Synthesis and Evaluation of O2-Derived Diazeniumdiolates Activatable via Bioorthogonal Chemistry Reactions in Living Cells

Bifunctional Sildenafil Diazeniumdiolates Acting as Phosphodiesterase 5 Inhibitors and Nitric Oxide Donors- Towards Wound Healing

Inefficient wound healing poses a global health challenge with a lack of efficient treatments. Wound healing issues often correlate with low endogenous nitric oxide (NO) levels. While exogenous delivery with NO-releasing compounds represents a promising therapeutic strategy, controlling the release of the highly reactive NO remains challenging. Phosphodiesterase 5 (PDE5) inhibitors, like sildenafil, have also been shown to promote wound healing. This study explores hybrid compounds, combining NO-releasing diazeniumdiolates with a sildenafil-derived PDE5 inhibitor. One compound demonstrated a favorable NO-release profile, triggered by an esterase (prodrug), and displayed in vitro nanomolar inhibition potency against PDE5 and thrombin-induced platelet aggregation. Both factors are known to promote blood flow and oxygenation. Thus, our findings unveil promising prospects for effective wound healing treatments.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Metal complexes for catalytic and photocatalytic reactions in living cells and organisms

The development of organometallic catalysis has greatly expanded the synthetic chemist toolbox compared to only exploiting "classical" organic chemistry. Although more widely used in organic solvents, metal-based catalysts have also emerged as efficient tools for developing organic transformations in water, thus paving the way for further development of bio-compatible reactions. However, performing metal-catalysed reactions within living cells or organisms induces additional constraints to the design of reactions and catalysts. In particular, metal complexes must exhibit good efficiency in complex aqueous media at low concentrations, good cell specificity, good cellular uptake and low toxicity. In this review, we focus on the presentation of discrete metal complexes that catalyse or photocatalyse reactions within living cells or living organisms. We describe the different reaction designs that have proved to be successful under these conditions, which involve very few metals (Ir, Pd, Ru, Pt, Cu, Au, and Fe) and range from in cellulo deprotection/decaging/activation of fluorophores, drugs, proteins and DNA to in cellulo synthesis of active molecules, and protein and organelle labelling. We also present developments in bio-compatible photo-activatable catalysts, which represent a very recent emerging area of research and some prospects in the field.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

Gasotransmitters like nitric oxide, carbon monoxide, and hydrogen sulfide with unique pleiotropic pharmacological effects in mammals are an emerging therapeutic modality for different human diseases including cancer, infection, ischemia-reperfusion injuries, and inflammation; however, their clinical translation is hampered by the lack of a reliable delivery form, which delivers such gasotransmitters to the action site with precisely controlled dosage. The external stimuli-responsive prodrug strategy has shown tremendous potential in developing gasotransmitter prodrugs, which affords precise temporospatial control and better dose control compared with endogenous stimuli-sensitive prodrugs. The promising external stimuli employed for gasotransmitter activation range from photo, ultrasound, and bioorthogonal click chemistry to exogenous enzymes. Herein, we highlight the recent development of external stimuli-mediated decaging chemistry for the temporospatial delivery of gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide and sulfur dioxide, and discuss the pros and cons of different designs.

A 5-FU Precursor Designed to Evade Anabolic and Catabolic Drug Pathways and Activated by Pd Chemistry In Vitro and In Vivo

5-Fluorouracil (5-FU) is an antineoplastic antimetabolite that is widely administered to cancer patients by bolus injection, especially to those suffering from colorectal and pancreatic cancer. Because of its suboptimal route of administration and dose-limiting toxicities, diverse 5-FU prodrugs have been developed to confer oral bioavailability and increase the safety profile of 5-FU chemotherapy regimens. Our contribution to this goal is presented herein with the development of a novel palladium-activated prodrug designed to evade the metabolic machinery responsible for 5-FU anabolic activation and catabolic processing. The new prodrug is completely innocuous to cells and highly resistant to metabolization by primary hepatocytes and liver S9 fractions (the main metabolic route for 5-FU degradation), whereas it is rapidly converted into 5-FU in the presence of a palladium (Pd) source. In vivo pharmokinetic analysis shows the prodrug is rapidly and completely absorbed after oral administration and exhibits a longer half-life than 5-FU. In vivo efficacy studies in a xenograft colon cancer model served to prove, for the first time, that orally administered prodrugs can be locally converted to active drugs by intratumorally inserted Pd implants.

In situ activation of therapeutics through bioorthogonal catalysis

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Tetrazine-mediated bioorthogonal removal of 3-isocyanopropyl groups enables the controlled release of nitric oxide in vivo

Bond cleavage bioorthogonal chemistry has been widely employed to restore or activate proteins or prodrugs. Nitric oxide (NO), as a free radical molecule, has joined the clinical arena of cancer therapy, since high levels of NO could produce a cancer cell growth inhibitory effect. However, the spatiotemporal controlled release of NO remains a great challenge, and bioorthogonal chemistry may open a new window. Herein, we described a class of O2-3-isocyanopropyl diazeniumdiolates 3a-f as new bioorthogonal NO precursors, which can be effectively uncaged via tetrazine-mediated bond cleavage reactions to liberate NO and acrolein in living cancer cells, exhibiting potent antiproliferative activity. Furthermore, 3a and tetrazine BTZ were respectively encapsulated into two liposomes. It was found that simultaneous administrations of the two liposomes could specifically release large amounts of NO in the implanted cancer cells in zebrafish, thus generating potent tumor suppression activity in vivo. Our findings indicate that the TZ-labile NO precursors could serve to expand the NO-based smart therapeutics and the scope of bioorthogonal chemistry utility in vivo in the near future.

General Strategy for Integrated Bioorthogonal Prodrugs: Pt(II)-Triggered Depropargylation Enables Controllable Drug Activation In Vivo

Bioorthogonal decaging reactions for controllable drug activation within complex biological systems are highly desirable yet extremely challenging. Herein, we find a new class of Pt(II)-triggered bioorthogonal cleavage reactions in which Pt(II) but not Pt(IV) complexes effectively trigger the cleavage of O/N-propargyl in a variety of ranges of caged molecules under biocompatible conditions. Based on these findings, we propose a general strategy for integrated bioorthogonal prodrugs and accordingly design a prodrug 16, in which a Pt(IV) moiety is covalently connected with an O²-propargyl diazeniumdiolate moiety. It is found that 16 can be specifically reduced by cytoplasmic reductants in human ovarian cancer cells to liberate cisplatin, which subsequently stimulates the cleavage of O²-propargyl to release large amounts of NO in situ, thus generating synergistic and potent tumor suppression activity in vivo. Therefore, Pt(II)-triggered depropargylation and the integration concept might provide a general strategy for broad applicability of bioorthogonal cleavage chemistry in vivo.

Transition metal catalysts for the bioorthogonal synthesis of bioactive agents

The incorporation of abiotic transition metal catalysis into the chemical biology space has significantly expanded the tool kit of bioorthogonal chemistries accessible for cell culture and in vivo applications. A rich variety of homogeneous and heterogeneous catalysts has shown functional compatibility with physiological conditions and biostability in complex environs, enabling their exploitation as extracellular or intracellular factories of bioactive agents. Current trends in the field are focusing on investigating new metals and sophisticated catalytic devices and toward more applied activities, such as the integration of subcellular, cell- and site-targeting capabilities or the exploration of novel biomedical applications. We present herein an overview of the latest advances in the field, highlighting the increasing role of transition metals for the controlled release of therapeutics.

Improved antiproliferative activity of novel Pulsatilla saponin D/α-hederin derivatives containing nitric oxide donors

Conjugates of PSD and furoxan exhibited synergetic effects on enhancing antiproliferative activity and reducing hemolytic toxicity.

Scalable Synthesis of Diazeniumdiolates: Application to the Preparation of MK-8150

Synthetic diazeniumdiolate (DAZD)-based nitric oxide is utilized to modulate the nitric oxide (NO) concentration in cellular environments and to control physiological processes, yet chemists are still struggling to find efficient and scalable methodologies that will enable them to access sufficient quantities of the high-energy diazeniumdiolate intermediates for biological studies. Now, a general, scalable, safer, and high-yielding new methodology adaptable to the large-scale synthesis of DAZDs has been developed.

Overview of Recent Strategic Advances in Medicinal Chemistry

Introducing novel strategies, concepts, and technologies that speed up drug discovery and the drug development cycle is of great importance both in the highly competitive pharmaceutical industry as well as in academia. This Perspective aims to present a "big-picture" overview of recent strategic innovations in medicinal chemistry and drug discovery.

Dissociative Bioorthogonal Reactions

Bioorthogonal reactions that proceed readily under physiological conditions without interference from biomolecules have found widespread application in the life sciences. Complementary to the bioorthogonal reactions that ligate two molecules, reactions that release a molecule or cleave a linker are increasingly attracting interest. Such dissociative bioorthogonal reactions have a broad spectrum of uses, for example, in controlling bio-macromolecule activity, in drug delivery, and in diagnostic assays. This review article summarizes the developed bioorthogonal reactions linked to a release step, outlines representative areas of the applications of such reactions, and discusses aspects that require further improvement.

Catalysis Concepts in Medicinal Inorganic Chemistry

Catalysis has strongly emerged in the field of medicinal inorganic chemistry as a suitable tool to deliver new drug candidates and to overcome drawbacks associated to metallodrugs. In this Concept article, we discuss representative examples of how catalysis has been applied in combination with metal complexes to deliver new therapy approaches. In particular, we explain key achievements in the design of catalytic metallodrugs that damage biomolecular targets and in the development of metal catalysis schemes for the activation of exogenous organic prodrugs. Moreover, we discuss our recent discoveries on the flavin-mediated bioorthogonal catalytic activation of metal-based prodrugs; a new catalysis strategy in which metal complexes are unconventionally employed as substrates rather than catalysts.

Bioorthogonal Uncaging of the Active Metabolite of Irinotecan by Palladium-Functionalized Microdevices

SN-38, the active metabolite of irinotecan, is released upon liver hydrolysis to mediate potent antitumor activity. Systemic exposure to SN-38, however, also leads to serious side effects. To reduce systemic toxicity by controlling where and when SN-38 is generated, a new prodrug was specifically designed to be metabolically stable and undergo rapid palladium-mediated activation. Blocking the phenolic OH of SN-38 with a 2,6-bis(propargyloxy)benzyl group led to significant reduction of cytotoxic activity (up to 44-fold). Anticancer properties were swiftly restored in the presence of heterogeneous palladium (Pd) catalysts to kill colorectal cancer and glioma cells, proving the efficacy of this novel masking strategy for aromatic hydroxyls. Combination with a Pd-activated 5FU prodrug augmented the antiproliferative potency of the treatment, while displaying no activity in the absence of the Pd source, which illustrates the benefit of achieving controlled release of multiple approved therapeutics-sequentially or simultaneously-by the same bioorthogonal catalyst to increase anticancer activity.