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

Advances in the Delivery, Activation and Therapeutics Applications of Bioorthogonal Prodrugs

Traditional prodrug strategies have been leveraged to overcome many inherent drawbacks of active native drugs in the drug research and development. However, endogenous stimuli such as specific microenvironment or enzymes are relied on to achieve the prodrug activation, resulting in unintended drug release and systemic toxicity. Alternatively, bioorthogonal cleavage reaction-enabled bioorthogonal prodrugs activation via exogenous triggers has emerged as a valuable approach, featuring spatiotemporally controlled drug release. Such bioorthogonal prodrug strategies would ensure targeted drug delivery and/or in situ generation, further circumventing systemic toxicity or premature elimination of active drugs. In recent years, metal-free bioorthogonal cleavage reactions with fast kinetics have boomed in the bioorthogonal prodrug design. Meanwhile, transition-metal-catalyzed and photocatalytic deprotection reactions have also been developed to trigger prodrug activation in biological systems. Besides traditional small molecule prodrugs, gasotransmitters have been successfully delivered to specific organelles or cells via bioorthogonal reactions, and nanosystems have been devised into bioorthogonal triggers as well. Herein, we present an overview of the latest advances in these bioorthogonally-uncaged prodrugs, focused on the delivery, activation and therapeutics applications.

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.

Dual-Locked Enzyme-Activatable Fluorescence Probes for Precise Bioimaging

Real-time visualization of endogenous enzymes not only helps reveal the underlying biological principles but also provides pathological information for cancer/disease diagnosis and even treatment guidance. To this end, enzyme-activatable fluorescence probes are frequently fabricated that turn their fluorescence signals "on" exclusively at the enzyme-rich region, thus enabling noninvasive and real-time imaging of enzymes of interest at the molecular level with superior sensitivity and selectivity. However, in a complex biological context, commonly used single enzyme-activatable (i.e., single-locked) probes may suffer from "false positive" signals at healthy tissues and be insufficient to accurately indicate the occurrence of certain diseases. Therefore, dual-locked fluorescence probes have been promoted to address these issues. Using dual enzymes (or an enzyme with another stimulus) as "keys", they permit simultaneous detection of distinct biomarkers, offering significantly enhanced imaging precision toward certain biological events. Considering that recent reviews on these probes remain scarce, we thus provide this review. We summarize the recent progress, particularly highlighting the breakthroughs in the last three years, and discuss the challenges in this field.

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.

Designing intelligent bioorthogonal nanozymes: Recent advances of stimuli-responsive catalytic systems for biomedical applications

Bioorthogonal nanozymes have emerged as a potent tool in biomedicine due to their unique ability to perform enzymatic reactions that do not interfere with native biochemical processes. The integration of stimuli-responsive mechanisms into these nanozymes has further expanded their potential, allowing for controlled activation and targeted delivery. As such, intelligent bioorthogonal nanozymes have received more and more attention in developing therapeutic approaches. This review provides a comprehensive overview of the recent advances in the development and application of stimuli-responsive bioorthogonal nanozymes. By summarizing the design outlines for anchoring bioorthogonal nanozymes with stimuli-responsive capability, this review seeks to offer valuable insights and guidance for the rational design of these remarkable materials. This review highlights the significant progress made in this exciting field with different types of stimuli and the various applications. Additionally, it also examines the current challenges and limitations in the design, synthesis, and application of these systems, and proposes potential solutions and research directions. This review aims to stimulate further research toward the development of more efficient and versatile stimuli-responsive bioorthogonal nanozymes for biomedical applications.

Stimulus-Responsive Nano-Prodrug Strategies for Cancer Therapy: A Focus on Camptothecin Delivery

Camptothecin (CPT) is a highly cytotoxic molecule with excellent antitumor activity against various cancers. However, its clinical application is severely limited by poor water solubility, easy inactivation, and severe toxicity. Structural modifications and nanoformulations represent two crucial avenues for camptothecin's development. However, the potential for further structural modifications is limited, and camptothecin nanoparticles fabricated via physical loading have the drawbacks of low drug loading and leakage. Prodrug-based CPT nanoformulations have shown unique advantages, including increased drug loading, reduced burst release, improved bioavailability, and minimal toxic side effects. Stimulus-responsive CPT nano-prodrugs that respond to various endogenous or exogenous stimuli by introducing various activatable linkers to achieve spatiotemporally responsive drug release at the tumor site. This review comprehensively summarizes the latest research advances in stimulus-responsive CPT nano-prodrugs, including preparation strategies, responsive release mechanisms, and their applications in cancer therapy. Special focus is placed on the release mechanisms and characteristics of various stimulus-responsive CPT nano-prodrugs and their application in cancer treatment. Furthermore, clinical applications of CPT prodrugs are discussed. Finally, challenges and future research directions for CPT nano-prodrugs are discussed. This review to be valuable to readers engaged in prodrug research is expected.

A Synergistic Chemoimmunotherapy System Leveraging PD-L1 Blocking and Bioorthogonal Prodrug Activation

Novel strategies to facilitate tumor-specific drug delivery and restore immune attacks remain challenging in overcoming the current limitations of chemoimmunotherapy. An antitumor chemoimmunotherapy system comprising bioorthogonal reaction-ready group tetrazine (TZ) modified with an anti-PD-L1 antibody (αPD-L1TZ) and TZ-activatable prodrug vinyl ether-doxorubicin (DOX-VE) for self-reinforced anti-tumor chemoimmunotherapy is proposed. The αPD-L1TZ effectively disrupts the PD-L1/PD-1 interaction and activates the DOX prodrug in situ through the bioorthogonal click reaction of TZ and VE. Conversely, the activated DOX upregulates PD-L1 on the surface of tumor cells, facilitating tumor accumulation of αPD-L1TZ and enhancing DOX-VE activation. Furthermore, the activated DOX-induced immunogenic cell death of tumor cells, substantially improving the response efficiency of αPD-L1 in an immune-suppressive tumor microenvironment. Thus, PD-L1 blocking and bioorthogonal in situ prodrug activation synergistically enhance the antitumor efficacy of the chemoimmunotherapy system. Therefore, the system significantly enhances αPD-L1 tumor accumulation and prodrug activation and induces a robust immunological memory effect to prevent tumor recurrence and metastasis. Thus, a feasible chemoimmunotherapy combination regimen is presented.

Bioorthogonal Bond Cleavage Chemistry for On-demand Prodrug Activation: Opportunities and Challenges

Time- and space-resolved drug delivery is highly demanded for cancer treatment, which, however, can barely be achieved with a traditional prodrug strategy. In recent years, the prodrug strategy based on a bioorthogonal bond cleavage chemistry has emerged with the advantages of high temporospatial resolution over drug activation and homogeneous activation irrespective of individual heterogeneity. In the past five years, tremendous progress has been witnessed in this field with one such bioorthogonal prodrug entering Phase II clinical trials. This Perspective aims to highlight these new advances (2019-2023) and critically discuss their pros and cons. In addition, the remaining challenges and potential strategic directions for future progress will also be included.

Camptothecin-based prodrug nanomedicines for cancer therapy

Camptothecin (CPT) is a cytotoxic alkaloid that attenuates the replication of cancer cells via blocking DNA topoisomerase 1. Despite its encouraging and wide-spectrum antitumour activity, its application is significantly restricted owing to its instability, low solubility, significant toxicity, and acquired tumour cell resistance. This has resulted in the development of many CPT-based therapeutic agents, especially CPT-based nanomedicines, with improved pharmacokinetic and pharmacodynamic profiles. Specifically, smart CPT-based prodrug nanomedicines with stimuli-responsive release capacity have been extensively explored owing to the advantages such as high drug loading, improved stability, and decreased potential toxicity caused by the carrier materials in comparison with normal nanodrugs and traditional delivery systems. In this review, the potential strategies and applications of CPT-based nanoprodrugs for enhanced CPT delivery toward cancer cells are summarized. We appraise in detail the chemical structures and release mechanisms of these nanoprodrugs and guide materials chemists to develop more powerful nanomedicines that have real clinical therapeutic capacities.