A High-Efficiency Bioorthogonal Tumor-Membrane Reactor for In Situ Selective and Sustained Prodrug Activation

The site-specific activation of bioorthogonal prodrugs has provided great opportunities for reducing the severe side effects of chemotherapy. However, the precise control of activation location, sustained drug production at the target site, and high bioorthogonal reaction efficiency in vivo remain great challenges. Here, we propose the construction of tumor cell membrane reactors in vivo to solve the above problems. Specifically, tumor-targeted liposomes with efficient membrane fusion capabilities are generated to install the bioorthogonal trigger, the amphiphilic tetrazine derivative, on the surface of tumor cells. These predecorated tumor cells act as many living reactors, transforming the tumor into a "drug factory" that in situ activates an externally delivered bioorthogonal prodrug, for example intratumorally injected transcyclooctene-caged doxorubicin. In contrast to the rapid elimination of cargo that is encapsulated and delivered by liposomes, these reactors permit stable retention of bioorthogonal triggers in tumor for 96 h after a single dose of liposomes via intravenous injection, allowing sustained generation of doxorubicin. Interestingly, an additional supplement of liposomes will compensate for the trigger consumed by the reaction and significantly improve the efficiency of the local reaction. This strategy provides a solution to the efficacy versus safety dilemma of tumor chemotherapy.

A Dual-Mechanism Targeted Bioorthogonal Prodrug Therapy

Bioorthogonal prodrug therapies offer an intriguing two-component system that features enhanced circulating stability and controlled activation on demand. Current strategies often deliver either the prodrug or its complementary activator to the tumor with a monomechanism targeted mechanism, which cannot achieve the desired antitumor efficacy and safety profile. The orchestration of two distinct and orthogonal mechanisms should overcome the hierarchical heterogeneity of solid tumors to improve the delivery efficiency of both components simultaneously for bio-orthogonal prodrug therapies. We herein developed a dual-mechanism targeted bioorthogonal prodrug therapy by integrating two orthogonal, receptor-independent tumor-targeting strategies. We first employed the endogenous albumin transport system to generate the in situ albumin-bound, bioorthogonal-caged doxorubicin prodrug with extended plasma circulation and selective accumulation at the tumor site. We then employed enzyme-instructed self-assembly (EISA) to specifically enrich the bioorthogonal activators within tumor cells. As each targeted delivery mode induced an intrinsic pharmacokinetic profile, further optimization of the administration sequence according to their pharmacokinetics allowed the spatiotemporally controlled prodrug activation on-target and on-demand. Taken together, by orchestrating two discrete and receptor-independent targeting strategies, we developed an all-small-molecule based bioorthogonal prodrug system for dual-mechanism targeted anticancer therapies to maximize therapeutic efficacy and minimize adverse drug reactions for chemotherapeutic agents.

Designing Bioorthogonal Reactions for Biomedical Applications

Bioorthogonal reactions are a class of chemical reactions that can be carried out in living organisms without interfering with other reactions, possessing high yield, high selectivity, and high efficiency. Since the first proposal of the conception by Professor Carolyn Bertozzi in 2003, bioorthogonal chemistry has attracted great attention and has been quickly developed. As an important chemical biology tool, bioorthogonal reactions have been applied broadly in biomedicine, including bio-labeling, nucleic acid functionalization, drug discovery, drug activation, synthesis of antibody-drug conjugates, and proteolysis-targeting chimeras. Given this, we summarized the basic knowledge, development history, research status, and prospects of bioorthogonal reactions and their biomedical applications. The main purpose of this paper is to furnish an overview of the intriguing bioorthogonal reactions in a variety of biomedical applications and to provide guidance for the design of novel reactions to enrich bioorthogonal chemistry toolkits.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Progress in controllable bioorthogonal catalysis for prodrug activation

Bioorthogonal catalysis, a class of catalytic reactions that are mediated by abiotic metals and proceed in biological environments without interfering with native biochemical reactions, has gained ever-increasing momentum in prodrug delivery over the past few decades. Albeit great progress has been attained in developing new bioorthogonal catalytic reactions and optimizing the catalytic performance of transition metal catalysts (TMCs), the use of TMCs to activate chemotherapeutics at the site of interest in vivo remains a challenging endeavor. To translate the bioorthogonal catalysis-mediated prodrug activation paradigm from flasks to animals, TMCs with targeting capability and stimulus-responsive behavior have been well-designed to perform chemical transformations in a controlled manner within highly complex biochemical systems, rendering on-demand drug activation to mitigate off-target toxicity. Here, we review the recent advances in the development of controllable bioorthogonal catalysis systems, with an emphasis on different strategies for engineering TMCs to achieve precise control over prodrug activation. Furthermore, we outline the envisaged challenges and discuss future directions of controllable bioorthogonal catalysis for disease therapy.

Targeted Molecular Construct for Bioorthogonal Theranostics of PD-L1-Expressing Cancer Cells

Molecular targeting of tumor-overexpressed oncoproteins can improve the selectivity and tolerability of anticancer therapies. The immunoinhibitory membrane protein programmed death ligand 1 (PD-L1) is highly expressed on certain tumor types, which masks malignant cells from T cell recognition and creates an optimal environment for the cancer to thrive and spread. We report here a ligand-tetrazine conjugate (LTzC) armed with a PD-L1 small molecule inhibitor to selectively target PD-L1-expressing cancer cells and inhibit PD-L1 function and conjugated to a tetrazine module and a lipoyl group to incorporate bioorthogonal reactivities and an oxidative stress enhancer into the construct. By pairing LTzC with an imaging probe, we have established a "track-&-tag" system for selective labeling of PD-L1 both on and in living cells using click chemistry. We have further shown the specificity and versatility of LTzC by click-to-release activation of prodrugs and selective killing of PD-L1-expressing breast cancer cells, offering a new multimodal approach to "track-&-treat" malignant cells that are capable of evading the immune system.

Metal-free bioorthogonal click chemistry in cancer theranostics

Bioorthogonal chemistry is a powerful tool to site-specifically activate drugs in living systems. Bioorthogonal reactions between a pair of biologically reactive groups can rapidly and specifically take place in a mild physiological milieu without perturbing inherent biochemical processes. Attributed to their high selectivity and efficiency, bioorthogonal reactions can significantly decrease background signals in bioimaging. Compared with metal-catalyzed bioorthogonal click reactions, metal-free click reactions are more biocompatible without the metal catalyst-induced cytotoxicity. Although a great number of bioorthogonal chemistry-based strategies have been reported for cancer theranostics, a comprehensive review is scarce to highlight the advantages of these strategies. In this review, recent progress in cancer theranostics guided by metal-free bioorthogonal click chemistry will be depicted in detail. The elaborate design as well as the advantages of bioorthogonal chemistry in tumor theranostics are summarized and future prospects in this emerging field are emphasized.

Derivatization based on tetrazine scaffolds: synthesis of tetrazine derivatives and their biomedical applications

The recent advances in tetrazine scaffold-based derivatizations have been summarized. The advantages and limitations of derivatization methods and applications of the developed tetrazine derivatives in bioorthogonal chemistry have been highlighted.

On-Demand Activation of a Bioorthogonal Prodrug of SN-38 with Fast Reaction Kinetics and High Releasing Efficiency In Vivo

Although a myriad of bioorthogonal prodrugs have been developed, very few of them present both fast reaction kinetics and complete cleavage. Herein, we report a new bioorthogonal prodrug strategy with both fast reaction kinetics (k₂: ∼10³ M^-1 s^-1) and complete cleavage (>90% within minutes) using the bioorthogonal reaction pair of N-oxide and boron reagent. Distinctively, an innovative 1,6-elimination-based self-immolative linker is masked by N-oxide, which can be bioorthogonally demasked by a boron reagent for the release of both amino and hydroxy-containing payload in live cells. Such a strategy was applied to prepare a bioorthogonal prodrug for a camptothecin derivative, SN-38, resulting in 10-fold weakened cytotoxicity against A549 cells, 300-fold enhanced water solubility, and "on-demand" activation upon a click reaction both in vitro and in vivo. This novel bioorthogonal prodrug strategy presents significant advances over the existing ones and may find wide applications in drug delivery in the future.

Antibody-drug conjugates: Recent advances in linker chemistry

Antibody–drug conjugates (ADCs) are gradually revolutionizing clinical cancer therapy. The antibody–drug conjugate linker molecule determines both the efficacy and the adverse effects, and so has a major influence on the fate of ADCs. An ideal linker should be stable in the circulatory system and release the cytotoxic payload specifically in the tumor. However, existing linkers often release payloads nonspecifically and inevitably lead to off-target toxicity. This defect is becoming an increasingly important factor that restricts the development of ADCs. The pursuit of ADCs with optimal therapeutic windows has resulted in remarkable progress in the discovery and development of novel linkers. The present review summarizes the advance of the chemical trigger, linker‒antibody attachment and linker‒payload attachment over the last 5 years, and describes the ADMET properties of ADCs. This work also helps clarify future developmental directions for the linkers.

Research fronts of Chemical Biology

Over the past decades, researchers have witnessed substantially increasing and ever-growing interests and efforts in Chemical Biology studies, thanks to the development of genome and epi-genome sequencing (revealing potential drug targets), synthetic chemistry (producing new medicines), bioorthogonal chemistry (chemistry in living systems) and high-throughput screening technologies (in vitro cell systems, protein binding assays and phenotypic assays). This report presents literature search results for current research in Chemical Biology, to explore basic principles, summarize recent advances, identify key challenges, and provide suggestions for future research (with a focus on Chemical Biology in the context of human health and diseases). Chemical Biology research can positively contribute to delivering a better understanding of the molecular and cellular mechanisms that accompany pathology underlying diseases, as well as developing improved methods for diagnosis, drug discovery, and therapeutic delivery. While much progress has been made, as shown in this report, there are still further needs and opportunities. For instance, pressing challenges still exist in selecting appropriate targets in biological systems and adopting more rational design strategies for the development of innovative and sustainable diagnostic technologies and medical treatments. Therefore, more than ever, researchers from different disciplines need to collaborate to address the challenges in Chemical Biology.

Unleashing the Power of Bond Cleavage Chemistry in Living Systems

Bioorthogonal cleavage chemistry has been rapidly emerging as a powerful tool for manipulation and gain-of-function studies of biomolecules in living systems. While the initial bond formation-centered bioorthogonal reactions have been widely adopted for labeling, tracing, and capturing biomolecules, the newly developed bond cleavage-enabled bioorthogonal reactions have opened new possibilities for rescuing small molecules as well as biomacromolecules in living systems, allowing multidimensional controls over biological processes in vitro and in vivo. In this Outlook, we first summarized the development and applications of bioorthogonal cleavage reactions (BCRs) that restore the functions of chemical structures as well as more complex networks, including the liberation of prodrugs, release of bioconjugates, and in situ reactivation of intracellular proteins. As we embarked on this fruitful progress, we outlined the unmet scientific needs and future directions along this exciting avenue. We believe that the potential of BCRs will be further unleashed when combined with other frontier technologies, such as genetic code expansion and proximity-enabled chemical labeling.

"One-stitch" bioorthogonal prodrug activation based on cross-linked lipoic acid nanocapsules

Bioorthogonal prodrug activation is fascinating but suffers from staggered administration of prodrug and trigger, which would not only reduce the therapeutic effect but bring great inconvenience for clinical application. Herein, we report a new cross-linked lipoic acid nanocapsules (cLANCs) based two-component bioorthogonal nanosystem for "one-stitch" prodrug activation. Due to the reversible stability of cLANCs, the loaded prodrug and trigger cannot release in advance while can react upon arrival in the tumor tissue. Moreover, the cLANCs would be degraded into dihydrolipoic acid in tumor cells to potentiate the anticancer effect of the drug synthesized in situ. The data showed that the new bioorthogonal system held a killing effect 1.63 times higher than that of parent drug 3 against human colorectal tumor cells (HT29) and a tumor inhibitory rate 34.2% higher than that of 3 against HT29 tumor xenograft model with negligible side effects. The biodistribution study showed that the "one-stitch" prodrug activation exhibited a selective accumulation of 3 in the tumor tissue compared with free 3 group (34.2 μg vs 3.56 μg of 3/g of tissue). This two-component bioorthogonal nanosystem based on cross-linked lipoic acid nanocapsules constitutes the first example of "one-stitch" bioorthogonal prodrug activation.

Activation and Delivery of Tetrazine-Responsive Bioorthogonal Prodrugs

Prodrugs, which remain inert until they are activated under appropriate conditions at the target site, have emerged as an attractive alternative to drugs that lack selectivity and show off-target effects. Prodrugs have traditionally been activated by enzymes, pH or other trigger factors associated with the disease. In recent years, bioorthogonal chemistry has allowed the creation of prodrugs that can be chemically activated with spatio-temporal precision. In particular, tetrazine-responsive bioorthogonal reactions can rapidly activate prodrugs with excellent biocompatibility. This review summarized the recent development of tetrazine bioorthogonal cleavage reaction and great promise for prodrug systems.

Coordinating bioorthogonal reactions with two tumor-microenvironment-responsive nanovehicles for spatiotemporally controlled prodrug activation

Precise activation of prodrugs in tumor tissues is critical to ensuring specific antitumor efficacy, meanwhile reducing the serious adverse effects. Here, a spatiotemporally controlled prodrug activation strategy was provided by integrating the inverse electron demand Diels-Alder (IEDDA) reaction with two tumor-microenvironment-responsive nanovehicles. The prodrug (Dox-TCO) and [4-(6-methyl-1,2,4,5-tetrazin-3-yl)phenyl]methanamine (Tz) were separately camouflaged into low pH and matrix metalloproteinase 2 (MMP-2) sensitive micellar nanoparticles. After systemic administration, only in the tumor tissues could both the nanovehicles dissociate via responding to two special tumor microenvironments, with Dox-TCO and Tz released and then immediately triggering the prodrug activation through the IEDDA reaction. The hierarchically regulated and locally confined Dox liberation led to dramatically decreased side-effects that were much lower than those of the clinical Doxorubicin Hydrochloride Liposomal Injection (Doxil), while the antitumor therapeutic effect was potent.