ent-Kaurane-based regio- and stereoselective inverse electron demand hetero-Diels-Alder reactions: synthesis of dihydropyran-fused diterpenoids

Design and synthesis of oridonin derivatives as cytotoxic agents

Oridonin is one of the ent-kaurane diterpenes that have been studied extensively for various bioactivities. In an effort to expand natural scaffold-based library as anticancer agents, we have designed and synthesised a number of novel oridonin derivatives and evaluated their bioactivities on a panel of human cancer cell lines (HCT116, A375, MCF-7, HepG2, and A549). Compound 4b bearing a 4-fluorophenyl moiety was found to be the most active compound with an IC50 value of 0.3 μM against MCF-7 cells, which was 7.4-fold more active than oridonin. This study could provide some insightful information for further synthesis of oridonin derivatives as anticancer agents.

Recent advances in oridonin derivatives with anticancer activity

Cancer is a leading cause of mortality responsible for an estimated 10 million deaths worldwide in 2020, and its incidence has been rapidly growing over the last decades. Population growth and aging, as well as high systemic toxicity and chemoresistance associated with conventional anticancer therapies reflect these high levels of incidence and mortality. Thus, efforts have been made to search for novel anticancer drugs with fewer side effects and greater therapeutic effectiveness. Nature continues to be the main source of biologically active lead compounds, and diterpenoids are considered one of the most important families since many have been reported to possess anticancer properties. Oridonin is an ent-kaurane tetracyclic diterpenoid isolated from Rabdosia rubescens and has been a target of extensive research over the last few years. It displays a broad range of biological effects including neuroprotective, anti-inflammatory, and anticancer activity against a variety of tumor cells. Several structural modifications on the oridonin and biological evaluation of its derivatives have been performed, creating a library of compounds with improved pharmacological activities. This mini-review aims to highlight the recent advances in oridonin derivatives as potential anticancer drugs, while succinctly exploring their proposed mechanisms of action. To wind up, future research perspectives in this field are also disclosed.

Spirolactone-type and enmein-type derivatives as potential anti-cancer agents derived from oridonin

Natural products (NPs) are always the important sources in the field of drug discovery, among which spirolactone-type and enmein-type compounds exhibit a wide range of biological activities, especially anti-tumor activity. Based on previous studies, the spirolactone-type and enmein-type compounds could be derived from natural oridonin (1) by several chemical reactions. Herein, a series of novel spirolactone-type and enmein-type derivatives with different aryl allyl ester substitutions at their C-14 hydroxyl group were designed and synthesized. The anti-tumor activity results showed that most of the compounds exhibited better anti-proliferative activities than parent compound oridonin, and the most potent compound had an IC₅₀ value of 0.40 μM in K562 cells. Further mechanistic studies revealed that the optimal compound could arrest K562 cells at G2/M phase by inhibiting cdc-2, cdc-25c and cyclin B1 expression. In addition, the optimal compound induced apoptosis in K562 cells through increasing ROS production and depolarizing mitochondrial membrane potential. Collectively, these valuable results suggested that the most potent compound could be an anti-tumor agent candidate and is worthy of further investigation.

Discovery of oridonin as a novel agonist for BRS-3

BACKGROUND

Bombesin Receptor Subtype-3 (BRS-3, Bombesin-like receptor, BB₃) is an orphan G-protein coupled receptor (GPCR). Recent studies have shown that BRS-3 played a vital role in glucose regulation, insulin secretion, and energy homeostasis. Therefore, discovering more novel exogenous ligands with diverse structures for BRS-3 will be of great importance for target validation and drug development.

PURPOSE

In this study, we aim to discover new agonists of BRS-3 from our natural compound libraries, providing a new probe to study the function of BRS-3.

STUDY DESIGN

Multiple cell-based assays and in vivo experiments were performed to identify the new ligand.

METHODS

BRS-3 overexpression cells were coupled with FLIPR assay, homogeneous time-resolved fluorescence (HTRF) IP-ONE assay, dynamic mass redistribution (DMR) assay, β-arrestin2 recruitment assay, and western blot to determine receptor activation and downstream signaling events. To further validate the target of BRS-3, a series of in vitro and in vivo experiences were conducted, including glucose uptake, glucose transporter type 4 (GLUT4) transportation in C2C12, and oral glucose tolerance test (OGTT) in mice.

RESULTS

We discovered and identified oridonin as a novel small molecule agonist of BRS-3, with a moderate affinity (EC₅₀ of 2.236 × 10^-7 M in calcium mobilization assay), specificity, and subtype selectivity. Further in vitro and in vivo tests demonstrated that oridonin exerted beneficial effects in glucose homeostasis through activating BRS-3.

CONCLUSIONS

Oridonin, as the discovered new ligand of BRS-3, provides a valuable tool compound to investigate BRS-3's function, especially for target validation in type 2 diabetes and obesity. Oridonin is promising as a lead compound in the treatment of metabolic disorders. Compared to the known agonists of BRS-3, we can take advantage of the multiple reported pharmacological activities of ODN as a natural product and assess whether these pharmacological activities are regulated by BRS-3. This may facilitate the discovery of novel functions of BRS-3.

Diastereoselective synthesis of chroman bearing spirobenzofuranone scaffolds via oxa-Michael/1,6-conjugated addition of para-quinone methides with benzofuranone-type olefins

A simple and convenient cyclization of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one type active olefins via oxa-Michael/1,6-conjugated addition has been developed, which afforded an easy access to enriched functionalized chroman-spirobenzofuran-2-one scaffolds with good to excellent yields (up to 90%) and diastereoselectivities (up to >19 : 1 dr). This reaction provided an efficient method for constructing desired spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Highly diastereoselective synthesis of spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Spiro ent-Clerodane Dimers: Discovery and Green Approaches for a Scalable Biomimetic Synthesis

Scospirosins A (1) and B (2), two unprecedented spiro ent-clerodane dimers with 6/6/10/6 and 6/6/6/6/6 ring systems, respectively, were isolated from Isodon scoparius. Their structures were unambiguously established by spectroscopic, X-ray crystallographic, and chemical approaches. A bioinspired protecting-group-free strategy for their synthesis was achieved on a gram scale and featured the application of green methods, including neat reaction, sensitized photooxygenation, and electrochemical oxidation. 2 exhibited selective immunosuppressive activity against the proliferation of T lymphocytes (IC₅₀ = 1.42 μM).

Recent Progress of Oridonin and Its Derivatives for the Treatment of Acute Myelogenous Leukemia

First stage human clinical trial (CTR20150246) for HAO472, the L-alanine-(14-oridonin) ester trifluoroacetate, was conducted by a Chinese company, Hengrui Medicine Co. Ltd, to develop a new treatment for acute myelogenous leukemia. Two patents, WO2015180549A1 and CN201410047904.X, covered the development of the I-type crystal, stability experiment, conversion rate research, bioavailability experiment, safety assessment, and solubility study. HAO472 hewed out new avenues to explore the therapeutic properties of oridonin derivatives and develop promising treatment of cancer originated from naturally derived drug candidates. Herein, we sought to overview recent progress of the synthetic, physiological, and pharmacological investigations of oridonin and its derivatives, aiming to disclose the therapeutic potentials and broaden the platform for the discovery of new anticancer drugs.

Dimeric and esterified sesquiterpenes from the liverwort Chiastocaulon caledonicum

This is the first chemical investigation of Chiastocaulon caledonicum, an endemic liverwort from New Caledonia. We herein present the isolation of thirteen compounds including seven undescribed sesquiterpenoids, namely four barbatane- and three myltaylane-type sesquiterpenes. The structures of these compounds were elucidated based on the interpretation of their chemical and spectroscopic/spectrometric data. Chiastocaulins A and B are the first examples of dimers based on two myltaylane units. The chemotaxonomic importance and the biosynthesis of the chiastocaulin structure are discussed. Terpenoid dimers formed via a Diels-Alder cyclization are thought to be specific to the Plagiochilaceae family.

Enantioselective Type II Cycloaddition of Alkynes via C-C Activation of Cyclobutanones: Rapid and Asymmetric Construction of [3.3.1] Bridged Bicycles

Synthesis of bridged scaffolds via Type II cyclization constitutes substantial challenges due to the intrinsic ring strain accumulated in reaction transition states. Catalytic enantioselective Type II-cyclization methods are even rarer. Here, we describe a detailed study of developing a Rh(I)-catalyzed enantioselective intramolecular Type II cyclization of alkynes via C-C activation of cyclobutanones. This method offers a rapid approach to access a wide range of functionalized [3.3.1]-bridged bicycles along with an exocyclic olefin and an all-carbon quaternary stereocenter. Excellent enantioselectivity has been achieved using a combination of cationic rhodium(I) and DTBM-segphos. Attributed to the redox neutral and strong acid/base-free reaction conditions, high chemoselectivity has also been observed. For the oxygen-tethered substrates, the reaction can proceed at room temperature. In addition, partial kinetic resolution has been achieved for substrates with existing stereocenters, forging interesting chiral tricyclic scaffolds. The methylalkyne-derived substrates gave unexpected dimeric structures in good yield with excellent enantioselectivity and complete diastereoselectivity. Furthermore, the bridged bicyclic products can be diversely functionalized through simple transformations. Finally, mechanistic studies reveal a surprising reaction pathway that involves forming a metal-stabilized anti-Bredt olefin intermediate.

Oridonin and its derivatives for cancer treatment and overcoming therapeutic resistance

Cancer is one of the diseases with high morbidity and mortality on a global scale. Chemotherapy remains the primary treatment option for most cancer patients, including patients with progressive, metastatic, and recurrent diseases. To date, hundreds of chemotherapy drugs are used to treat various cancers, however, the anti-cancer efficacy and outcomes are largely hampered by chemotherapy-associated toxicity and acquired therapeutic resistance. The natural product (NP) oridonin has been extensively studied for its anti-cancer efficacy. More recently, oridonin has been shown to overcome drug resistance through multiple mechanisms, with yet-to-be-defined bona fide targets. Hundreds of oridonin derivative analogs (oridonalogs) have been synthesized and screened for improved potency, bioavailability, and other drug properties. Particularly, many of these oridonalogs have been tested against oridonin for tumor growth inhibition, potential for overcoming therapeutic resistance, and immunity modulation. This concise review seeks to summarize the advances in this field in light of identifying clinical-trial level drug candidates with the promise for treating progressive cancers and reversing chemoresistance.

Total Synthesis of Nominal ent-Chlorabietol B

The nominal enantiomer of chlorabietol B was regio- and stereoselectively synthesized from (-)-abietic acid in 13 steps. Key features of the synthesis involved an oxidative [3+2] cycloaddition to install the dihydrobenzofuran moiety and an Aldol reaction, followed by elimination and reduction steps to introduce the long chain with three cis double bonds. However, obvious differences in the NMR spectra of the synthetic and natural samples suggested that the proposed structure of chlorabietol B should be revised carefully.

Oridonin derivatives as potential anticancer drug candidates triggering apoptosis through mitochondrial pathway in the liver cancer cells

The biological function of the natural ent-kaurene diterpenoid isolated from genus Isodon, oridonin, has been intensively studied. However, its mechanism studies and clinical applications were hampered by its moderate biological activities. In order to enlarge the applied range of oridonin and explore its mechanism of action, a series of derivatives were designed and synthesized based on the structure of oridonin. Some of the derivatives were significantly more potent than oridonin against four cancer cell lines. Especially, the most potent compound 20 markedly inhibited the proliferation of well differentiated HepG2 and poorly differentiated PLC/PRF/5 cells, with IC₅₀ values as low as 1.36 μM and 0.78 μM respectively, while the IC₅₀ values of oridonin are 8.12 μM and 7.41 μM. We found that compound 20 inhibited liver cancer cell proliferation via arresting cell cycle at G1 phase. Moreover, it induced liver cancer cell apoptosis by decreasing the mitochondrial membrane potential, increasing intracellular reactive oxygen species level and inducing the expression of apoptosis-related proteins. Furthermore, compound 20 significantly inhibited growth of PLC/PRF/5 xenograft tumors in nude mice and had no observable toxic effect. Altogether, these results indicated that compound 20 is a promising lead for liver cancer therapeutics.

Recent Development of Oridonin Derivatives with Diverse Pharmacological Activities

Oridonin is one of the major components isolated from Isodon rubescens, a traditional Chinese medicine, and it has been confirmed to exhibit many kinds of biological activities including anticancer, anti-inflammation, antibacterial and so on. However, the poor pharmaceutical property limits the clinical applications of oridonin. So many strategies have been explored in the purpose of improving the potencies of oridonin, and structure modification is one thus way. This review outlines the landscape of the recent development of oridonin derivatives with diverse pharmacological activities, mainly focusing on the biological properties, structure-activity relationships, and mechanism of actions.

Regio- and Stereospecific Synthesis of Oridonin D-Ring Aziridinated Analogues for the Treatment of Triple-Negative Breast Cancer via Mediated Irreversible Covalent Warheads

Covalent drug discovery has undergone a resurgence in recent years due to comprehensive optimization of the structure-activity relationship (SAR) and the structure-reactivity relationship (SRR) for covalent drug candidates. The natural product oridonin maintains an impressive pharmacological profile through its covalent enone warhead on the D-ring and has attracted substantial SAR studies to characterize its potential in the development of new molecular entities for the treatment of various human cancers and inflammation. Herein, for the first time, we report the excessive reactivity of this covalent warhead and mediation of the covalent binding capability through a Rh₂(esp)₂-catalyzed mild and concise regio- and stereospecific aziridination approach. Importantly, aziridonin 44 (YD0514), with a more-druglike irreversible covalent warhead, has been identified to significantly induce apoptosis and inhibit colony formation against triple-negative breast cancer with enhanced antitumor effects in vitro and in vivo while displaying lower toxicity to normal human mammary epithelial cells in comparison to oridonin.

Therapeutic Potential of Oridonin and Its Analogs: From Anticancer and Antiinflammation to Neuroprotection

Oridonin, a diterpenoid natural product commonly used in East Asian herbal medicine, is garnering increased attention in the biomedical community due to its extensive biological activities that include antitumor, anti-inflammatory, antimicrobial, hepatic fibrosis prevention, and neurological effects. Over the past decade, significant progress has been made in structure activity relationship and mechanism of action studies of oridonin for the treatment of cancer and other diseases. This review provides a brief summary on oridonin and its analogs in cancer drug discovery and antiinflammation and highlights its emerging therapeutic potential in neuroprotection applications.

Targeting XBP1-mediated β-catenin expression associated with bladder cancer with newly synthetic Oridonin analogues

Conventional chemotherapy is commonly used for advanced stages of transitional cell carcinoma (TCC) with modest success and high morbidity; however, TCC eventually develops resistance. Muscle invasive bladder cancer (MIBC) is recognized as a lethal disease due to its poor response to traditional chemotherapy. Numerous studies have implicated β-catenin, a critical effector in Wnt–mediated pathway associated with epithelial-mesenchymal transition and cancer stem cell, is involved in TCC progression, and furthermore closely associated with chemo-resistance. In this study, we discovered a novel natural product analogue CYD 6-17 that has a potent inhibitory effect on TCC cells exhibiting drug resistance to various chemotherapeutics, with an IC₅₀ at nM range. Delivery of CYD 6-17 significantly inhibited the tumor growth using xenograft model but without detectable side effects. Mechanistically, it targeted β-catenin gene transcription by decreasing the binding of XBP1 to the promoter region, which appeared to be a new regulatory mechanism for β-catenin gene expression. Clinically, XBP1 expression correlated with the poor overall survival of patients. Overall, this study unveils unique mechanism of β-catenin gene regulation in advanced TCC and also offers a potential rational therapeutic regimen to MIBC.

Design and synthesis of novel oridonin analogues as potent anticancer agents

To identify anticancer agents with higher potency and lower toxicity, a series of oridonin derivatives with substituted benzene moieties at the C17 position were designed, synthesised, and evaluated for their antiproliferative properties. Most of the derivatives exhibited antiproliferative effects against AGS, MGC803, Bel7402, HCT116, A549, and HeLa cells. Compound 2p (IC₅₀= 1.05 µM) exhibited the most potent antiproliferative activity against HCT116 cells; it was more potent than oridonin (IC₅₀ = 6.84 µM) and 5-fluorouracil (5-FU) (IC₅₀= 24.80 µM). The IC₅₀ value of 2p in L02 cells was 6.5-fold higher than that in HCT116 cells. Overall, it exhibited better selective antiproliferative activity and specificity than oridonin and 5-FU. Furthermore, compound 2p arrested HCT116 cells at the G2 phase of the cell cycle and increased the percentage of apoptotic cells to a greater extent than oridonin.

Structural modification of oridonin via DAST induced rearrangement

A novel and concise synthetic approach for the preparation of 6,20-epoxy ent-kaurane diterpenoid from oridonin was established.

Oridonin, a Promising ent-Kaurane Diterpenoid Lead Compound

Oridonin belongs to ent-kaurane tetracyclic diterpenoid and was first isolated from Isodon species. It exhibits inhibitory activities against a variety of tumor cells, and pharmacological study shows that oridonin could inhibit cell proliferation, DNA, RNA and protein synthesis of cancer cells, induce apoptosis and exhibit an antimutagenic effect. In addition, the large amount of the commercially-available supply is also very important for the natural lead oridonin. Moreover, the good stability, suitable molecular weight and drug-like property guarantee its further generation of a natural-like compound library. Oridonin has become the hot molecule in recent years, and from the year 2010, more than 200 publications can be found. In this review, we summarize the synthetic medicinal chemistry work of oridonin from the first publication 40 years ago and share our research experience of oridonin for about 10 years, which may provide useful information to those who are interested in this research field.

Discovery and development of natural product oridonin-inspired anticancer agents

Natural products have historically been, and continue to be, an invaluable source for the discovery of various therapeutic agents. Oridonin, a natural diterpenoid widely applied in traditional Chinese medicines, exhibits a broad range of biological effects including anticancer and anti-inflammatory activities. To further improve its potency, aqueous solubility and bioavailability, the oridonin template serves as an exciting platform for drug discovery to yield better candidates with unique targets and enhanced drug properties. A number of oridonin derivatives (e.g. HAO472) have been designed and synthesized, and have contributed to substantial progress in the identification of new agents and relevant molecular mechanistic studies toward the treatment of human cancers and other diseases. This review summarizes the recent advances in medicinal chemistry on the explorations of novel oridonin analogues as potential anticancer therapeutics, and provides a detailed discussion of future directions for the development and progression of this class of molecules into the clinic.

Probing the Anticancer Action of Oridonin with Fluorescent Analogues: Visualizing Subcellular Localization to Mitochondria

Oridonin (1) is a complex ent-kaurane diterpenoid exhibiting remarkable antitumor activity. However, the detailed mechanism or cellular target that underlies this activity has not yet been identified. Herein, we report an efficient approach for exploring the anticancer mechanism of oridonin through development of the potent fluorescent analogues. A series of novel fluorescent oridonin probes linked with coumarin moieties were designed, synthesized, and characterized. Fluorescence microscopy and confocal imaging studies suggested that fluorescent oridonin probe 17d was rapidly taken up into tumor cells and the mitochondrion was the main site of its accumulation. Moreover, we confirmed that cytochrome c played an important role in oridonin induced mitochondrion-mediated apoptosis and α,β-unsaturated ketone is the active moiety of oridonin, which is crucial to its uptake, localization, and cytotoxicity. Our results provide new insights on the molecular mechanism of oridonin and would be useful for its further development into an antitumor agent.

Enhanced anti-fibrogenic effects of novel oridonin derivative CYD0692 in hepatic stellate cells

Oridonin, isolated from Rabdosia rubescens, has been proven to possess various anti-neoplastic and anti-inflammatory properties. Previously, we reported the anti-fibrogenic effects of oridonin for liver in vitro. In the present study, we investigated the effects of a newly designed analog CYD0692 in vitro. Cell viability was measured by Alamar Blue assay. Cell apoptosis was assessed by Cell Death ELISA and Yo-Pro-1 staining. Western blots were performed for cellular proteins. Flow cytometry was used to measure cell cycle regulation. CYD0692 significantly inhibited LX-2 cells proliferation in a dose- and time-dependent manner with an IC50 value of ~0.7 μM for 48 h, ~tenfold greater potency than oridonin. Similar results were observed in HSC-T6 cells. In contrast, on the human hepatocyte cell line C3A, only 12 % of the cell growth was inhibited with 5 μM of CYD0692 treatment for 48 h, while 30 % inhibited at 10 μM. After CYD0692 treatment on LX-2 cells, apoptosis and S-phase cell cycle arrest were induced; cleaved-PARP, p21, and p53 were activated while cyclin-B1 levels declined. In addition, α-smooth muscle actin, type I Collagen, and fibronectin (FN) were markedly down regulated. Transforming growth factor β1 (TGF β1) has been identified as a dominant stimulator for ECM production in HSC. Our results indicated that pretreatment with CYD0692 blocked TGF β1-induced FN expression, thereby decreasing the downstream factors of TGF β1 signaling, such as Phospho-Smad2/3 and phospho-ERK. In comparison with oridonin, its novel derivative CYD0692 has demonstrated to be a more potent and potentially safer anti-fibrogenic agent for the treatment of hepatic fibrosis.

Enhanced effects of novel oridonin analog CYD0682 for hepatic fibrosis

BACKGROUND

Activated hepatic stellate cells (HSCs) are responsible for excess extracellular matrix (ECM) protein deposition in liver fibrosis. Previously, our group reported that the natural compound oridonin induces apoptosis, inhibits cell proliferation, and downregulates ECM proteins in activated HSC. In this study, the antifibrogenic effects of oridonin derivative CYD0682 on the activated human LX-2 and rat HSC-T6 stellate cell lines were investigated.

METHODS

Cell proliferation was measured by alamarBlue assay. Apoptosis was detected by Cell Death ELISA and staining of Yo-Pro-1 and propidium iodide. Cell cycle was determined by flow cytometry. Immunoblot and immunofluorescence staining were performed for cellular protein expression.

RESULTS

CYD0682 treatment significantly inhibited LX-2 cell proliferation in a dose- and time-dependent manner with an IC50 value of 0.49 μM for 48 h, ∼10-fold greater potency than oridonin. Similar results were observed in HSC-T6 cells. In contrast, 2.5 μM of CYD0682 treatment had no significant effects on proliferation of the human hepatocyte cell line C3A. CYD0682 treatment induced LX-2 cell apoptosis and S-phase cell cycle arrest and was associated with activation of p53, p21, and cleaved caspase-3. The myofibroblast marker protein α-smooth muscle actin and major ECM proteins type I collagen and fibronectin were markedly suppressed in a time- and dose-dependent fashion by CYD0682. Furthermore, pretreatment with CYD0682 blocked transforming growth factor-β-induced type I collagen and fibronectin production.

CONCLUSIONS

In comparison with oridonin, its novel derivative CYD0682 may act as a more potent antihepatic fibrosis agent.

Evolution in medicinal chemistry of ursolic acid derivatives as anticancer agents

Currently, there is a renewed interest in common dietaries and plant-based traditional medicines for the prevention and treatment of cancer. In the search for potential anticancer agents from natural sources, ursolic acid (UA), a pentacyclic triterpenoid widely found in various medicinal herbs and fruits, exhibits powerful biological effects including its attractive anticancer activity against various types of cancer cells. However, the limited solubility, rapid metabolism and poor bioavailability of UA restricted its further clinical applications. In the past decade, with substantial progress toward the development of new chemical entities for the treatment of cancer, numerous UA derivatives have been designed and prepared to overcome its disadvantages. Despite extensive effort, discovery of effective UA derivatives has so far met with only limited success. This review summarizes the current status of the structural diversity and evolution in medicinal chemistry of UA analogues and provides a detailed discussion of future direction for further research in the chemical modifications of UA.

Effect of natural and semisynthetic pseudoguianolides on the stability of NF-κB:DNA complex studied by agarose gel electrophoresis

The nuclear factor κB (NF-κB) is a promising target for drug discovery. NF-κB is a heterodimeric complex of RelA and p50 subunits that interact with the DNA, regulating the expression of several genes; its dysregulation can trigger diverse diseases including inflammation, immunodeficiency, and cancer. There is some experimental evidence, based on whole cells studies, that natural sesquiterpene lactones (Sls) can inhibit the interaction of NF-κB with DNA, by alkylating the RelA subunit via a Michael addition. In the present work, 28 natural and semisynthetic pseudoguianolides were screened as potential inhibitors of NF-κB in a biochemical assay that was designed using pure NF-κB heterodimer, pseudoguianolides and a ~1000 bp palindromic DNA fragment harboring two NF-κB recognition sequences. By comparing the relative amount of free DNA fragment to the NF-κB - DNA complex, in a routine agarose gel electrophoresis, the destabilizing effect of a compound on the complex is estimated. The results of the assay and the following structure-activity relationship study, allowed the identification of several relevant structural features in the pseudoguaianolide skeleton, which are necessary to enhance the dissociating capacity of NF-κB-DNA complex. The most active compounds are substituted at C-3 (α-carbonyl), in addition to having the α-methylene-γ-lactone moiety which is essential for the alkylation of RelA.