Design, synthesis and biological evaluation of novel C-29 carbamate celastrol derivatives as potent and selective cytotoxic compounds

Synthesis and Evaluation of Isosteviol Derivatives: Promising Anticancer Therapies for Colon Cancer

Background: Isosteviol, a tetracyclic diterpenoid with a beyerene-type skeleton, exhibited wide pharmacological activities and an inhibitory impact on tumor proliferation in colon cancer; Methods: 22 isosteviol derivatives were synthesized by modifying the C-16 and C-19 position of isosteviol, and then the inhibitory activities of derivatives 2-22 were evaluated by CCK8 method. Next, the structure-activity relationships (SARs) of these isosteviol derivatives in HCT116 cells were discussed in detail. Network pharmacology was employed to predict and analyze the targets of isosteviol in the treatment of colon cancer; Results: The results indicated derivative 8 possessed stronger inhibitory activity against HCT116 and HepG2 cells (IC50 = 6.20 ± 0.61 μM for HCT116, and IC50 = 39.84 ± 0.43 μM for HepG2). Additionally, cell cycle analysis indicated that derivative 8 arrested HCT116 cells at the G1 phase and increased the percentage of apoptotic cells. Moreover, the molecular docking showed that derivative 8 could interact with TP53 through its Tyr-1600 and Leu-1534 residues (docking energy: -11.84 kcal/mol); Conclusions: With these results, we can conclude that derivative 8 may be a promising candidate for anticancer chemotherapy.

Synthesis of a celastrol derivative as a cancer stem cell inhibitor through regulation of the STAT3 pathway for treatment of ovarian cancer

Accumulating evidence suggests that the root of drug chemoresistance in ovarian cancer is tightly associated with subpopulations of cancer stem cells (CSCs), whose activation is largely associated with signal transducer and activator of transcription 3 (STAT3) signaling. Recently, celastrol has shown a significant anti-cancer effect on ovarian cancer, but its clinical translation is very challenging due to its oral bioavailability and high organ toxicity. In this study, a celastrol derivative (Cel-N) was synthesized to augment the overall efficacy, and its underlying mechanisms were also explored. Different ovarian cancer cells, SKOV3 and A2780, were used to evaluate and compare the anticancer effects. Cel-N displayed potent activities against all the tested ovarian cancer cells, with the lowest IC50 value of 0.14-0.25 μM. Further studies showed that Cel-N effectively suppressed the colony formation and sphere formation ability, decreased the percentage of CD44+CD24- and ALDH+ cells, and induced ROS production. Furthermore, western blot analysis indicated that Cel-N significantly inhibited both Tyr705 and Ser727 phosphorylation and reduced the protein expression of STAT3. In addition, Cel-N could dramatically induce apoptosis and cell cycle arrest, and inhibit migration and invasion. Importantly, Cel-N showed a potent antitumor efficacy with no or limited systemic toxicity in mice xenograft models. The anticancer effect of Cel-N is stronger than celastrol. Cel-N attenuates cancer cell stemness, inhibits the STAT3 pathway, and exerts anti-ovarian cancer effects in cell and mouse models. Our data support that Cel-N is a potent drug candidate for ovarian cancer.

Design, synthesis, and evaluation of antitumor activity of novel C-6 sulfhydryl-substituted and 20-substituted derivatives of celastrol

Celastrol has been identified as a potential candidate for anticancer drug development. In this study, 28 novel celastrol derivatives with C-6 sulfhydryl substitution and 20-substitution were designed and synthesized, and their antiproliferative activity against human cancer cells and non-malignant human cells was evaluated, with cisplatin and celastrol being used as controls. The results showed that most of the derivatives had enhanced in vitro anticancer activity compared to the parent compound celastrol. Specifically, derivative 2f demonstrated the most potent inhibitory potential and selectivity against HOS with an IC50 value of 0.82 μM. Our study provides new insights into the structure-activity relationship of celastrol and suggests that compound 2f may be a promising drug candidate for the treatment of osteosarcoma.

Design, synthesis, and biological evaluation of new arjunolic acid derivatives as anticancer agents

Arjunolic acid (AA) is a pentacyclic triterpenoid with promising anticancer properties. A series of novel AA derivatives containing a pentameric A-ring with an enal moiety, combined with additional modifications at C-28, were designed and prepared. The biological activity on the viability of human cancer and non-tumor cell lines was evaluated in order to identify the most promising derivatives. Additionally, a preliminary study of the structure-activity relationship was carried out. The most active derivative, derivative 26, also showed the best selectivity between malignant cells and non-malignant fibroblasts. For compound 26, the anticancer molecular mechanism of action in PANC-1 cells was further studied and the results showed that this derivative induced a cell-cycle arrest at G0/G1 phase and significantly inhibited the wound closure rate of PANC-1 cancer cells in a concentration-dependent manner. Additionally, compound 26 synergistically increased the cytotoxicity of Gemcitabine, especially at a concentration of 0.24 μM. Moreover, a preliminary pharmacological study indicated that at lower doses this compound did not demonstrate toxicity in vivo. Taken together, these findings suggest that compound 26 may be a valuable compound for the development of new pancreatic anticancer treatment, and further studies are needed to explore its full potential.

Discovery of semisynthetic celastrol derivatives exhibiting potent anti-ovarian cancer stem cell activity and STAT3 inhibition

The hallmark of ovarian cancer is its high mortality rate attributed to the existence of cancer stem cells (CSCs) subpopulations which result in therapy recurrence and metastasis. A series of C-29-substituted and/or different A/B ring of celastrol derivatives were synthesized and displayed potential inhibition against ovarian cancer cells SKOV3, A2780 and OVCAR3. Among them, compound 6c exhibited the most potent anti-proliferative activity and selectivity, gave superior anti-CSC effects through inhibition of the sphere formation and downregulation of the percentage of CD44⁺CD24^- and ALDH⁺ cells. Further mechanism research demonstrated that compound 6c could attenuate the expression of STAT3 and p-STAT3. The results suggested that the inhibition of celastrol derivative 6c on ovarian cancer cells may be related to resistance to cancer stem-like characters and regulation of STAT3 pathway.

Synthesis of Four Steroidal Carbamates with Antitumor Activity against Mouse Colon Carcinoma CT26WT Cells: In Vitro and In Silico Evidence

Colorectal cancer (CRC) is one of the most lethal cancers worldwide. If detected on time, surgery can expand life expectations of patients up to five more years. However, if metastasis has grown deliberately, the use of chemotherapy can play a crucial role in CRC control. Moreover, the lack of selectivity of current anticancer drugs, plus mutations that occur in cancerous cells, demands the development of new chemotherapeutic agents. Several steroids have shown their potentiality as anticancer agents, while some other compounds, such as Taxol and its derivatives bearing a carbamate functionality, have reached the market. In this article, the synthesis, characterization, and antiproliferative activity of four steroidal carbamates on mouse colon carcinoma CT26WT cells are described. Carbamate synthesis occurred via direct reaction between diosgenin, its B-ring modified derivative, and testosterone with phenyl isocyanate under a Brønsted acid catalysis. All obtained compounds were characterized by ¹H and ¹³C Nuclear Magnetic Resonance (NMR), High Resolution Mass Spectroscopy (HRMS); their melting points are also reported. Results obtained from antiproliferative activity assays indicated that carbamates compounds have inhibitory effects on the growth of this colon cancer cell line. A molecular docking study carried out on Human Prostaglandin E Receptor (EP4) showed a high affinity between carbamates and protein, thus providing a valuable theoretical explanation of the in vitro results.

Discovery of Novel Celastrol-Imidazole Derivatives with Anticancer Activity In Vitro and In Vivo

To discover celastrol (CEL) derivatives with enhanced Hsp90-Cdc37 inhibition, C-20-COOH was introduced with various substituted imidazoles, which might affect the Michael addition of CEL by nucleophilic attack. The most potent compound 9, which showed higher antiproliferation, covalent-binding ability, and Hsp90-Cdc37 inhibition than CEL, was selected from 28 new target compounds. Then, the binding sites and the docking mode of 9 to Hsp90 and Cdc37 were studied. Furthermore, the activity of 9 sharply decreased or even disappeared in the Hsp90- and/or Cdc37-overexpressing A549 cells, indicating that the activity was related to its combination with Hsp90 and Cdc37. Moreover, 9 could more effectively induce apoptosis and inhibit tumor growth than CEL in vivo. This study first found that imidazoles linked to C-20 of CEL might affect its Michael addition, which will provide support of CEL or even the other Michael acceptors for the development as antitumor agents.

Synthesis and biological evaluation of celastrol derivatives as potential anti-glioma agents by activating RIP1/RIP3/MLKL pathway to induce necroptosis

Celastrol, a quinone methide triterpenoid, possesses potential anti-glioma activity. However, its relatively low activity limit its application as an effective agent for glioma treatment. In search for effective anti-glioma agents, this work designed and synthesized two series of celastrol C-3 OH and C-20 COOH derivatives 4a-4o and 6a-6o containing 1, 2, 3-triazole moiety. Their anti-glioma activities against four human glioma cell lines (A172, LN229, U87, and U251) were then evaluated using MTT assay in vitro. Results showed that compound 6i (IC₅₀ = 0.94 μM) exhibited substantial antiproliferative activity against U251 cell line, that was 4.7-fold more potent than that of celastrol (IC₅₀ = 4.43 μM). In addition, compound 6i remarkably inhibited the colony formation and migration of U251 cells. Further transmission electron microscopy and mitochondrial depolarization assays in U251 cells indicated that the potent anti-glioma activity of 6i was attributed to necroptosis. Mechanism investigation revealed that compound 6i induced necroptosis mainly by activating the RIP1/RIP3/MLKL pathway. Additionally, compound 6i exerted acceptable BBB permeability in mice and inhibited U251 cell proliferation in an in vivo zebrafish xenograft model, obviously. In summary, compound 6i might be a promising lead compound for potent celastrol derivatives as anti-glioma agents.

Synthesis and biological evaluation of celastrol derivatives as potent antitumor agents with STAT3 inhibition

Using STAT3 inhibitors as a potential strategy in cancer therapy have attracted much attention. Recently, celastrol has been reported that it could directly bind to and suppress the activity of STAT3 in the cardiac dysfunction model. To explore more effective STAT3 inhibiting anti-tumour drug candidates, we synthesised a series of celastrol derivatives and biologically evaluated them with several human cancer cell lines. The western blotting analysis showed that compound 4 m, the most active derivative, could suppress the STAT3's phosphorylation as well as its downstream genes. SPR analysis, molecular docking and dynamics simulations' results indicated that the 4m could bind with STAT3 protein more tightly than celastrol. Then we found that the 4m could block cell-cycle and induce apoptosis on HCT-116 cells. Furthermore, the anti-tumour effect of 4m was verified on colorectal cancer organoid. This is the first research that discovered effective STAT3 inhibitors as potent anti-tumour agents from celastrol derivatives.

Design, synthesis of novel celastrol derivatives and study on their antitumor growth through HIF-1α pathway

Four series of hypoxia-inducible factor-1 alpha (HIF-1α) functioning derivatives stemming from modifications to the C-29 carboxyl group of celastrol were designed and synthesized, and their anticancer activities were evaluated. To address the structure and activity relationship of each derivative, extensive structural changes were made. HRE luciferase reporter assay demonstrated that 12 modified compounds showed superior HIF-1α inhibitory activity. Among them, compound C6 exhibited the best features: firstly, the strongest HIF-1α inhibitory activity (IC₅₀ = 0.05 μM, 5-fold higher than that of celastrol); secondly, lower cytotoxicity (22-fold lower, C6-16.85 μM vs celastrol-0.76 μM). Thus, the safety factor of C6 was about 112 times higher than that of celastrol. Western blot assay indicated that C6 may inhibit the expression of HIF-1α protein in cells. Additionally, C6 hindered tumor cell cloning, migration and induced cell apoptosis. It is worth mentioning that in the mouse tumor xenograft model, C6 (10 mg/kg) displayed good antitumor activity in vivo, showing a better inhibition rate (74.03%) than the reference compound 5-fluorouracil (inhibition rate, 59.58%). However, the celastrol treatment group experienced collective death after four doses of the drug. Moreover, C6 minimally affected the mouse weight, indicating that its application in vivo has little toxic effect. H&E staining experiments show that it could also exacerbate the degree of tumor cell damage. The results of water solubility experiment show that the solubility of C6 is increased by 1.36 times than that of celastrol. In conclusion, C6 is a promising antitumor agent through HIF-1α pathway.

Discovery of novel celastrol-triazole derivatives with Hsp90-Cdc37 disruption to induce tumor cell apoptosis

To enhance the disruption of Hsp90-Cdc37, we designed and synthesized a series (27) of CEL-triazole derivatives. Most of the target compounds showed enhanced anti-proliferative activity on four cancer cell lines (MDA-MB-231, MCF-7, HepG2 and A459). Among them, compound 6 showed the best anti-proliferation (IC₅₀ = 0.34 ± 0.01 μM) on MDA-MB-231. Pharmacological studies had found that compound 6 showed a higher ability to disrupt Hsp90-Cdc37 interaction in cells and inhibited the expression of the key Hsp90-Cdc37 clients in a concentration-dependent manner. Further studies indicated that an enhanced covalent binding between compound 6 and thiols (cysteine) might be one of the reasons for the increased activity. Furthermore, compound 6 arrested cells in the G₀/G₁ phase and induced tumor cell apoptosis significantly. Overall, for cancer treatment, compound 6 was worth further exploring.

Biosynthesis, total synthesis, structural modifications, bioactivity, and mechanism of action of the quinone-methide triterpenoid celastrol

Celastrol, a quinone-methide triterpenoid, was extracted from Tripterygium wilfordii Hook. F. in 1936 for the first time. Almost 70 years later, it is considered one of the molecules most likely to be developed into modern drugs, as it exhibits notable bioactivity, including anticancer and anti-inflammatory activity, and exerts antiobesity effects. In addition, the molecular mechanisms underlying its bioactivity are being widely studied, which offers new avenues for its development as a pharmaceutical reagent. Owing to its potential therapeutic effects and unique chemical structure, celastrol has attracted considerable interest in the fields of organic, biosynthesis, and medicinal chemistry. As several steps in the biosynthesis of celastrol have been revealed, the mechanisms of key enzymes catalyzing the formation and postmodifications of the celastrol scaffold have been gradually elucidated, which lays a good foundation for the future heterogeneous biosynthesis of celastrol. Chemical synthesis is also an effective approach to obtain celastrol. The total synthesis of celastrol was realized for the first time in 2015, which established a new strategy to obtain celastroid natural products. However, owing to the toxic effects and suboptimal pharmacological properties of celastrol, its clinical applications remain limited. To search for drug-like derivatives, several structurally modified compounds were synthesized and tested. This review focuses primarily on the latest research progress in the biosynthesis, total synthesis, structural modifications, bioactivity, and mechanism of action of celastrol. We anticipate that this paper will facilitate a more comprehensive understanding of this promising compound and provide constructive references for future research in this field.

Synthesis of celastrol derivatives as potential non-nucleoside hepatitis B virus inhibitors

A series of para-quinone methide (pQM) moiety and C-20- modified derivatives of celastrol were synthesized and evaluated for their inhibitory effect on the secretion of HBsAg and HBeAg as well as the inhibitory effect against HBV DNA replication. The results suggested that amidation of C-20 carboxylic group could generate derivatives with good anti-HBV profile, among them compound 14 showed the best inhibitory activity on the secretion of HBsAg (IC₅₀  = 11.9 µμ) and HBeAg (IC₅₀  = 13.1 µμ) with SI of 3.3 and 3.0, respectively. In addition, 14 also showed potent inhibitory effect against HBV DNA replication (48.5 ± 15.1%, 25 µM). This is, to our knowledge, the first report of celastrol derivatives as potential non-nucleoside HBV inhibitors.

Design, Synthesis and Biological Activity of (20S,21S)-7-Cyclohexyl-21-fluorocamptothecin Carbamates as Potential Antitumor Agents

(20S,21S)-7-Cyclohexyl-21-fluorocamptothecin was discovered by a fluorine drug design strategy with potent antitumor activity and increased metabolic stability. In continuous efforts to find novel antitumor agents derived from natural product camptothecin, 20-carbamates of the active compound (20S,21S)-7-cyclohexyl-21-fluorocamptothecin have been designed and synthesized. Among them, one compound with the diethylamino group showed greater antiproliferative activity than the other 20-carbamate derivatives. The following biological activity assays indicated that the above compound is a valuable lead compound with excellent Topo I inhibitory activity and solution stability.

Exploration of (3-benzyl-5-hydroxyphenyl)carbamates as new antibacterial agents against Gram-positive bacteria

A series of (3-benzyl-5-hydroxyphenyl)carbamates were evaluated as new antibacterial agents. Several compounds showed potent inhibitory activity against sensitive and drug-resistant Gram-positive bacteria. The compounds are ineffective against all tested Gram-negative bacteria. The structure of the ester group exerted a profound effect on antibacterial activity. 4,4-Dimethylcyclohexanyl carbamate 6h exhibited the most potent inhibitory activity against the standard and clinically isolated Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis (minimum inhibitory concentration = 4-8 µg/ml) strains. The preliminary experimental evidence indicated that these carbamates target the bacterial cell wall and share a similar mechanism of action with vancomycin.

Exploring New Structural Features of the 18β-Glycyrrhetinic Acid Scaffold for the Inhibition of Anaplastic Lymphoma Kinase

Novel 18β-glycyrrhetinic acid derivatives possessing a carbamate moiety and structurally similar ester derivatives were developed and evaluated for their efficacy as antitumor inhibitors. In the cellular assays, most of the N-substituted carbamate derivatives at the C3-position exhibited potent activities. The results of SAR investigation revealed that the introduction of the morpholine group at the C30-COOH led to a significant loss of the inhibitory potency. Among the ester derivatives, the ester group at C3-position also determined a noticeable reduction in the efficacy. Compound 3j exhibited the most prominent antiproliferative activity against six human cancer cells (A549, HT29, HepG2, MCF-7, PC-3, and Karpas299). Furthermore, compound 3j exerted a moderate inhibiting effect on the ALK. The results of molecular docking analyses suggested that it could bind well to the active site of the receptor ALK, which was consistent with the biological data. These results might inspire further structural optimization of 18β-glycyrrhetinic acid aiming at the development of potent antitumor agents. The structures 4d, 4g, 4h, 4j, and 4n were studied by X-ray crystallographic analyses.

Synthesis and biological evaluation of celastrol derivatives as anti-ovarian cancer stem cell agents

Ovarian cancer is associated with a high percentage of recurrence of tumors and resistance to chemotherapy. Cancer stem cells (CSCs) are responsible for cancer progression, tumor recurrence, metastasis, and chemoresistance. Thus, developing CSC-targeting therapy is an urgent need in cancer research and clinical application. In an attempt to achieve potent and selective anti-CSC agents, a series of celastrol derivatives with cinnamamide chains were synthesized and evaluated for their anti-ovarian cancer activities. Most of the compounds exhibited stronger antiproliferative activity than celastrol, and celastrol derivative 7g with a 3,4,5-trimethoxycinnamamide side chain was found to be the most potent antiproliferative agent against ovarian cancer cells with an IC₅₀ value of 0.6 μM. Additionally, compound 7g significantly inhibited the colony formation ability and reduced the number of tumor spheres. Furthermore, compound 7g decreased the percentage of CD44⁺, CD133⁺ and ALDH⁺ cells. Thus, compound 7g is a promising anti-CSC agent and could serve as a candidate for the development of new anti-ovarian cancer drugs.

Synthesis and Biological Evaluation of New Madecassic Acid Derivatives Targeting ERK Cascade Signaling

In the present study, a series of novel madecassic acid derivatives was synthesized and screened against the National Cancer Institute's 60 human cancer cell line panel. Among them, compounds 5, 12, and 17 displayed potent and highly differential antiproliferative activity against 80% of the tumor cells harboring the B-Raf^V600E mutation within the nanomolar range. Structure-activity analysis revealed that a 5-membered A ring containing an α,β-unsaturated aldehyde substituted at C-23 with a 2-furoyl group seems to be crucial to produce this particular growth inhibition signature. In silico analysis of the cytotoxicity pattern of these compounds identified two highly correlated clinically approved drugs with known B-Raf^V600E inhibitory activity. Follow-up analysis revealed inhibition of the ERK signaling pathway through the reduction of cellular Raf protein levels is a key mechanism of action of these compounds. In particular, 17 was the most potent compound in suppressing tumor growth of B-Raf^V600E-mutant cell lines and displayed the highest reduction of Raf protein levels among the tested compounds. Taken together, this study revealed that modifications of madecassic acid structure can provide molecules with potent anticancer activity against cell lines harboring the clinically relevant B-Raf^V600E mutation, with compound 17 identified as a promising lead for the development of new anticancer drugs.