Ionizing radiation-induced drug release is a combined chemoradiation therapy, which aims to reduce the systemic toxicity of chemotherapeutics. Radiation is used for both radiotherapy and to trigger the release of a chemotherapeutic. To understand radiation-induced drug activation and to design new radiation-sensitive chemotherapeutics, it is important to become familiar with the underlying reaction mechanisms. Here, we provide an overview of the crucial process of water radiolysis induced by ionizing radiation and the mechanisms of reactive species generation. We also discuss the reactivity of these species with cellular components and chemical functional groups, to give insight into selective drug activation in complex cellular environments. Finally, we discuss recent progress on radiation-induced drug release focusing on the reaction of water radiolysis products with drug caging groups and the yield of released drugs. We aim to bridge the gap between basic chemical processes in water radiolysis and their relevance for drug release and provide suggestions on the design of radiation-sensitive prodrugs or nanocarriers.

Photosensitizers have significant potential as radiosensitizers in cancer treatment, yet the mechanism of ionizing-radiation-induced singlet oxygen (¹O₂) generation remains unclear. Here, we systematically investigated ¹O₂ production by the photosensitizer Chlorin e6 (Ce6) using the Singlet Oxygen Sensor Green probe and imidazole/ p -nitroso- N , N -dimethylaniline detection methods, evaluating the effects of photon energy (X-rays up to 310 kV and ⁶⁰Co gamma rays at 1.17 and 1.33 MeV), dose, and dose rate. Ce6 produced more ¹O₂ with increasing photon energy. At 5 Gy, the lowest dose rate (0.005 Gy/min) yielded significantly more ¹O₂ than higher dose rates (7–0.05 Gy/min). Scavenging experiments identified superoxide anions (·O₂⁻) as a key intermediate. We propose that, unlike classical triplet-state photosensitization, ionizing radiation induces Ce6 radical cations (Ce6^⋅+), which react with radiation-induced ·O₂⁻ to generate ¹O₂. These findings suggest potential for photosensitizer-radiation combinations in low-dose-rate therapies, although further biological validation and consideration of tumor redox status are required.

Boronic acid and ester-caged prodrugs have been widely investigated in cellular-generated hydrogen peroxide triggered release. Although it is well-known that ionizing radiation generates hydrogen peroxide in aqueous solution, using this approach to activate boronic acid or ester-based prodrugs suffers from low H₂ O₂ yields and thus low uncaging efficiency. However, the organochloride peroxyl radical formed from irradiating an aqueous solution of an organochloride may increase the uncaging efficiency. In this study, we used a boronic acid-caged coumarin derivative to quantify the yield of oxidation induced by clinical doses of radiation (less than 8 Gy), and boronic acid-caged gemcitabine to assess the activation of a prodrug upon irradiation. Irradiation of the coumarin derivative in phosphate buffered saline shows a low yield of 0.048 µM per Gy, and the prodrug after irradiation has only limited toxicity to the U87 cell line, indicating limited uncaging. The oxidation of boronic acid can be greatly enhanced by the peroxyl radical generated from irradiation of dilute PBS-organochloride solutions, with the yield increasing to 0.13 µM per Gy. Moreover, the oxidation by peroxyl radical can be catalyzed by N,N-dimethylaniline derivatives, increasing the yield to 0.19 µM per Gy. Clinical dose irradiation of the caged gemcitabine derivative in a solution of PBS with trichloroethanol and 2-(dimethylamino)benzoic acid shows efficient tumor cell killing and a comparable toxicity with that of the parent drug, indicating efficient uncaging.

Dynamic covalent (DCv) ureas are highly diverse chemical moieties and have become one of the most used motifs in self-healing materials and beyond. This review summarizes the historical development, properties, and applications of DCv ureas in different fields of organic chemistry, materials chemistry, and biomedical applications and provides guidance on the design of different DCv ureas depending on stability and dynamicity requirements.

The controlled release of drugs using local ionizing radiation presents a promising approach for targeted cancer treatment, particularly when applied in concurrent radio-chemotherapy. In these approaches, radiation-generated reactive species often play an important role. However, the reactive species that can be used to trigger release have low yield and lack selectivity. Here, we demonstrate the generation of highly oxidative species when aqueous solutions containing low concentrations of organochlorides (such as chloroform) are irradiated with ionizing radiation at therapeutically relevant doses. These reactive species were identified as peroxyl radicals, which formed in a reaction cascade between organochlorides and aqueous electrons. We employed stilbene-based probes to investigate the oxidation process, showing double bond oxidation and cleavage. To translate this reactivity into a radiation-sensitive material, we synthesized a micelle-forming amphiphilic block copolymer that has stilbene as the linker between two blocks. Upon exposure to ionizing radiation, the oxidation of stilbene led to the cleavage of the polymer, which induces the dissociation of the block-copolymer micelles and the release of loaded drugs.

Dextran-based hydrogels are promising therapeutic materials for drug delivery, tissue regeneration devices, and cell therapy vectors, due to their high biocompatibility, along with their ability to protect and release active therapeutic agents. This report describes the synthesis, characterization, and application of a new dynamic covalent dextran hydrogel as an injectable depot for peptide vaccines. Dynamic covalent crosslinks based on double Michael addition of thiols to alkynones impart the dextran hydrogel with shear-thinning and self-healing capabilities, enabling hydrogel injection. These injectable, non-toxic hydrogels show adjuvant potential and have predictable sub-millimolar loading and release of the peptide antigen SIINFEKL, which after its release is able to activate T-cells, demonstrating that the hydrogels deliver peptides without modifying their immunogenicity. This work demonstrates the potential of dynamic covalent dextran hydrogels as a sustained-release material for the delivery of peptide vaccines.

Living organisms are capable of dynamically changing their structures for adaptive functions through sophisticated reaction-diffusion processes. Here we show how active supramolecular hydrogels with programmable lifetimes and macroscopic structures can be created by relying on a simple reaction-diffusion strategy. Two hydrogel precursors (poly(acrylic acid) PAA/CaCl₂ and Na₂CO₃) diffuse from different locations and generate amorphous calcium carbonate (ACC) nanoparticles at the diffusional fronts, leading to the formation of hydrogel structures driven by electrostatic interactions between PAA and ACC nanoparticles. Interestingly, the formed hydrogels are capable of autonomously disintegrating over time because of a delayed influx of electrostatic-interaction inhibitors (NaCl). The hydrogel growth process is well explained by a reaction-diffusion model which offers a theoretical means to program the dynamic growth of structured hydrogels. Furthermore, we demonstrate a conceptual access to dynamic information storage in soft materials using the developed reaction-diffusion strategy. This work may serve as a starting point for the development of life-like materials with adaptive structures and functionalities.

Over the last few decades, the study of more complex, chemical systems closer to those found in nature, and the interactions within those systems, has grown immensely. Despite great efforts, the need for new, versatile, and robust chemistry to apply in CRNs remains. In this Feature Article, we give a brief overview over previous developments in the field of systems chemistry and how β′-substituted Michael acceptors (MAs) can be a great addition to the systems chemist's toolbox. We illustrate their versatility by showcasing a range of examples of applying β′-substituted MAs in CRNs, both as chemical signals and as substrates, to open up the path to many applications ranging from responsive materials, to pathway control in CRNs, drug delivery, analyte detection, and beyond.

Low-molecular-weight hydrogels are attractive scaffolds for drug delivery applications because of their modular and facile preparation starting from inexpensive molecular components. The molecular design of the hydrogelator results in a commitment to a particular release strategy, where either noncovalent or covalent bonding of the drug molecule dictates its rate and mechanism. Herein, we demonstrate an alternative approach using a reaction-coupled gelator to tune drug release in a facile and user-defined manner by altering the reaction pathway of the low-molecular-weight gelator (LMWG) and drug components through an acylhydrazone-bond-forming reaction. We show that an off-the-shelf drug with a reactive handle, doxorubicin, can be covalently bound to the gelator through its ketone moiety when the addition of the aldehyde component is delayed from 0 to 24 h, or noncovalently bound with its addition at 0 h. We also examine the use of an l-histidine methyl ester catalyst to prepare the drug-loaded hydrogels under physiological conditions. Fitting of the drug release profiles with the Korsmeyer-Peppas model corroborates a switch in the mode of release consistent with the reaction pathway taken: increased covalent ligation drives a transition from a Fickian to a semi-Fickian mode in the second stage of release with a decreased rate. Sustained release of doxorubicin from the reaction-coupled hydrogel is further confirmed in an MTT toxicity assay with MCF-7 breast cancer cells. We demonstrate the modularity and ease of the reaction-coupled approach to prepare drug-loaded self-assembled hydrogels in situ with tunable mechanics and drug release profiles that may find eventual applications in macroscale drug delivery.

In the quest for stimuli-responsive materials with specific, controllable functions, coacervate hydrogels have become a promising candidate, featuring sensitive responsiveness to environmental signals enabling control over sol-gel transitions. However, conventional coacervation-based materials are regulated by relatively non-specific signals, such as temperature, pH or salt concentration, which limits their possible applications. In this work, we constructed a coacervate hydrogel with a Michael addition-based chemical reaction network (CRN) as a platform, where the state of coacervate materials can be easily tuned by specific chemical signals. We designed a pyridine-based ABA triblock copolymer, whose quaternization can be regulated by an allyl acetate electrophile and an amine nucleophile, leading to gel construction and collapse in the presence of polyanions. Our coacervate gels showed not only highly tunable stiffness and gelation times, but excellent self-healing ability and injectability with different sized needles, and accelerated degradation resulting from chemical signal-induced coacervation disruption. This work is expected to be a first step in the realization of a new class of signal-responsive injectable materials.

Living systems can respond to their environment through signal transduction cascades. In these cascades, original stimuli are amplified and translated into reaction or assembly events. In an effort to instill synthetic materials with biomimetic responsivity, we report an aggregation process forming a supramolecular network held together by host-guest interactions that is responsive to nucleophilic chemical signals through a chemical reaction-assembly cascade. In particular, we developed a signal-induced switch between cucurbit[8]uril binary and ternary complexes with cationic bipyridine derivatives where the charge on the bipyridine can be changed through an allylic substitution reaction with the nucleophilic signal. When applied to a multitopic bipyridine guest, the reaction with the nucleophile signals leads to supramolecular network formation where the aggregation rates and final structure depend on the nucleophilicity of the signal. This work opens the door to new opportunities for signal-responsive synthetic materials and interactions with biological systems.

Shunts, alternative pathways in chemical reaction networks (CRNs), are ubiquitous in nature, enabling adaptability to external and internal stimuli. We introduce a CRN in which the recovery of Michael-accepting species is driven by oxidation chemistry. Using weak oxidants can enable access to two shunts within this CRN with different kinetics and a reduced number of side reactions compared to the main cycle that is driven by strong oxidants. Furthermore, we introduce a strategy to recycle one of the main products under flow conditions to partially reverse the CRN and control product speciation throughout time. These findings introduce new levels of control over artificial CRNs, driven by redox chemistry, narrowing the gap between synthetic and natural systems.

We present an approach for detecting thiol analytes through a self-propagating amplification cycle that triggers the macroscopic degradation of a hydrogel scaffold. The amplification system consists of an allylic phosphonium salt that upon reaction with the thiol analyte releases a phosphine, which reduces a disulfide to form two thiols, closing the cycle and ultimately resulting in exponential amplification of the thiol input. When integrated in a disulfide cross-linked hydrogel, the amplification process leads to physical degradation of the hydrogel in response to thiol analytes. We developed a numerical model to predict the behavior of the amplification cycle in response to varying concentrations of thiol triggers and validated it with experimental data. Using this system, we were able to detect multiple thiol analytes, including a small molecule probe, glutathione, DNA, and a protein, at concentrations ranging from 132 to 0.132 μM. In addition, we discovered that the self-propagating amplification cycle could be initiated by force-generated molecular scission, enabling damage-triggered hydrogel destruction.

The redox balance in tumor and diseased cells often leads to the production of reactive oxygen species (ROS). Many ROS-responsive materials based on sulfur oxidation have been reported with the goal of achieving controlled delivery at the tumor. However, these materials often lack responsiveness to low ROS concentrations present in the tumor environment. To address this, we use organocatalysis to achieve an enhanced response of thioether-based nanocarriers triggered by low concentrations of ROS. Using block copolymer micelles that can disassemble through thioether oxidation followed by ester hydrolysis, this work shows how an in-situ-formed imine oxidation catalyst can enhance disassembly kinetics at low millimolar hydrogen peroxide concentrations. The results show that with organocatalysis, Nile Red-loaded micelles release their cargo twice as fast compared to uncatalyzed conditions. This study highlights the potential of organocatalysis as a valuable strategy to enhance the responsiveness of biomarker-triggered delivery systems.

Combination of therapies is a common strategy in cancer treatment. Such combined therapies only have merit provided that there is superior therapeutic outcome with fewer side effects, compared to single therapies. Here, this work explores the possibility to combine chemotherapy with radionuclide therapy using polymeric micelles as a delivery vehicle. For this purpose, this work prepares poly(ε-caprolactone-b-ethylene oxide) (PCL-PEO) micelles and load them simultaneously with paclitaxel (PTX) and ¹⁷⁷Lu(III). This work chooses a 3D tumor spheroid composed of glioblastoma cells (U87) to evaluate the combined treatment. The diffusion of the micelles in the spheroid is investigated by confocal laser scanning microscopy (CLSM) and light-sheet fluorescence microscopy (LSFM). The results show that the micelles are able to penetrate deep into the spheroid within 24 h of incubation and mainly accumulated around or in the lysosomes once in the cell. Subsequently, this work evaluates the cell killing efficiency of the single treatments (PTX or ¹⁷⁷Lu(III)) versus combined treatment (PTX + ¹⁷⁷Lu(III)) by measuring the growth of the spheroids as well as by performing a cell-viability assay. The results indicate that the combined therapy achieves a superior therapeutic outcome with better cell growth inhibition and cell killing efficiency compared to the single treatments.

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

Polycationic carriers promise low cost and scalable gene therapy treatments, however inefficient intracellular unpacking of the genetic cargo has limited transfection efficiency. Charge-reversing polycations, which transition from cationic to neutral or negative charge, can offer targeted intracellular DNA release. We describe a new class of charge-reversing polycation which undergoes a cationic-to-neutral conversion by a reaction with cellular nucleophiles. The deionization reaction is relatively slow with primary amines, and much faster with thiols. In mammalian cells, the intracellular environment has elevated concentrations of amino acids (∼10×) and the thiol glutathione (∼1000×). We propose this allows for decationization of the polymeric carrier slowly in the extracellular space and then rapidly in the intracellular milleu for DNA release. We demonstrate that in a lipopolyplex formulation this leads to both improved transfection and reduced cytotoxicity when compared to a non-responsive polycationic control.

In living systems adaptive regulation requires the presence of nonlinear responses in the underlying chemical networks. Positive feedbacks, for example, can lead to autocatalytic bursts that provide switches between two stable states or to oscillatory dynamics. The stereostructure stabilized by hydrogen bonds provides an enzyme its selectivity, rendering pH regulation essential for its functioning. For effective control, triggers by small concentration changes play roles where the strength of feedback is important. Here we show that the interaction of acid-base equilibria with simple reactions with pH-dependent rate can lead to the emergence of a positive feedback in hydroxide ion concentration during the hydrolysis of some Schiff bases in the physiological pH range. The underlying reaction network can also support bistability in an open system.