In Vivo Targeting Using Arylboronate/Nopoldiol Click Conjugation

Boron enabled bioconjugation chemistries

Novel bioconjugation reactions have been heavily pursued for the past two decades. A myriad of conjugation reactions have been developed for labeling molecules of interest in their native context as well as for constructing multifunctional molecular entities or stimuli-responsive materials. A growing cluster of bioconjugation reactions were realized by tapping into the unique properties of boron. As a rare element in human biology, boronic acids and esters exhibit remarkable biocompatibility. A number of organoboron reagents have been evaluated for bioconjugation, targeting the reactivity of either native biomolecules or those incorporating bioorthogonal functional groups. Owing to the dynamic nature of B-O and B-N bond formation, a significant portion of the boron-enabled bioconjugations exhibit rapid reversibility and accordingly have found applications in the development of reversible covalent inhibitors. On the other hand, stable bioconjugations have been developed that display fast kinetics and significantly expand the repertoire of bioorthogonal chemistry. This contribution presents a summary and comparative analysis of the recently developed boron-mediated bioconjugations. Importantly, this article seeks to provide an in-depth discussion of the thermodynamic and kinetic profiles of these boron-enabled bioconjugations, which reveals structure-reactivity relationships and provides guidelines for bioapplications.

Boronate Ester Hydrogels for Biomedical Applications: Challenges and Opportunities

Boronate ester (BE) hydrogels are increasingly used for biomedical applications. The dynamic nature of these molecular networks enables bond rearrangement, which is associated with viscoelasticity, injectability, printability, and self-healing, among other properties. BEs are also sensitive to pH, redox reactions, and the presence of sugars, which is useful for the design of stimuli-responsive materials. Together, BE hydrogels are interesting scaffolds for use in drug delivery, 3D cell culture, and biofabrication. However, designing stable BE hydrogels at physiological pH (≈7.4) remains a challenge, which is hindering their development and biomedical application. In this context, advanced chemical insights into BE chemistry are being used to design new molecular solutions for material fabrication. This review article summarizes the state of the art in BE hydrogel design for biomedical applications with a focus on the materials chemistry of this class of materials. First, we discuss updated knowledge in BE chemistry including details on the molecular mechanisms associated with BE formation and breakage. Then, we discuss BE hydrogel formation at physiological pH, with an overview of the main systems reported to date along with new perspectives. A last section covers several prominent biomedical applications of BE hydrogels, including drug delivery, 3D cell culture, and bioprinting, with critical insights on the design relevance, limitations and potential.

Insight into ortho-boronoaldehyde conjugation via a FRET-based reporter assay

Ortho-boronoaldehydes react with amine-based nucleophiles with dramatically increased rates and product stabilities, relative to unfunctionalised benzaldehydes, leading to exciting applications across biological and material chemistry. We have developed a novel Förster resonance energy transfer (FRET)-based assay to provide key new insights into the reactivity of these boronoaldehydes, allowing us to track conjugation with unprecedented sensitivity and accuracy under standardised conditions. Our results highlight the key role played by reaction pH, buffer additives, and boronoaldehyde structure in controlling conjugation speed and stability, providing design criteria for further innovations and applications in the field.

Upconversion Nanoparticle-Based Cell Membrane-Coated cRGD Peptide Bioorthogonally Labeled Nanoplatform for Glioblastoma Treatment

Glioblastoma is hard to be eradicated partly because of the obstructive blood-brain barrier (BBB) and the dynamic autophagy activities of glioblastoma. Here, hydroxychloroquine (HDX)-loaded yolk-shell upconversion nanoparticle (UCNP)@Zn0.5Cd0.5S nanoparticle coating with the cyclic Arg-Gly-Asp (cRGD)-grafted glioblastoma cell membrane for near-infrared (NIR)-triggered treatment of glioblastoma is prepared for the first time. UCNPs@Zn0.5Cd0.5S (abbreviated as YSN, yolk-shell nanoparticle) under NIR radiation will generate reactive oxygen species for imposing cytotoxicity. HDX, the only available autophagy inhibitor in clinical studies, can enhance cytotoxicity by preventing damaged organelles from being recycled. The cRGD-decorated cell membrane allowed the HDX-loaded nanoparticles to efficiently bypass the BBB and specifically target glioblastoma cells. Exceptional treatment efficacy of the NIR-triggered chemotherapy and photodynamic therapy was achieved in U87 cells and in the mouse glioblastoma model as well. Our results provided proof-of-concept evidence that HDX@YSN@CCM@cRGD could overcome the delivery barriers and achieve targeted treatment of glioblastoma.

Biomaterial-targeted precision nanoparticle delivery to the injured spinal cord

Drug delivery requires precision in timing, location, and dosage to achieve therapeutic benefits. Challenges in addressing all three of these critical criteria result in poor temporal dexterity, widespread accumulation and off-target effects, and high doses with the potential for toxicity. To address these challenges, we have developed the BiomatErial Accumulating Carriers for On-demand Nanotherapy (BEACON) platform that utilizes an implantable biomaterial to serve as a target for systemically delivered nanoparticles (NPs). With the BEACON system, administered NPs are conjugated with a ligand that has high affinity for a receptor in the implanted biomaterial. To test BEACON, an in vivo spinal cord injury (SCI) model was used as it provides an injury model where the three identified criteria can be tested as it is a dynamic and complicated injury model with no currently approved therapies. Through our work, we have demonstrated temporal dexterity in NP administration by injecting 6 days post-SCI, decreased off-target accumulation with a significant drop in liver accumulation, and retention of our NPs in the target biomaterial. The BEACON system can be applied broadly, beyond the nervous system, to improve systemically delivered NP accumulation at an implanted biomaterial target. STATEMENT OF SIGNIFICANCE: Targeted drug delivery approaches have the potential to improve therapeutic regimens for patients on a case-by-case basis. Improved localization of a therapeutic to site of interest can result in increased efficacy and limit the need for repeat dosing. Unfortunately, targeted strategies can fall short when receptors on cells or tissues are too widespread or change over the course of disease or injury progression. The BEACON system developed herein eliminates the need to target a cell or tissue receptor by targeting an implantable biomaterial with location-controllable accumulation and sustained presentation over time. The targeting paradigm presented by BEACON is widely applicable throughout tissue engineering and regenerative medicine without the need to retool for each new application.

Engineered Polymer-Supported Biorthogonal Nanocatalysts Using Flash Nanoprecipitation

Transition-metal catalysts (TMCs) effect bioorthogonal transformations that enable the generation of therapeutic agents in situ, minimizing off-target effects. The encapsulation of insoluble TMCs into polymeric nanoparticles to generate "polyzymes" has vastly expanded their applicability in biological environments by enhancing catalyst solubility and stability. However, commonly used precipitation approaches provide limited encapsulation efficiency in polyzyme fabrication and result in a low catalytic activity. Herein, we report the creation of polyzymes with increased catalyst loading and optimized turnover efficiency using flash nanoprecipitation (FNP). Polyzymes with controlled size and catalyst loading were fabricated by tuning the process conditions of FNP. The biological applicability of polyzymes was demonstrated by efficiently transforming a non-toxic prodrug into the active drug within cancer cells.

On-Demand Drug Release from Click-Refillable Drug Depots

Stimuli-responsive, on-demand release of drugs from drug-eluting depots could transform the treatment of many local diseases, providing intricate control over local dosing. However, conventional on-demand drug release approaches rely on locally implanted drug depots, which become spent over time and cannot be refilled or reused without invasive procedures. New strategies to noninvasively refill drug-eluting depots followed by on-demand release could transform clinical therapy. Here we report an on-demand drug delivery paradigm that combines bioorthogonal click chemistry to locally enrich protodrugs at a prelabeled site and light-triggered drug release at the target tissue. This approach begins with introduction of the targetable depot through local injection of chemically reactive azide groups that anchor to the extracellular matrix. The anchored azide groups then capture blood-circulating protodrugs through bioorthogonal click chemistry. After local capture and retention, active drugs can be released through external light irradiation. In this report, a photoresponsive protodrug was constructed consisting of the chemotherapeutic doxorubicin (Dox), conjugated to dibenzocyclooctyne (DBCO) through a photocleavable ortho-nitrobenzyl linker. The protodrug exhibited excellent on-demand light-triggered Dox release properties and light-mediated in vitro cytotoxicity in U87 glioblastoma cell lines. Furthermore, in a live animal setting, azide depots formed in mice through intradermal injection of activated azide-NHS esters. After i.v. administration, the protodrug was captured by the azide depots with intricate local specificity, which could be increased with multiple refills. Finally, doxorubicin could be released from the depot upon light irradiation. Multiple rounds of depot refilling and light-mediated release of active drug were accomplished, indicating that this system has the potential for multiple rounds of treatment. Taken together, these in vitro and in vivo proof of concept studies establish a novel method for in vivo targeting and on-demand delivery of cytotoxic drugs at target tissues.

Supramolecular "Click Chemistry" for Targeting in the Body

The fields of precision imaging and drug delivery have revealed a number of tools to improve target specificity and increase efficacy in diagnosing and treating disease. Biological molecules, such as antibodies, continue to be the primary means of assuring active targeting of various payloads. However, molecular-scale recognition motifs have emerged in recent decades to achieve specificity through the design of interacting chemical motifs. In this regard, an assortment of bioorthogonal covalent conjugations offer possibilities for in situ complexation under physiological conditions. Herein, a related concept is discussed that leverages interactions from noncovalent or supramolecular motifs to facilitate in situ recognition and complex formation in the body. Classic supramolecular motifs based on host-guest complexation offer one such means of facilitating recognition. In addition, synthetic bioinspired motifs based on oligonucleotide hybridization and coiled-coil peptide bundles afford other routes to form complexes in situ. The architectures to include recognition of these various motifs for targeting enable both monovalent and multivalent presentation, seeking high affinity or engineered avidity to facilitate conjugation even under dilute conditions of the body. Accordingly, supramolecular "click chemistry" offers a complementary tool in the growing arsenal targeting improved healthcare efficacy.

Boronic acid based dynamic click chemistry: recent advances and emergent applications

Recently, reversible click reactions have found numerous applications in chemical biology, supramolecular chemistry, and biomedical applications. Boronic acid (BA)-mediated cis-diol conjugation is one of the best-studied reactions among them. An excellent understanding of the chemical properties and biocompatibility of BA-based compounds has inspired the exploration of novel chemistries using boron to fuel emergent sciences. This topical review focuses on the recent progress of iminoboronate and salicylhydroxamic-boronate constituted reversible click chemistries in the past decade. We highlight the mechanism of reversible kinetics and its applications in chemical biology, medicinal chemistry, biomedical devices, and material chemistry. This article also emphasizes the fundamental reactivity of these two conjugate chemistries with assorted nucleophiles at variable pHs, which is of utmost importance to any stimuli-responsive biological and material chemistry explorations.