Evaluation of the pharmacophoric motif of the caged Garcinia xanthones

Caged xanthone derivatives to promote mitochondria-mediated apoptosis in breast cancer cells

Caged xanthones represent a class of natural secondary metabolites exhibiting significant potential as antitumor agents. These compounds are characterized by their distinct cage-like structures, which offer novel and compelling frameworks for drug design. Nonetheless, there exists a dearth of research focused on the structural modification of these compounds, particularly in relation to their cage-like architectures. This study aims to address this gap by introducing an innovative synthetic method for constructing a novel caged structure that incorporates a widely employed maleimide group. Drawing upon the well-established synthetic approach for dihydroxanthones previously developed within our research group, we successfully synthesized 13 new caged xanthones using the Diels-Alder reaction. Subsequently, we evaluated their anti-proliferative activity against HepG2, A549, and MDA-MB-231 cell lines. The results revealed that compound 10i exhibited IC50 values of 15.86 µM ± 1.29, 19.27 µM ± 1.58, and 12.96 µM ± 0.09 against these cell lines, respectively. Further investigations into the mechanism of action of 10i demonstrated its ability to induce G2/M cell cycle arrest and initiate mitochondria-mediated apoptosis in breast cancer cells.

Design, Synthesis and Anti-cancer Evaluation of Nitrogen-containing Derivatives of 30-Carboxyl of Gambogic Acid

BACKGROUND

Gambogic acid (GA) is a natural product from the resin of the Garcinia species, which showed significant activity in the induction of apoptosis. .t can be one promising lead compound for the design and synthesis of new anticancer drugs.

OBJECTIVE

The objective of the current study is to design novel nitrogen-contained GA derivatives with better anti-cancer activities and study the effect of the introduction of different nitrogen-contained groups on the activity of GA.

METHODS

The designed 15 derivatives were synthesized via esterification or amidation of 30-carboxylate. The synthetic compounds were characterized via different spectroscopic techniques, including X-ray single crystal diffraction, MS and NMR. The cytotoxic activity of the designed derivatives was evaluated in vitro against A549, HepG-2, and MCF-7 cell lines using methyl thiazolyl tetrazolium (MTT) test.

RESULTS

15 nitrogen-contained GA derivatives were successfully synthesized and established. Based on the IC50 values, compounds 9, 10, 11 and 13 showed stronger inhibitory effects on A549, HepG-2, MCF-7 cell lines than GA, while 9 is the most active compound with IC50 value of 0.64-1.49 μM. Most derivatives of GA with esterification of C-30 including cyano-benzene ring were generally weaker than those of pyrimidinyl-substituted derivatives. In addition, length of alkyl linkers between C-30 of GA and nitrogen-contained group produced different effects on A549, HepG-2 and MCF-7 cell lines.

CONCLUSION

The structure-activity relationship results show that aromatic substituent and linker length play important roles to improve the anticancer activities, while compound 9 with pyrimidine substituent and C-C-C linkers is the most active derivative against tested cell lines, and is a promising anti-cancer agent for further development.

Research Progress in the Field of Gambogic Acid and Its Derivatives as Antineoplastic Drugs

Gambogic acid (GA) is a natural product with a wide range of pharmacological properties. It plays an important role in inhibiting tumor growth. A large number of GA derivatives have been designed and prepared to improve its shortcomings, such as poor water solubility, low bioavailability, poor stability, and adverse drug effects. So far, GA has been utilized to develop a variety of active derivatives with improved water solubility and bioavailability through structural modification. This article summarized the progress in pharmaceutical chemistry of GA derivatives to provide a reference and basis for further study on structural modifications of GA and expansion of its clinical applications.

Synthesis and biological evaluation of cajaninstilbene acid and amorfrutins A-D as cytotoxic agents against human pancreatic carcinoma PANC-1 cells

A concise synthesis of cajaninstilbene acid was achieved in 7 steps from (E)-3,5-dimethoxystilbene in 8.6% overall yield via the Claisen rearrangement of an aryl reverse-prenyl ether as the key step. Cytotoxic activities against human pancreatic carcinoma PANC-1 cells of cajaninstilbene acid and amorfrutins A-D were also evaluated.

Asymmetric hydrogenation and transfer hydrogenation in the enantioselective synthesis of flavonoids

In this review, we explore the applications of Asymmetric Hydrogenation (AH) and Asymmetric Transfer Hydrogenation (ATH) in the total synthesis of natural flavonoids and their analogues, highlighting the limitations and opportunities in the field.

Discovery of a Highly Potent and Novel Gambogic Acid Derivative as an Anticancer Drug Candidate

BACKGROUND AND PURPOSE

Gambogic Acid (GA), a promising anti-cancer agent isolated from the resin of Garcinia species in Southeast Asia, exhibits high potency in inhibiting a wide variety of cancer cells' growth. Moreover, the fact that it is amenable to chemical modification makes GA an attractive molecule for the development of anti-cancer agents.

METHODS

Gambogic acid-3-(4-pyrimidinyloxy) propyl ester (compound 4) was derived from the reaction between 4-hydroxypropoxy pyrimidine and GA. Its structure was elucidated by comprehensive analysis of ESIMS, HRESIMS, 1 D NMR data. Anti-tumor activities of compound 4 and GA in vitro against HepG-2, A549 and MCF-7 cells were investigated by MTT assay. FITC/PI dye was used to test apoptosis. The binding affinity difference of compound 4 and GA binding to IKKβ was studied by using Discovery Studio 2016.

RESULTS

Compound 4 was successfully synthesized and showed strong inhibitory effects on HepG-2, A549 and MCF-7 cells lines with an IC₅₀ value of 1.49±0.11, 1.37±0.06 and 0.64±0.16μM, respectively. Molecular docking study demonstrated that four more hydrogen bonds were established between IKKβ and compound 4, compared with GA.

CONCLUSION

Our results suggested that compound 4 showed significant effects in inducing apoptosis. Further molecular docking study indicated that the introduction of pyrimidine could improve GA's binding affinity to IKKβ. Compound 4 may serve as a potential lead compound for the development of new anti-cancer drugs.

Facile Synthesis of Kwakhurin, a Marker Compound of Pueraria mirifica and Its Quantitative NMR Analysis for Standardization as a Reagent

The side effects of kwao keur dietary supplements (obtained from the tuberous root of Pueraria mirifica) have recently been reported by the Ministry of Health, Labour and Welfare, Japan. To control the quality of kwao keur products, its ingredients need to be maintained by characteristic marker compounds, such as miroestrol, deoxymiroestrol, and kwakhurin (KWA). In this study, we described the facile synthesis of KWA, a marker compound of P. mirifica. Our revised synthetic method produced KWA with shorter steps and higher yield than the reported method. Furthermore, the absolute purity of KWA was determined by quantitative NMR analysis for standardization as a reagent, and its purity was 92.62 ± 0.12%.

Unified total synthesis of amorfrutins A and C via the Claisen rearrangement

A concise, unified total synthesis of the two prenylated aromatic polyketides amorfrutins A and C, which exhibit various medicinally important biological profiles such as antimicrobial, PPARγ modulating and quorum sensing inhibitory activities, has been achieved from commercially available 3,5-dimethoxybenzaldehyde in 38% and 10% overall yields through nine and ten steps, respectively. The key transformation for the synthesis of amorfrutin A was the Claisen rearrangement of a mono-O-(1,1-dimethylallyl)resorcinol derivative to install the C3-prenyl substituent, while that for the synthesis of amorfrutin C was the double Claisen rearrangement of a di-O-(1,1-dimethylallyl)resorcinol derivative to introduce the two prenyl groups at the C3 and C5 positions all at once.

Marine Spirotetronates: Biosynthetic Edifices That Inspire Drug Discovery

Spirotetronates are actinomyces-derived polyketides that possess complex structures and exhibit potent and unexplored bioactivities. Due to their anticancer and antimicrobial properties, they have potential as drug hits and deserve further study. In particular, abyssomicin C and tetrocarcin A have shown significant promise against antibiotic-resistant S. aureus and tuberculosis, as well as for the treatment of various lymphomas and solid tumors. Improved synthetic routes to these compounds, particularly the class II spirotetronates, are needed to access sufficient quantities for structure optimization and clinical applications.

Synthetic Chiral Derivatives of Xanthones: Biological Activities and Enantioselectivity Studies

Many naturally occurring xanthones are chiral and present a wide range of biological and pharmacological activities. Some of them have been exhaustively studied and subsequently, obtained by synthesis. In order to obtain libraries of compounds for structure activity relationship (SAR) studies as well as to improve the biological activity, new bioactive analogues and derivatives inspired in natural prototypes were synthetized. Bioactive natural xanthones compromise a large structural multiplicity of compounds, including a diversity of chiral derivatives. Thus, recently an exponential interest in synthetic chiral derivatives of xanthones (CDXs) has been witnessed. The synthetic methodologies can afford structures that otherwise could not be reached within the natural products for biological activity and SAR studies. Another reason that justifies this trend is that both enantiomers can be obtained by using appropriate synthetic pathways, allowing the possibility to perform enantioselectivity studies. In this work, a literature review of synthetic CDXs is presented. The structures, the approaches used for their synthesis and the biological activities are described, emphasizing the enantioselectivity studies.

Enantioselective Total Synthesis of Natural Isoflavans: Asymmetric Transfer Hydrogenation/Deoxygenation of Isoflavanones with Dynamic Kinetic Resolution

A concise and highly enantioselective synthesis of structurally diverse isoflavans from a single chromone is described. The key transformation is a single-step conversion of racemic isoflavanones into virtually enantiopure isoflavans by domino asymmetric transfer hydrogenation/deoxygenation with dynamic kinetic resolution.

Exodermis and endodermis are the sites of xanthone biosynthesis in Hypericum perforatum roots

Xanthones are specialized metabolites with antimicrobial properties, which accumulate in roots of Hypericum perforatum. This medicinal plant provides widely taken remedies for depressive episodes and skin disorders. Owing to the array of pharmacological activities, xanthone derivatives attract attention for drug design. Little is known about the sites of biosynthesis and accumulation of xanthones in roots. Xanthone biosynthesis is localized at the transcript, protein, and product levels using in situ mRNA hybridization, indirect immunofluorescence detection, and high lateral and mass resolution mass spectrometry imaging (AP-SMALDI-FT-Orbitrap MSI), respectively. The carbon skeleton of xanthones is formed by benzophenone synthase (BPS), for which a cDNA was cloned from root cultures of H. perforatum var. angustifolium. Both the BPS protein and the BPS transcripts are localized to the exodermis and the endodermis of roots. The xanthone compounds as the BPS products are detected in the same tissues. The exodermis and the endodermis, which are the outermost and innermost cell layers of the root cortex, respectively, are not only highly specialized barriers for controlling the passage of water and solutes but also preformed lines of defence against soilborne pathogens and predators.

The value of pyrans as anticancer scaffolds in medicinal chemistry

Pyran-based heterocycles are promising for anticancer drug discovery.

Caged Garcinia Xanthones, a Novel Chemical Scaffold with Potent Antimalarial Activity

Caged Garcinia xanthones (CGXs) constitute a family of natural products that are produced by tropical/subtropical trees of the genus Garcinia CGXs have a unique chemical architecture, defined by the presence of a caged scaffold at the C ring of a xanthone moiety, and exhibit a broad range of biological activities. Here we show that synthetic CGXs exhibit antimalarial activity against Plasmodium falciparum, the causative parasite of human malaria, at the intraerythrocytic stages. Their activity can be substantially improved by attaching a triphenylphosphonium group at the A ring of the caged xanthone. Specifically, CR135 and CR142 were found to be highly effective antimalarial inhibitors, with 50% effective concentrations as low as ∼10 nM. CGXs affect malaria parasites at multiple intraerythrocytic stages, with mature stages (trophozoites and schizonts) being more vulnerable than immature rings. Within hours of CGX treatment, malaria parasites display distinct morphological changes, significant reduction of parasitemia (the percentage of infected red blood cells), and aberrant mitochondrial fragmentation. CGXs do not, however, target the mitochondrial electron transport chain, the target of the drug atovaquone and several preclinical candidates. CGXs are cytotoxic to human HEK293 cells at the low micromolar level, which results in a therapeutic window of around 150-fold for the lead compounds. In summary, we show that CGXs are potent antimalarial compounds with structures distinct from those of previously reported antimalarial inhibitors. Our results highlight the potential to further develop Garcinia natural product derivatives as novel antimalarial agents.

Gambogic acid identifies an isoform-specific druggable pocket in the middle domain of Hsp90β

Because of their importance in maintaining protein homeostasis, molecular chaperones, including heat-shock protein 90 (Hsp90), represent attractive drug targets. Although a number of Hsp90 inhibitors are in preclinical/clinical development, none strongly differentiate between constitutively expressed Hsp90β and stress-induced Hsp90α, the two cytosolic paralogs of this molecular chaperone. Thus, the importance of inhibiting one or the other paralog in different disease states remains unknown. We show that the natural product, gambogic acid (GBA), binds selectively to a site in the middle domain of Hsp90β, identifying GBA as an Hsp90β-specific Hsp90 inhibitor. Furthermore, using computational and medicinal chemistry, we identified a GBA analog, referred to as DAP-19, which binds potently and selectively to Hsp90β. Because of its unprecedented selectivity for Hsp90β among all Hsp90 paralogs, GBA thus provides a new chemical tool to study the unique biological role of this abundantly expressed molecular chaperone in health and disease.

Molecular Cloning and Characterization of a Xanthone Prenyltransferase from Hypericum calycinum Cell Cultures

In plants, prenylation of metabolites is widely distributed to generate compounds with efficient defense potential and distinct pharmacological activities profitable to human health. Prenylated compounds are formed by members of the prenyltransferase (PT) superfamily, which catalyze the addition of prenyl moieties to a variety of acceptor molecules. Cell cultures of Hypericum calycinum respond to elicitor treatment with the accumulation of the prenylated xanthone hyperxanthone E. A cDNA encoding a membrane-bound PT (HcPT) was isolated from a subtracted cDNA library and transcript preparations of H. calycinum. An increase in the HcPT transcript level preceded hyperxanthone E accumulation in cell cultures of H. calycinum treated with elicitor. The HcPT cDNA was functionally characterized by expression in baculovirus-infected insect cells. The recombinant enzyme catalyzed biosynthesis of 1,3,6,7-tetrahydroxy-8-prenylxanthone through regiospecific C-8 prenylation of 1,3,6,7-tetrahydroxyxanthone, indicating its involvement in hyperxanthone E formation. The enzymatic product shared significant structural features with the previously reported cholinesterase inhibitor γ-mangostin. Thus, our findings may offer a chance for semisynthesis of new active agents to be involved in the treatment of Alzheimer's disease.

The Fe-S cluster-containing NEET proteins mitoNEET and NAF-1 as chemotherapeutic targets in breast cancer

Identification of novel drug targets and chemotherapeutic agents is a high priority in the fight against cancer. Here, we report that MAD-28, a designed cluvenone (CLV) derivative, binds to and destabilizes two members of a unique class of mitochondrial and endoplasmic reticulum (ER) 2Fe-2S proteins, mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1), recently implicated in cancer cell proliferation. Docking analysis of MAD-28 to mNT/NAF-1 revealed that in contrast to CLV, which formed a hydrogen bond network that stabilized the 2Fe-2S clusters of these proteins, MAD-28 broke the coordinative bond between the His ligand and the cluster's Fe of mNT/NAF-1. Analysis of MAD-28 performed with control (Michigan Cancer Foundation; MCF-10A) and malignant (M.D. Anderson-metastatic breast; MDA-MB-231 or MCF-7) human epithelial breast cells revealed that MAD-28 had a high specificity in the selective killing of cancer cells, without any apparent effects on normal breast cells. MAD-28 was found to target the mitochondria of cancer cells and displayed a surprising similarity in its effects to the effects of mNT/NAF-1 shRNA suppression in cancer cells, causing a decrease in respiration and mitochondrial membrane potential, as well as an increase in mitochondrial iron content and glycolysis. As expected, if the NEET proteins are targets of MAD-28, cancer cells with suppressed levels of NAF-1 or mNT were less susceptible to the drug. Taken together, our results suggest that NEET proteins are a novel class of drug targets in the chemotherapeutic treatment of breast cancer, and that MAD-28 can now be used as a template for rational drug design for NEET Fe-S cluster-destabilizing anticancer drugs.

Synthesis and Bioevaluation of Novel Oxa-Caged Garcinia Xanthones as Anti-Tumour Agents

Gambogic acid (GA), a special category of Garcinia xanthones, has attracted great attention owing to its striking bioactivities and unique structure. To further explore its structure–activity relationship, we prepared seven novel oxa-caged Garcinia xanthones that were for the first time varied at the C-2 position of B ring and at the C-21/22 or C-23 position of the prenyl group in the caged scaffold. Some compounds exhibited strong anti-proliferation activities in different cancer cell lines. Particularly, compound 8 showed more potent cytotoxic activity and better selectivity towards the A549 cell line than GA. Oxa-caged xanthones 8 was identified as an A549 cell apoptosis inducer through observations of morphological changes and Annexin-V/PI double-staining assay. Additionally, the structure–activity relationships of these new analogues were discussed.

A-ring oxygenation modulates the chemistry and bioactivity of caged Garcinia xanthones

Natural products of the caged Garcinia xanthones (CGX) family are characterized by a unique chemical structure, potent bioactivities and promising pharmacological profiles. We have developed a Claisen/Diels-Alder reaction cascade that, in combination with a Pd(0)-catalyzed reverse prenylation, provides rapid and efficient access to the CGX pharmacophore, represented by the structure of cluvenone. To further explore this pharmacophore, we have synthesized various A-ring oxygenated analogues of cluvenone and have evaluated their bioactivities in terms of growth inhibition, mitochondrial fragmentation, induction of mitochondrial-dependent cell death and Hsp90 client inhibition. We found that installation of an oxygen functionality at various positions of the A-ring influences significantly both the site-selectivity of the Claisen/Diels-Alder reaction and the bioactivity of these compounds, due to remote electronic effects.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

A practical access to highly enantiomerically pure flavanones by catalytic asymmetric transfer hydrogenation

A surprisingly selective, non-enzymatic kinetic resolution of readily available, racemic β-chiral ketones enabled the title process, which was applied to a rapid synthesis of several bioactive flavanones in virtually enantiopure form (see scheme; MOM=methoxymethyl, Ts=p-toluenesulfonyl).

Studies on chemical-structure modification and structure-activity relationship of gambogic acid derivatives at carbon(34)

Gambogic acid (GA), a natural product, was identified as a promising antitumor agent. To further explore the structure-activity relationship of GA and discover novel GA derivatives as antitumor agents, 19 novel GA derivatives modified at C(34) were synthesized and evaluated against A549, BGC-823, U251, HepG2, and MB-231 cancer cell lines by cellular assays. Among them, 15 compounds were found to be more potent than GA against some cancer cell lines. Notably, compound 3 possessed potent inhibitory activities against five cell lines with IC(50) values ranging between 0.24 and 1.09 μM. Compounds 9 and 18 were seven to eightfold more active than GA against A549 cell line. Chemical modification at C(34) of GA by introducing of hydrophilic aliphatic amines resulted in increased activity and improved drug-like properties. These findings will enhance our understanding of the SAR of GA and can lead to the discovery of novel GA derivatives as potential antitumor agents.

Garcinia xanthones as orally active antitumor agents

Using a newly developed strategy whose key step is the regioselective propargylation of hydroxyxanthone substrates, 99 structurally diverse Garcinia natural-product-like xanthones based on gambogic acid were designed and synthesized and their in vitro antitumor activity was evaluated. A set of 40 related compounds was chosen for determination of their physicochemical properties including polar surface area, log D₇.₄, aqueous solubility, and permeability at pH 7.4. In the light of the in vitro antitumor activity and the physicochemical properties, two compounds were advanced into in vivo efficacy experiments. The antitumor activity of compound 112, administered po, showed more potent in vivo oral antitumor activity than gambogic acid.

Synthesis and bioconversions of notoamide T: a biosynthetic precursor to stephacidin A and notoamide B

In an effort to further elucidate the biogenesis of the stephacidin and notoamide families of natural products, notoamide T has been identified as the likely precursor to stephacidin A. The total synthesis of notoamide T is described along with it is C-6-epimer, 6-epi-notoamide T. The chemical conversion of stephacidin A to notoamide T by reductive ring opening is described as well as the oxidative conversion of notoamide T to stephacidin A. Furthermore, [(13)C](2)-notoamide T was synthesized and provided to Aspergillus versicolor and Aspergillus sp. MF297-2, in which significant incorporation was observed in the advanced metabolite, notoamide B.

Synthesis, biological evaluation, and live cell imaging of novel fluorescent duocarmycin analogs

For a better understanding of the mode of action of duocarmycin and its analogs, the novel fluorescent duocarmycin derivatives 13-15 and 17b-19b were synthesized, and their bioactivity as well as their cellular uptake investigated using confocal laser scanning microscopy (CLSM) in live-cell imaging experiments.

Subcellular localization and activity of gambogic acid

The natural product gambogic acid (GA) has shown significant potential as an anticancer agent as it is able to induce apoptosis in multiple tumor cell lines, including multidrug-resistant cell lines, as well as displaying antitumor activity in animal models. Despite the fact that GA has entered phase I clinical trials, the primary cellular target and mode of action of this compound remain unclear, although many proteins have been shown to be affected by it. By thorough analysis of several cellular organelles, at both the morphological and functional levels, we demonstrate that the primary effect of GA is at the mitochondria. We found that GA induces mitochondrial damage within minutes of incubation at low-micromolar concentrations. Moreover, a fluorescent derivative of GA was able to localize specifically to the mitochondria and was displaced from these organelles after competition with unlabeled GA. These findings indicate that GA directly targets the mitochondria to induce the intrinsic pathway of apoptosis, and thus represents a new member of the mitocans.

Studies on gambogic acid (IV): Exploring structure-activity relationship with IκB kinase-beta (IKKβ)

Previously we have reported a series of gambogic acid's analogs and have identified a compound that possessed comparable in vitro growth inhibitory effect as gambogic acid. However, their target protein as well as the key pharmacophoric motifs on the target have not been identified yet. Herein we report that gambogic acid and its analogs inhibit the activity of IκB Kinase-beta (IKKβ) through suppressing the activation of TNFα/NF-κB pathway, which in turn induces A549 and U251 cell apoptosis. IKKβ can serve as one of gambogic acid's targets. The preparation of the compounds was carefully discussed in the article. Caged 4-oxa-tricyclo[4.3.1.0(3,7)]dec-2-one xanthone, which was identified as the pharmacophoric scaffold, represents a promising therapeutic agent for cancer and useful probe against NF-κB pathway.

Microwave-Assisted Claisen Rearrangement/Diels–Alder Cascade reaction for the Synthesis of Caged Garcinia Natural Products and Analogues

A microwave-assisted Claisen rearrangement/Diels–Alder cascade reaction was used to synthesise caged Garcinia natural products and analogues containing the unique 4-oxa-tricyclo[4.3.1.03,7]decan-2-one scaffold from xanthone-based and chromone-based substrates. Forbesione and cluvenone as well as two new and six known caged analogues were obtained. Compared with the traditional thermal method, the reaction time was reduced dramatically and the yield was greatly improved under microwave irradiation. In addition, the regioselectivity observed in the cascade reaction has also been rationalised. Garcinia have long been used in folk medicines for their healing, antibacterial, and cytotoxic activities.

Cluvenone induces apoptosis via a direct target in mitochondria: a possible mechanism to circumvent chemo-resistance?

The synthetic caged Garcinia xanthone, cluvenone, has potent and selective cytotoxicity against numerous cancer cell lines including those that are multi-drug resistant. The direct target of this structurally and functionally unique agent is unknown and that of the parent natural product, gambogic acid (GA), presently in clinical trials, is not yet entirely clear. For the first time, using fluorescently labeled GA (GA-Bodipy), we determined that GA-Bodipy localized in mitochondria and was effectively displaced by cluvenone in competition experiments indicating that the direct target of cluvenone resided in mitochondria and was shared by GA. In agreement with these findings, treatment of HeLa cells with cluvenone or GA resulted in disruption of mitochondrial morphology within 4 h. Furthermore, experiments using the potential sensitive JC-1 dye demonstrated that cells treated with 1 μM cluvenone for 1 h had significant loss of MMP compared to control cells. Examination of Cyt c levels in leukemia cells treated with 1 μM cluvenone resulted in a 4-fold increase in levels of both cytosolic and mitochondrial Cyt c. In agreement with Cyt c release, caspase 9 activity was increased 2.6-fold after treatment of cells for 5 h with 1 μM cluvenone. Remarkably, the caspase-9 inhibitor, Z-LEHD-FMK, blocked cluvenone-induced apoptosis in a dose-dependent manner with apoptosis being completely blocked by 10 μM of the inhibitor. In conclusion, cluvenone, an agent with potent cytotoxicity against multi-drug resistant tumor cells, has direct targets in mitochondria thus setting precedence for drug discovery efforts against these targets in the treatment of refractory cancers.

Absolute configuration of (-)-gambogic acid, an antitumor agent

(-)-Gambogic acid (1), a biologically active "caged xanthone" from gamboge, the dried resin of Garcinia hanburyi, is of interest as a potential anticancer agent. The planar structure of (-)-gambogic acid has been determined previously by analysis of its detailed NMR data and confirmed by single-crystal X-ray diffraction, with the absolute configuration at C-13 deduced as R through a series of chemical degradations. Using (-)-morellic acid (2), an analogue of (-)-gambogic acid, as a model compound, the 5R, 7S, 10aS, 13R, 27S absolute configuration of (-)-gambogic acid was determined for the first time by comparison of physical and spectroscopic data, especially experimental and calculated electronic circular dichroism.

Studies on chemical modification and biology of a natural product, gambogic acid (III): determination of the essential pharmacophore for biological activity

Caged 4-oxa-tricyclo[4.3.1.0(3,7)]dec-2-one structural motifs are found in Garcinia natural products that demonstrate anti-tumor activity. Gambogic acid (GA, 1), the most abundant caged Garcinia xanthones, has been reported to be a promising anti-cancer agent. To identify the essential pharmacophore for its anti-tumor activity, a series of GA analogues that address potential key structural features for biological activity were synthesized, among which compound 11a displayed comparable in vitro anti-tumor activity as GA. Mechanistic studies on 11a determined that the compound induces apoptosis as well as arrests the G2/M phase of the cell cycle in HepG2 cells. The determination of the essential part of the scaffold found in GA to maintain anti-tumor effects, and the SAR based on the caged pharmacophore are reported and will provide key information for future anti-cancer drug development studies.

The synthetic caged garcinia xanthone cluvenone induces cell stress and apoptosis and has immune modulatory activity

Several caged Garcinia xanthone natural products have potent bioactivity and a documented value in traditional Eastern medicine. Previous synthesis and structure activity relationship studies of these natural products resulted in the identification of the pharmacophore represented by the structure of cluvenone. In the current study, we examined the anticancer activity of cluvenone and conducted gene expression profiling and pathway analyses. Cluvenone was found to induce apoptosis in T-cell acute lymphoblastic leukemia cells (EC₅₀ = 0.25 μmol/L) and had potent growth-inhibitory activity against the NCI60 cell panel, including those that are multidrug-resistant, with a GI₅₀ range of 0.1 to 2.7 μmol/L. Importantly, cluvenone was approximately 5-fold more potent against a primary B-cell acute lymphoblastic leukemia compared with peripheral blood mononuclear cells from normal donors, suggesting that it has significant tumor selectivity. Comparison of cluvenone's growth-inhibitory profile to those in the National Cancer Institute database revealed that compounds with a similar profile to cluvenone were mechanistically unlike known agents, but were associated with cell stress and survival signaling. Gene expression profiling studies determined that cluvenone induced the activation of mitogen-activated protein kinase and NrF2 stress response pathways. Furthermore, cluvenone was found to induce intracellular reactive oxygen species formation. Lastly, the modulation in the expression of several genes associated with T cell and natural killer cell activation and function by cluvenone suggests a role as an immune-modulator. The current work highlights the potential of cluvenone as a chemotherapeutic agent and provides support for further investigation of these intriguing molecules with regard to mechanism and targets.

Chemistry and biology of the caged Garcinia xanthones

Natural products have been a great source of many small molecule drugs for various diseases. In spite of recent advances in biochemical engineering and fermentation technologies that allow us to explore microorganisms and the marine environment as alternative sources of drugs, more than 70 % of the current small molecule therapeutics derive their structures from plants used in traditional medicine. Natural-product-based drug discovery relies heavily on advances made in the sciences of biology and chemistry. Whereas biology aims to investigate the mode of action of a natural product, chemistry aims to overcome challenges related to its supply, bioactivity, and target selectivity. This review summarizes the explorations of the caged Garcinia xanthones, a family of plant metabolites that possess a unique chemical structure, potent bioactivities, and a promising pharmacology for drug design and development.