Synthesis, biological evaluation and structure-activity relationships of glycyrrhetinic acid derivatives as novel anti-hepatitis B virus agents

Application of 18β-glycyrrhetinic acid in the structural modification of natural products: a review

18β-Glycyrrhetinic acid (GA) is an oleane-type pentacyclic triterpene saponin obtained from glycyrrhizic acid by removing 2 glucuronic acid groups. GA and its analogues are active substances of glycyrrhiza aicd, with similar structure and important pharmacological effects such as anti-inflammatory, anti-diabetes, anti-tumor and anti-fibrosis. Although GA combined compounds are in the clinical trial stages, its application potential is severely restricted by its low bioavailability, water solubility and membrane permeability. In this article, synthetic methods and structure-activity relationships (SARs) of GA derivatives from 2018 to present are reviewed based on pharmacological activity. It is hoped that this review can provide reference for the future development of potential GA preclinical candidate compounds, and furnish ideas for the development of pentacyclic triterpenoid lead compounds.

Discovery, Biomanufacture, and Derivatization of Licorice Triterpenoids

Triterpenoids are the major active constituents of licorice, a well-known traditional medicinal herb. Licorice triterpenoids, represented by glycyrrhizin and glycyrrhetic acid, have a high structural diversity and are excellent lead compounds for the development of potent pharmaceuticals. However, their further application can be limited by insufficient activities, low bioavailability, and the presence of side effects, as well as the inefficiency of traditional plant extraction processes for compound production. To address these issues, researchers are focusing on rare triterpenoid components in the genus Glycyrrhiza and developing derivatives to preserve or enhance the original physiological activities with improved bioavailability and reduced side effects. At the same time, synthetic biology offers opportunities to shorten the production cycle, create eco-friendly manufacturing processes, and reduce the cost of producing licorice triterpenoids. Although much progress has been achieved in this field in recent years, there is still a lack of a comprehensive review to summarize the overall characteristics of licorice triterpenoids rather than glycyrrhizin and glycyrrhetinic acid. Based on this, our review comprehensively outlines the structures, origins, and pharmacological activities of licorice triterpenoids and predicts their pharmacological activities using the drugCIPHER algorithm. Furthermore, this paper reviews the advances and strategies for the biomanufacturing of licorice triterpenoids using synthetic biology methods and outlines the perspectives and structure-activity relationships for the derivatization of licorice triterpenoids. This review provides new insights into the discovery and synthesis of pharmaceuticals derived from natural triterpenes.

A four-step biosynthetic pathway involving C-3 oxidation-reduction reactions from cycloastragenol to astragaloside IV in Astragalus membranaceus

Astragaloside IV is a significant chemical component derived from the medicinal plant Astragalus membranaceus. Despite the characterization of several glycosyltransferases from A. membranaceus, the complete biosynthetic pathway of astragaloside IV has not been fully elucidated. In this study, we propose a biosynthetic pathway for astragaloside IV that involves a sequence of oxidation-reduction reactions. The biosynthesis pathway from cycloastragenol to astragaloside IV encompasses four key steps: C-3 oxidation, 6-O-glucosylation, C-3 reduction, and 3-O-xylosylation. We identified a hydroxysteroid dehydrogenase AmHSD1 from A. membranaceus. AmHSD1 catalyzes the C-3 oxidation of cycloastragenol, yielding cycloastragenol-3-one, and the C-3 reduction of cycloastragenol-3-one-6-O-glucoside, resulting in cycloastragenol-6-O-glucoside. Additionally, the glycosyltransferases AmGT8 and AmGT1, previously reported by our groups, were identified as catalyzing the 6-O-glucosylation and 3-O-xylosylation steps, respectively. Astragaloside IV was successfully synthesized in transient expression in Nicotiana benthamiana using the combination of AmHSD1, AmGT8 and AmGT1. These results support the proposed four-step biosynthetic pathway and suggest that AmHSD1 probably plays a crucial role in the biosynthesis of astragaloside IV within A. membranaceus.

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.

Glycyrrhizin Production in Licorice Hairy Roots Based on Metabolic Redirection of Triterpenoid Biosynthetic Pathway by Genome Editing

Glycyrrhizin, a type of the triterpenoid saponin, is a major active ingredient contained in the roots of the medicinal plant licorice (Glycyrrhiza uralensis, G. glabra and G. inflata), and is used worldwide in diverse applications, such as herbal medicines and sweeteners. The growing demand for licorice threatens wild resources and therefore a sustainable method of supplying glycyrrhizin is required. With the goal of establishing an alternative glycyrrhizin supply method not dependent on wild plants, we attempted to produce glycyrrhizin using hairy root culture. We tried to promote glycyrrhizin production by blocking competing pathways using CRISPR/Cas9-based gene editing. CYP93E3 CYP72A566 double-knockout (KO) and CYP93E3 CYP72A566 CYP716A179 LUS1 quadruple-KO variants were generated, and a substantial amount of glycyrrhizin accumulation was confirmed in both types of hairy root. Furthermore, we evaluated the potential for promoting further glycyrrhizin production by simultaneous CYP93E3 CYP72A566 double-KO and CYP88D6-overexpression. This strategy resulted in a 3-fold increase (∼1.4 mg/g) in glycyrrhizin accumulation in double-KO/CYP88D6-overexpression hairy roots, on average, compared with that of double-KO hairy roots. These findings demonstrate that the combination of blocking competing pathways and overexpression of the biosynthetic gene is important for enhancing glycyrrhizin production in G. uralensis hairy roots. Our findings provide the foundation for sustainable glycyrrhizin production using hairy root culture. Given the widespread use of genome editing technology in hairy roots, this combined with gene knockout and overexpression could be widely applied to the production of valuable substances contained in various plant roots.

Antiviral opportunities of Mannich bases derived from triterpenic N-propargylated indoles

Oleanolic and glycyrrhetic acids alkyne derivatives were synthesized as a result of propargylation of the indole NH-group condensed with the triterpene A-ring, the following aminomethylation led to a series of Mannich bases. The synthesized compounds were tested for their potential inhibition of influenza A/PuertoRico/8/34 (H1N1) virus in Madin-Darby canine kidney (MDCK) cell culture and SARS-CoV-2 pseudovirus in baby hamster kidney-21-human angiotensin-converting enzyme 2 (BHK-21-hACE2) cells. Mannich bases of oleanolic and glycyrrhetic acids N-propargylated indoles 7, 8, and 12 were the most efficacious against influenza virus A with IC50 7-10 μM together with a low toxicity (CC50  > 145 μM) and high selectivity index SI value 20. Indolo-oleanolic acid morpholine amide Mannich base holding N-methylpiperazine moiety 9 showed anti-SARS-CoV-2 pseudovirus activity with EC50 value of 14.8 μM. Molecular docking and dynamics modeling investigated the binding mode of the compounds 7 and 12 into the binding pocket of influenza A virus M2 protein and compound 9 into the RBD domain of SARS-CoV-2 spike glycoprotein.

Application of Quinoline Ring in Structural Modification of Natural Products

Natural compounds are rich in pharmacological properties that are a hot topic in pharmaceutical research. The quinoline ring plays important roles in many biological processes in heterocycles. Many pharmacological compounds, including saquinavir and chloroquine, have been marketed as quinoline molecules with good anti-viral and anti-parasitic properties. Therefore, in this review, we summarize the medicinal chemistry of quinoline-modified natural product quinoline derivatives that were developed by several research teams in the past 10 years and find that these compounds have inhibitory effects on bacteria, viruses, parasites, inflammation, cancer, Alzheimer's disease, and others.

18β-glycyrrhetinic acid inhibits proliferation of gastric cancer cells through regulating the miR-345-5p/TGM2 signaling pathway

BACKGROUND

Gastric cancer (GC) is a common gastrointestinal malignancy worldwide. Based on cancer-related mortality, the current prevention and treatment strategies for GC still show poor clinical results. Therefore, it is important to find effective drug treatment targets.

AIM

To explore the molecular mechanism of 18β-glycyrrhetinic acid (18β-GRA) regulating the miR-345-5p/TGM2 signaling pathway to inhibit the proliferation of GC cells.

METHODS

CCK-8 assay was used to determine the effect of 18β-GRA on the survival rate of GES-1 cells and AGS and HGC-27 cells. Cell cycle and apoptosis were detected by flow cytometry, cell migration was detected by a wound healing assay, the effect of 18β-GRA on subcutaneous tumor growth in BALB/c nude mice was investigated, and the cell autophagy level was determined by MDC staining. TMT proteomic analysis was used to detect the differentially expressed autophagy-related proteins in GC cells after 18β-GRA intervention, and then the protein-protein interaction was predicted using STRING (https://string-db.org/). MicroRNAs (miRNAs) transcriptome analysis was used to detect the miRNA differential expression profile, and use miRBase (https://www.mirbase/) and TargetScan (https://www.targetscan.org/) to predict the miRNA and complementary binding sites. Quantitative real-time polymerase chain reaction was used to detect the expression level of miRNA in 18β-GRA treated cells, and western blot was used to detect the expression of autophagy related proteins. Finally, the effect of miR-345-5p on GC cells was verified by mir-345-5p overexpression.

RESULTS

18β-GRA could inhibit GC cells viability, promote cell apoptosis, block cell cycle, reduce cell wound healing ability, and inhibit the GC cells growth in vivo. MDC staining results showed that 18β-GRA could promote autophagy in GC cells. By TMT proteomic analysis and miRNAs transcriptome analysis, it was concluded that 18β-GRA could down-regulate TGM2 expression and up-regulate miR-345-5p expression in GC cells. Subsequently, we verified that TGM2 is the target of miR-345-5p, and that overexpression of miR-345-5p significantly inhibited the protein expression level of TGM2. Western blot showed that the expression of autophagy-related proteins of TGM2 and p62 was significantly reduced, and LC3II, ULK1 and AMPK expression was significantly increased in GC cells treated with 18β-GRA. Overexpression of miR-345-5p not only inhibited the expression of TGM2, but also inhibited the proliferation of GC cells by promoting cell apoptosis and arresting cell cycle.

CONCLUSION

18β-GRA inhibits the proliferation of GC cells and promotes autophagy by regulating the miR-345-5p/TGM2 signaling pathway.

Polymorphism of Butyl Ester of Oleanolic Acid—The Dominance of Dispersive Interactions over Electrostatic

Oleanolic (OA) and glycyrrhetinic acids (GE), as well as their derivatives, show a variety of pharmacological properties. Their crystal structures provide valuable information related to the assembly modes of these biologically active compounds. In the known-to-date crystals of OA esters, their 11-oxo derivatives, and GE ester crystals, triterpenes associate, forming different types of ribbons and layers whose construction is based mainly on van der Waals forces and weak C-H···O interactions. New crystal structures of 11-oxo OA methyl ester and the polymorph of OA butyl ester reveal an alternative aggregation mode. Supramolecular architectures consist of helical chains which are stabilized by hydrogen bonds of O-H···O type. It was found that two polymorphic forms of butyl OA ester (layered and helical) are related monotropically. In a structure of metastable form, O-H···O hydrogen bonds occur, while the thermodynamically preferred phase is governed mainly by van der Waals interactions. The intermolecular interaction energies calculated using CrystalExplorer, PIXEL, and Psi4 programs showed that even in motifs formed through O-H···O hydrogen bonds, the dispersive forces have a significant impact.

Cheminformatics-Based Study Identifies Potential Ebola VP40 Inhibitors

The Ebola virus (EBOV) is still highly infectious and causes severe hemorrhagic fevers in primates. However, there are no regulatorily approved drugs against the Ebola virus disease (EVD). The highly virulent and lethal nature of EVD highlights the need to develop therapeutic agents. Viral protein 40 kDa (VP40), the most abundantly expressed protein during infection, coordinates the assembly, budding, and release of viral particles into the host cell. It also regulates viral transcription and RNA replication. This study sought to identify small molecules that could potentially inhibit the VP40 protein by targeting the N-terminal domain using an in silico approach. The statistical quality of AutoDock Vina’s capacity to discriminate between inhibitors and decoys was determined, and an area under the curve of the receiver operating characteristic (AUC-ROC) curve of 0.791 was obtained. A total of 29,519 natural-product-derived compounds from Chinese and African sources as well as 2738 approved drugs were successfully screened against VP40. Using a threshold of −8 kcal/mol, a total of 7, 11, 163, and 30 compounds from the AfroDb, Northern African Natural Products Database (NANPDB), traditional Chinese medicine (TCM), and approved drugs libraries, respectively, were obtained after molecular docking. A biological activity prediction of the lead compounds suggested their potential antiviral properties. In addition, random-forest- and support-vector-machine-based algorithms predicted the compounds to be anti-Ebola with IC50 values in the micromolar range (less than 25 μM). A total of 42 natural-product-derived compounds were identified as potential EBOV inhibitors with desirable ADMET profiles, comprising 1, 2, and 39 compounds from NANPDB (2-hydroxyseneganolide), AfroDb (ZINC000034518176 and ZINC000095485942), and TCM, respectively. A total of 23 approved drugs, including doramectin, glecaprevir, velpatasvir, ledipasvir, avermectin B1, nafarelin acetate, danoprevir, eltrombopag, lanatoside C, and glycyrrhizin, among others, were also predicted to have potential anti-EBOV activity and can be further explored so that they may be repurposed for EVD treatment. Molecular dynamics simulations coupled with molecular mechanics Poisson–Boltzmann surface area calculations corroborated the stability and good binding affinities of the complexes (−46.97 to −118.9 kJ/mol). The potential lead compounds may have the potential to be developed as anti-EBOV drugs after experimental testing.

Potential of glycyrrhizic and glycyrrhetinic acids against influenza type A and B viruses: A perspective to develop new anti-influenza compounds and drug delivery systems

Despite the recent dynamic development of medicine, influenza is still a significant epidemiological problem for people around the world. The growing resistance of influenza viruses to currently available antiviral drugs makes it necessary to search for new compounds or drug forms with potential high efficacy against human influenza A and B viruses. One of the methods of obtaining new active compounds is to chemically modify privileged structures occurring in the natural environment. The second solution, that is gaining more and more interest, is the use of modern drug carriers, which significantly improve physicochemical and pharmacokinetic parameters of the transported substances. Molecules known from the earliest times for their numerous therapeutic properties are glycyrrhizinic acid (GA) and glycyrrhetinic acid (GE). Both compounds constitute the main active agents of the licorice (Glycyrrhiza glabra, Leguminosae) root and, according to a number of scientific reports, show antiviral properties against both DNA and RNA viruses. The above information prompted many scientific teams around the world to obtain and test in vitro and/or in vivo new synthetic GA and GE derivatives against influenza A and B viruses. Similarly, in recent years, a significant amount of GA and GE-based drug delivery systems (DDS) such as nanoparticles, micelles, liposomes, nanocrystals, and carbon dots has been prepared and tested for antiviral activity, including those against influenza A and B viruses. This work systematizes the attempts undertaken to study the antiviral activity of new GA and GE analogs and modern DDS against clinically significant human influenza viruses, at the same time indicating the directions of their further development.

Research progress on the antiviral activities of natural products and their derivatives: Structure–activity relationships

Viruses spread rapidly and are well-adapted to changing environmental events. They can infect the human body readily and trigger fatal diseases. A limited number of drugs are available for specific viral diseases, which can lead to non-efficacy against viral variants and drug resistance, so drugs with broad-spectrum antiviral activity are lacking. In recent years, a steady stream of new viral diseases has emerged, which has prompted development of new antiviral drugs. Natural products could be employed to develop new antiviral drugs because of their innovative structures and broad antiviral activities. This review summarizes the progress of natural products in antiviral research and their bright performance in drug resistance issues over the past 2 decades. Moreover, it fully discusses the effect of different structural types of natural products on antiviral activity in terms of structure–activity relationships. This review could provide a foundation for the development of antiviral drugs.

Single-crystal-to-single-crystal phase transition of 18β-glycyrrhetinic acid isopropyl ester

Due to the destruction of the integrity of the parent crystal, single-crystal-to-single-crystal phase transition in organic compounds is still a relatively rare phenomenon. The phase transition in glycyrrhetinic acid isopropyl ester is triggered by temperature change. The increasing volume of the isopropyl substituent as a result of increasing temperature forces a remodelling of the structural motifs. These changes cause a single-crystal-to-single-crystal phase transition. The low-temperature form is isostructural with glycyrrhetinic acid methanol solvate, while the high-temperature phase is isostructural with the ethyl ester of this acid.

Glycyrrhetinic acid restricts mitochondrial energy metabolism by targeting SHMT2

Glycyrrhetinic acid (GA) is a natural product of licorice with mitochondria targeting properties and shows broad anticancer activities, but its targets and underlying mechanisms remain elusive. Here, we identified the mitochondrial enzyme serine hydroxymethyltransferase 2 (SHMT2) as a target of GA by using chemical proteomics. Binding to and inhibiting the activity of SHMT2 by GA were validated in vitro and in vivo. Knockout of SHMT2 or inhibiting SHMT2 with GA restricts mitochondrial energy supplies by downregulating mitochondrial oxidative phosphorylation (OXPHOS) and fatty acid β-oxidation, and consequently suppresses cancer cell proliferation and tumor growth. Crystal structures of GA derivatives indicate that GA occupies SHMT2 folate-binding pocket and regulates SHMT2 activity. Modifications at GA carboxylic group with diamines significantly improved its anticancer potency, demonstrating GA as a decent structural template for SHMT2 inhibitor development.

Novel β-Cyclodextrin-Based Heptavalent Glycyrrhetinic Acid Conjugates: Synthesis, Characterization, and Anti-Influenza Activity

In our continuing efforts toward the design of novel pentacyclic triterpene derivatives as potential anti-influenza virus entry inhibitors, a series of homogeneous heptavalent glycyrrhetinic acid derivatives based on β-cyclodextrin scaffold were designed and synthesized by click chemistry. The structure was unambiguously characterized by NMR, IR, and MALDI-TOF-MS measurements. Seven conjugates showed sufficient inhibitory activity against influenza virus infection based on the cytopathic effect reduction assay with IC50 values in the micromolar range. The interactions of conjugate 37, the most potent compound (IC50 = 2.86 μM, CC50 > 100 μM), with the influenza virus were investigated using the hemagglutination inhibition assay. Moreover, the surface plasmon resonance assay further confirmed that compound 37 bound to the influenza HA protein specifically with a dissociation constant of 5.15 × 10−7 M. Our results suggest the promising role of β-cyclodextrin as a scaffold for preparing a variety of multivalent compounds as influenza entry inhibitors.

Phytomedicines to Target Hepatitis B Virus DNA Replication: Current Limitations and Future Approaches

Hepatitis B virus infection (HBV) is one of the most common causes of hepatitis, and may lead to cirrhosis or hepatocellular carcinoma. According to the World Health Organization (WHO), approximately 296 million people worldwide are carriers of the hepatitis B virus. Various nucleos(t)ide analogs, which specifically suppress viral replication, are the main treatment agents for HBV infection. However, the development of drug-resistant HBV strains due to viral genomic mutations in genes encoding the polymerase protein is a major obstacle to HBV treatment. In addition, adverse effects can occur in patients treated with nucleos(t)ide analogs. Thus, alternative anti-HBV drugs of plant origin are being investigated as they exhibit excellent safety profiles and have few or no side effects. In this study, phytomedicines/phytochemicals exerting significant inhibitory effects on HBV by interfering with its replication were reviewed based on different compound groups. In addition, the chemical structures of these compounds were developed. This will facilitate their commercial synthesis and further investigation of the molecular mechanisms underlying their effects. The limitations of compounds previously screened for their anti-HBV effect, as well as future approaches to anti-HBV research, have also been discussed.

A Mini-Review on Structure-Activity Relationships of Glycyrrhetinic Acid Derivatives with Diverse Bioactivities

Pentacyclic triterpenoids, consisting of six isoprene units, are a kind of natural active substance. At present, numerous pentacyclic triterpene have been observed and classified into four subgroups of oleanane, ursane, lupane, and xylene on the basis of the carbon skeleton. Among them, oleanane is the most popular due to its rich backbone and diverse bioactivities. 18β-Glycyrrhetinic acid (GA), an oleanane-type pentacyclic triterpene isolated from licorice roots, possesses diverse bioactivities including antitumor, anti-inflammatory, antiviral, antimicrobial, enzyme inhibitor, hepatoprotective and so on. It has received more attention in medicinal chemistry due to the advantages of easy-to-access and rich bioactivity. Thus, numerous novel lead compounds were synthesized using GA as a scaffold. Herein, we summarize the structure-activity relationship and synthetic methodologies of GA derivatives from 2010 to 2020 as well as the most active GA derivatives. Finally, we anticipate that this review can benefit future research on structural modifications of GA to enhance bioactivity and provide an example for developing pentacyclic triterpene-based novel drugs.

Combination of natural antivirals and potent immune invigorators: A natural remedy to combat COVID-19

The flare-up in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that emerged in December 2019 in Wuhan, China, and spread expeditiously worldwide has become a health challenge globally. The rapid transmission, absence of anti-SARS-CoV-2 drugs, and inexistence of vaccine are further exacerbating the situation. Several drugs, including chloroquine, remdesivir, and favipiravir, are presently undergoing clinical investigation to further scrutinize their effectiveness and validity in the management of COVID-19. Natural products (NPs) in general, and plants constituents specifically, are unique sources for various effective and novel drugs. Immunostimulants, including vitamins, iron, zinc, chrysin, caffeic acid, and gallic acid, act as potent weapons against COVID-19 by reinvigorating the defensive mechanisms of the immune system. Immunity boosters prevent COVID-19 by stimulating the proliferation of T-cells, B-cells, and neutrophils, neutralizing the free radicals, inhibiting the immunosuppressive agents, and promoting cytokine production. Presently, antiviral therapy includes several lead compounds, such as baicalin, glycyrrhizin, theaflavin, and herbacetin, all of which seem to act against SARS-CoV-2 via particular targets, such as blocking virus entry, attachment to host cell receptor, inhibiting viral replication, and assembly and release.

Anti-hepatitis B activities of Myanmar medicinal plants: a narrative review of current evidence

Hepatitis B is one of the major burdens for health services and is the leading cause of morbidity and mortality from cirrhosis of liver and hepatocellular carcinoma. Current treatment strategies using nucleos(t)ide analogue reverse-transcriptase inhibitors or interferons are targeted for the long-term suppression of hepatitis B DNA. However, functional cure of hepatitis B infection (HBsAg clearance) was difficult to attain with such treatments. Therefore, new treatment strategies or innovative treatments are urgently needed. The new treatments should focus on the potential therapeutic targets such as covalently closed circular DNA which may be important for the HBsAg clearance. Plant based medicines have been used in different traditional medicine practices and these natural products/compounds serve as a good source of information or clues for use in drug discovery and design. Many natural products were found to be effective against hepatitis B virus and some even have better therapeutic activities than currently used compounds. This review summarizes the current evidence of Myanmar medicinal plants in basic and clinical research which shows promising potential for the development of novel therapeutic agents for the treatment of hepatitis B.

Glycyrrhetinic acid: a promising scaffold for the discovery of anticancer agents

INTRODUCTION

Oleanane-type pentacyclic triterpenes named glycyrrhetinic acids (GAs) featuring a C-30 carboxylic acid group, are extracted from the licorice (Glycyrrhiza uralensis). Numerous biological properties of GA have been reported and have attracted researchers from all over the world in recent years due to the peculiar GA scaffold-based semisynthetic cytotoxic effects.

AREAS COVERED

This review represents the applications of semisynthetic derivatives of GA for the development of future cancer treatments. Included in the review are important structural features of the semisynthetic GAs crucial for cytotoxic effects.

EXPERT OPINION

Numerous semisynthetic GA derivatives illustrated excellent cytotoxic effects toward various cancer cells. Notably the C-₃(OH) at ring A along with C_30-CO₂H at ring E as vital structural features, make GA very appealing as a lead scaffold for medicinal chemistry, since these two groups permit the creation of further chemical diversity geared toward improved cytotoxic effects. Furthermore, numerous GA derivatives have been synthesized and indicate that compounds featuring cyanoenone moieties in ring A, or compounds having the amino group or nitrogen comprising heterocycles and hybrids thereof, illustrate more potent cytotoxicity. Furthermore, GA has a great capability to be conjugated with other anticancer molecules to synergistically enhance their combined cytotoxicity.

Anti-SARS-CoV-2 Natural Products as Potentially Therapeutic Agents

Severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2), a β-coronavirus, is the cause of the recently emerged pandemic and worldwide outbreak of respiratory disease. Researchers exchange information on COVID-19 to enable collaborative searches. Although there is as yet no effective antiviral agent, like tamiflu against influenza, to block SARS-CoV-2 infection to its host cells, various candidates to mitigate or treat the disease are currently being investigated. Several drugs are being screened for the ability to block virus entry on cell surfaces and/or block intracellular replication in host cells. Vaccine development is being pursued, invoking a better elucidation of the life cycle of the virus. SARS-CoV-2 recognizes O-acetylated neuraminic acids and also several membrane proteins, such as ACE2, as the result of evolutionary switches of O-Ac SA recognition specificities. To provide information related to the current development of possible anti-SARS-COV-2 viral agents, the current review deals with the known inhibitory compounds with low molecular weight. The molecules are mainly derived from natural products of plant sources by screening or chemical synthesis via molecular simulations. Artificial intelligence-based computational simulation for drug designation and large-scale inhibitor screening have recently been performed. Structure-activity relationship of the anti-SARS-CoV-2 natural compounds is discussed.

Glycyrrhetinic acid derivatives as Zika virus inhibitors: Synthesis and antiviral activity in vitro

Zika virus (ZIKV) is an arbovirus of the Flaviviridae family (Flavivirus genus), causing serious neurological complications, such as Guillain-Barre Syndrome (GBS) in adults and fetal microcephaly. Licensed vaccines or specific antiviral agents against ZIKV do not currently exist. Therefore, the search and development of anti-ZIKV agents are particularly relevant and necessary. Glycyrrhetinic (3β-hydroxy-11-oxo-18βH-Olean-12-en-30-oic acid) (GA) 1 is one of the well-known pentacyclic triterpenoids isolated from licorice root (Glycyrrhiza glabra L., Gl. uralensis Fisher) (Leguminosae) possessing many biological features, including antiviral activity. This paper is devoted to the synthesis and studies of a number of nitrogen and sulfur-containing GA derivatives as ZIKV inhibitors. Sixteen GA and related triterpenoids (3β-hydroxy-18βH-Olean-12-en-30-oic acid and 3β-hydroxy-11-oxo-18βH-Olean-12(13),18(19)-dien-30-oic acid) derivatives were synthesized (amides, semi- and thiosemicarbazones, and 1,2,3-thiadiazoles) and antiviral activity against ZIKV was studied in vitro, including the inhibitory assays on cytopathic effect (CPE), viral protein synthesis, and replication stages. Four active compounds were found among GA derivatives tested, 13 (3-O-acetyl-30-aminopyridine GA), 16 (3-semicarbazone-30-butyl GA), 18 (1,2,3-thiadiazole-30-methyl GA), and 19 (1,2,3-thiadiazole-30-butyl GA) with IC₅₀ < 1 μM against ZIKV replication. These compounds had a stronger inhibitory activity on ZIKV-induced CPE and viral protein translation in infected cells as compared to derivatives of 11-desoxo-GA. The most active compound was amide 13 (IC₅₀ 0.13 μM, TI ˃ 384). Time-of-addition assays indicated that 1,2,3-thiadiazole ring is important for inhibiting viral entry stage (compounds 18 and 19), while the 30-butyl ester group influenced on post-entry stage (compound 19). The molecular docking analysis demonstrated that lead compounds 13 and 19 forms a hydrogen-bond interaction with the catalytic triad (His51-Asp75-Ser135) of ZIKV NS2B-NS3 protease. Therefore, the active GA derivatives are promising for developing new antiviral agents against ZIKV infection.

Design, Synthesis, Antibacterial Evaluation, and Induced Apoptotic Behaviors of Epimeric and Chiral 18β-Glycyrrhetinic Acid Ester Derivatives with an Isopropanolamine Bridge against Phytopathogens

Because only a handful of agrochemicals can manage bacterial infections, the discovery and development of innovative, inexpensive, and high-efficiency antibacterial agents targeting these infections are challenging. Herein, a series of novel epimeric and chiral 18β-glycyrrhetinic acid (GA) ester derivatives with various tertiary amine pendants were designed, synthesized, and screened for pharmacological activity. Results showed that some of the title compounds were conferred with significantly enhanced antibacterial activity toward phytopathogens Xanthomonas oryzae pv oryzae (A₂, B₁-B₃, and C₁, EC₅₀ values within 3.81-4.82 μg/mL) and Xanthomonas axonopodis pv citri (B₁, EC₅₀ = 3.18 μg/mL; B₂, EC₅₀ = 2.76 μg/mL). These activities are superior to those of GA (EC₅₀ > 400 μg/mL), thiodiazole copper, and bismerthiazol. Pharmacophore studies revealed that the synergistic combination of GA skeleton and tertiary amine scaffolds contributed to the biological actions. In vivo experiments displayed their promising applications in controlling bacterial infections. Antibacterial mechanism studies revealed that the title compounds could trigger apoptosis in the tested pathogens, evident by bacteria morphological changes observed in scanning electron microscopy images. This outcome should motivate the development of various apoptosis inducers against plant bacterial diseases by a novel mode of action compared to that of existing agricultural chemicals.

Bioactive Candy: Effects of Licorice on the Cardiovascular System

Licorice, today chiefly utilized as a flavoring additive in tea, tobacco and candy, is one of the oldest used herbs for medicinal purposes and consists of up to 300 active compounds. The main active constituent of licorice is the prodrug glycyrrhizin, which is successively converted to 3β-monoglucuronyl-18β-glycyrrhetinic acid (3MGA) and 18β-glycyrrhetinic acid (GA) in the intestines. Despite many reported health benefits, 3MGA and GA inhibit the 11-β-hydrogenase type II enzyme (11β-HSD2) oxidizing cortisol to cortisone. Through activation of mineralocorticoid receptors, high cortisol levels induce a mild form of apparent mineralocorticoid excess in the kidney and increase systemic vascular resistance. Continuous inhibition of 11β-HSD2 related to excess licorice consumption will create a state of hypernatremia, hypokalemia and increased fluid volume, which can cause serious life-threatening complications especially in patients already suffering from cardiovascular diseases. Two recent meta-analyses of 18 and 26 studies investigating the correlation between licorice intake and blood pressure revealed statistically significant increases both in systolic (5.45 mmHg) and in diastolic blood pressure (3.19/1.74 mmHg). This review summarizes and evaluates current literature about the acute and chronic effects of licorice ingestion on the cardiovascular system with special focus on blood pressure. Starting from the molecular actions of licorice (metabolites) inside the cells, it describes how licorice intake is affecting the human body and shows the boundaries between the health benefits of licorice and possible harmful effects.

Validation of an LC-MS/MS method for simultaneous detection of diverse components of Qinxing Qingre Zhike Granule in rat plasma and its application to pharmacokinetic study after oral administration to rats

A sensitive and validated method of liquid chromatography-tandem mass spectrometry (LC-MS/MS) was established to test the plasma concentrations of active ingredients in Qinxing Qingre Zhike Granule, namely geniposide, liquiritin, isoliquiritin, baicalin, wogonoside, baicalein, liquiritigenin, isoliquiritigenin and glycyrrhetinic acid. The analysis was performed on an Ultimate XB-C₁₈ column at the flow rate of 0.4 mL min^-1 in a single run of 18 min. The mobile phase was composed of 0.05% formic acid in water and acetonitrile with gradient elution. Positive and negative scanning and selected multiple reaction monitoring modes were applied for quantization. The proposed method showed good linearity in the given ranges from 0.6800-340.0 to 3.920-1960 ng mL^-1 with r²  > 0.9917 for all the analytes. The precision (RSD) was no more than 12%, and the accuracy (RE) was less than ±11% for intra- and inter-day. The extract recovery and matrix effect were acceptable for the requirements of biological sample analysis. Moreover, the developed method was effectively applied to the pharmacokinetic investigation of Qinxing Qingre Zhike Granule after oral administration in rats.

Synthesis and structure-activity relationship studies of water-soluble β-cyclodextrin-glycyrrhetinic acid conjugates as potential anti-influenza virus agents

Glycyrrhetinic acid (GA) is a major constituent of the herb Glycyrrhiza glabra, and many of its derivatives demonstrate a broad spectrum of antiviral activities. In the current study, 18 water-soluble β-cyclodextrin (CD)-GA conjugates, in which GA was covalently coupled to the primary face of β-CD using 1,2,3-triazole moiety along with varying lengths of linker, were synthesized via copper-catalyzed azide-alkyl cycloaddition reaction. Benefited from the attached β-CD moiety, all these conjugates showed lower hydrophobicity (AlogP) compared with their parent compound GA. With the exception of per-O-methylated β-CD-GA conjugate (35), all other conjugates showed no significant cytotoxicity to MDCK cells, and these conjugates were then screened against A/WSN/33 (H1N1) virus using the cytopathic effect assay. The preliminary results indicated that six conjugates showed promising antiviral activity, and the C-3 and C-30 of GA could tolerate some modifications. Our findings suggested that GA could be used as a lead compound for the development of potential anti-influenza virus agents.

Natural-Product-Tailored Polyurethane: Size-Dictated Construction of Polypseudorotaxanes with Cyclodextrin-Triterpenoid Pairs

Cyclodextrin (CD)-based polyrotaxanes (PRs) and polypseudorotaxanes (PPRs) have attracted considerable attention due to their unique topological structures and functions. However, limited by the simple chemical structures and the single functionalization of guest polymer units like poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG), to date the construction of CD-based PRs and PPRs with precisely controllable supramacromolecular structures is fairly rare. In this work, two kinds of molecular necklace-like PPRs with CD-triterpenoid pairs were prepared via the size-dictated construction, where the threaded guest polymer was a natural product-tailored polyurethane (PU-PEG-GA) with the alternating structure of triterpenoid and PEG segments via a simple step-growth polymerization. Taking advantage of the differentiation in host-guest interactions between β/γ-CD and triterpenoid pairs, β-CD simultaneously located on both PEG segments and triterpenoid units in PU-PEG-GA, while γ-CD selectively recognized triterpenoid units. Consequently, the assembly morphology of PU-PEG-GA was adjusted hierarchically from micelles to worms and vesicles upon addition of β-CD, whereas they gradually collapsed to disappear in the presence of γ-CD. Our biocompatible PPRs with precisely controllable supramacromolecular structures may lead to the exploration on understanding and simulating macromolecular recognition using natural products.

Design, synthesis and biological evaluation of seco-A-pentacyclic triterpenoids-3,4-lactone as potent non-nucleoside HBV inhibitors

A series of seco-A-pentacyclic triterpenoids-3,4-lactone were synthesized and the anti-HBV activities were evaluated in vitro. Several compounds inhibited the secretion of HBV antigen and the replication of HBV DNA in micromolar level. Compounds D7 and D10, seco-A-oleanane-3,4-lactone, suppressed the HBeAg secretion with IC₅₀ values of 0.14 μM and 0.86 μM respectively, and the inhibitory activities were also confirmed by detecting the fluorescence intensity of FITC-labeled monoclonal mouse HBeAg antibody via flow cytometry. Compounds D7 and D10 as well as B4, ring-A cleaved 3,30-dioic acid, also displayed remarkable inhibition on both HBV DNA replication at the concentration of 25 μM and HBV cccDNA (covalently closed circularDNA) replication with IC₅₀ values of 33.5 μM, 32.7 μM and 12.3 μM respectively.

Synthesis and Biological Evaluation of Glycyrrhetic Acid Derivatives as Potential VEGFR2 Inhibitors

Vascular endothelial growth factor receptor 2 (VEGFR2) has been proven to play a major role in the regulation of tumor angiogenesis. A series of novel glycyrrhetic acid derivatives were synthesized and evaluated for their VEGFR2 inhibitory activity as well as their antiproliferative properties against four cancer cell lines (MCF-7, HeLa, HepG2, and A549). In vitro biological evaluations against these human tumor cell lines indicate that most of the prepared compounds have antiproliferative activities; compound 3 a (3β-hydroxy-30-(4-phenyl-1-piperazinyl)olean-12-ene-11,30-dione) exhibited the best inhibitory activity against MCF-7 cells, with an IC₅₀ value of 1.08 μm. Compound 3 a also showed the most potent inhibitory activity against VEGFR2 tyrosine kinase, with an IC₅₀ value of 0.35 μm. Docking simulations were performed with the aim of discovering the binding mode of compound 3 a, and the results indicate that 3 a could bind at the VEGFR2 active site.

An Overview of Structurally Modified Glycyrrhetinic Acid Derivatives as Antitumor Agents

Glycyrrhetinic Acid (GA), a triterpenoid aglycone component of the natural product glycyrrhizinic acid, was found to possess remarkable anti-proliferative and apoptosis-inducing activity in various cancer cell lines. Though GA was not as active as other triterpenes, such as betulinic acid and oleanolic acid, it could trigger apoptosis in tumor cells and it can be obtained easily and cheaply, which has stimulated scientific interest in using GA as a scaffold to synthesize new antitumor agents. The structural modifications of GA reported in recent decades can be divided into four groups, which include structural modifications on ring-A, ring-C, ring-E and multiple ring modifications. The lack of a comprehensive and recent review on this topic prompted us to gather more new information. This overview is dedicated to summarizing and updating the structural modification of GA to improve its antitumor activity published between 2005 and 2016. We reviewed a total of 210 GA derivatives that we encountered and compiled the most active GA derivatives along with their activity profile in different series. Furthermore, the structure activity relationships of these derivatives are briefly discussed. The included information is expected to be of benefit to further studies of structural modifications of GA to enhance its antitumor activity.

Naturally derived anti-hepatitis B virus agents and their mechanism of action

Despite that some approved drugs and genetically engineered vaccines against hepatitis B virus (HBV) are available for HBV patients, HBV infection is still a severe public health problem in the world. All the approved therapeutic drugs (including interferon-alpha and nucleoside analogues) have their limitations. No drugs or therapeutic methods can cure hepatitis B so far. Therefore, it is urgently needed to discover and develop new anti-HBV drugs, especially non-nucleoside agents. Naturally originated compounds with enormous molecular complexity and diversity offer a great opportunity to find novel anti-HBV lead compounds with specific antiviral mechanisms. In this review, the natural products against HBV are discussed according to their chemical classes such as terpenes, lignans, phenolic acids, polyphenols, lactones, alkaloids and flavonoids. Furthermore, novel mode of action or new targets of some representative anti-HBV natural products are also discussed. The aim of this review is to report new discoveries and updates pertaining to anti-HBV natural products in the last 20 years, especially novel skeletons and mode of action. Although many natural products with various skeletons have been reported to exhibit potent anti-HBV effects to date, scarcely any of them are found in the list of conventional anti-HBV drugs worldwide. Additionly, in anti-HBV mechanism of action, only a few references reported new targets or novel mode of action of anti-HBV natural products.

Synthesis of erythrocentaurin derivatives as a new class of hepatitis B virus inhibitors

Twenty-four derivatives of erythrocentaurin (ET) were synthesized and evaluated for their anti-HBV activities on HepG 2.2.15 cell line in vitro. Eight compounds 1, 2, 5, 8, 9, 1e, 1k, and 1m increased activity against HBV DNA replication with the SI values higher than 11. In particular, derivatives 1e and 1k exhibited the most potent inhibition on HBV DNA replication with the IC50 values of 0.026 mM (SI>70.8) and 0.045 mM (SI>36.0), respectively. The primary structure-activity relationships (SARs) of ET derivatives were summarized for exploring potent anti-HBV agents.

Antiviral natural products and herbal medicines

Viral infections play an important role in human diseases, and recent outbreaks in the advent of globalization and ease of travel have underscored their prevention as a critical issue in safeguarding public health. Despite the progress made in immunization and drug development, many viruses lack preventive vaccines and efficient antiviral therapies, which are often beset by the generation of viral escape mutants. Thus, identifying novel antiviral drugs is of critical importance and natural products are an excellent source for such discoveries. In this mini-review, we summarize the antiviral effects reported for several natural products and herbal medicines.

Synthesis, structure-activity relationships and biological evaluation of dehydroandrographolide and andrographolide derivatives as novel anti-hepatitis B virus agents

Dehydroandrographolide and andrographolide, two natural diterpenoids isolated from Andrographis paniculata possessed activity against HBV DNA replication with IC50 values of 22.58 and 54.07μM and low SI values of 8.7 and 3.7 in our random assay. Consequently, 48 derivatives of dehydroandrographolide and andrographolide were synthesized and evaluated for their anti-HBV properties to yield a series of active derivatives with lower cytotoxicity, including 14 derivatives against HBsAg secretion, 19 derivatives against HBeAg secretion and 38 derivatives against HBV DNA replication. Interestingly, compound 4e could inhibit not only HBsAg and HBeAg secretions but also HBV DNA replication with SI values of 20.3, 125.0 and 104.9. Furthermore, the most active compound 2c with SI value higher than 165.1 inhibiting HBV DNA replication was revealed with the optimal logP value of 1.78 and logD values. Structure-activity relationships (SARs) of the derivatives were disclosed for guiding the future research toward the discovery of new anti-HBV drugs.

In vitro antiviral activity and underlying molecular mechanisms of dipotassium glycyrrhetate against porcine reproductive and respiratory syndrome virus

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused large economic losses in the swine industry. Currently, there is no effective way to prevent PRRSV infection. In this study, we investigated the inhibitory effect of dipotassium glycyrrhetate (DG), a derivative of glycyrrhetinic acid, on PRRSV infection ability.

METHODS

The cytotoxicity of DG was measured by MTT assay, and the effects of DG on PRRSV N gene/protein were investigated using real-time PCR, western blot and immunofluorescence assay. In addition, the effect of DG on cell apoptosis was analysed by fluorescence staining.

RESULTS

Our results indicated that DG could effectively inhibit virus replication and N gene expression in MARC-145 cells infected with PRRSV. When the infected cells received DG, the numbers of apoptotic cells were decreased, and the cleaved caspase-3 contents were decreased dramatically.

CONCLUSIONS

Our study demonstrates that DG could effectively inhibit the PRRS virus via multiple pathways including inhibition of virus replication and N gene expression and reduction of apoptotic cells. DG can serve as a potential chemical for PRRSV prevention and control.

Human viral diseases: what is next for antiviral drug discovery?

For the treatment of human immunodeficiency virus (HIV) infections for which there are ample drugs available, the immediate future lies in a once-daily combination pill containing three or four active ingredients. This strategy may also be envisaged for the treatment of hepatitis C virus (HCV) infections as soon as we have at hand the appropriate direct-acting antiviral agents (DAAs) to be combined. A combination drug therapy is generally not entertained for other viruses. Yet, new drugs are at the horizon for the treatment of herpes simplex virus (HSV), varicella-zoster virus (VZV), poxvirus, hepatitis B virus (HBV), influenza and enveloped viruses-at-large.