemical threat agents for terrorists because of ease of synthesis. Sulfur mustard and monofunctional analogues (half-mustards, 2-[chloroethyl] alkyl sulfides) react as electrophiles, damaging cellular macromolecules, and thus are potentially subject to scavenging by nucleophilic agents. We have determined rate constants for the reaction of four purine derivatives that contain nucleophilic thiol moieties with several sulfur-half-mustards. Three of these compounds, 2,6-dithiopurine, 2,6-dithiouric acid, and 9-methyl-6-mercaptopurine, exhibit facile reaction with the electrophilic mustard compounds. At near neutral pH, these thiopurines are much better nucleophilic scavengers of mustard electrophiles than other low molecular weight thiols such as N-acetyl cysteine and glutathione. Progress curves calculated by numerical integration techniques indicate that equimolar concentrations of thiopurine provide significant reductions in the overall exposure to the episulfonium ions, which are the major reactive, electrophiles produced when sulfur mustards are dissolved in aqueous solution.