Glioblastoma (GBM) is the most common primary malignant brain tumor in adults. According to current data, the median survival of patients from the date of diagnosis ranges 12–14 months, with the proportion of GBM patients living longer than five years being 6.8%. The latter figure strongly depends on the age at which the diagnosis was made [1]. The standard treatment for GBM involves three stages: surgical tumor resection followed by radiation and chemotherapy. Currently, temozolomide (TMZ) is the primary chemotherapeutic agent for first-line treatment of GBM, the use of which has increased the median patient survival by only two months, from 12.1 to 14.6 months. Thus, the current therapy does not show high effectiveness, mainly due to high inter- and intra-tumoral heterogeneity, the invasive nature of the tumor, its molecular-genetic plasticity, as well as its pronounced resistance to radiation and chemotherapy [2][3].

The cytotoxic mechanism of TMZ action is based on the process of DNA alkylation. Thanks to its lipophilic nature, TMZ can cross the blood–brain barrier and is hydrolyzed with the release of an active metabolite that methylates guanine and adenine in DNA. Particularly significant is the O-methylation of guanine at the sixth position (O⁶), forming O⁶-methylguanine, which pairs with thymine instead of cytosine. The mismatch repair system in this case replaces O⁶-methylguanine with adenine, thereby introducing mutations. Furthermore, excessive activity of repair systems leads to single- and double-strand breaks, significantly inhibiting gene expression and replication. This destabilizes the genetic material and disrupts the cell cycle, triggering apoptosis and tumor cell death [4]. However, almost invariably, after a short period of time, the tumor resumes its growth and acquires a more aggressive phenotype due to the presence of intrinsic or acquired mechanisms of TMZ resistance. As a result, patients with GBM inevitably experience tumor recurrence [5].

One of the most well-studied mechanisms of TMZ resistance is mediated by the O⁶-methylguanine-DNA methyltransferase (MGMT), an enzyme that removes the methyl group from O⁶-methylguanine. This enzyme neutralizes DNA damage caused by alkylating agents and promotes the development of tumor cell resistance to these agents [5]. However, MGMT expression is suppressed when the promoter of its gene is methylated. The use of TMZ for treating GBM patients with a methylated MGMT promoter has increased their median survival to 21.2 months, compared to 14 months for patients with an unmethylated promoter. Thus, the methylation status of the MGMT promoter is one of the recognized and clinically applicable prognostic markers for GBM treatment [6]. However, although some patients showed initial improvement after TMZ treatment, tumor growth was observed to resume, leading to recurrence due to the development of acquired resistance [7].

The repair activity of MGMT is an important, but not the sole, factor that contributes to the development of TMZ resistance. In recent years, many other molecular mechanisms of GBM chemoresistance to TMZ have been studied [8]. One such mechanism is the adaptation of redox metabolism in cancer cells in response to TMZ treatment [9]. Cellular models of GBM have shown that TMZ-induced damage of genetic material triggers an increased production of reactive oxygen species (ROS) and leads to the development of oxidative stress, DNA damage, lipid membrane damage, and protein damage, as well as the activation of pro-apoptotic cascades [10]. Tumor cells, in turn, can enhance the activity of antioxidant systems, reducing the level of oxidative stress [11]. In this regard, targeting redox metabolism with the purpose of shifting the balance toward oxidative stress and cancer cell death may become an effective approach for anticancer therapy.

In this study, we carry out a review of the redox-dependent mechanisms of chemoresistance development in GBM, as well as analyze the potential of drugs that destabilize redox homeostasis for glioblastoma therapy.

The search for scientific literature was conducted in scientific databases (PubMed, Google Scholar) using the following keywords: glioblastoma, chemotherapy resistance, oxidative stress, antioxidant system, redox metabolism. As a result, 60 publications were selected and analyzed, including review and original research articles, as well as clinical studies published between 2004–2025 in international peer-reviewed journals.

ROS production is an integral part of cellular metabolism. ROS are formed as a result of the partial reduction of molecular oxygen O2. Two main types of ROS with the greatest physiological significance are the superoxide anion radical (O2⁻) and hydrogen peroxide (H2O2), from which all other forms originate. The superoxide anion radical O2⁻ is generated in cells due to the leakage of electrons from complexes I and III in the mitochondria, as well as through the directed and controlled production of O2⁻ by the members of NADPH oxidase (NOX) family [12]. The superoxide anion radical O2⁻ itself exhibits a relatively low reactivity; however, its interaction with other reactive species, such as H2O2 or nitric oxide (II) (NO), can form highly reactive compounds, including hydroxyl radical OH⁻ or peroxynitrite OONO⁻. These compounds can destabilize the structures of proteins, DNA, and lipids [12]. The superoxide anion radical, O2⁻, dismutates either spontaneously or with the help of superoxide dismutase (SOD) enzymes into O2 and H2O2, and this pathway serves as the primary source of intracellular H2O2. The relative stability of H2O2 compared to other ROS and its ability to diffuse over long distances make this molecule a participant in various intracellular signaling and regulatory processes. At the same time, H2O2 predominantly interacts with SH-groups of cysteine residues in proteins, altering their conformation and functional activity [13]. The accepted physiological concentration of H2O2 in the cytoplasm ranges 1–10 nM. When the intracellular concentration of H2O2 increases to 50–70 nM, it activates the processes related to proliferation, survival, and migration, while H2O2 levels exceeding 100 nM for an extended period of time trigger cell death [13]. Excessive concentrations of H2O2 can interact with other ROS, forming highly reactive compounds, and irreversibly oxidize proteins, leading to oxidative stress and cell death. In response, cells have developed diverse mechanisms for multi-level regulation of intracellular ROS content [14].

Important enzymes that metabolize ROS are superoxide dismutase (SOD), which catalyzes the dismutation of O2⁻ into O2 and H2O2, and catalase, which converts excess H2O2 into O2 and H2O. These enzymes neutralize high levels of ROS rather rapidly and are effective in combating significant oxidative stress [15]. For more precise regulation of H2O2 levels, enzymatic systems which are sensitive to low concentrations of H2O2 have developed additional levels of activity regulation. These include the glutathione-dependent and thioredoxin-dependent antioxidant systems [16].

The key enzymes of the glutathione-dependent antioxidant defense system are glutathione peroxidases (GPX). They reduce H2O2 to H2O, using glutathione (GSH) as an electron donor. The oxidation of GSH leads to the condensation of SH-groups from cysteine residues of two GSH molecules, forming an oxidized form of glutathione (GSSG). The reduction of GSSG is performed by the FAD-containing enzyme glutathione reductase using NADPH [17].

Glutathione is a tripeptide consisting of glycine, cysteine, and glutamate. The synthesis of this antioxidant is highly dependent on the availability of cysteine, which is mediated by the cystine/glutamate antiporter SLC7A11 (xCT). The intracellular glutathione content ranges 1–15 mM, which helps maintain the SH-groups of most intracellular proteins in their reduced state [18]. In addition to reducing the active sites of GPX, glutathione conjugates with cysteine residues of various proteins through the action of glutathione S-transferase enzymes, while the removal of glutathione and the reduction of protein cysteines are carried out by the low-molecular-weight oxidoreductases referred to as glutaredoxins. These processes, on the one hand, protect protein SH-groups from oxidative stress, and on the other hand, provide redox-dependent regulation of many protein kinases, transcription factors, signaling proteins, and metabolic enzymes [19].

The thioredoxin-based antioxidant defense system relies on the functioning of peroxiredoxins (PRDX). Under normal conditions, they can neutralize up to 90% of ROS, reducing H2O2 to H2O through the oxidation of cysteine residues in the active sites. Their reduction requires a special type of low-molecular-weight reductases — thioredoxins. In turn, thioredoxin reduction is performed by thioredoxin reductases (TrxR), which accept electrons from NADPH [20].

The functioning of glutathione and thioredoxin antioxidant systems depends on an adequate level of NADPH, which is primarily maintained by the activity of pentose phosphate pathway enzymes, such as glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase [21]. Furthermore, in addition to the main antioxidant systems, there are proteins that, rather than directly inactivating ROS, either limit their production or eliminate metabolites formed with ROS involvement in the cell. These proteins include enzymes that metabolize xenobiotics, such as aldehyde dehydrogenases (ALDH), aldo-keto reductases (AKR), carbonyl reductases (CBR), and NADPH:quinone reductases (NQO), which mitigate the consequences of ROS cytotoxicity by preventing the accumulation of lipid oxidation products, disrupting the electron transport chain, and depleting the glutathione pool [22].

Cellular systems for maintaining stable ROS levels are strictly controlled by a complex network of transcription factors. One of the primary regulators of cellular redox homeostasis is the nuclear factor NRF2. Its activity is strictly regulated by the KEAP1 protein, which targets NRF2 for proteasomal degradation in the absence of oxidative stress. NRF2 activation occurs in response to increased ROS levels, which destabilize the KEAP1 structure, allowing the released NRF2 to translocate to the nucleus. In the nucleus, NRF2 interacts with sequences in the promoter regions of its target genes, known as antioxidant response elements (ARE), and activates genes that encode ROS scavengers, such as catalase, glutathione peroxidases, and peroxiredoxins, as well as proteins involved in glutathione metabolism and the pentose phosphate pathway. This enables a multifaceted response to elevated ROS levels in cells [23].

Among other significant regulators of cellular redox processes are various transcription factors and master regulators activated in response to stress stimuli, such as hypoxia (hypoxia-inducible factor, HIF-1α), endoplasmic reticulum stress and unfolded protein accumulation (heat shock protein, HSP1), inflammation, ATP deficiency, and genetic material damage (nuclear factor NF-κB). To counteract oxidative stress, they shift the metabolism from oxidative phosphorylation to glycolysis, reducing ROS production by mitochondrial electron transport chain complexes, enhance glutathione synthesis and reduction, and activate antioxidant defense systems in mitochondria and the endoplasmic reticulum. At the same time, other regulators, such as p53, Jun, Fos, ATF, and FOXO proteins, which are responsible for regulating the cell cycle, proliferation, differentiation, and apoptosis, are also capable of activating the expression of antioxidant systems in response to induced oxidative stress [22]. Although all these factors respond to different threshold levels of ROS and are unlikely to be activated simultaneously during oxidative stress, they coordinate their activities in response to various types of stress.

Thus, cells regulate the maintenance of moderate ROS levels through multiple mechanisms. It is known that the activity of many proteins involved in the antioxidant systems, as well as transcription factors ensuring their high expression, are enhanced in tumors, including GBM, indicating their role in tumor growth, invasiveness, and therapy resistance [22].

In a response to the cytotoxic effects of chemotherapy, GBM utilizes several mechanisms involving antioxidant systems and mitochondria [24].

Chemotherapy-treated GBM cells show an increased expression of NRF2. The overexpression of NRF2 is observed in recurrent tumors, being correlated with GBM therapy resistance and low patient survival [25]. Genome-wide CRISPR/Cas9 screening of genes conferring GBM chemoresistance has also confirmed NRF2 to be one of the key factors ensuring tumor survival when exposed to TMZ [26].

As mentioned previously, the antioxidant function of NRF2 is associated with the activation of the glutathione and thioredoxin antioxidant systems, as well as the system of enzymes maintaining sufficient NADPH concentration. These three systems contribute to the development and maintenance of a resistant phenotype in GBM. For example, the disruption of glutathione synthesis by suppressing NRF2 expression or the inhibition of γ-glutamylcysteine ligase can sensitize cells to TMZ [11]. It was also shown that an increase in the expression of the cystine/glutamate antiporter xCT, which imports cysteine for glutathione synthesis, correlates with the expression of markers of a more aggressive GBM phenotype and reprograms tumor cells into a chemotherapy-resistant subtype [27]. Enzymes that help maintain cellular redox homeostasis and use glutathione as a cofactor also contribute to the development of a chemoresistant phenotype in GBM. Thus, the high activity of glutathione reductase, which replenishes the reduced glutathione pool, corresponded to a low tumor sensitivity to TMZ [28].

Furthermore, high expression of thioredoxin reductase and thioredoxin correlates with low survival rates in patients with various brain tumors, including GBM [29]. Inhibition of thioredoxin sensitizes the tumor to the action of chemotherapeutic agents [30].

It was been observed that GBM can overcome TMZ-induced oxidative stress by utilizing a mechanism of mitochondrial metabolic switching. It is known that TMZ can methylate not only nuclear but also mitochondrial DNA. In response to this stress, the GBM mitochondria remodel the electron transport chain (ETC). For instance, in TMZ-resistant cells, a reduced activity of complex I and ATP synthase, coupled with an enhanced activity of complexes III and IV, was observed. This ensures a more efficient electron flow through the ETC for ATP energy production and reduced ROS generation [31]. In addition, increased expression of the mitochondrial isoform of superoxide dismutase, SOD2, was demonstrated in TMZ-resistant GBM cells. This expression was not coordinated by NRF2, but rather by Sp1, another transcription factor, which mediates stress response. This finding reveals a new mechanism for the emergence of redox-associated resistance in GBM [32].

Finally, enzymes of detoxification systems, such as aldehyde dehydrogenases (ALDH), play a significant role in the development of GBM chemoresistance. Enzymes of this family catalyze the oxidation of intracellular aldehydes to their corresponding carboxylic acids, neutralizing lipid peroxidation products, such as malondialdehyde and 4-hydroxy-2-nonenal, and thereby preventing their interaction with enzyme active sites and destabilization of protein homeostasis [33]. At the same time, increased expression of one of the ALDH isoforms, ALDH1A3, is an important factor in maintaining the stem-like properties of tumor cells, serving as a marker of a more aggressive mesenchymal phenotype in GBM [34].

The presented data indicate that increased expression of antioxidant systems in GBM is a significant factor contributing to GBM resistance to TMZ chemotherapy. Therefore, the identification of compounds that destabilize antioxidant activity in GBM, for their use in combination with established chemotherapeutic agents as a therapy, is a promising solution for overcoming tumor resistance.

Currently, there exist numerous synthetic and natural compounds with a pro-oxidant effect induced by the production of ROS or inhibition of tumor cell antioxidant systems. Drugs based on some of these compounds are undergoing preclinical and clinical trials for GBM therapy.

Since the intracellular redox homeostasis of GBM largely depends on the quantity and status of glutathione within the cell, destabilization of the synthesis of this tripeptide and disruption of the delivery of its precursor molecules appear to be an effective way to block the glutathione-dependent antioxidant system. Reducing glutathione content in cells induces ferroptosis, a type of non-apoptotic type of cell death, associated with lipid peroxidation, which is often observed in malignant tumor cells [35]. One of the components that directly suppresses the maintenance of the glutathione pool is L-buthionine sulfoximine (BSO), which irreversibly inhibits γ-glutamyl cysteine synthetase, an enzyme that catalyzes the rate-limiting step of glutathione synthesis. Although BSO has not demonstrated significant efficacy in monotherapy, its use has enhanced the effect of cisplatin and carboplatin in experiments on GBM cell cultures, suggesting that BSO may be a potential candidate for use in combination with primary GBM therapy [36].

Another promising strategy for suppressing the glutathione system in GBM involves reducing the availability of cysteine for glutathione synthesis, e.g., by inhibiting the activity of the xCT cystine/glutamate antiporter. Sulfasalazine was used to inhibit xCT, effectively targeting this transporter in GBM cells. However, clinical trials of this drug have currently been discontinued due to its high systemic toxicity [37]. Another xCT inhibitor that has been tested in GBM models is erastin, which, in addition to suppressing cysteine import into cells, destabilizes mitochondrial voltage-dependent anion channels (VDAC) and disrupts the integrity of the mitochondrial outer membrane [38]. Erastin also inhibits cystathionine-γ-lyase, a key enzyme in sulfur-containing amino acid metabolism that catalyzes the conversion of cystathionine into cysteine, thus completely blocking the possibility of obtaining cysteine for glutathione synthesis [39]. Erastin has demonstrated a more potent cytotoxic effect than sulfasalazine, while also sensitizing GBM to TMZ [39]. Finally, RSL-3, another ferroptosis inducer that targets GPX4 inhibition, is currently being tested as a potential GBM therapy. This compound effectively suppressed GBM growth in vitro and enhanced the efficacy of TMZ [40].

The thioredoxin antioxidant system has been recognized as a promising target for combating malignant tumors. The activity of peroxiredoxins, the most abundant antioxidant proteins in cells, is almost entirely dependent on the amount of reduced thioredoxin, the availability of which is regulated by TrxR. Therefore, impairing the activity of such a crucial enzyme can lead to an almost complete destabilization of cellular redox homeostasis. The selenocysteine-containing active site of TrxR is susceptible to irreversible inhibition by alkylating agents, certain natural compounds, such as curcumin and piperlongumine, as well as metal-organic complexes based on mercury, gold, silver, platinum, and gadolinium [41]. One such gold complex is auranofin, an anti-inflammatory drug. The pronounced pro-oxidant and antitumor effect of auranofin was repeatedly demonstrated on GBM cell cultures, including those with a therapy-resistant phenotype [42][43].

Drugs based on compounds that induce enhanced generation and accumulation of ROS have shown antitumor effects in GBM in preclinical and clinical studies. One such drug is arsenic trioxide (As2O3), which triggers ROS release and suppresses the activity of various antioxidant enzymes. During phase I and II clinical trials of the efficacy of a combination of As2O3 with radiation therapy and TMZ, no significant side effects were identified. An increase in the overall and relapse-free survival was demonstrated in GBM patients [44].

High-dose ascorbate is a promising pro-oxidant agent for tumor therapy. Its use induces enhanced generation of H2O2 in tumor cells, leading to their death. Ascorbate also increases tumor sensitivity to radiation and chemotherapy. The combination of ascorbate with TMZ was shown to be effective in clinical trials for patients with primary GBM [45]. Phytocompounds, such as resveratrol [46] and cannabidiol [27], also sensitize tumors to TMZ through ROS production, as demonstrated in various in vitro and in vivo GBM models.

Another promising target for destabilizing redox homeostasis in GBM is NADPH:quinone oxidoreductase 1 (NQO1). This is an enzyme of the xenobiotic detoxification and antioxidant defense system, often overexpressed in tumors [47]. Certain natural compounds from the quinone class, such as β-lapachone or tanshindiol [48], are substrates of NQO1. Their metabolism leads to the depletion of the NADPH pool, which is necessary for the function of antioxidant systems. This results in oxidative stress, which sensitizes tumors to TMZ [49]. In cell cultures and GBM xenograft models, the use of quinone derivatives, such as menadione [50] or coenzyme Q [51], in combination with ascorbate, was shown to selectively affect tumor cells and suppress their growth and proliferation through the induction of oxidative stress.

Thus, preclinical and clinical studies of drugs based on compounds that destabilize the redox homeostasis of cancer cells have shown that their use in combination with conventional radiation therapy and TMZ chemotherapy can reduce tumor resistance to chemotherapy, hamper the transformation of the tumor into a more aggressive phenotype, and improve survival rates in patients with GBM.

The adaptation of redox metabolism to stress conditions induced by chemotherapy, mediated by the activation of antioxidant systems, is a key mechanism of GBM resistance. Therefore, the development of therapeutic strategies aimed at inhibiting these systems for effectively inducing oxidative stress is a highly relevant task. In this field, three main directions can be distinguished: development of selective inhibitors of antioxidant system components, optimization of local delivery of these inhibitors, and selection of personalized approaches to GBM therapy based on the data on the expression of antioxidant systems and the assessment of redox status in tumors.

Currently, approaches aimed at selective inhibition of such components of the antioxidant system as thioredoxin reductase are being actively developed. Understanding the molecular structure of the TrxR active site and its interaction mechanisms with inhibitors opens possibilities for designing and discovering new drugs. As was discussed above, auranofin, a gold-containing drug, irreversibly interacts with the selenocysteine residue in the TrxR active site. Along with that, organic mercury compounds, such as thimerosal (TmHg) and its metabolite ethylmercury (EtHg), are also considered as inhibitors. These substances exhibit a high affinity for selenocysteine residues in the active site of TrxR, while also having the ability to cross the blood–brain barrier. These properties could enable effective GBM therapy. Studies into GBM cell cultures showed that the exposure to micromolar concentrations of TmHg and EtHg leads to a significant increase in oxidative stress and cell death via apoptosis. Furthermore, exhibiting no significant toxicity, TmHg is widely used as a preservative in many drug components, making its use as an antitumor agent feasible [52]. Future research should focus on both investigating the drugs mentioned above and on searching for new selective TrxR inhibitors.

Many inhibitors of antioxidant systems have a rather limited effect due to poor penetration through the blood–brain barrier (BBB) and the inability to achieve effective concentrations in brain tissues. Strategies for improving drug delivery across the BBB include enhancing the properties of the substance itself and its formulation, e.g., increasing its solubility and stability in liquid media by adding polysorbates or ethanol. Direct chemical modification of the substance by adding lipophilic groups can also be used. One promising direction consists in encapsulating the active substance to form nanoparticles [53]. For this purpose, polymer and PLGA particles, as well as such inorganic nanoparticles as magnetic or gold silicon nanoparticles are employed. However, the most popular method for delivering active agents to the brain is the use of cationic liposomes. Another promising strategy is the application of active targeting implying the attachment of ligands to nanoparticles that specifically bind to the BBB receptors, such as transferrin or insulin [41]. PLGA nanoparticles with transferrin loaded with drugs like doxorubicin and paclitaxel significantly suppressed glioma growth in in vivo experiments [54]. Preclinical studies demonstrated the efficacy of methods that temporarily and reversibly increase BBB permeability using focused ultrasound (FUS) or osmotically active components, such as mannitol [41]. Almost all the aforementioned strategies can be applied to deliver the inhibitors of antioxidant systems; however, they require adaptation of the methods to specific molecules.

Development of individualized approaches to GBM therapy requires the assessment of parameters that reflect the redox homeostasis status of tumor cells [55]. As mentioned previously, increased NRF2 expression correlates with GBM therapy resistance and low patient survival [9]. At the same time, high NRF2 expression in GBM is associated with increased sensitivity of tumor cells to ferroptosis. Therefore, the use of ferroptosis-inducing drugs may sensitize tumors with high NRF2 expression to therapy [56]. In order to determine NRF2 activity in tumor cells and, accordingly, predict the effectiveness of therapy for a specific patient, the expression of target genes controlled by this transcription factor can be evaluated, such as the catalytic (GCLC) and modifier (GCLM) subunits of γ-glutamyl cysteine ligase, which catalyze the rate-limiting step of glutathione synthesis; NADPH:quinone oxidoreductase 1 (NQO1); heme oxygenase 1 (HMOX1); thioredoxin reductase 1 (TXNRD1); and sulfiredoxin 1 (SRXN1). Overexpression of these genes suggests hyperactivation of the NRF2 pathway and may indicate the “redox sensitivity” of the tumor [57].

When predicting the response to chemotherapy and, therefore, developing personalized approaches to treating various oncological diseases with redox agents, the status of the glutathione system in tumor cells can serve as a predictive marker [58]. Assessment of the expression of genes related to glutathione metabolism [59], as well as direct measurement of the total glutathione content and the ratio of its oxidized and reduced forms in tumor tissue, can provide valuable information about the antioxidant reserves of the tumor. A low basal level of intracellular glutathione may indicate a potential vulnerability to glutathione synthesis inhibitors [43]. The activity of glutathione peroxidases, glutathione reductase, and glutathione S-transferases provides additional information about functioning of this antioxidant system [60].

Combination therapy including both TrxR inhibitors and GSH synthesis inhibitors may be more effective than standard therapy for patients with high NRF2 expression and its target genes, as well as a low GSH/GSSG ratio.

The development of an effective therapy for glioblastoma, one of the most aggressive tumors, remains a significant challenge. Conventional therapy for such tumors based on the use of temozolomide only slightly improves clinical outcomes in patients. The high activity of the antioxidant systems in tumor cells restrains oxidative stress induced by standard chemotherapy agents, ensuring the survival and chemoresistance of the tumor. The conducted review of available data on redox-dependent mechanisms of chemoresistance development in GBM, as well as the analysis of prospects for drugs that destabilize tumor redox homeostasis in GBM therapy, has been shown that the use of drugs aimed at destabilizing the redox homeostasis of GBM cells, either as monotherapy or in combination with conventional chemotherapeutic agents, can improve survival rates in GBM patients. Nevertheless, the development of more effective therapeutic regimens using inhibitors of antioxidant systems or pro-oxidants is still necessary. The search for more selective and stable inhibitors of key antioxidant enzymes, the optimization of approaches for their delivery to the tumor, as well as the stratification of GBM patients based on biomarkers associated with redox homeostasis, open prospects for more effective GBM therapy.

Authors’ contributions. All the authors confirm that they meet the ICMJE criteria for authorship. The most significant contributions were as follows: Timofei K. Bulgakov — original draft writing; Lada A. Startseva — investigation; Maria Yu. Kordyukova — conceptualization, writing and editing; Evgeny K. Shevchenko — project administration; Vsevolod V. Belousov — funding acquisition, supervision.