Therapeutic efficacy of IL-2-loaded hydrogels in a mouse tumor model

Protein click chemistry and its potential for medical applications

A revolution in chemical biology occurred with the introduction of click chemistry. Click chemistry plays an important role in protein chemistry modifications, providing specific, sensitive, rapid, and easy-to-handle methods. Under physiological conditions, click chemistry often overlaps with bioorthogonal chemistry, defined as reactions that occur rapidly and selectively without interfering with biological processes. Click chemistry is used for the posttranslational modification of proteins based on covalent bond formations. With the contribution of click reactions, selective modification of proteins would be developed, representing an alternative to other technologies in preparing new proteins or enzymes for studying specific protein functions in different biological processes. Click-modified proteins have potential in diverse applications such as imaging, labeling, sensing, drug design, and enzyme technology. Due to the promising role of proteins in disease diagnosis and therapy, this review aims to highlight the growing applications of click strategies in protein chemistry over the last two decades, with a special emphasis on medicinal applications.

Engineered Cytokines for Cancer and Autoimmune Disease Immunotherapy

Cytokine signaling is critical to a range of biological processes including cell development, tissue repair, aging, and immunity. In addition to acting as key signal mediators of the immune system, cytokines can also serve as potent immunotherapies with more than 20 recombinant products currently Food and Drug Administration (FDA)-approved to treat conditions including hepatitis, multiple sclerosis, arthritis, and various cancers. Yet despite their biological importance and clinical utility, cytokine immunotherapies suffer from intrinsic challenges that limit their therapeutic potential including poor circulation, systemic toxicity, and low tissue- or cell-specificity. In the past decade in particular, methods have been devised to engineer cytokines in order to overcome such challenges and here, the myriad strategies are reviewed that may be employed in order to improve the therapeutic potential of cytokine and chemokine immunotherapies with applications in cancer and autoimmune disease therapy, as well as tissue engineering and regenerative medicine. For clarity, these strategies are collected and presented as they vary across size scales, ranging from single amino acid substitutions, to larger protein-polymer conjugates, nano/micrometer-scale particles, and macroscale implants. Together, this work aims to provide readers with a timely view of the field of cytokine engineering with an emphasis on early-stage therapeutic approaches.

Injectable Hydrogels as Local Depots at Tumor Sites for Antitumor Immunotherapy and Immune-Based Combination Therapy

Despite the encouraging clinical responses of several human cancers to immunotherapy, the efficacy of this treatment remains limited by variable objective response rates and severe systemic immune-related adverse events. To overcome these issues, injectable hydrogels have been developed as local depots that permit the sustained release of single or multiple immunotherapy agents, including traditional immunomodulatory factors, immune checkpoint blocking antibodies, and exogenous immune cells. The antitumor efficacy of immunotherapy can also be enhanced by its combination with other therapeutic approaches, including chemotherapy, radiotherapy, and phototherapy. Despite local treatment strategies, potent systemic antitumor immune responses with low systemic toxicity can be obtained, leading to significant local and abscopal tumor-killing, reduced tumor metastasis, and the prevention of tumor recurrence. This review highlights recent progress in injectable hydrogel-based local depots for tumor immunotherapy and immune-based combination therapy. Moreover, the proposed mechanisms responsible for these antitumor effects are discussed.

Advances and trends of hydrogel therapy platform in localized tumor treatment: A review

Due to limitations of treatment and the stubbornness of infiltrative tumor cells, the outcome of conventional antitumor treatment is often compromised by a variety of factors, including severe side effects, unexpected recurrence, and massive tissue loss during the treatment. Hydrogel-based therapy is becoming a promising option of cancer treatment, because of its controllability, biocompatibility, high drug loading, prolonged drug release, and specific stimuli-sensitivity. Hydrogel-based therapy has good malleability and can reach some areas that cannot be easily touched by surgeons. Furthermore, hydrogel can be used not only as a carrier for tumor treatment agents, but also as a scaffold for tissue repair. In this review, we presented the latest researches in hydrogel applications of localized tumor therapy and highlighted the recent progress of hydrogel-based therapy in preventing postoperative tumor recurrence and improving tissue repair, thus proposing a new trend of hydrogel-based technology in localized tumor therapy. And this review aims to provide a novel reference and inspire thoughts for a more accurate and individualized cancer treatment.

Cell and tissue engineering in lymph nodes for cancer immunotherapy

In cancer, lymph nodes (LNs) coordinate tumor antigen presentation necessary for effective antitumor immunity, both at the levels of local cellular interactions and tissue-level organization. In this review, we examine how LNs may be engineered to improve the therapeutic outcomes of cancer immunotherapy. At the cellular scale, targeting the LNs impacts the potency of cancer vaccines, immune checkpoint blockade, and adoptive cell transfer. On a tissue level, macro-scale biomaterials mimicking LN features can function as immune niches for cell reprogramming or delivery in vivo, or be utilized in vitro to enable preclinical testing of drugs and vaccines. We additionally review strategies to induce ectopic lymphoid sites reminiscent of LNs that may improve antitumor T cell priming.

Polymeric nanoparticles and microparticles for the delivery of peptides, biologics, and soluble therapeutics

Biologically derived therapeutics, or biologics, are the most rapidly growing segment of the pharmaceutical marketplace. However, there are still unmet needs in improving the delivery of biologics. Injectable polymeric nanoparticles and microparticles capable of releasing proteins and peptides over time periods as long as weeks or months have been a major focus in the effort to decrease the frequency of administration. These particle systems fit broadly into two categories: those composed of hydrophilic and those composed of hydrophobic polymeric scaffolds. Here we review the factors that contribute to the slow and controlled release from each class of particle, as well as the effects of synthesis parameters and product design on the loading, encapsulation efficiency, biologic integrity, and release profile. Generally, hydrophilic scaffolds are ideal for large proteins while hydrophobic scaffolds are more appropriate for smaller biologics without secondary structure. Here we also introduce a Flash NanoPrecipitation method that has been adopted for encapsulating biologics in nanoparticles (40-200nm) at high loadings (50-75wt.%) and high encapsulation efficiencies. The hydrophilic gel interior and hydrophobic shell provide an opportunity to combine the best of both classes of injectable polymeric depots.

Effectiveness of slow-release systems in CD40 agonistic antibody immunotherapy of cancer

Slow-release delivery has great potential for specifically targeting immune-modulating agents into the tumor-draining area. In prior work we showed that local treatment of slowly delivered anti-CD40 antibody induced robust anti-tumor CD8+ T cell responses without systemic toxicity. We now report on the comparison of two slow-release delivery systems for their use in antibody-based immunotherapy of cancer. Anti-CD40 agonistic antibody delivered locally in mineral oil Montanide ISA 51 or in dextran-based microparticles activated tumor-specific T cell activation. Both slow-release formulations significantly decreased systemic side-effects compared to systemic administration of anti-CD40 antibody. However, dextran-based microparticles caused serious local inflammation associated with unwanted rapid outgrowth of tumors instead of the tumor clearance observed with delivery in Montanide. We therefore conclude that Montanide ISA 51 is to be preferred as a slow-release agent for CD40 agonist immunotherapy of cancer.

Biodegradable dextran hydrogels for protein delivery applications

The rapid development of protein-based pharmaceuticals over recent decades has tremendously increased the need for suitable delivery systems, guaranteeing a safe and controlled delivery of proteinacious drugs. Hydrogels offer good opportunities as protein delivery systems or tissue engineering scaffolds owing to an inherent biocompatibility. Their hydrophilic, soft and rubbery nature ensures minimal tissue irritation and a low tendency of cells and proteins to adhere to the hydrogel surface. A variety of both natural and synthetic polymers have been used for the design of hydrogels, in which network formation is established by chemical or physical crosslinking. This review introduces the general features of hydrogels and focuses on dextran hydrogels in particular. Chemically and physically crosslinked systems are described and their potential suitability as protein delivery systems, as well as tissue engineering scaffolds are discussed. Special attention is given to network properties, protein delivery, degradation behavior and biocompatibility.

Click chemistry in peptide-based drug design

Click chemistry is an efficient and chemoselective synthetic method for coupling molecular fragments under mild reaction conditions. Since the advent in 2001 of methods to improve stereochemical conservation, the click chemistry approach has been broadly used to construct diverse chemotypes in both chemical and biological fields. In this review, we discuss the application of click chemistry in peptide-based drug design. We highlight how triazoles formed by click reactions have been used for mimicking peptide and disulfide bonds, building secondary structural components of peptides, linking functional groups together, and bioconjugation. The progress made in this field opens the way for synthetic approaches to convert peptides with promising functional leads into structure-minimized and more stable forms.

Hydrogels for protein delivery in tissue engineering

Tissue defects caused by diseases or trauma present enormous challenges in regenerative medicine. Recently, a better understanding of the biological processes underlying tissue repair led to the establishment of new approaches in tissue engineering which comprise the combination of biodegradable scaffolds and appropriate cells together with specific environmental cues, such as growth or adhesive factors. These factors (in fact proteins) have to be loaded and sustainably released from the scaffolds in time. This review provides an overview of the various hydrogel technologies that have been proposed to control the release of bioactive molecules of interest for tissue engineering applications. In particular, after a brief introduction on bioactive protein drugs that have remarkable relevance for tissue engineering, this review will discuss their release mechanisms from hydrogels, their encapsulation and immobilization methods and will overview the main classes of hydrogel forming biomaterials used in vitro and in vivo to release them. Finally, an outlook on future directions and a glimpse into the current clinical developments are provided.

Facile formation of dynamic hydrogel microspheres for triggered growth factor delivery

Dynamic hydrogels have emerged as an important class of biomaterials for temporal control over growth factor delivery. In this study we formed dynamic hydrogel microspheres from protein-polymer conjugates using an aqueous two-phase suspension polymerization process. This polymerization process enabled rapid microsphere formation without the use of an organic phase, surfactants, mechanical strain or toxic radical initiators. The microspheres' size distribution was modulated by varying the protein-polymer conformation in the pre-polymer solution. Notably, the protein's ligand-induced, nanometer-scale conformational change translated to maximum hydrogel volume changes of 76±10%. The magnitude of the microspheres' volume change was tuned by varying the crosslinking time and ligand identity. After characterizing the microspheres' dynamic properties, we encapsulated two important therapeutic proteins, vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2), in the hydrogel microspheres and characterized how the microspheres' dynamic properties controlled their release. Significantly, the aqueous two-phase suspension polymerization process enabled high encapsulation efficiencies (65.8±4.8% and 79.5±3.0% for VEGF and BMP-2, respectively). Also, the microspheres' ligand-induced volume change triggered VEGF and BMP-2 release at specific, predetermined times. There are hundreds of proteins that undergo well-characterized conformational changes that could be processed into hydrogel microspheres via aqueous two-phase suspension polymerizations. Therefore, this approach could be used to form dynamic, growth-factor-releasing hydrogel microspheres that respond to a broad range of specific biochemical ligands.

A review on composite liposomal technologies for specialized drug delivery

The combination of liposomes with polymeric scaffolds could revolutionize the current state of drug delivery technology. Although liposomes have been extensively studied as a promising drug delivery model for bioactive compounds, there still remain major drawbacks for widespread pharmaceutical application. Two approaches for overcoming the factors related to the suboptimal efficacy of liposomes in drug delivery have been suggested. The first entails modifying the liposome surface with functional moieties, while the second involves integration of pre-encapsulated drug-loaded liposomes within depot polymeric scaffolds. This attempts to provide ingenious solutions to the limitations of conventional liposomes such as short plasma half-lives, toxicity, stability, and poor control of drug release over prolonged periods. This review delineates the key advances in composite technologies that merge the concepts of depot polymeric scaffolds with liposome technology to overcome the limitations of conventional liposomes for pharmaceutical applications.

Liquid-liquid two-phase systems for the production of porous hydrogels and hydrogel microspheres for biomedical applications: A tutorial review

Macroporous hydrogels may have direct applications in regenerative medicine as scaffolds to support tissue formation. Hydrogel microspheres may be used as drug-delivery vehicles or as building blocks to assemble modular scaffolds. A variety of techniques exist to produce macroporous hydrogels and hydrogel microspheres. A subset of these relies on liquid-liquid two-phase systems. Within this subset, vastly different types of polymerization processes are found. In this review, the history, terminology and classification of liquid-liquid two-phase polymerization and crosslinking are described. Instructive examples of hydrogel microsphere and macroporous scaffold formation by precipitation/dispersion, emulsion and suspension polymerizations are used to illustrate the nature of these processes. The role of the kinetics of phase separation in determining the morphology of scaffolds and microspheres is also delineated. Brief descriptions of miniemulsion, microemulsion polymerization and ionotropic gelation are also included.

Modular injectable matrices based on alginate solution/microsphere mixtures that gel in situ and co-deliver immunomodulatory factors

Biocompatible polymer solutions that can crosslink in situ following injection to form stable hydrogels are of interest as depots for sustained delivery of therapeutic factors or cells, and as scaffolds for regenerative medicine. Here, injectable self-gelling alginate formulations obtained by mixing alginate microspheres (as calcium reservoirs) with soluble alginate solutions were characterized for potential use in immunotherapy. Rapid redistribution of calcium ions from microspheres into the surrounding alginate solution led to crosslinking and formation of stable hydrogels. The mechanical properties of the resulting gels correlated with the concentration of calcium-reservoir microspheres added to the solution. Soluble factors such as the cytokine interleukin-2 were readily incorporated into self-gelling alginate matrices by simply mixing them with the formulation prior to gelation. Using alginate microspheres as modular components, strategies for binding immunostimulatory CpG oligonucleotides onto the surface of microspheres were also demonstrated. When injected subcutaneously in the flanks of mice, self-gelling alginate formed soft macroporous gels supporting cellular infiltration and allowing ready access to microspheres carrying therapeutic factors embedded in the matrix. This in situ gelling formulation may thus be useful for stimulating immune cells at desired locales, such as solid tumors or infection sites, as well as for other soft tissue regeneration applications.

A chitosan hydrogel-based cancer drug delivery system exhibits synergistic antitumor effects by combining with a vaccinia viral vaccine

Cancer treatment combining chemotherapy and immunotherapy has been vigorously exploited to further improve cancer therapeutic efficacy. This study investigated a new chemoimmunotherapy approach utilizing hydrogel as a local anti-cancer drug delivery system. Chitosan hydrogel containing doxorubicin (CH-DOX) and vaccinia virus vaccine expressing Sig/E7/LAMP-1 (Vac-Sig/E7/LAMP-1) were used as chemoimmunotherapeutic agents. It was found that intratumoral injection of CH-DOX effectively inhibited tumor growth itself and, in addition, exhibited a synergistic antitumor effect in combination with a vaccinia virus-based vaccine. This combination did not decrease but rather increased the number of tumor-specific CD8(+) T cells primed by vaccinia virus-mediated vaccination; the resulting antitumor effects were further improved up to 60 days as compared with monotherapy after tumor challenge, and the survival of tumor-bearing mice was dramatically prolonged. This study is a pioneer report that demonstrates the use of a biodegradable hydrogel system as an anti-cancer drug delivery system for successful chemoimmunotherapy. It is hoped that, this study can provide a foundation for a rational approach to improve antitumor efficacy of chemoimmunotherapy.

In vitro and in vivo protein delivery from in situ forming poly(ethylene glycol)–poly(lactide) hydrogels

Previous studies have shown that stereocomplexed hydrogels are rapidly formed in situ by mixing aqueous solutions of eight-arm poly(ethylene glycol)-poly(L-lactide) and poly(ethylene glycol)-poly(D-lactide) star block copolymers (denoted as PEG-(PLLA)(8) and PEG-(PDLA)(8), respectively). In this study, in vitro and in vivo protein release from stereocomplexed hydrogels was investigated. These hydrogels were fully degradable under physiological conditions. Proteins could be easily loaded into the stereocomplexed hydrogels by mixing protein containing aqueous solutions of PEG-(PLLA)(8) and PEG-(PDLA)(8) copolymers. The release of the relatively small protein lysozyme (d(h)=4.1 nm) followed first order kinetics and approximately 90% was released in 10 days. Bacteria lysis experiments showed that the released lysozyme had retained its activity. The relatively large protein IgG (d(h)=10.7 nm) could be released from stereocomplexed hydrogels with nearly zero order kinetics, wherein up to 50% was released in 16 days. The in vitro release of the therapeutic protein rhIL-2 from stereocomplexed hydrogels also showed nearly zero order kinetics, wherein up to 45% was released in 7 days. The therapeutic efficacy of stereocomplexed hydrogels loaded with 1x10(6) IU of rhIL-2 was studied using SL2-lymphoma bearing DBA/2 mice. The PEG-(PLLA)(8)/PEG-(PDLA)(8)/rhIL-2 mixture could be easily injected intratumorally. The released rhIL-2 was therapeutically effective as the tumor size was reduced and the cure rate was 30%, whereas no therapeutic effect was achieved when no rhIL-2 was given. However, the cure rate of rhIL-2 loaded stereocomplexed hydrogels was lower, though not statistically significant, compared to that of a single injection with 1x10(6) IU of free rhIL-2 at the start of the therapy (cure rate=70%). The therapeutic effect of rhIL-2 loaded stereocomplexed hydrogels was retarded for approximately 1-2 weeks compared to free rhIL-2, most likely due to a slow, constant release of rhIL-2 from the hydrogels.

Pharmacokinetic and pharmacodynamic evaluation of a novel in situ forming poly(ethylene glycol)-based hydrogel for the controlled delivery of the camptothecins

Inadequate drug delivery, due to problems associated with achieving constant therapeutic blood levels, has hampered the use of anticancer agents of the camptothecin (CPT) class. The objective of the current studies was to develop a depot delivery system for the water-soluble analog of CPT, topotecan (TPT). In this study, a 2-phase drug depot consisting of TPT-loaded liposomes entrapped in a poly(ethylene glycol) hydrogel was designed. Physically entrapped unaltered TPT displayed a rapid release rate from the hydrogel. Controlled release was demonstrated in vitro and in vivo from the 2-phase system with constant blood levels being achieved for several days in rats. Cytotoxicity and antitumor activity were also evaluated in rats inoculated with syngeneic MAT B III breast cancer cells. Rats treated with the liposome-loaded hydrogel displayed significantly longer tumor growth suppression and did not exhibit body weight loss compared to those treated with other delivery modes. These experiments constitute a proof-of-principle of the 2-phase depot concept and its potential value for enhancing safety and efficacy in chemotherapy.

Effect of polymerization conditions on the network properties of dex-HEMA microspheres and macro-hydrogels

Dextran-hydroxy-ethyl-methacrylate (dex-HEMA) hydrogels in the form of microspheres are an attractive system for the controlled delivery of protein drugs. In this work, the microspheres were prepared by a water-in-water emulsion polymerization process. The polymerization reaction was initiated by potassium peroxodisulfate (KPS) and catalyzed by N,N,N',N'-tetramethylethylenediamine (TEMED). The effect of the initiator concentration, reaction temperature and pH on the mechanical and network properties of the microspheres were investigated. The size and size distribution of the microspheres, equilibrium water content, and methacrylate conversion were also determined. The mechanical properties of single microspheres were measured by a micromanipulation technique and the rheological characteristics of the same material in the form of macroscopic hydrogel slabs were determined by a controlled stress rheometer. The results showed that the Young's moduli of the microspheres and of macroscopic slabs measured by these two methods were in good agreement. Higher KPS initiator concentrations resulted in a more rapid polymerization with a shorter gelation and lag time, and a higher Young's modulus of the gels. An increase in temperature also resulted in a more rapid polymerization with a shorter gelation and lag time. However, the Young's modulus of the gels decreased with an increase in polymerization temperature. The pH had no significant effect on the mechanical properties of the microspheres. This study demonstrates that the network properties of dex-HEMA hydrogels can be tailored by the polymerization conditions, which opens the possibility to modulate the release rate of entrapped compounds.

In situ crosslinked biodegradable hydrogels loaded with IL-2 are effective tools for local IL-2 therapy

We investigated the therapeutic efficacy of recombinant human interleukin-2 (rhIL-2)-loaded, in situ gelling, physically crosslinked dextran hydrogels, locally applied to SL2 lymphoma in mice. The physical crosslinking was established by stereocomplex formation between d-lactic acid oligomers and l-lactic acid oligomers grafted separately to dextrans. The stereocomplex hydrogel as described in our manuscript has several favourable characteristics, which enables its use as system for the controlled release of pharmaceutically active proteins. Firstly, the hydrogel system is a physically crosslinked system. In physically crosslinked gels, the use of chemical crosslinking agents is avoided. Such agents can potentially inactivate the protein and can covalently link the protein to the hydrogel network. Secondly, the hydrogel formation takes place at room temperature and physiological pH, and, importantly, in an all-aqueous environment. All factors are important to preserve the three-dimensional structure, and thus the biological activity, of the protein to be entrapped and released from the gels. Thirdly, the gel formation does not occur instantaneously. This means that a liquid formulation can be injected which solidifies after injection (in situ gel formation is possible). Fourthly, no pH drop during degradation is expected during degradation. As a control, free rhIL-2 was administered locally in either a single injection or at five consecutive days. All mice received the same total dose of rhIL-2. The rhIL-2-loaded hydrogels released most IL-2 over a period of about 5 days. The biocompatibility and biodegradability of the gels were excellent, as there were no acute or chronic inflammatory reaction and as the gels were replaced completely by fibroblasts after 15 days. The therapeutic efficacy of rhIL-2-loaded in situ gelled hydrogels is very good, as was demonstrated in DBA/2 mice bearing SL2. The therapeutic effect of a single application of gels loaded with 1 x 10(6) IU rhIL-2 is at least comparable to the therapeutic effect of injection of an equal dose of free rhIL-2. All mice cured with rhIL-2-loaded hydrogels survived a subsequent challenge, rejecting 10(6) intraperitoneal (i.p.) injected SL2 cells. In conclusion, this study demonstrates that in situ gelling, physically crosslinked dextran hydrogels slowly release encapsulated rhIL-2 in such a way that it is intact and biologically and therapeutically active. These hydrogels may greatly enhance the clinical applicability of rhIL-2 immunotherapy as only a single treatment is required and as these hydrogels are completely biodegradable.

Measurement of cytotoxic activity in experimental cancer

Several tumor cell systems have been developed for investigating the cytotoxic activity of different substances. The Ehrlich ascytic tumor (EAT) has been studied extensively and used to increase our comprehension of immunotherapy (Yamamoto and Naraparaju. Cancer Res 57:2187-2192, 1997). In this study, we evaluated the ability of an enzymatic method to assess cytotoxic tumor activity in vitro. To prepare cytotoxic cells, lymphoid cells isolated from the lymph nodes of C57BL/6 mice were activated with interleukin-2 (IL-2). The cytotoxic activity to EAT cells was assessed by an enzymatic test using lactate dehydrogenase (LDH), an enzyme widely distributed in mammalian tissues which, in the presence of NAD/NADH, converts lactate to pyruvate. IL-2 treatment of lymph node cells induced a greater percentage of lysis (75.47%) than effector cells that were not treated with IL-2 (42.38%). This suggests that IL-2 directly activates effector cells and not tumor cells. The results demonstrate that the LDH release-measurement method can be utilized to assay cytotoxic cell-mediated activity in which murine tumor cells act as the target. Levels of IL-2 appear to have a direct correlation to cellular immunity and treatment with these substances may strengthen immune function and decrease the pace of disease development.

IL-2 loaded dextran microspheres with attractive histocompatibility properties for local IL-2 cancer therapy

Biodegradable dextran microspheres (MS) were developed as a slow-release system for interleukin-2 (IL-2) to apply them for local IL-2 therapy of cancer. We describe the tissue reactions induced by these MS without or with IL-2 in rats. Dextran MS stain bright red-purple with the periodic acid Schiff (PAS), visualising the exact spot of IL-2 release and its relation to the histological reaction pattern. Subcutaneously injected MS always form a well-circumscribed deposit. In the first 2 days there is a PMN inflammation within the MS-deposit, but the surroundings show only a scanty inflammatory reaction. The PMN reaction is replaced by an abundant macrophage reaction in particular in the MS-deposit. At day 21 a fibrous capsule of about 50 mum surrounds the deposit. The effect of IL-2 administered in its free form is mainly vascular, with vascular dilatation, vascular leakage and oedema. It is remarkable that lymphocytes are present in the injection area already at day 2. When IL-2 releasing MS were used, the various reactions induced by IL-2 and MS were amplified leading to local necrosis. We conclude that neither placebo MS nor IL-2 leads to necrosis after subcutaneous injection in rats. In contrast, when IL-2 was released from MS, then massive necrosis was induced. This might be due to increased phagocytosis or changes in the micro-niche due to the release of humoral factors by the infiltrating cells. This is probably fortuitous for local IL-2 therapy of cancer, as massive necrosis of tumour cells can be expected to lead to an increased antitumour reaction.