Towards Targeted and Tunable Release of Hydrogen Sulfide

Abstract

Hydrogen sulfide (H2S) has emerged as an important gaseous signaling molecule with diverse physiological roles. Endogenously produced H2S regulates the homeostasis of various physiological processes such as cardiovascular, neuronal, renal, gastrointestinal etc. Due to its increased physiological relevance, the role of H2S as a potential therapeutic agent has been extensively studied. For example, diminished levels of H2S are associated with various pathological conditions such as inflammation, gastrointestinal disorders, cardiovascular disorders etc. Exogenous administration of H2S under these conditions has shown beneficial effects. However, the therapeutics of H2S largely depends on the concentration and the rate at which it is produced. Therefore, methods for achieving controlled generation and dissipation of H2S assume importance. Also, modulating the rate of H2S release is needed for better understanding its implications in a diseased state. Due to its gaseous nature, reliable intracellular enhancement of H2S is challenging. Although numerous methodologies for H2S delivery have been reported in the past, but they suffer from limitations such as lack of specificity, no structural handle to tune the rate of release and lack of well-defined negative controls. Here we describe synthesis and evaluation of site directed and tunable H2S donors with potential therapeutic applications. Carbonyl sulfide (COS) is a naturally occurring gas that is hydrolyzed to H2S by a widely prevalent enzyme, carbonic anhydrase (CA). This work focuses on controlled generation of COS in response to various biological stimuli. First, esterase activated COS/H2S donors were designed and synthesized. In order to study the effects on rate of COS release after activation by esterase, carbonothioates (containing alcohols as leaving groups) and carbamothioates (with amines as leaving groups) were developed. The donors upon activation by esterase released COS which was further hydrolyzed to H2S by CA. The rate of H2S release from these donors was found to be dependent on the basicity of the amine. Due to the established roles of hydrogen sulfide as a protective agent against oxidative stress, the next class of COS donors were synthesized to be activated by elevated hydrogen peroxide, a condition that is frequently encountered during oxidative stress. In the presence of hydrogen peroxide these compounds decomposed and released H2S. The half-lives of H2S release from these donors ranged from 24 min to 208 min. Cellular assays revealed the anti-inflammatory effects of these donors by depletion of ROS levels. Gastrointestinal tract (GIT) is one of the most affected systems by excessive usage of non-steroidal anti-inflammatory drugs (NSAIDs) that cause erosions and further leads to peptic ulcers. H2S in conjugation with NSAIDs shows a remarkable increase in the efficacy of the drug and also protects the tissues against NSAID induced damage. Therefore, in the subsequent chapter, colon targeted delivery of H2S and H2S – NSAID hybrid donors was proposed. NAD(P)H quinone oxidoreductase (NQO1) is a 2e- reducing cytosolic enzyme over expressed in tissues such as colon epithelium which catalyzes the reduction of quinone to hydroquinone, thereby acting as a detoxifying agent in cells. Therefore, NQO1 activated H2S and H2S-NSAID hybrid donors were synthesized. The compounds were selective towards activation by NQO1 and showed cytoprotective effects against xenobiotic induced stress. The cytoprotective effects of H2S were dependent on the extent of persulfidation induced by these donors. Thus, a correlation between the persulfidating ability and cytoprotective effects of the donors was established. Since the effects of H2S were found to be dependent on the persulfidation ability of the donors, therefore, in the last chapter, NQO1 activated persulfide donors were synthesized and their cytoprotective effects were evaluated. In summary, our results address important challenges associated with site directed delivery and tunable release of H2S. Targeted delivery of H2S, H2S-NSAID hybrid and persulfide donors using the molecules that we have developed herein is possible.