A review of typical biological activities of glycyrrhetinic acid and its derivatives

Glycyrrhetinic acid, a triterpenoid compound primarily sourced from licorice root, exhibits noteworthy biological attributes, including anti-inflammatory, anti-tumor, antibacterial, antiviral, and antioxidant effects. Despite these commendable effects, its further advancement and application, especially in clinical use, have been hindered by its limited druggability, including challenges such as low solubility and bioavailability. To enhance its biological activity and pharmaceutical efficacy, numerous research studies focus on the structural modification, associated biological activity data, and underlying mechanisms of glycyrrhetinic acid and its derivatives. This review endeavors to systematically compile and organize glycyrrhetinic acid derivatives that have demonstrated outstanding biological activities over the preceding decade, delineating their molecular structures, biological effects, underlying mechanisms, and future prospects for assisting researchers in finding and designing novel glycyrrhetinic acid derivatives, foster the exploration of structure–activity relationships, and aid in the screening of potential candidate compounds.


Introduction
Natural products play a crucial role in the exploration of new drugs as they possess broad-spectrum activity against bacteria, fungi, viruses, cancer, and other diseases, and they exhibit a vast array of chemically diverse structures, which hold the potential to serve as lead compounds in drug discovery.2][3][4][5] Among these natural products, glycyrrhetinic acid is the triterpenoid aglycone constituent of glycyrrhizinic acid (Fig. 1), derived from the roots of the licorice plant (Glycyrrhiza glabra). 6,7There are two isomers of glycyrrhetinic acid (GA), one is (3b,18b)-3-hydroxy-11oxoolean-12-en-30-oic acid, oen called 18b-glycyrrhetinic acid or enoxolone, denoted by 18b-GA.Another one is (3b,18a)-3hydroxy-11-oxoolean-12-en-29-oic acid, known as 18a-glycyrrhetinic acid, denoted by 18a-GA, as shown in Fig. 2. 18b-GA is the major bioactive constituent of Glycyrrhiza glabra and has been investigated to possess a wide range of biological activities, including anti-inammatory, antitumor, antibacterial, antiviral, and antioxidant.Apart from these characteristic

Liang Chen
Liang Chen received his B.S. degree from Xuzhou Medical University.He is a graduate student at Hainan Medical University.He is a graduate student working in associate Professor Shuojin Wang's group at Hainan Medical University.activities, glycyrrhetinic acid has been observed to exhibit additional properties, such as anti-diabetic, anticoagulant, immunoregulatory, anti-cholinesterase, antiarrhythmic, and anti-tetanus toxin actions. 80][11][12] To improve the pharmacokinetic properties and enhance the bioactivity, various structural modications of glycyrrhetinic acid have been carried out to develop novel derivatives for making them attractive candidates for further development as potential drug leads; in the process, extensive studies on the structure-activity relationship (SAR) of 18b-GA and its derivatives have been extensively investigated. 13Furthermore, these modications focused on altering the chemical structure, including the introduction of functional groups, changes in stereochemistry, and modications of the aglycone skeleton.Studies on the pharmacological activities of 18b-GA derivatives have shown their potential as therapeutics for various diseases, such as inammatory diseases, cancer, bacterial and viral infections, diabetes, and liver diseases, especially in the past two years.
The references incorporated in this review were exclusively sourced from the databases of Google Scholar, PubMed, and Web of Science.The compilation focusing on 18b-GA and its derivatives was based in works published within the temporal span of 2000 to 2023.Signicantly, the majority of these citations were published within the most recent half-decade, highlighting the contemporaneity of our curated selection.In   addition, we meticulously scrutinized 266 compounds with signicant biological activity from a pool of over 500 derivatives sourced from these cited references.To provide a more comprehensive and organized overview, we have compiled tables summarizing the chemical structures and effects or mechanisms of the typical biological activities of 18b-GA and its derivatives, including anti-inammatory, anti-tumor, antibacterial, antiviral and antioxidant effects.The labeling scheme for the modication sites of all 18b-GA derivatives is described in the form of a diagram.Please refer to Fig. 3 for a visual representation of the labeling scheme.

Anti-inflammatory activity
Inammation is considered to be a driver of many diseases, including arteriosclerosis, cancer, autoimmunity, and chronic infections. 146][17][18][19][20][21] Therefore, the design and optimization of drugs become more complicated.The presence of active ingredients in natural products opens up new opportunities for the development of anti-inammatory drugs.Extensive research has shown that 18b-GA demonstrates anti-inammatory effects and holds signicant potential as a therapeutic agent for various ailments. 22For instance, 18b-GA inhibits the expression of various inammatory mediators, such as intercellular adhesion molecule-1 (ICAM-1), tumor necrosis factor-alpha (TNF-a), cyclooxygenase-2 (Cox-2), and inducible nitric oxide synthase (iNOS), by inhibiting the activity of the nuclear factor-kB (NF-kB) pathway. 23Additionally, 18b-GA has been found to reduce the production of inammatory cytokines by inhibiting the activity of NF-kB and phosphoinositide 3-kinase (PI3K) and inhibiting the production of NO, prostaglandin E 2 (PGE 2 ), and reactive oxygen species (ROS) under lipopolysaccharide (LPS) stimulation. 24However, in an Ana-1 mouse macrophage model, 18b-GA induced the expression of Toll-like receptor 4 and activated the TLR-4 signaling pathway via the myeloid differentiation primary response 88 (MYD88) pathway. 25n recent years, the research of 18b-GA on anti-inammation has been deepened.18b-GA (40 mg kg −1 day −1 ) has been found to effectively improve lung function in ovalbumin (OVA)induced asthma mouse model, reduce lung inammation and inammatory cell inltration, and inhibit the phosphorylation of NF-kB in the treatment of airway allergic inammation.These effects are achieved through a decrease in the levels of interleukin-5 (IL-5) by approximately 40%, interleukin-13 (IL-13) by approximately 30%, and TNF-a by approximately 70%.Additionally, there is an increase in the levels of nuclear factor erythroid 2-related factor2 (Nrf2) by approximately 50% and heme oxygenase 1 (HO-1) by approximately 50%. 26Gupta et al. found that 18b-GA has potential therapeutic effects in treating depression.Specically, it can improve symptoms caused by chronic unpredictable mild stress by activating the brainderived neurotrophic factor (BDNF)/Tropomyosin receptor kinase B (TrkB) signaling pathway in the prefrontal cortex (PFC) and hippocampus.This activation leads to a reduction in neu-roinammation, liver biomarkers, and stress hormones while increasing the body weight and brain neurotransmitter concentrations. 27dditionally, the complex of 18b-GA also exhibits remarkable anti-inammatory activity.Ishida et al. demonstrated that the complex of 18b-GA and hydroxypropyl-b-cyclodextrin can mitigate indomethacin-induced small intestinal injury by reducing TNF-a expression by 27.5%, interleukin-6 (IL-6) by 16.2%, and interleukin-1b (IL-1b) by 17.9% compared to indomethacintreated tissue. 28The salt of 18b-GA and L-arginine can be formed through a co-solvent evaporation reaction, and a solid dispersion called 18b-GA-SD can be created by adding a polymer solvent, Soluplus®, with a hydrophilic-hydrophobic chemical structure.18b-GA-SD has higher solubility, cell utilization rate, and bioavailability than 18b-GA itself.Following treatment with 18b-GA-SD, enzyme-linked immunosorbent assay (ELISA) analysis revealed an increase in LPS-induced secretion levels of cytokines such as IL-1b, IL-6, macrophage inammatory protein-1 (MCP-1), TNF-a, interleukin-23 (IL-23), and interleukin-17A (IL-17A) in RAW 264.7 cells; meanwhile, there was a decrease in the levels of interleukins-4 (IL-4) and -10 (IL-10). 11n the context of COVID-19, 18b-GA has been found to affect the disease by inhibiting the interleukin-17 (IL-17), IL-6, and TNF-a signaling pathways, thereby holding potential as a treatment strategy. 29Another study found that a combination of 18b-GA and vitamin C (VC) treatment for COVID-19 was associated with an increase in immunity and a decrease in inammatory stress, as well as activation of the T cell receptor signaling pathway, regulation of Fc gamma R-mediated phagocytosis, ErbB signaling pathway, and vascular endothelial growth factor signaling pathway. 30Furthermore, highly biocompatible 18b-GA nanoparticles have been synthesized and have shown promise as a treatment strategy for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. 31Zhou et al. demonstrated that 18b-GA inhibited the expression of intercellular adhesion molecule-1 (ICAM-1), TNF-a, COX-2, and iNOS, which was attributed to the inhibition of NF-kB expression and the attenuation of NF-kB nuclear translocation. 32oreover, another study discovered that 18a-GA suppressed the invasion on Matrigel-coated transwells of DU145 prostate cancer cells by regulating the expression of nu NF-kB (p65), vascular endothelial growth factor (VEGF), and metalloproteinase-9 (MMP-9).18a-GA also augmented the expression of non-steroidal anti-inammatory gene-1 (NAG-1) in DU-145 cells, thereby indicating its capacity for anti-inammatory activity against prostate cancer cells. 33The mechanisms underlying the anti-inammatory effects of GA discussed above are graphically depicted in Fig. 4. In the realm of hepatoprotective activity, 18b-GA has been shown to mitigate hepatic inammatory injury caused by hepatitis virus infection by blocking the release of the high mobility group box 1 (HMGB1) cytokine and inhibiting its activity. 34,35Furthermore, 18b-GA has potential as a hepatoprotective agent through activating of Nuclear factor erythroid 2-related factor 2 (Nrf2) and peroxisome proliferator-activated receptor gamma (PPAR-g), and subsequent suppression of NF-kB, and 18b-GA has been shown to protect the liver from cholestatic liver injury induced by lithocholic acid (LCA) by inhibiting the TLR2/NF-kB pathway and upregulating hepatic farnesoid X receptor (FXR) expression, while reducing inammation and promoting bile excretion.][38][39] Since the hepatic protection effect of 18b-GA is not only realized through the anti-inammatory mechanism but could also through the antioxidant mechanism, the review about hepatic protection discussion is in the antioxidant part; Fig. 6 depicts all relevant studies.
In other investigations, various compounds derived from 18b-GA, such as 1-15 (Table 1), have exhibited anti-inammatory effects.For instance, Ma et al. identied three major metabolites (compounds 1-3) produced by the microbial transformation of 18b-GA.These metabolites exhibited potent anti-inammatory activity by inhibiting LPS-induced NO production in mouse microglia BV2 cells. 40The structure and inhibitory activity are shown in Table 1.Another investigation found that compound 4 showed improved pharmacokinetic properties and reduced toxicity in a similar way to fungal metabolism and LPS-induced mouse models. 41][47] Among these compounds, compound 12 was identied as the most potent anti-inammatory agent, exhibiting a signicant reduction in inammatory cytokine levels in the mouse model of AKI by inhibiting TNF-a and IL-6 in a dose-dependent manner.Compound 13 also has anti-inammatory activity, and studies have shown that it interacts with proteins in the inammatory process, such as matrix metalloproteinase MMP9, neutrophil elastase, and thrombin. 48Tu et al. focus on the anti-inammatory activity of novel 18b-GA derivatives.The study evaluated the derivatives' activity in mouse models of acute inammation induced by carrageenan.The results showed that Fig. 4 Anti-inflammatory mechanisms of glycyrrhetinic acid and its derivatives.

Table 1
Chemical structure and anti-inflammation activity of glycyrrhetinic acid and its derivatives 1-21    MAPKs: mitogen-activated protein kinases.MIP-1a: macrophage inammatory protein-1 alpha.11b-HSD: 11b-hydroxysteroid dehydrogenase several compounds demonstrated signicant inhibition of paw edema and leukocyte inltration. 49The results obtained from both in vitro and in vivo experiments indicate that compound 14 and compound 15 exhibit anti-inammatory effects by reducing the expression of NO, pro-inammatory cytokines, and chemokines, such as IL-1b, IL-6, IL-12, TNF-a, MCP-1, and macrophage inammatory protein-1 alpha (MIP-1a) while increasing the expression of anti-inammatory cytokine IL-10.Wang et al.

Compounds
introduced Soluplus®-glycyrrhetinic acid solid dispersion, which signicantly improves the bioavailability and anti-inammatory activity of 18b-GA.The solubility of 18b-GA increased with the addition of Soluplus®, and the bioavailability was enhanced 2.61-fold.The anti-inammatory activity of 18b-GA was also improved by 32.3%. 11Compounds 16-21 have been structurally modied at the C-2 and C-30 carboxyl positions of 18b-GA.][52] In conclusion, 18b-GA has potential therapeutic applications for various conditions due to its anti-inammatory effects.Although more research is required, the use of 18b-GA and its derivatives may provide new avenues for treating inammationrelated diseases.

Antitumor activity
Cancer ravages and cripples the earth's inhabitants, ranking among the foremost destroyers of life. 55For countless years, scholars have been devoting themselves to the quest for a cure for tumors.Presently, the globe is awash with more than 80 conventional anti-tumor medications, ranging from cytotoxic drugs and hormones, to biological response modiers (BRMs) and monoclonal antibodies. 56The majority of anticancer medications exhibit notable toxicity and necessitate administration in periodic cycles to mitigate adverse effects and impede the emergence of drug resistance.However, the excellent vitality of natural compounds adds new impetus to the research and development of anticancer drugs. 57And within this pantheon of treatment options stands the 18b-GA compound-a veritable powerhouse in its ability to vanquish cancerous cells from any part of the human body with unrivaled efficacy.Scores of meticulous studies attest to the fact that this drug is a gamechanger in the ght against various forms of cancer.The sterling performance against malignant cells has been proven time and time again, and it holds immense potential as an agent in the battle against cancer.Wang et al. demonstrated that 18b-GA has potent inhibitory effects on colorectal cancer cell proliferation in vitro and in vivo.This study showed that 18b-GA treatment resulted in a signicant reduction in cell migration, invasion, and wound healing capability, accompanied by the downregulation of matrix metalloproteinase (MMP) expression.Moreover, 18b-GA decreased the protein levels of phosphorylated PI3K, protein kinase B (AKT), Signal Transducer and Activator of Transcription 3 (STAT3), c-Jun N-terminal Kinase (JNK), p38 mitogen-activated protein kinase (p38), and NF-kB p65, where the phosphorylation of PI3K and STAT3 decreased as early as 2 h aer 18b-GA treatment. 58Luo et al. found that 18b-GA-induced apoptosis and G2/M cell cycle arrest and inhibited migration via the ROS/MAPK/STAT3/NF-kB signaling pathways in A549 lung cancer cells.They also found that 18b-GA could reduce tumor growth in a mouse xenogra model.In breast cancer treatment, 59 Shi et al. found that a combination of 18b-GA and doxorubicin enhanced cytotoxicity, apoptosis, and loss of mitochondrial membrane potential via the upregulation of a mitochondrial-dependent apoptosis pathway against MCF-7 (breast adenocarcinoma cell line) cells. 60In recent years, 18b-GA has also been found to have potential in liver cancertargeted therapy.Speciale et al. provided a comprehensive review of the topic. 61he derivatives of 18b-GA have been unearthed to harbor even more potent cancer properties in comparison to the progenitor compound.One of the most remarkable advantages of 18b-GA lies in its all-encompassing efficacy in targeting a myriad of cancer types.It has conspicuously showcased outstanding effectiveness against cancers of the digestive tract, liver, nervous system, reproductive system, immune system, thyroid, and other organ-related cancers.This renders it an invaluable weapon in the war against cancer. 62,63The 18b-GA's anti-cancer effects are believed to stem from its capacity to incite apoptosis, a process of purposeful cell death, in cancer cells.Additionally, it also exhibits anti-inammatory and antioxidant properties that can shield cells from harm and amplify the growth of healthy cells.As demonstrated in Table 2, we have amassed an extensive collection of 18b-GA derivatives with extraordinary anticancer activity.
In the realm of liver cancer treatment, researchers have discovered that 18b-GA holds signicant potential due to its ability to exhibit toxicity against multiple liver cancer cell lines.A study conducted by Lai et al. found that 18b-GA derivatives 46-60 demonstrated selective cell toxicity against human hepatocellular carcinoma, hepG2 (hepatocellular carcinoma cell line) cells, and BEL-7402 (hepatocellular carcinoma cell line) cells. 644][75][76] Researchers also discovered the complex of 18b-GA-conjugated-b-cyclodextrin and emodin's superior cell toxicity against hep3B (hepatocellular carcinoma cell line) cells when compared to emodin alone. 77n the domain of gastrointestinal cancers, encompassing those that affect the mouth, esophagus, colon, and stomach, the extraordinary cytotoxicity of 18b-GA and its derivatives has been strikingly demonstrated, particularly against colon cancer cell lines.The literature is replete with evidence of 18b-GA's potent effects on HCT-116 (colorectal carcinoma cell line), HCT-8 (colorectal adenocarcinoma cell line), DLD-1 (colorectal adenocarcinoma cell line), and HT-29 (colorectal adenocarcinoma cell line) cells.For instance, derivatives 152-154 and 45 exhibit toxicity towards HCT-116, with derivative 45 also affecting HCT-8 cells and DLD-1.9][80] Moreover, Seribian et al.'s study unveiled the high cytotoxicity of 18b-GA 1,9-peroxide on numerous human tumor cell lines,           including HT-29 cells. 81Compounds 22-29 manifest substantial activity against Panc-1 (pancreatic carcinoma-1 cell line) and Panc-28 (pancreatic carcinoma-28 cell line) cells, and compounds 109-114 have been established as inhibitors of MIAPaca2 (pancreatic carcinoma cell line) cells. 66,67,82,83][86][87] In the context of prostate cancer cell lines such as PC-3 (androgen-independent) and LN-Cap, compounds 61-62, 86-90, and 128-143 have demonstrated signicant inhibitory effects. 75,76,97In ovarian cancer cell lines like A2780, compounds 64-71 exhibited inhibitory activity up to 1.5 mM. 90,91Notably, compounds 109-114, 103, 106,102, 144, and 146 displayed notable inhibitory activity against HeLa cells (cervical cancer cell line). 70,71,81,98Additionally, compounds 152-156 showed strong inhibitory activity against MCF-9 breast cancer cell line. 79,101eyond these realms, GA and its derivatives have also exhibited their anticancer activity in other areas.Prior research has established that GA and its derivatives have the ability to inhibit Neurosystem-associated cancer cell lines, such as SH-SY5Y (human neuroblastoma cell line) and SK-N-MC (human neuroblastoma cell line). 66,84In the investigation conducted by Csuk et al. conducted an investigation, which found that GA and its derivatives displayed robust activity against thyroid cancer. 91i et al. found that 18b-GA exert anticancer effects as pin1 inhibitors. 959,[91][92][93][94]96 In conclusion, 18b-GA and its derivatives have shown promising anti-tumor properties in various types of cancer, including colorectal, breast, lung, and liver.The cytotoxic effects of 18b-GA have been attributed to its ability to induce apoptosis, cell cycle arrest, inhibit migration, and downregulate various signaling pathways involved in cancer progression.In addition, 18b-GA has been shown to enhance the cytotoxicity of conventional chemotherapeutic agents, making it a potential adjuvant therapy for cancer treatment.Although 18b-GA and its derivatives have shown potential as anti-tumor agents, further studies are needed to fully understand their mechanisms of action and to optimize their pharmacological properties for clinical applications.

Antibacterial activity
The emergence and spread of drug-resistant bacteria pose a signicant threat to global health.Conventional antibiotics are oen rendered ineffective against these resistant strains, leading to prolonged and complicated treatment regimens, as well as increased morbidity and mortality rates.Consequently, there is a critical need to identify novel antibiotics that can effectively target and eliminate these drug-resistant bacteria. 102esearchers have turned their attention to natural compounds as potential sources of new antibiotics.Natural compounds have long been recognized for their diverse chemical structures       Review RSC Advances and biological activities.By studying and modifying these compounds, scientists hope to develop more potent and effective antibiotics.Among the natural compounds explored for their antibacterial properties, 18b-GA and related compounds have shown promise.These compounds have exhibited antibacterial effects against various bacterial strains, suggesting their potential as therapeutic agents.Further investigations are underway to elucidate the mechanisms of action and optimize the activity of these compounds. 103he antimicrobial properties of 18b-GA, a compound extracted from the licorice plant, have been extensively studied by various researchers.Kim et al. discovered that 18b-GA has the ability to disrupt bacterial cell membranes, leading to the eradication of these microorganisms.This nding has generated signicant interest in the potential of 18b-GA as a novel antibacterial agent. 104Salari et al. further supported the antibacterial activity of 18b-GA against periodontopathogenic and capnophilic bacteria, while another investigation found that this natural compound can inhibit the growth of Helicobacter pylori. 105,106In a comprehensive study, Schrader et al. explored the antibacterial properties of various natural plant compounds, including 18b-GA and 18a-GA, and evaluated their efficacy against common pathogens found in pond-cultured channel catsh. 107It has been demonstrated that 18b-GA can effectively combat antibiotic-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA), by inhibiting their survival and virulence gene expression. 108Furthermore, this compound has shown potential in preventing the growth and formation of supragingival plaque bacteria and treating H. pylori infections. 109,110In the ght against opportunistic nosocomial P. aeruginosa, 18b-GA has proven to be a valuable ally. 111Additionally, 18b-GA has been investigated for its ability to enhance the activity of tobramycin and polymyxin B against MRSA. 112In the quest to combat opportunistic nosocomial P. aeruginosa, 18b-GA has been found to be a valuable ally. 113Moreover, 18b-GA has been used in combination with nanoparticles and hydrogels to combat bacterial infections.Darvishi et al. developed and evaluated the antibacterial activity of 18b-GA-loaded PL18b-GA nanoparticles, which demonstrated signicant antibacterial activity against both Gram-positive and Gram-negative bacteria. 114Similarly, Zhao et al. engineered an injectable moldable hydrogel assembled from natural glycyrrhizic acid, which exhibited remarkable antibacterial activity against both types of bacteria. 115Recently, the remarkable antibacterial capabilities of 18b-GA derivatives have come to light.These derivatives have shown promising inhibitory effects against various bacterial strains, making them potential candidates for combating bacterial infections. 116In this review, our objective is to classify and elucidate the antibacterial activities of different 18b-GA derivatives against specic bacterial species.18b-GA and its derivatives, as shown in Table 3, have demonstrated signicant potential in inhibiting pathogens.
Compounds 157-163 have emerged as potent inhibitors of Streptomyces scabies, a notorious plant pathogen.These derivatives have exhibited remarkable inhibitory activity, suggesting their potential application in managing plant bacterial diseases. 117Compound 161 has demonstrated superior inhibitory activity against Bacillus subtilis, Staphylococcus aureus, and MRAS compared to conventional antibiotics such as ampicillin, streptomycin, and vancomycin.This nding highlights the potential of 18b-GA derivatives as effective alternatives for combating drug-resistant bacterial strains.
Furthermore, compounds 164-166, compounds 177-178, compounds 183-187, and compounds 196-225 have displayed robust inhibitory activity against Xanthomonas oryzae pv.oryzae (Xoo) and X. axonopodis pv.20][121] 123 Additionally, compounds 173-176 have exhibited high activity against various bacteria, particularly showing enhanced antibacterial effects against Micrococcus luteus compared to gentamicins. 79ropical bovine theileriosis (TBT) is one of the progressive and lymphoproliferative tick-borne diseases caused by Theileria annulata.Buvanesvaragurunathan et al. investigated the effect of 18b-GA esters (compounds 179-184) on the growth of Theileria annulata and found that they induced apoptosis in parasite cells.Among these esters, the isopropyl ester of 18b-GA (compound 183) showed improved anti-theileriosis efficacy than other 18b-GA derivatives. 124n conclusion, the rise of drug-resistant bacteria necessitates the discovery of novel antibiotics that can effectively combat these resilient strains mentioned above.Natural compounds, such as 18b-GA and its derivatives, offer a promising avenue for antibiotic development.Future research efforts should focus on understanding the mode of action of these compounds and optimizing their efficacy against drug-resistant bacteria.

Antiviral activity
Over the past two decades, the potencies have been extensively investigated for pentacyclic triterpenoids, such as asiatic acid, betulinic acid, boswellic acid, glycyrrhizin, 18b-GA, lupeol, oleanolic acid, and ursolic acid, and their analogs and derivatives, as potent antitumor and antiviral agents.These triterpenoids have displayed remarkable cytotoxic activity against various tumor cell lines and exhibit antiviral properties, in particular, anti-HIV activity. 126The main active constituents of licorice are triterpenoids, which have shown inhibitory effects on several viruses, including SARS-CoV-2. 127It has been revealed that these compounds achieve their antiviral effects through various mechanisms such as inhibiting virus replication, directly inactivating viruses, halting inammation mediated by HMGB1/TLR4, preventing b-chemokines, reducing the binding of HMGB1 to DNA to weaken virus activity, and inhibiting reactive oxygen species formation. 128,129While these natural products offer great potential as anti-viral and anti-microbial agents, they comprise complex mixtures of organic molecules, making it difficult to determine their exact effectiveness.Hence, further research is required to gain an intricate understanding of their mechanisms of action and their potential for use as food or herbal medicine.Additionally, it is vital to carefully consider the pleiotropic effects of these compounds to avoid potential negative consequences.
Several studies have shown that 18b-GA inhibit several viruses (Fig. 5), for example, Sato et al. reported that 18b-GA inhibits hepatitis B virus (HBV) by suppressing surface antigens, 130 while Hardy et al. showed that 18b-GA exhibits signicant antiviral activity against rotavirus replication in vitro. 131][134][135] In recent years, researchers have also worked on the antiviral properties of 18b-GA derivatives (Table 4).Baltina et al. synthesized a series of 18b-GA derivatives.They found that compounds 227-230 exert the most signicant antiviral activity (IC 50 = 0.13 mM) against ZIKV, with compound 227 demonstrating promising potential as an antiviral agent against ZIKV infection. 136Similarly, Zígolo et al. reported that compound 231 exhibited signicant antiviral activity against TK+ and TK− strains of herpes simplex virus type 1 (HSV-1). 137Liang et al. found that water-soluble b-cyclodextrin-18b-GA (compounds 232-237) showed promising antiviral activity against the inuenza A/WSN/33 (H1N1) virus. 138,139More recently, Ding et al.
suggested that 18b-GA and its derivatives (compounds 238-241) could alleviate the symptoms of COVID-19 patients. 140Additionally, Wang et al. synthesized several compounds and observed that compounds 242-243 exhibited signicant inhibitory activities against HBV DNA replication. 73These ndings highlight the potential of 18b-GA and its derivatives as potent antiviral agents with remarkable antiviral activity against numerous viral infections.
In summary, the research on pentacyclic triterpenoids, including 18b-GA and its derivatives, suggests their immense potential as effective and safe antiviral agents.These compounds have demonstrated varying degrees of antiviral activity against numerous viral infections, making them a promising area of ongoing research.However, further studies are necessary to comprehensively investigate their mechanisms of action and how they can be effectively used as food or herbal medicine while considering the possible negative consequences of their pleiotropic effects.The biochemical variables of rats return to normal.The ndings provide sufficient evidence to demonstrate that 18b-GA possesses the capability to suppress the production of oxygen species and reinstate the antioxidant mechanisms in diabetic rats afflicted with acrylamide-induced liver and kidney cytotoxicity. 141Similarly, Melekoglu et al. discovered that the antioxidant defense system parameters, encompassing malondialdehyde (MDA), reduced glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT), were signicantly higher in the ovarian tissues of rats treated with 18b-GA (100 mg kg −1 day −1 ) compared to those subjected to ischemiareperfusion (I/R) alone. 142These ndings suggest that 18b-GA may have protective effects against oxidative stress in a variety of tissues and systems.In addition to its potential antioxidant properties, recent research has also explored the potential therapeutic applications of 18b-GA in the context of viral infections.nding suggests that 18b-GA may have the potential as a therapeutic agent in the ght against COVID-19. 143e have discovered that a signicant number of studies on the antioxidant properties of 18b-GA focus on its hepatoprotective function.In the mouse model of carbon tetrachloride (CCl 4 )-induced chronic liver brosis, it was observed that CCl 4 inhibited the expression of Nrf2 regulatory genes, including CAT, glutathione peroxidase 2 (GPX2), and superoxide dismutase 3 (SOD3).However, 18b-GA was found to protect the mouse liver from oxidative stress by potentially activating the nuclear trans of Nrf2, enhancing the expression of its target genes, and increasing the activity of antioxidant enzymes. 37Furthermore, 18b-GA was also found to have the ability to inhibit the activity of xanthine oxidase (XO) signicantly.XO is responsible for reducing O 2 to superoxide anionic radical O 2 , leading to oxidative stress. 144In a mouse model of methotrexate (MTX)-induced liver injury, Mahmoud et al. discovered that 18b-GA was able to reverse the signicant manifestations of Nrf2, hemooxygenase-1, and PPARg induced by MTX, thus restoring antioxidant defense. 38Another study demonstrated that 18b-GA signicantly reduced alphanaphthylisothiocyanate (ANIT)-induced liver damage primarily by increasing the expression of nuclear factors (such as Sirt1, FXR, and Nrf2) and their targeted excretion transporters in the liver, which play a crucial role in maintaining bile acidosis in hepatocytes.The plasma levels of ALT, AST, ALP, g-glutamyl transpeptidase (GGT), and total bilirubin (TBIL) were signicantly elevated by 31.2-,33.4-, 5.1-, 5.0-, and 91.3-fold, respectively, in rats induced with ANIT (P < 0.0001).However, for 18b-GA (60 mg kg −1 for 7 days treatment), all of these levels showed a signicant reduction of 62.0%, 38.5%, 45.7%, 51.6%, and 39.7%, respectively (P < 0.05). 145Moreover, the study also revealed that 18b-GA exerts its hepatoprotective effects against RTS-induced liver damage through the phosphatidylinositol 3kinase (PI3K)/protein kinase B (AKT) pathway and enhanced glycogen synthase kinase 3 beta (GSK3b) pathway, which promotes the Nrf2-mediated antioxidant system. 146Fig. 6 briey illustrates the hepatoprotective effect of 18b-GA based on anti-inammatory and antioxidant mechanisms.][149][150] 18b-GA derivatives (Table 5) also demonstrated signicant antioxidant activity.It was discovered that compounds 244-247 exhibited robust antioxidant activity and inhibited ROS activity by up to 41%. 151   Aer treatment with compound activated HSC-T6, the expression levels of the two biomarkers were down-regulated.Second, both compounds downregulated the expression levels of COX-2 and transforming growth factor beta1 (TGF-b 1 ) and reduced ROS levels in a concentration-dependent manner.This suggests that they inhibit HSC-T6 activation and may also be due to downregulation of COX-2 levels, inhibition of the TGF-b1 signaling pathway, and reduction of ROS levels.Overall, while the study of oxidative stress and its effects on the body is complex, recent research has shed light on the potential benets of compounds like 18b-GA in combatting this process.By exploring the mechanisms of these compounds and their effects on various tissues and systems, we can better understand how to combat oxidative stress and its associated health risks.

Discussion
Experience has imparted the understanding that when a compound manifests a biological activity characterized by an IC 50 value lower than 10 mM, it may be classied as potential biological efficacy.Additionally, in the process of scrutinizing lead and candidate compounds, it is importance to consider both cost-effectiveness and the intricacy of synthetic routes.Keeping these pivotal factors in consideration, the investigation unveiled that compounds 16-21 exhibited noteworthy inhibitory activities against 11b-HSD2 within the sub-micromolar (nM) range.Particularly remarkable is compound 16, which boasts an exceptionally modest synthetic complexity, necessitating a single-step reaction initiated from 18b-GA.The incorporation of amide and hydroxyl groups at the C-30 position has substantially augmented the solubility of 18b-GA.Compounds of this kind exhibit tremendous promise for further in-depth exploration.Moreover, numerous studies have demonstrated that the majority of structural alterations to 18b-GA revolve around rigid ve-ring skeleton structure, encompassing the  Comparatively, few studies explore the strategy, such as scaffold hopping and changes in the skeleton itself to the biological activity.Reports about compounds 38-41, 116-122, 227-230, and 248-259 have discernible indicated that brought about a substantial augmentation in the anti-tumor, antiviral, and antioxidant properties of 18b-GA through the processes of ring opening and ring expansion.The modications in 18b-GA from the complexity of the derivative structure is mainly due to addition rather than subtraction.It may be connected with that there are few reaction methods for removing carbon atoms in the rigid alkyl skeleton.
It is particularly noteworthy that compounds 227-230 demonstrate an inhibitory activity against the ZIKA virus within the sub-micromolar (nM) range.Perhaps designing modications that involve adding or reducing rings could provide excellent solutions for enhancing the target binding strength, selectivity, bioavailability, selective tissue distribution, and metabolic stability of 18b-GA derivatives.However, further studies are necessary to comprehensively reveal their mechanisms or the target protein to further guide the modication of compounds.Moreover, 18b-GA derivatives that self-assemble, including gels, micelles, nanoparticles, and liposomes, hold potential for application in food additives and intelligent drug delivery due to availability, biocompatibility, and controllable degradability. 154Additionally, while the mainstream research direction focuses on the aforementioned topics, shiing the focus to other biologically active research areas such as antidiabetes, anti-coagulation, and neuroprotection, could prove worthwhile, as the studies in these areas are still relatively scarce.This could further broaden the development prospects of 18b-GA derivatives and increase their role in various elds.

Conclusions
In conclusion, the past decade has yielded promising research on the therapeutic potential of 18b-GA and its derivatives for various diseases, including cancer, inammation, bacterial infection, hepatic diseases, and viral infections.Pharmacological effects have been observed through a variety of pathways, including inammation-related signaling, immune response modulation, and gene expression regulation.However, it is unfortunate that no derivatives have entered clinical trials (from https://www.clinicaltrials.gov)due to their poor pharmacological properties, low bioavailability, signicant toxic side effects, and other factors.
The review of over 200 chemical structures and key activity data in this review article serves as a valuable data resource for pharmaceutical chemists and also provides future research directions.Future research, except self-assembling derivatives, as well as exploring other related elds should more focus on revealing the mechanisms of action or the target protein and the relationship with the SAR of derivates and to further guide the structural modications.With further research and optimization, 18b-GA derivatives will address the above crucial issues that hold great promise as potential therapeutic agents for various diseases.

Fig. 3
Fig. 3 Modification of C-3 sites are labeled in pink, modification of C-2 sites are labeled in red, and modification of C-11 to C-13 sites modification are labeled in fluorescent green.The C-20 carboxyl sites are labeled in blue, while the other sites are labeled in fluorescent blue.
Li et al. found that compound 5 decreased the expression of iNOS, COX-2, and mitogen-activated protein kinases (MAPKs) as well as the activation of NF-kB in LPS-stimulated RAW 264.7 cells. 42More recently, Yang et al. investigated the anti-inammatory effects of compound 6 on ear edema in mice and LPS-stimulated RAW 264.7 macrophages, respectively. 43Compound 6 was shown to decrease approximately 59.69% of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ear edema with a gavage treatment of 40.0 mg mL −1 , and immunohistochemistry results revealed that this effect was related to the inhibition of TPA-induced upregulation of TNF-a.Compound 7 effectively inhibited the protein and mRNA expression of iNOS and the mRNA expression of TNF-a, IL-6, and IL-1b in LPS-stimulated RAW 264.7 macrophages.Bian et al. investigated the anti-inammatory effects of compound 8 on LPS-induced RAW 264.7 cells and found that it suppressed the expression of pro-inammatory cytokines including IL-6, TNF-a, and NO. 44Compounds 9-12

− 1
NO inhibitory assay in RAW 264.7:IC 50 = 10.13 mM Inhibited iNOS, COX-2, MAPKs, and NF-kB in the LPS-stimulated RAW 264Inhibited LPS-induced NO production.Inhibited iNOS, TNF-a, IL-6, and IL-1b in LPSstimulated RAW 264.7 macrophages Inhibited TPA-induced up regulation of the pro-inammatory cytokines TNF-a and IL-1b and decreased the expression level of p65 in the NF-kB signaling pathway Inhibition at 50 mM: 99.08% NO inhibitory assay in RAW 264.7:IC 50 = 18.5 mM NO and IL-6 inhibitory activity in RAW 264.7NO and IL-6 inhibitory activity in RAW 264.7:IC 50 = 15.5 mM NO inhibitory assay in RAW 264.7Inhibit inammatory response (10-50 mM) induced by IFNg in macrophages in vitro and carrageenan in murine models in vivo, probably by primary interactions with active sites of MMP9, neutrophil elastase, and thrombin X = Cl, IC 50 = 53.0mM 15: X = F, IC 50 = 55.4 mM Anti-inammatory activities through the downregulation of NO, pro-inammatory cytokines and chemokines (IL-1b, IL-6, IL-12, TNF-a, MCP-1, and MIP-1a) and upregulation of anti-inammatory cytokines (IL-10).IC 50 of NO inhibitory assay in microglia BV2 cells Table 1

Zhang
et al. found that compounds 259-263 hindered the proliferation of activated hepatic stellate cells (HSC)-T6 cells by inducing apoptosis and arresting them in the G0/G1 phase.They used rat hepatic stellate cell line T6 cells activated by transforming growth factor-b-1 as the cell model and as the 18b-GA control.The IC 50 value of the compound on cell proliferation was determined by tetrazolium salt colorimetry.It was found that the inhibitory effect of compounds 259-263 on activated HSC-T6 was stronger than that of GA (IC 50 = 78.4± 2.3 mM).153Numerous studies have demonstrated a strong association between COX-2 and the activation of hepatic stellate cells (HSCs), thereby facilitating the initiation and progression of hepatic brosis.Among them, compounds 262 and 265 strongly inhibit the activation of HSC-T6 cells by downregulating the expression of alpha-smooth muscle actin (a-SMA) and type I collagen (Col1) proteins, which are biomarkers of liver brosis.

Table 3 (
Contd. ) Xiang et al. particularly emphasized the potency of compounds 164 and 165.In vivo trials have further conrmed the potential of these compounds in managing rice bacterial blight disease, with control efficacy ranging between 50.57% and 53.70% at 200 mg mL −1 . 118Moreover, Yang et al. discovered that derivatives of 18b-GA (compounds 167-176, 190-195, and 226) exhibit potent antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, and MRAS. 43,122Compound 172, as identied by Guo et al., has demonstrated robust antibacterial properties and has been used to prepare supramolecular self-assembly hydrogels with exceptional thermodynamic stability and high melting temperatures.

Table 4
Chemical structure and antiviral activity of 230-246

Table 4 (
Contd. ) Maitraie et al. observed that compounds 249-258 displayed both anti-inammatory and antioxidant properties, with compound 254 specically exerting inhibitory effects on NO and superoxide anions in RAW 246.7 cells. 152Moreover,