Advancements in Light-Activated Hydrogen Sulfide (H₂S) Delivery for Therapeutic Applications

Introduction

Delivery of hydrogen sulfide has emerged as a promising strategy for regulating inflammation in various pathological processes. Chronic diabetic wound as one of the severe complications requires specific and coordinated wound care, which remains an on-going challenge. Recently, a novel controllable H₂S donor named PRO-F has been recently developed to address persistent inflammation and counteract cell damage caused by Reactive Oxygen Species (ROS), which aims to promote wound healing effectively.

Description

The objective

The management of chronic diabetic wounds poses a significant challenge due to persistent inflammation and the accumulation of ROS, resulting in oxidative stress. Recently, several clinical approaches have been developed to promote wound healing, including cell therapy, advanced wound dressing, and targeted drug delivery systems. However, among these approaches, the direct modulation of inflammation and regulation of redox homeostasis process using signalling molecules hold immense promise in improving wound conditions.

Sulfur-rich waters, with Hydrogen Sulfide (H₂S) concentrations ranging from 0.3 to 8 mM, has long been employed as a folk medicine procedure for the effective treatment and alleviation of various skin conditions, including wound healing. Recent advancements in understanding the signal pathway of H₂S with cells have brought forth valuable insights into the underlying mechanism involved. Notably, this includes the distinct effect it has on intracellular signalling proteins and transcription factors, which play a pivotal role in resolving inflammation and safeguarding cells against damage. Consequently, the modulation of H₂S levels with cells represents a promising avenue for gaining a better comprehension of H₂S's mechanisms of action, and it may even lead to innovative therapeutic interventions.

Multiple strategies have been devised to modulate cellular H₂S levels, encompassing selective knockout of H₂S producing enzymes, employment of enzyme-related inhibitors or stimulators, and administration of synthetic H₂S donors. Among these approaches, H₂S donors are a series of compounds that can release H₂S directly or under specific stimulus, thereby altering the local concentration. A crucial consideration lies in mimicking the endogenous production of H₂S while achieving precise spatio-temporal resolution in delivery. Furthermore, the inclusion of a real-time tracking system for monitoring the release process would represent a favourable choice.

The solution

We present the development of a photo-activated H₂S donor, PRO-F, which containing a photo removable group as the cage component and a thiocarbonate core as the source of sulfur. Furthermore, a Near-Infrared (NIR) fluorescence reporting system has been integrated into PRO-F. The O-nitrobenzyl group has been carefully chosen as the trigger for the donor activation, providing selective responsiveness to endogenous substance while ensuring efficient photolysis. Notably, light, as a non-invasive stimulus, can be precisely controlled, thereby minimizing disturbances to the cellular environment and facilitating the achievement of high spatio-temporal resolution delivery. In addition to the activation strategy, careful consideration has been given to the production pathway of H₂S. By utilizing Carbonyl Sulfide (COS) as a precursor to H₂S, instead of directly generating H₂S, we have sought to mimic the endogenous production pathway. In the presence of carbonic anhydrase, a ubiquitous mammalian enzyme, COS can be rapidly converted into H₂S. Importantly, the accompanying fluorescent signal allows for the tracking the H₂S release process.

Cell imaging studies have demonstrated the successful delivery H₂S using PRO-F in a cellular environment under light activation. The tracking of release process, verified through changes in the fluorescence signal, further supports the observation. Since H₂S has multiple cellular protective effects, we initially investigated the ROS scavenging ability of PRO-F in cells, which serves as the foundation for its antioxidant properties. Additionally, in an experiment examining PMA-induced apoptosis, the anti-apoptotic potential of PRO-F was observed. In its dark state, PRO-F showed minimal inflammation resolution effects. However, upon activating, the levels of inflammatory factors were down-regulated in a concentration-dependent manner. We attribute this effect to the slow and continuous release profile of PRO-F. With these findings established, we sought to determine whether PRO-F would perform similarly effects in an in vivo setting. In a mouse model of diabetes induced by intraperitoneal injection of streptozotocin, PRO-F demonstrated promising results in accelerating wound healing, comparable to the reported angiogenesis and collagen deposition-promoting effects of simvastatin. These findings underscore the therapeutic significance of PRO-F in inflammation-related conditions.

Future direction

Our findings highlight the advantageous effects of delivering the gasotransmitter, H₂S, for improving inflammation conditions and mitigating oxidative stress-induced damage in chronic wound healing. The incorporation of a fluorescent report system enables real-time monitoring, thereby facilitating the potential application of H₂S donors in vivo. While the debate surrounding the physiological role of H₂S has diminished, it is worth noting that the release profile of H₂S donor also plays a crucial role in H₂S-based therapeutics. However, the lack of tunability and necessary control compounds impedes further exploration of the physiological effects of H₂S. To address these limitations, our laboratory is currently engaged in the development of a library of light-activated H₂S donor with tunable release profile. We believe that expanding the repertoire of practical H₂S donors will provide valuable insights into the mechanisms underlying H₂S's cellular signaling activity.

Conclusion

A major concern is the potential photo damage to tissue associated with the use of ultraviolet light for delivery. Moreover, the limited tissue penetration restricts the application of H₂S-based therapeutics to superficial areas, such as the skin or superficial tumors. Employing near-infrared activation may be offer a more promising approach to overcome these disadvantages. Additionally, exploring alternative activation methods with good remote-controllable and high precision delivery is an area of great interest.