Cancer cell-targeted cisplatin prodrug delivery in vivo via metabolic labeling and bioorthogonal click reaction

Selective activation of prodrugs in breast cancer using metabolic glycoengineering and the tetrazine ligation bioorthogonal reaction

Increasing the selectivity of chemotherapies by converting them into prodrugs that can be activated at the tumour site decreases their side effects and allows discrimination between cancerous and non-cancerous cells. Herein, the use of metabolic glycoengineering (MGE) to selectively label MCF-7 breast cancer cells with tetrazine (Tz) activators for subsequent activation of prodrugs containing the trans-cyclooctene (TCO) moiety by a bioorthogonal reaction is demonstrated. Three novel Tz-modified monosaccharides, Ac4ManNTz 7, Ac4GalNTz 8, and Ac4SiaTz 16, were used for expression of the Tz activator within sialic-acid rich breast cancer cells' surface glycans through MGE. Tz expression on breast cancer cells (MCF-7) was evaluated versus the non-cancerous L929 fibroblasts showing a concentration-dependant effect and excellent selectivity with ≥35-fold Tz expression on the MCF-7 cells versus the non-cancerous L929 fibroblasts. Next, a novel TCO-N-mustard prodrug and a TCO-doxorubicin prodrug were analyzed in vitro on the Tz-bioengineered cells to probe our hypothesis that these could be activated via a bioorthogonal reaction. Selective prodrug activation and restoration of cytotoxicity were demonstrated for the MCF-7 breast cancer cells versus the non-cancerous L929 cells. Restoration of the parent drug's cytotoxicity was shown to be dependent on the level of Tz expression where the Ac4ManNTz 7 and Ac4GalNTz 8 derivatives (20 µM) lead to the highest Tz expression and full restoration of the parent drug's cytotoxicity. This work suggests the feasibility of combining MGE and tetrazine ligation for selective prodrug activation in breast cancer.

Stromal and tumor immune microenvironment reprogramming through multifunctional cisplatin-based liposomes boosts the efficacy of anti-PD-1 immunotherapy in pancreatic cancer

The dense stromal barrier in pancreatic cancer tissues blocks intratumoral delivery and distribution of chemotherapeutics and therapeutic antibodies, causing poor chemoimmunotherapy responses. We designed a multi-targeted pH-sensitive liposome which encapsulates cisplatin (Pt) in its water core (denoted as ATF@Pt Lps) and shows high affinity for uPAR receptors in pancreatic cancer cells, tumor-associated macrophages, and cancer-associated fibroblasts. Systemic administration of ATF@Pt Lps enabled overcoming the central stromal cellular barrier and effective drug delivery into tumor cells, resulting in a strong therapeutic response in a Panc02 cell derived transplanted tumor mouse model. More importantly, ATF@Pt Lps degradation of collagen contributes to the infiltration of CD8+ T cells into tumors as well as an enhanced accumulation of anti PD-1 monoclonal antibodies. Furthermore, the killing of tumor cells by Pt also leads to the release of tumor antigens, which promote the proliferation of immune cells, especially CD83+ cells, Th1 CD4+ cells, and CD8+ cytotoxic T cells, that converted an immunoscore "cold" pancreatic cancer into a pro-immune "hot" tumor. A further combination with an immune checkpoint agent, anti PD-1 antibodies that inhibit PD-1, can enhance tumor specific cytotoxic T cell response. Accordingly, ATF@Pt Lps displays multi-targeting, controlled drug release, stromal disruption, enhanced penetration, killing of cancer cells, modification of the immunosuppressive microenvironment, and enhancement of immunity. This study provides important mechanistic information for the further development of a combination of ATF@Pt Lps and anti PD-1 antibodies for the effective treatment of pancreatic cancer.

Bioorthogonal activation of prodrugs, for the potential treatment of breast cancer, using the Staudinger reaction

Selective prodrug activation at a tumor site is crucial to maximise the efficiency of chemotherapy approaches and minimise side effects due to off-site activation. In this paper, a new prodrug activation strategy is reported based on the bioorthogonal Staudinger reaction. The feasibility of this prodrug activation strategy was initially demonstrated using 9-azido sialic acid 4 as a trigger and two novel triphenylphosphine-modified N-mustard-PRO 10 and doxorubicin-PRO 12 prodrugs in an HPLC-monitored release study. Then, the azide reporter group was introduced on cancer cells' surfaces through metabolic glycoengineering of sialic acid-rich surface glycans using azide-modified monosaccharides (9-azido sialic acid 4, tetra-O-acetylated-9-azido sialic acid 5 and tetra-O-acetyl azidomannosamine). Next, the N-mustard-PRO 10 and doxorubicin-PRO 12 prodrugs were employed in vitro with the bioengineered cells, and activation of the prodrugs, which allowed selective release of the cytotoxic moiety at the tumour cell, was assessed. Release of the parent drugs from the prodrugs was shown to be dependent on the level of metabolic labelling, where tetra-O-acetyl azidomannosamine allowed the highest level of azide reporter generation in tumor cells and led to full recovery of the parent cytotoxic drug's potency. The selectivity of azide expression on breast cancer MCF-7 cells versus normal fibroblast L929 cells was also probed, with the 9-azido sialic acid and tetra-O-acetylated-9-azido sialic acid showing ∼17-fold higher azide expression on the former. Taken together, these data demonstrate the feasibility of the Staudinger reaction for selective activation of prodrugs targeted to the MCF-7 breast cancer cells.

Cell-based Relay Delivery Strategy in Biomedical Applications

The relay delivery strategy is a two-step targeting approach based on two distinct modules in which the first step with an initiator is to artificially create a target/environment which can be targeted by the follow-up effector. This relay delivery concept creates opportunities to amplify existing or create new targeted signals through deploying initiators to enhance the accumulation efficiency of the following effector at the disease site. As the "live" medicines, cell-based therapeutics possess inherent tissue/cell homing abilities and favorable feasibility of biological and chemical modifications, endowing them the great potential in specifically interacting with diverse biological environments. All these unique capabilities make cellular products great candidates that can serve as either initiators or effectors for relay delivery strategies. In this review, we survey recent advances in relay delivery strategies with a specific focus on the roles of various cells in developing relay delivery systems.

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

Recent applications of CBT-Cys click reaction in biological systems

Click chemistry is a hot topic in many research fields. A biocompatible reaction from fireflies has attracted increasing attention since 2009. Herein, we focus on the firefly-sourced click reaction between cysteine (Cys) and 2-cyanobenzothiazole (2-CBT). This reaction has many excellent properties, such as rapidity, simplicity and high selectivity, which make it successfully applied in protein labeling, molecular imaging, drug discovery and other fields. Meanwhile, its unique ability to form nanoparticles expands its applications in biological systems. We review its principle, development, and latest applications in the past 5 years and hope this review provides more profound and comprehensive insights to its further application.

Smart Nanogatekeepers for Tumor Theranostics

Nanoparticulate drug delivery systems (nano-DDSs) are required to reliably arrive and persistently reside at the tumor site with minimal off-target side effects for clinical theranostics. However, due to the complicated environment and high interstitial pressure in tumor tissue, they can return to the bloodstream and cause secondary side effects in normal organs. Recently, a number of nanogatekeepers have been engineered via structure-transformable/stable strategies to overcome this undesirable dilemma. The emerging structure-transformable nanogatekeepers for tumor imaging and therapy are first overviewed here, particularly for nanogatekeepers undergoing structural transformation in tumor microenvironments, cell membranes, and organelles. Thereafter, intelligent structure-stable nanogatekeepers through reversible activation and artificial individualization receptors are overviewed. Finally, the ongoing challenges and prospects of nanogatekeepers for clinical translation are briefly discussed.

EGFR-targeted prodrug activation using bioorthogonal alkene-azide click-and-release chemistry

Epidermal growth factor receptor (EGFR) is overexpressed in many cancers and therefore serves as an excellent target for prodrug activation. Functionalised trans-cyclooctenes (TCO) were conjugated to an EGFR antibody (cetuximab), providing a reagent for pre-targeting and localisation of the bioorthogonal reagent. The TCOs react with a 4-azidobenzyl carbamate doxorubicin prodrug via a [3 + 2]-cycloaddition and subsequent self-immolation leads to release of doxorubicin (click-and-release). In vitro cell-based assays demonstrated proof-of-concept, that cetuximab conjugated to highly strained TCO (AB-d-TCO) could bind to the EGFR in a melanoma cell line, and selectively activate the doxorubicin prodrug. In a non-EGFR expressing melanoma cell line, no significant prodrug activation was observed. In vivo experiments using this combination of AB-d-TCO and the azido-doxorubicin prodrug in a murine melanoma model revealed no significant anti-tumour activity or increased survival, suggesting there was insufficient prodrug activation and drug release at the tumour site.

Cytosolic protein delivery via metabolic glycoengineering and bioorthogonal click reactions

Cytosolic protein delivery holds great potential for the development of protein-based biotechnologies and therapeutics. Currently, cytosolic protein delivery is mainly achieved with the assistance of various carriers. Herein, we present a universal and effective strategy for carrier-free cytosolic protein delivery via metabolic glycoengineering and bioorthogonal click reactions. Ac₄ManNAz (AAM), an azido-modified N-acetylmannosamine analogue, was first employed to label tumor cell surfaces with abundant azido groups via glycometabolism. Then, proteins including RNase A, cytochrome C (Cyt C), and bovine serum albumin (BSA) were covalently modified with dibenzocyclooctyne (DBCO). Based on the highly efficient bioorthogonal click reactions between DBCO and azido, DBCO-modified proteins could be efficiently internalized by azido-labeled cancer cells. RNase A-DBCO could largely maintain its enzymatic activity and, thus, led to notable anti-tumor efficacy in HeLa and B16F10 cells in vitro and in B16F10 xenograft tumors in vivo. This study therefore provides a simple and powerful approach for carrier-free protein delivery and would have broad applicability in anti-tumor protein therapy.

Understanding selectivity of metabolic labelling and click-targeting in multicellular environments as a route to tissue selective drug delivery

Cancer cells generally exhibit higher metabolic demands relative to that of normal tissue cells. This offers great possibilities to exploit metabolic glycoengineering in combination with bio-orthogonal chemistry reactions to achieve tumour site-targeted therapeutic delivery. This work addresses the selectivity of metabolic glycan labelling in diseased (i.e., cancer) versus normal cells grown in a multicellular environment. Dibenzocylooctyne (DBCO)-bearing acetylated-d-mannosamine (Ac4ManNDBCO) was synthesised to metabolically label three different types of cell lines originating from the human lung tissues: A549 adenocarcinomic alveolar basal epithelial cells, MeT5A non-cancerous mesothelial cells, and MRC5 non-cancerous fibroblasts. These cell lines displayed different labelling sensitivity, which trended with their doubling time in the following order: A549 ≈ MeT5A > MRC5. The higher metabolic labelling efficiency inherently led to a higher extent of specific binding and accumulation of the clickable N3-conjugated gold nanoparticles (N3-AuNps, core diameter = 30 nm) in the DBCO-glycan modified A549 and MeT5A cells, but to a less prominent effect in MRC5 cells. These findings demonstrate that relative rates of cell metabolism can be exploited using metabolic labelling to recruit nanotherapeutics whilst minimising non-specific targeting of surrounding tissues.