Synthesis, Properties, and Biology of Advanced H2S-Releasing Materials

Abstract

Hydrogen sulfide (H2S) is an endogenously produced signaling gas involved in numerous cellular functions. At the appropriate concentration, exogenous administration of this gasotransmitter regulates vasodilation, promotes angiogenesis of endothelial cells, and generally exhibits beneficial effects as an anti-inflammatory and antioperoxidative agent. H2S is also capable of acting as a gaseous chemotherapeutic agent. Therefore, the therapeutic potential of exogenous delivery of H2S is vast. The delivery of H2S is complicated by its gaseous nature. Under physiologically relevant conditions, H2S is rapidly depleted from solution by oxidation and/or degassing. Therefore, direct exogenous delivery is difficult. To date, most studies have employed Na2S as a convenient H2S source. However, the rapid surge in H2S concentration upon Na2S dissolution followed by its rapid decline poorly mimics the sustained production of low concentrations of H2S that occurs in biological systems. We synthesized a library of S-aroylthiooximes (SATOs)—H2S-releasing compounds that more aptly mimic in vivo H2S concentrations. SATOs are synthesized via reaction of a S-aroylthiohydroxylamine and an aldehyde or ketone. SATOs release H2S in response to a thiol functionality. H2S release from SATOs could be controlled, with H2S release half-lives on the order of minutes to hours. SATO chemistry was utilized to prepare H2S-releasing polymers. Copolymers prepared using RAFT polymerization could be functionalized with SATOs with conversions > 99%, and these polymers released H2S on a similar timescale to our small molecule donors, confirming the viability of SATO formation as a post-polymerization modification strategy. SATO-functionalized polymer amphiphiles were prepared that self-assembled into micelles or vesicles based on their composition. H2S was released from these polymer assemblies more slowly than from the small molecules and statistical polymers. These H2S-releasing micelles were employed in in vitro cytotoxicity studies. H2S released from the micelles was found to be selectively toxic to human colon cancer cells compared with healthy fibroblasts. These polymeric micelle donors outperformed existing H2S donors in terms of their toxicity towards cancer cells. The observed enhanced toxicity was suspected to arise from the slow and sustained release of H2S from the micelles.