Bioorthogonal Cleavage of Tetrazine-Caged Ethers and Esters Triggered by trans-Cyclooctene

Fast Release of Carboxylic Acid inside Cells

Delivering carboxylic acid functions into cells is challenging due to their poor permeability across lipophilic membranes at physiological pH, where they are ionized. Masking carboxylic acids as esters improves cell entry, but once inside the cell, its rapid release is essential to maintain spatiotemporal control which can be beneficial for therapeutic and diagnostic applications. This study evaluates the 2-hydroxyethyl-dithio-benzyl ester functional group which undergoes selective and rapid cleavage of the disulfide bond by thioredoxin (Trx), triggering rapid self-immolation of the thio-benzyl ester releasing the carboxylic acid. Fluorescence-based assays using the pro-fluorescent BODIPY structure have demonstrated the rapid intracellular release of carboxylic acids within minutes in both eukaryotic and prokaryotic cells. The approach was tested on antibiotics, and among them, levofloxacin ester prodrug, having the 2-hydroxyethyl-dithio-benzyl ester functional group, showed significantly enhanced antimicrobial activity against resistant and intracellular bacteria compared to its methyl ester analogue.

Bioorthogonal catalytic microvesicle-mediated prodrug activation against liver cancer

OBJECTIVE

Exploring the role and mechanism of a novel bioorthogonal system using transition metals as catalysts in the treatment of hepatocellular carcinoma (HCC).

METHODS

Initially, a catalytic ruthenium (Ru) complex and the substrate alloc-RH 110 were synthesized, followed by the identification of their structures utilizing mass spectrometry and nuclear magnetic resonance (NMR) techniques. The catalytic efficacy of the Ru complex was then assessed using a fluorescence spectrophotometer. Subsequently, employing HepG2 cells as the cellular source, cell-derived vesicles encapsulating the Ru complexes, designated as EVs@Ru, were prepared. The EVs@Ru were characterized by measuring their particle size and Zeta potential, observing morphological features under transmission electron microscopy (TEM), and detecting specific protein expressions via Western blot analysis. Drug loading within the EVs@Ru was quantified using inductively coupled plasma mass spectrometry (ICP-MS), and their catalytic efficiency was evaluated. In vitro, the low-activity prodrug alloc-DOX was synthesized and its toxicity, along with the drug concentration in EVs@Ru, was determined. Further, the catalytic cytotoxicity of alloc-DOX against HepG2 cells encapsulated in EVs@Ru was analyzed through microscopic observation, CCK-8 assays, and apoptosis experiments. For in vivo studies, a tumor-bearing mouse model was established using human liver cancer HepG2 cells to observe the antitumor effects. Finally, the primary organs of each group of tumor-bearing mice were assessed for in vivo safety.

RESULTS

ESI-MS and 1H NMR confirmed the accurate structure of Ru complexes and alloc-RH 110. The Ru complexes achieved full catalytic conversion of alloc-RH 110 within 24 hours. EVs and EVs@Ru exhibited particle sizes of ∼116.85 nm and ∼281.88 nm, respectively, with Zeta potentials of ∼-20.86 mV and ∼-25.89 mV, both appearing quasi-circular under TEM. WB analysis verified the presence of vesicle-specific marker proteins in both, confirming their cell-derived nature. ICP-MS determined a drug loading of 21.90 μg/mL for EVs@Ru, with an encapsulation efficiency of ∼24.86%. Fluorescence spectrophotometry demonstrated 100% catalytic efficiency for EVs@Ru. Synthetic alloc-DOX validated by 1H NMR and ESI-MS matched literature data. MTT and CCK-8 assays confirmed low toxicity for alloc-DOX and Ru complexes, setting the experimental drug concentration at 4μM. In vitro, the EVs@Ru+alloc-DOX group exhibited potent HepG2 cell killing and apoptosis. In vivo, this group significantly inhibited tumor growth in tumor-bearing mice, with no observed toxicity to vital organs, indicating good biosafety.

CONCLUSION

The integration of bio-derived microvesicles (MVs) with transition metal catalysts has resulted in a biologically orthogonal system for efficient Ru complex delivery to tumor sites. This system facilitates controlled release of the Ru complexes, enabling tumor cell elimination. This innovative strategy holds great promise for enhancing tumor immunity and targeted therapeutic approaches.

Bioorthogonal chemistry-based prodrug strategies for enhanced biosafety in tumor treatments: current progress and challenges

Cancer is a significant global health challenge, and while chemotherapy remains a widely used treatment, its non-specific toxicity and broad distribution can lead to systemic side effects and limit its effectiveness against tumors. Therefore, the development of safer chemotherapy alternatives is crucial. Prodrugs hold great promise, as they remain inactive until they reach the cancer site, where they are selectively activated by enzymes or specific factors, thereby reducing side effects and improving targeting. However, subtle differences in the microenvironments between tumors and normal tissue may still result in unintended cytotoxicity. Bioorthogonal reactions, known for their selectivity and precision without interfering with natural biochemical processes, are gaining attention. When combined with prodrug strategies, these reactions offer the potential to create highly effective chemotherapy drugs. This review examines the safety and efficacy of prodrug strategies utilizing various bioorthogonal reactions in cancer treatment.

Enabling Universal Access to Rapid and Stable Tetrazine Bioorthogonal Probes through Triazolyl-Tetrazine Formation

Despite the immense potential of tetrazine bioorthogonal chemistry in biomedical research, the in vivo performance of tetrazine probes is challenged by the inverse correlation between the physiological stability and reactivity of tetrazines. Additionally, the synthesis of functionalized tetrazines is often complex and requires specialized reagents. To overcome these issues, we present a novel tetrazine scaffold-triazolyl-tetrazine-that can be readily synthesized from shelf-stable ethynyl-tetrazines and azides. Triazolyl-tetrazines exhibit improved physiological stability along with high reactivity. We showcase the effectiveness of this approach by creating cell-permeable probes for protein labeling and live cell imaging, as well as efficiently producing 18F-labeled molecular probes for positron emission tomography imaging. By utilizing the readily available pool of functionalized azides, we envisage that this modular approach will provide universal accessibility to tetrazine bioorthogonal tools, facilitating applications in biomedicine and materials science.

Photoinduced Charge Centralization Quenches the Fluorescence of Conjugation-Fused Tetrazine Labels with Red-to-Near-Infrared Emissions

Tetrazine-derived fluorogenic labels are extensively studied for their potential in biological and medical imaging. Nonetheless, the fluorescence quenching mechanism in numerous precursors continues to be debated, particularly as the wavelengths extend into the red and near-infrared (NIR) regions. This challenge poses obstacles to systematically optimizing their fluorogenicity, i.e., achieving red-shifted wavelengths and improved fluorescence turn-on signals through click reactions. This paper highlights the significance of photoinduced charge centralization (PCC), a quenching mechanism observed in tetrazine-fused fluorogenic labels with integrated π-conjugations. PCC is primarily responsible for the quenching effects observed in such labels emitting in the red-to-NIR spectrum. Drawing from structure-property relationships, this study proposes two molecular design strategies that incorporate the PCC mechanism and constitutional isomerization to develop high-performance tetrazine-based labels. These strategies facilitate multiplex fluorescence imaging following click reactions, promising significant advancements in bio-orthogonal imaging techniques.

An all-in-one tetrazine reagent for cysteine-selective labeling and bioorthogonal activable prodrug construction

The prodrug design strategy offers a potent solution for improving therapeutic index and expanding drug targets. However, current prodrug activation designs are mainly responsive to endogenous stimuli, resulting in unintended drug release and systemic toxicity. In this study, we introduce 3-vinyl-6-oxymethyl-tetrazine (voTz) as an all-in-one reagent for modular preparation of tetrazine-caged prodrugs and chemoselective labeling peptides to produce bioorthogonal activable peptide-prodrug conjugates. These stable prodrugs can selectively bind to target cells, facilitating cellular uptake. Subsequent bioorthogonal cleavage reactions trigger prodrug activation, significantly boosting potency against tumor cells while maintaining exceptional off-target safety for normal cells. In vivo studies demonstrate the therapeutic efficacy and safety of this prodrug design approach. Given the broad applicability of functional groups and labeling versatility with voTz, we foresee that this strategy will offer a versatile solution to enhance the therapeutic range of cytotoxic agents and facilitate the development of bioorthogonal activatable biopharmaceuticals and biomaterials.

A bioorthogonal cell sorting strategy for isolation of desired cell phenotypes

Here we describe a cost-effective and simplified cell sorting method using tetrazine bioorthogonal chemistry. We successfully isolated SKOV3 cells from complex mixtures, demonstrating efficacy in separating mouse lymphocytes expressing interferon and HeLa cells expressing virally transduced green fluorescent protein post-infection.

Ligand-Directed Photocatalysts and Far-Red Light Enable Catalytic Bioorthogonal Uncaging inside Live Cells

Described are ligand-directed catalysts for live-cell, photocatalytic activation of bioorthogonal chemistry. Catalytic groups are localized via a tethered ligand either to DNA or to tubulin, and red light (660 nm) photocatalysis is used to initiate a cascade of DHTz oxidation, intramolecular Diels-Alder reaction, and elimination to release phenolic compounds. Silarhodamine (SiR) dyes, more conventionally used as biological fluorophores, serve as photocatalysts that have high cytocompatibility and produce minimal singlet oxygen. Commercially available conjugates of Hoechst dye (SiR-H) and docetaxel (SiR-T) are used to localize SiR to the nucleus and microtubules, respectively. Computation was used to assist the design of a new class of redox-activated photocage to release either phenol or n-CA4, a microtubule-destabilizing agent. In model studies, uncaging is complete within 5 min using only 2 μM SiR and 40 μM photocage. In situ spectroscopic studies support a mechanism involving rapid intramolecular Diels-Alder reaction and a rate-determining elimination step. In cellular studies, this uncaging process is successful at low concentrations of both the photocage (25 nM) and the SiR-H dye (500 nM). Uncaging n-CA4 causes microtubule depolymerization and an accompanying reduction in cell area. Control studies demonstrate that SiR-H catalyzes uncaging inside the cell, and not in the extracellular environment. With SiR-T, the same dye serves as a photocatalyst and the fluorescent reporter for microtubule depolymerization, and with confocal microscopy, it was possible to visualize microtubule depolymerization in real time as the result of photocatalytic uncaging in live cells.

Dual stimulus-triggered bioorthogonal nanosystem for spatiotemporally controlled prodrug activation and near-infrared fluorescence imaging

In this study, we combined low pH and cathepsin B dual-stimulus-triggered delivery carriers with a bioorthogonal reaction-activated prodrug to achieve regulated activation of the prodrug. A workable method for precise tumor therapy and imaging is provided by the bioorthogonal reaction, which activates the prodrug and fluorescent probe.