Controlled activation is a critical component in prodrug development. Herein we report a concentration-sensitive platform approach for bioorthogonal prodrug activation by taking advantage of reaction kinetics. Using two “click and release” systems, we demonstrate enrichment and prodrug activation specifically in mitochondria to demonstrate the principle of this approach. In both cases, the payload (doxorubicin or carbon monoxide) was released inside the mitochondrial matrix upon the enrichment-initiated click reaction. Furthermore, mitochondria-targeted delivery yielded substantial augmentation of functional biological and therapeutic effects in vitro and in vivo, as compared to controls that did not result in enrichment. This method is thus a platform for targeted drug delivery amenable to conjugation with a variety of molecules and not limited to cell-surface delivery. Taken together, these two click and release pairs clearly demonstrate the concept of enrichment-triggered drug release and critical feasibility of treating clinically relevant diseases such as acute liver injury and cancer.

This review summarizes recent developments in using bioorthogonal chemistry in prodrug design for the delivery of traditional small molecule- and gasotransmitter-based therapeutics.

Prodrugs that release hydrogen sulfide upon esterase-mediated cleavage of an ester group followed by lactonization are described herein. By modifying the ester group and thus its susceptibility to esterase, and structural features critical to the lactonization rate, H2S release rates can be tuned. Such prodrugs directly release hydrogen sulfide without the involvement of perthiol species, which are commonly encountered with existing H2S donors. Additionally, such prodrugs can easily be conjugated to another non-steroidal anti-inflammatory agent, leading to easy synthesis of hybrid prodrugs. As a biological validation of the H2S prodrugs, the anti-inflammatory effects of one such prodrug were examined by studying its ability to inhibit LPS-induced TNF-α production in RAW 264.7 cells. This type of H2S prodrugs shows great potential as both research tools and therapeutic agents.

Prodrug strategies have been proven to be a very effective way of addressing delivery problems. Much of the chemistry in prodrug development relies on the ability to mask an appropriate functional group, which can be removed under appropriate conditions. However, developing organic prodrugs of gasotransmitters represent unique challenges. This is especially true with carbon monoxide, which does not have an easy "handle" for bioreversible derivatization. By taking advantage of an intramolecular Diels-Alder reaction, we have developed a prodrug strategy for preparations of organic CO prodrugs that are stable during synthesis and storage, and yet readily release CO with tunable release rates under near physiological conditions. The effectiveness of the CO prodrug system in delivering a sufficient quantity of CO for possible therapeutic applications has been studied using a cell culture anti-inflammatory assay and a colitis animal model. These studies fully demonstrate the proof of concept, and lay a strong foundation for further medicinal chemistry work in developing organic CO prodrugs.

Carbon monoxide (CO) is an endogenously produced gasotransmitter in mammals, and may have signaling roles in bacteria as well. It has many recognized therapeutic effects. A significant challenge in this field is the development of pharmaceutically acceptable forms of CO delivery with controllable and tunable release rates. Herein, we describe the structure-release rate studies of the first class of organic CO-prodrugs that release CO in aqueous solution at neutral pH.

Carbon monoxide prodrugs with triggered release profiles are highly desirable for targeted CO delivery to minimize their untoward side-effects. Herein, we describe a series of pH-sensitive metal-free CO prodrugs which are stable under acidic conditions and yet begin to release CO in response to increases in pH with tunable and predictable release rates.

A bottleneck for developing CO-based therapeutics is the lack of a safe and controllable delivery form. Herein, we describe efforts toward organic CO prodrugs with dual-responsive endogenous triggers. One representative CO prodrug showed significant anti-inflammatory effects both in vitro and in a LPS-simulated systemic inflammation model. These results firmly establish such CO prodrugs as either research tools or candidate compounds for the treatment of systemic inflammation or inflammation related organ injuries.

CO prodrugs with triggered release mechanisms are highly desirable for targeted delivery. Herein described are organic CO prodrugs that are activated by ROS and thus can be used to selectively deliver CO to cells with elevated ROS levels. Such CO prodrugs can serve as powerful tools for targeted delivery to disease sites with elevated ROS levels and to explore the therapeutic applications of CO.