A “natural” approach: Synthesis and cytoxicity of monodesmosidic glycyrrhetinic acid glycosides

Design, synthesis, and in vivo and in vitro biological screening of pseudolaric acid B derivatives as potential anti-tumor agents

Pseudolaric Acid B (PAB), a natural product with remarkable anti-tumor activity, is a starting point for new anticancer therapeutics. We designed and synthesized 27 PAB derivatives and evaluated their anti-proliferative activities against four cancer cell lines: MCF-7, HCT-116, HepG2, and A549. Compared with unmodified PAB, the PAB derivatives showed stronger anti-proliferative activity. The ability of compound D3 (IC50 = 0.21 μM) to inhibit HCT-116 cells was approximately 5.3 times that of PAB (IC50 = 1.11 μM) and the antiproliferative action was unrelated to cytotoxicity (SI=20.38), indicating its superior safety profile (PAB; SI=0.95). Compound D3 effectively suppressed the EdU-positive rate and reduced colony formation, arrested HCT-116 cells in the S and G2/M phases and induced apoptosis. In vivo experiments further demonstrated low toxicity of compound D3 while suppressing tumor growth in mice. In summary, given its strong anti-proliferative effect and relative safety, further development of compound D3 is warranted.

Synthesis and Antiviral and Antitumor Activities of Novel 18β-Glycyrrhetinic Acid Derivatives

A series of novel derivatives of 18β-glycyrrhetinic acid (GA) were synthesized by introducing aromatic or heterocyclic structures to extend the side chain, thereby enhancing their interaction with amino acid residues in the active pocket of the target protein. These compounds were structurally characterized using 1H NMR, 13C NMR, and HRMS. The compounds were subsequently evaluated for their inhibitory effects on HIV-1 protease and cell viability in the human cancer cell lines K562 and HeLa and the mouse cancer cell line CT26. Towards HIV-1 protease, compounds 28 and 32, which featured the introduction of heterocyclic moieties at the C3 position of GA, exhibited the highest inhibition, with inhibition rates of 76% and 70.5%, respectively, at 1 mg/mL concentration. Further molecular docking suggests that a 3-substituted polar moiety would be likely to enhance the inhibitory activity against HIV-1 protease. As for the anti-proliferative activities of the GA derivatives, incorporation of a thiazole heterocycle at the C3- position in compound 29 significantly enhanced the effect against K562 cells with an IC50 value of 8.86 ± 0.93 µM. The introduction of electron-withdrawing substituents on the C3-substituted phenyl ring augmented the anti-proliferative activity against Hela and CT26 cells. Compound 13 exhibited the highest inhibitory activity against Hela cells with an IC50 value of 9.89 ± 0.86 µM, whereas compound 7 exerted the strongest inhibition against CT26 cells with an IC50 value of 4.54 ± 0.37 µM. These findings suggest that further modification of GA is a promising path for developing potent novel anti-HIV and anticancer therapeutics.

Optimization, and biological evaluation of 3-O-β-chacotriosyl betulinic acid amide derivatives as novel small-molecule Omicron

SARS-CoV-2 Omicron viruses possess a high antigenic shift, and the approved anti-SARS-CoV-2 drugs are extremely limited, which makes the development of new antiviral drugs for the clinical treatment and prevention of SARS-CoV-2 outbreaks imperative. We have previously discovered a new series of markedly potent small-molecule inhibitors of SARS-CoV-2 virus entry, exampled by the hit compound 2. Here, we report a further study of bioisosteric replacement of the eater linker at the C-17 position of 2 with a variety of aromatic amine moieties, followed by a focused structure-activity relationship study, leading to the discovery of a series of novel 3-O-β-chacotriosyl BA amide derivatives as small-molecule Omicron fusion inhibitors with improved potency and selectivity index. Particularly, our medicinal chemistry efforts have resulted in a potent, and efficacious lead compound S-10 with appreciable pharmacokinetic properties, which exhibited broad-spectrum potency against Omicron and other variants with EC₅₀ values ranging from 0.82 to 5.45 μM. Mutagenesis studies confirmed that inhibition of Omicron viral entry was mediated by the direct interaction with S in the prefusion state. These results reveal that S-10 is suitable for further optimization as Omicron fusion inhibitors, with the potential to be developed as therapeutic agents for the treatment and control of SARS-CoV-2 ant its variants infections.

Glycyrrhetinic acid nanoparticles combined with ferrotherapy for improved cancer immunotherapy

Programmed cell death protein 1 (PD-1)/Programmed Cell Death Ligand 1 (PD-L1) blockade immunotherapy has emerged as a promising strategy to treat both solid and hematological malignancies. Despite the considerable therapeutic effects obtained in pre-clinical and clinical studies, PD-1/PD-L1 blockade therapy is still limited by the low benefit rates and a large number of patients still do not respond to this treatment. In this study, we developed a highly efficient and cancer-specific immunogenic cell death nanoinducer for effective tumor immunotherapy. A leukocyte membrane coated poly (lactic-co-glycolic acid) encapsulating glycyrrhetinic acid (GCMNPs) was developed to enhance targeting, tumor-homing capacity, and reduce toxicity in vivo. GCMNPs could induce ferroptosis in acute myeloid leukemia and colorectal cancer cells by downregulating glutathione-dependent peroxidases 4, leading to increased lipid peroxidation levels. Moreover, GCMNPs and ferumoxytol could synergistically enhance Fe-dependent cytotoxicity through the Fenton reaction. Finally, in vivo studies showed that GCMNPs synergized with ferumoxytol and anti-PD-L1 synergistically improve T-cell immune response against leukemia and colorectal tumor. This study anticipated that the combination of glycyrrhetinic acid-based nanomaterials and ferrotherapy would provide further insights into anti-cancer immune response to PD-1/PD-L1 blockade for both solid and hematological malignancies. STATEMENT OF SIGNIFICANCE: Despite the considerable therapeutic effects obtained in pre-clinical and clinical studies, PD-1/PD-L1 blockade therapy is still limited by the low benefit rates and a large number of patients still do not respond to this treatment. We designed a glycyrrhetinic acid-based nanoplatform as a new ICD inducer (GCMNPs), with high cancer cell specificity and reduced toxicity to AML and CRC. GCMNPs cooperates with ferumoxytol to promote a Fenton reaction and induce ferroptosis. Moreover, the combination of GCMNPs and ferumoxytol enhanced the blockage of PD-1/PD-L1 to activate T cells, subsequently generating a systemic immune response in CRC and AML mouse models. This pre-clinical findings provide the proof-of-concept of combination of glycyrrhetinic acid-based nanomaterials and ferrotherapy as an "ICD nano-inducer" and immunotherapeutic agent for treating cancer.

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.

Application of Amino Acids in the Structural Modification of Natural Products: A Review

Natural products and their derivatives are important sources for drug discovery; however, they usually have poor solubility and low activity and require structural modification. Amino acids are highly soluble in water and have a wide range of activities. The introduction of amino acids into natural products is expected to improve the performance of these products and minimize their adverse effects. Therefore, this review summarizes the application of amino acids in the structural modification of natural products and provides a theoretical basis for the structural modification of natural products in the future. The articles were divided into six types based on the backbone structures of the natural products, and the related applications of amino acids in the structural modification of natural products were discussed in detail.

Cytotoxic and Anti-inflammatory Terpenoids from the Whole Plant of Vaccinium emarginatum

Two new Δ¹² ursene-type triterpenoid coumaroyl esters (1: and 2: ), one new Δ^7,15 isopimarane-type diterpenoid glycoside (20: ), and two new irido-δ-lactone-type iridoids (21: and 22: ), together with 17 known pentacyclic triterpenoids (3:  - 19: ), were isolated during the phytochemical investigation of a methanol extract of the whole plant of Vaccinium emarginatum. Their structures were determined by detailed analysis of standard spectroscopic data (MS, IR, 1D, and 2D NMR) and comparison with data of known analogs. The isolates were evaluated for their cytotoxicity against the PC-3 and Du145 prostate cancer cell lines (as assessed by an MTT cell proliferation assay), as well as for their anti-inflammatory activity via the inhibition of nitric oxide production in lipopolysaccharide-induced murine macrophage RAW 264.7 cells. Among the isolates, the triterpenoid coumaroyl and feruloyl esters (1, 3: , and 4: ) exhibited strong cytotoxicity against PC-3 prostate cancer cells, with 85.6 - 90.2% inhibition at 10.0 µg/mL. The pomolic acid coumaroyl and feruloyl esters (1: and 3: ) also showed moderate anti-inflammatory activity against nitric oxide production in lipopolysaccharide-induced RAW 264.7 cells, with 59.2 (± 1.0) and 47.1% (± 0.2) inhibition at 12.5 µg/mL, respectively.

Synthesis and inhibition of α-glucosidase of methyl glycyrrhetinate glycosides

The synthesis of the methyl glycyrrhetinate glycosides and inhibition of α-glucosidase were studied. The carboxyl group of glycyrrhetinic acid was methylated, and glucose and galactose were introduced into the hydroxyl group to obtain compounds 7 and 12. Compound 1, 2, 7, 12 and glycyrrhizic acid (GL) were evaluated for their inhibitory activities against α-glucosidase. As a result, Compound 1, 2, 7, 12 and GL all showed significant α-glucosidase inhibitory activity and IC₅₀ values were 0.465, 1.352, 0.759, 0.687 and 2.085 mM, respectively, and acted as non-competitive inhibitors. The activity of the compound 2, 7, 12 was lower than compound 1, but significantly higher than GL. Therefore, it was concluded that the change of structure in glycyrrhetinic acid by chemical modification had certain effect on bioactivity, and the change of carboxyl group, hydroxyl group and the type of monosaccharide introduced were the influencing factors.

Therapeutic potential of glycyrrhetinic acids: a patent review (2010-2017)

INTRODUCTION

Glycyrrhetinic acids (GAs) viz., 18β-glycyrrhetinic acid and 18α-glycyrrhetinic acid, are oleanane-type triterpenes having a carboxylic acid group at C-30, and are extracted from the Chines herbal medicine licorice (Glycyrrhiza uralensis). Although the pharmacological properties of GAs have long been known, attention to them has greatly increased in recent times due to their cytotoxic activity.

AREAS COVERED

This review represents the patents granted about natural and synthetic glycyrrhetinic acid analogs from January 2010 to December 2017, the advances made by research groups in conjunction with pharmaceutical companies in the discovery of new natural or synthetic glycyrrhetinic acid analogs.

EXPERT OPINION

GAs demonstrate excellent cytotoxic, antimicrobial, enzyme inhibitory, antiinflammatory, antioxidant, analgesic, and antiviral effects. It is interesting to note that the C-₃(OH) and C_30-CO₂H functional groups make GAs very attractive lead structures for medicinal scientists since these functionalities allow the generation of further chemical diversity for improved pharmacological effects. Moreover, various GA analogues have been prepared via modification of the C_30-CO₂H. It is noteworthy that the C-30 amide of GA demonstrated better cytotoxic effects compared to the parent compounds. In addition, GAs have the capability to conjugate with other anticancer drugs or be converted into their halo or amino analogs which is expected to stimulate medicinal chemist to synthesize new lead compounds in cancer drug discovery.

Recent progress in the structural modification and pharmacological activities of ligustrazine derivatives

Ligustrazine is a main active fraction of the traditional medicine known as Ligusticum chuanxiong hort, which has been used as clinical medication for cerebral thrombosis, coronary heart disease and stenocardia recently. The rapid metabolism and short half-life of ligustrazine seriously limits its application in clinical practice. Therefore, derivatives of ligustrazine are designed and synthesized in our and other labs, including piperazine, cinnamic acid, styrene, acylguanidine, amides, curcumin and triterpenes derivatives of ligustrazine. Most of these compounds present better pharmacodynamics activities and more favorable pharmacokinetic properties compared to the parent compound. Besides, some new biological activities of these compounds are discovered. Hence, this review continues the previous review of our group as well as aims to highlight recent prominent advances in this field in the past ten years.

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.

Combination of amino acid/dipeptide with ligustrazine-betulinic acid as antitumor agents

The lead compound TBA, 3β-Hydroxy-lup-20(29)-ene-28-oic acid-3, 5, 6-trimethylpyrazin-2-methyl ester, which exhibited promising antitumor activity and induced tumor cell apoptosis in various cancer cell lines, had previously been reported. Moreover, reports have revealed that the introduction of amino acid to betulinic acid could improve selective cytotoxicity as well as water solubility. Thus, a series of novel TBA amino acid and dipeptide derivatives were designed, synthesized and screened for selective cytotoxic activity against five cancer cell lines (HepG2, HT-29, Hela, BCG-823 and A549) and the not malignant cell line MDCK by standard MTT assay. Most of the tested TBA-amino acid and dipeptide analogues showed stronger anti-proliferative activity against all tested tumor cell lines than TBA. Among them, BA-25 exhibited the greatest cytotoxic activity on tumor cell lines (mean IC₅₀ = 2.31 ± 0.78 μM), that was twofold than the positive drug cisplatin (DDP), while it showed lower cytotoxicity on MDCK cell line than DDP. Further cell apoptosis analyses indicated BA-25-induced apoptosis was associated with loss of mitochondrial membrane potential and increase of intracellular free Ca²⁺ concentration.

Design, synthesis, and biological evaluation of the novel glycyrrhetinic acid-cinnamoyl hybrids as anti-tumor agents

Background

Glycyrrhetinic acid (GA) derivatives had shown not only cytotoxicity but also could trigger apoptosis in various human cancer cell lines. Moreover, cinnamic acid (CA) and its phenolic analogues as potent antitumor agents were employed in the design of anti-tumor drugs. To further improve the anti-tumor activity of GA and CA derivatives, a series of novel compounds were designed and synthesized using GA and CA derivatives fragments.

Results

The result showed that all the novel glycyrrhetinic acid-cinnamoyl (GA–CA) hybrids presented higher antitumor activity on the tumor cell lines of HepG2, HT-29, Hela and lower cytotoxicity on three non-tumor cells lines MDCK, HY926, H9C2 than the parent compounds (IC₅₀ > 50 μM). It was worth noting that 8a had a superior cytotoxicity effect on Hela cells (IC₅₀ = 8.54 μM) than on other cancer cell lines (IC₅₀ > 15 μM). And it also indicated that 8a showed lower cytotoxicity (IC₅₀ > 27 μM) towards MDCK, HY926 and H9C2 cells than cisplatin (DDP, IC₅₀ < 10 μM). Moreover, according to the acute toxicity, it could be indicated that the LD₅₀ of 8a exceeded 3.0 g/kg by oral administration in mice. The further research using Giemsa, H33342 staining, flow cytometric analysis and caspase-3 assay showed that 8a could cause Hela cell damage, nuclei lysis and apoptosis. In addition, the structure–activity relationships of these hybrids were briefly discussed.

Conclusions

Compared with GA, target compounds demonstrated better anti-tumor activity, among which 8a was the most active one. What’s more, structure–activity relationship analysis also revealed that hybrids with trans olefinic bond group show higher antitumor activity than those without olefinic bond, such as 1a > 1b, 6a > 2b, 8a > 3b, 9a > 4b. In addition, it was found that the methoxy substituent might enhance selectivity of GA–CA hybrids towards regular non-cancerous cells MDCK, HY926 and H9C2, such as 4a, 6a, 7a, 8a. However, there might be less relationship between the cytotoxicity and the quantity, position of methoxy moiety. Hence, it is urgent need to synthesize efficient, low toxicity and multi-target anti-tumor compounds based on the structure combination principle.

Synthesis and Anticancer Activities of Glycyrrhetinic Acid Derivatives

A total of forty novel glycyrrhetinic acid (GA) derivatives were designed and synthesized. The cytotoxic activity of the novel compounds was tested against two human breast cancer cell lines (MCF-7, MDA-MB-231) in vitro by the MTT method. The evaluation results revealed that, in comparison with GA, compound 42 shows the most promising anticancer activity (IC₅₀ 1.88 ± 0.20 and 1.37 ± 0.18 μM for MCF-7 and MDA-MB-231, respectively) and merits further exploration as a new anticancer agent.

Natural product-based design, synthesis and biological evaluation of Albiziabioside A derivatives that selectively induce HCT116 cell death

A series of Albiziabioside A coupled substituents of cinnamoyl derivatives were designed and synthesized. The synthesized compounds were screened for anticancer activity against a panel of six human cancer cell lines using a MTT assay. Synthetic derivatives showed excellent selectivity, as they were toxic against only HCT116 cell line. Some compounds exhibited better anti-cancer activity against HCT116 compared to positive controls, such as 5-fluorouracil and Albiziabioside A. Compound 8n was the most active derivative. Importantly, it was also found that the anti-proliferative activity of 8n could be attributed to the induction of cell cycle arrest and apoptosis in HCT116 cells.

A Series of New Ligustrazine-Triterpenes Derivatives as Anti-Tumor Agents: Design, Synthesis, and Biological Evaluation

A series of novel ligustrazine-triterpenes derivatives was designed, synthesized and screened for their cytotoxicity against five cancer cell lines (Bel-7402, HepG2, HT-29, Hela, and MCF-7) and Madin-Darby canine kidney (MDCK). Current study suggested that most of the ligustrazine-triterpenes conjunctions showed better cytotoxicity than the starting materials. In particular, compound 4a exhibited better cytotoxic activity (IC₅₀ < 5.23 μM) against Bel-7402, HT-29, MCF-7, Hela, and HepG2 than the standard anticancer drug cisplatin (DDP). The cytotoxicity selectivity detection revealed that 4a exhibited low cytotoxicity (IC₅₀ > 20 μM) towards MDCK cells. A combination of fluorescence staining observation and flow cytometric analysis indicated that 4a could induce HepG2 cell apoptosis. Further studies suggested that 4a-induced apoptosis is mediated through depolarization of the mitochondrial membrane potential and increase of intracellular free Ca²⁺ concentration. In addition, the structure-activity relationships of these derivatives were briefly discussed.

Synthesis and biological evaluation of betulonic acid derivatives as antitumor agents

Structural modification was performed at the C-28 position of betulonic acid (BetA). Twenty-five BetA derivatives were synthesized, and evaluated for their antitumor activities against MGC-803, PC3, Bcap-37, A375, and MCF-7 human cancer cell lines by MTT assay. Among the derivatives, most of the derivatives had significant antiproliferative ability (IC50 < 19 μM). Compound 3k, the most active compound, showed IC50 values of 3.6, 5.6, 4.2, 7.8, and 5.2 μM on the five cancer cell lines respectively, and was selected to investigate cell apoptosis by subsequent florescence staining and flow cytometry analysis. The results revealed that compound 3k could induce apoptosis in MGC-803 cell lines, and the apoptosis ratios reached 28.33% after 36 h of treatment at 10 μM. In addition, the study of cancer cell apoptotic signaling pathway indicated that the apoptosis of MGC-803 cells induced by compound 3k could be through the mitochondrial intrinsic pathway.

Structure-activity relationships of 3-O-β-chacotriosyl ursolic acid derivatives as novel H5N1 entry inhibitors

A series of methyl ursolate 3-O-β-chacotrioside analogs have been designed, synthesized and evaluated as H5N1 entry inhibitors based on a small molecule inhibitor saponin 3 previously discovered by us. Detailed structure-activity relationships (SARs) studies on the aglycone of compound 3 indicated that both the type of pentacyclic triterpene and the subtle modification of ursolic acid as an aglycon had key influences on the antiviral activity. These results suggested that either the introduction of a disubstituted amide structure at the 17-COOH of ursolic acid or alteration of the C-3 configuration of ursolic acid from 3β-to 3α-forms was helpful to significantly improve the selective index while keeping their antiviral activities.