Cytotoxic prenylxanthones from Garcinia bracteata

A new xanthone isolated from Garcinia bracteata and its important effect on NO levels

A new xanthone, allanxanthone F (1), and 10 known compounds were isolated from the ethanol extract of Garcinia bracteata. The structure of compound 1 was elucidated based on spectroscopic methods (UV, IR, HR-ESI-MS, and NMR). In addition, compounds 1-9 were assessed for their anti-inflammatory activities based on the expression of nitric oxide (NO) levels on lipopolysaccharide (LPS)-induced RAW264.7 macrophages, and compounds 1-3, 4 and 6-9 suggested potential anti-inflammatory activities.

Structurally Diverse Cytotoxic Polyphenols from Garcinia gracilis

Thirty-five diverse polyphenols, belonging to seven structure classes, were isolated from Garcinia gracilis, a medicinal and edible plant sampled from Laos. The structures of nine new compounds, gargarcilones A-I (1-3, 5-7, 10, 12, and 17), were established using spectroscopic, X-ray diffraction, and experimental and calculated ECD methods. Additionally, we revised the stereochemical assignment of cochinchinoxanthone and cochinchinoxanthone C. The compounds were evaluated for antiproliferative activity against five human tumor cell lines (HL-60, A549, SMMC-7721, MDA-MB-231, and SW480). Compounds 1-4, 7, and 8 exhibited cytotoxic activity with IC50 values of 0.5-8.9 μM. Compound 3 significantly induced apoptosis in SMMC-7721 cells.

A review on natural products with cage-like structure

Natural products with diverse structures and significant biological activities are essential sources of drug lead compounds, and play an important role in the research and development of innovative drugs. Cage-like compounds have various structures and are widely distributed in nature, especially caged xanthones isolated from Garcinia genus, paeoniflorin and its derivatives isolated from Paeonia lactiflora Pall, tetrodotoxin (TTX) and its derivatives, and so on. In recent years, the development and utilization of cage-like compounds have been a research hotspot in chemistry, biology and other fields due to their special structures and remarkable biological activities. In this review, we mainly summarized the cage-like compounds with various structures found and isolated from natural drugs since 1956, summarized its broad biological activities, and introduced the progress in the biosynthesis of some compounds, so as to provide a reference for the discovery of more novel compounds, and the development and application of innovative drugs.

Six new polyoxygenated xanthones from Garcinia cowa and their neuroprotective effects on glutamate-mediated hippocampal neuronal HT22 cell death

Six new polyoxygenated xanthones, garcicowanones F-H (1-3), norcowanol A-B (4-5), and garcinone F (6) along with twelve known compounds 7-18 were obtained from the latex of Garcinia cowa Roxb. ex Choisy. All new compounds have a 1,3,7-trioxygenated or 1,3,6,7-tetraoxygenated xanthone nucleus and differ from majority of xanthones from G. cowa by hydrated side chains. Compounds 1, 7, 8 and 18 exhibited significant neuroprotective effects on glutamate-mediated hippocampal neuronal HT22 cell death. In particular, compound 1 exhibited the most potent neuroprotective effect with >80% cell viability in the concentration range of 2.9-115 µM. Further studies on compound 1 showed that it decreased cellular Ca2+ influx and inhibits cellular reactive oxygen species generation in HT22 cells. A Western blot analysis showed that MAPK phosphorylation, Bax, and AIF translocation dramatically increased upon treatment with 5 mM glutamate and decreased upon a co-treatment with compound 1.

The natural compound neobractatin inhibits cell proliferation mainly by regulating the RNA binding protein CELF6

The fruits of Garcinia bracteata can be eaten raw or processed into spices, which are considered to possess nutritional and medicinal value. Neobractatin (NBT) is a natural compound isolated from Garcinia bracteate. This study showed that NBT showed antitumor effect by upregulation of CELF6. CELF6, an RNA-binding protein of the CELF family, is involved in cancer cell proliferation. However, the role of CELF6 in human cervical cancer remains unknown. Here, we showed that CELF6 overexpression significantly suppressed HeLa cell proliferation. Mechanistically, the RNA immunoprecipitation sequencing (RIP-seq) results suggested that CELF6 physically targeted the cyclin D1 transcript, affecting protein stability. Overexpression of CELF6 increased the degradation of cyclin D1. Consistent results were obtained for the effect of NBT, which increased the expression of CELF6 at both the mRNA and protein levels. An in vivo study further confirmed the regulatory effect of NBT on CELF6 and cyclin D1 levels in a HeLa xenograft model. Similar effects of NBT on CELF6 were also shown in K562 cells in vitro and in vivo. In conclusion, our findings identified CELF6 as a tumor suppressor and a novel therapeutic target in cervical cancer. The upregulation of CELF6 expression by NBT and its antiproliferative effect on HeLa cells indicated that NBT from G. bracteata might be a small-molecule compound targeting CELF6.

Acylphloroglucinol Derivatives with a Tricyclo-[4.4.1.11,4] Dodecane Skeleton from Garcinia bracteata Fruits

Two novel polycyclic polyprenylated acylphloroglucinols (PPAPs), garcibractinones A (1) and B (2), as well as three known analogues doitunggarcinones A-B (3-4) and garcibracteatone (5) were isolated from Garcinia bracteata fruits. Their structures were elucidated by comprehensive spectroscopic methods and single crystal X-ray diffraction. Compounds 1 and 2 possess an unprecedented caged tricyclo-[4.4.1.1^1,4] dodecane skeleton, and their biosynthetic pathways are also proposed. Compounds 1-2 were tested for their inhibitory effects on lipopolysaccharide (LPS)-induced nitric oxide (NO) production in RAW 264.7 macrophages.

The Natural Compound Neobractatin Induces Cell Cycle Arrest by Regulating E2F1 and Gadd45α

The complexity and multi-target feature of natural compounds have made it difficult to elucidate their mechanism of action (MoA), which hindered the development of lead anticancer compounds to some extent. In this study, we applied RNA-Seq and GSEA transcriptome analysis to rapidly and efficiently evaluate the anticancer mechanisms of neobractatin (NBT), a caged prenylxanthone isolated from the Chinese herb Garcinia bracteata. We found that NBT exerted anti-proliferative effect on various cancer cells and caused both G1/S and G2/M arrest in synchronized cancer cells through its effects on the expression of E2F1 and GADD45α. The in vivo animal study further suggested that NBT could reduce tumor burden in HeLa xenograft model with no apparent toxicity. By demonstrating the biological effect of NBT, we provided evidences for further investigations of this novel natural compound with anticancer potential.

Raistrickiones A-E from a Highly Productive Strain of Penicillium raistrickii Generated through Thermo Change

Three new diastereomers of polyketides (PKs), raistrickiones A−C (1⁻3), together with two new analogues, raistrickiones D and E (4 and 5), were isolated from a highly productive strain of Penicillium raistrickii, which was subjected to an experimental thermo-change strategy to tap its potential of producing new secondary metabolites. Metabolites 1 and 2 existed in a diastereomeric mixture in the crystal packing according to the X-ray data, and were laboriously separated by semi-preparative HPLC on a chiral column. The structures of 1⁻5 were determined on the basis of the detailed analyses of the spectroscopic data (UV, IR, HRESIMS, 1D, and 2D NMR), single-crystal X-ray diffractions, and comparison of the experimental and calculated electronic circular dichroism spectra. Compounds 1⁻5 represented the first case of 3,5-dihydroxy-4-methylbenzoyl derivatives of natural products. Compounds 1⁻5 exhibited moderate radical scavenging activities against 1,1-diphenyl-2-picrylhydrazyl radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH).

Bioassay- and Chemistry-Guided Isolation of Scalemic Caged Prenylxanthones from the Leaves of Garcinia bracteata

With bioassay- and chemistry-guided fractionation, seven new caged prenylxanthones including two scalemic mixtures, epiisobractatin (1), 13-hydroxyisobractatin (2), 13-hydroxyepiisobractatin (3), 8-methoxy-8,8a-dihydrobractatin (4), 8-ethoxy-8,8a-dihydrobractatin (5), garcibracteatone (6), and 8-methoxy-8,8a-dihydroneobractiatin (7), and the eight known compounds 8-15 were isolated from the leaves of Garcinia bracteata. The structures were unambiguously elucidated through analysis of spectroscopic data. The 2D structures and relative configurations of 1 and 5 were confirmed by X-ray crystallographic analysis. The separation of the enantiomers of 1-5 was accomplished by chiral-phase HPLC. The absolute configurations of the enantiomers of 1 and 5 were assigned by comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. The absolute configurations of the related compounds were determined via comparisons of their ECD data with those of the enantiomers of 1 and 5, respectively. Notably, compound 7, with a neo caged skeleton, is the first representative of a novel type of caged xanthone lacking a Δ^8(8a) double bond. The isolated compounds exhibited significant cell growth inhibitory activities in vitro against human leukemic HL-60 and K562 cell lines, with GI₅₀ values ranging from 0.2 to 8.8 μM. A preliminary structure-activity relationship is discussed.

Scalemic Caged Xanthones Isolated from the Stem Bark Extract of Garcinia propinqua

Seven new caged xanthones, doitunggarcinones E-K (1-7), all as scalemic mixtures and 10 known compounds (8-17), were isolated from the stem bark extract of Garcinia propinqua. The structures were elucidated on the basis of spectroscopic methods. The separation of the enantiomers of 1-6 was achieved by semipreparative chiral HPLC. The absolute configuration of compound (+)-1 was determined by single-crystal X-ray crystallographic analysis using Cu Kα radiation. The absolute configurations of the other related compounds were determined from comparisons of their ECD spectra with that of compound (+)-1. Compounds (-)-6 and 7 showed cytotoxicity against a colon cancer cell line with IC₅₀ values of 14.23 and 23.95 μM, respectively.

Xanthones from the Pericarp of Garcinia mangostana

Mangosteen (Garcinia mangostana L.) is one of the most popular tropical fruits (called the "Queen of Fruits"), and is a rich source of oxygenated and prenylated xanthone derivatives. In the present work, phytochemical investigation has resulted in one new prenylated xanthone and 13 known xanthones isolated from the pericarp of G. mangostana. Their structures were established by spectroscopic data analysis, including X-ray diffraction. The new one was further tested for cytotoxic activity against seven cancer cell lines (CNE-1, CNE-2, A549, H490, PC-3, SGC-7901, U87), displaying the half maximal inhibitory concentration (IC₅0) values 3.35, 4.01, 4.84, 7.84, 6.21, 8.09, and 6.39 μM, respectively. It is noteworthy that the new compound can promote CNE-2 cells apoptosis in late stage, having a remarkable inhibition effect on the side population growth of CNE-2 at 1.26 μM. The bioactive compound was also detected in extract from fresh mangosteen flesh, which indicated that the popular fruit could have potential cytotoxic activity for cancer cell lines.

Neobraclactones A–C, three unprecedented chaise longue-shaped xanthones from Garcinia bracteata

Three novel caged-polyprenylated xanthone derivatives, featuring an unusual scaffold with significant antitumor activity, were isolated.

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.

Xanthones from Garcinia Propinqua Roots

Phytochemical investigation of Garcinia propinqua roots led to the isolation and identification of a new xanthone, doitunggarcinone D (1), together with 15 known compounds (2–16). Their structures were elucidated by intensive analysis of spectroscopic data. Compounds 3, 6, 7, 14, 15 and 16 exhibited strong antibacterial activity against Bacillus subtilis TISTR 088 with MIC values in the range of 1–4 μg/mL. Compounds 3, 7, 10 and 14 also showed good antibacterial activity against B. cereus TISTR 688 with MIC values ranging from 4–8 μg/mL.

Investigation of Vietnamese plants for potential anticancer agents

Higher plants continue to afford humankind with many new drugs, for a variety of disease types. In this review, recent phytochemical and biological progress is presented for part of a collaborative multi-institutional project directed towards the discovery of new antitumor agents. The specific focus is on bioactive natural products isolated and characterized structurally from tropical plants collected in Vietnam. The plant collection, identification, and processing steps are described, and the natural products isolated from these species are summarized with their biological activities.

Three types of cytotoxic natural caged-scaffolds: pure enantiomers or partial racemates

Two rare new natural products, the neocaged-xanthone pruniflorone T (1) and the rearranged caged-xanthone pruniflorone U (3), and the known caged-xanthone cochinchinone C (2) were isolated from the roots of Cratoxylum formosum ssp. pruniflorum. The unique structures of 1-3 were determined by analysis of NMR and X-ray diffraction data. The X-ray data of 1-3 revealed that they all exist with both enantiomers in their crystal packing. Separation of 1-3 by chiral HPLC led to the isolation of three pairs of enantiomers, (-)-1/(+)-1, (-)-2/(+)-2, and (-)-3/(+)-3, and their absolute configurations were determined by analysis of single-crystal X-ray diffraction and ECD spectroscopic data. A 1:1 mixture of 1 and 3 showed potent in vitro cytotoxicity against an MCF-7 human breast cancer cell line with an IC50 value of 0.11 μg/mL.

In vitro cytotoxic, antioxidant, and antimicrobial activities of Mesua beccariana (Baill.) Kosterm., Mesua ferrea Linn., and Mesua congestiflora extracts

The in vitro cytotoxicity tests on the extracts of Mesua beccariana, M. ferrea, and M. congestiflora against Raji, SNU-1, HeLa, LS-174T, NCI-H23, SK-MEL-28, Hep-G2, IMR-32, and K562 were achieved using MTT assay. The methanol extracts of Mesua beccariana showed its potency towards the proliferation of B-lymphoma cell (Raji). In addition, only the nonpolar to semipolar extracts (hexane to ethyl acetate) of the three Mesua species indicated cytotoxic effects on the tested panel of human cancer cell lines. Antioxidant assays were evaluated using DPPH scavenging radical assay and Folin-Ciocalteu method. The methanol extracts of M. beccariana and M. ferrea showed high antioxidant activities with low EC₅₀ values of 12.70 and 9.77  μg/mL, respectively, which are comparable to that of ascorbic acid (EC₅₀ = 5.62  μg/mL). Antibacterial tests were carried out using four Gram positive and four Gram negative bacteria on Mesua beccariana extracts. All the extracts showed negative results in the inhibition of Gram negative bacteria. Nevertheless, methanol extracts showed some activities against Gram positive bacteria which are Bacillus cereus, methicillin-sensitive Staphylococcus aureus (MSSA), and methicillin-resistant Staphylococcus aureus (MRSA), while the hexane extract also contributed some activities towards Bacillus cereus.

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.

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.

Xanthones from the stem bark of Garcinia bracteata with growth inhibitory effects against HL-60 cells

Five xanthones, 1,4,5,6-tetrahydroxyxanthone (1) and bracteaxanthones III-VI (2-5) together with twenty-six known compounds (6-31), were isolated from the ethanol extract of the stem bark of Garcinia bracteata. Their structures were elucidated via spectroscopic analyses. Growth inhibitory activities of these compounds against the human leukaemic HL-60 cell line were measured in vitro. Compounds 7, 11, and 29 exhibited moderate activities with GI(50) values of 2.8, 3.4, and 3.1 μM, respectively, and a preliminary structure-activity relationship is discussed.

Design and synthesis of gambogic acid analogs as potent cytotoxic and anti-inflammatory agents

Prenyl- and pyrano-xanthones derived from 1,3,6-trihydroxy-9H-xanthen-9-one, a basic backbone of gambogic acid (GA), were synthesized and evaluated for in vitro cytotoxic effects against four human cancer cell lines (KB, KBvin, A549, and DU-145) and anti-inflammatory activity toward superoxide anion generation and elastase release by human neutrophils in response to fMLP/CB. Among them, prenylxanthones 7-13 were generally less active than pyranoxanthones 14-21 in both anticancer and anti-inflammatory assays. Furthermore, two angular 3,3-dimethypyranoxanthones (16 and 20) showed the greatest and selective activity against the KBvin multidrug resistant (MDR) cell line with IC(50) values of 0.9 and 0.8 μg/mL, respectively. An angular 3-methyl-3-prenylpyranoxanthone (17) selectively inhibited elastase release with 200 times more potency than phenylmethylsulfonyl fluoride (PMSF), the positive control.

Prenylated caged xanthones: chemistry and biology

CONTEXT

Prenylated caged xanthones are "privileged structure" characterized by the presence of the unusual 4-oxo-tricyclo[4.3.1.0(3,7)]dec-8-en-2-one scaffold. The natural sources of these compounds confines mainly in the Garcinia genus in the family of Guttiferae. Gambogic acid is the most abundant substance and most of the studies have been done on this compound, particularly as a new potential antitumor agent. The history, sources, structural diversity, and biological activities of these compounds are covered.

OBJECTIVE

This review is written with the intention to provide additional aspects from what have been published of prenylated caged xanthones, including history, sources, structural diversity, and biological activities.

METHODS

This review has been compiled using information from a number of reliable references mainly from major databases including SciFinder, ScienceDirect, and PubMed.

RESULTS

More than 120 prenylated caged xanthones have been found in the plant genera Garcinia, Cratoxylum, and Dascymaschalon. These compounds exhibited various potentially useful biological activities such as anticancer, anti-HIV-1, antibacterial, anti-inflammatory, and neurotrophic activities.

CONCLUSIONS

Prenylated caged xanthones, both naturally occurring and synthetic analogues, have been identified as promising bioactive compounds, especially for anticancer agents. Gambogic acid has been demonstrated to be a highly valuable lead compound for antitumor chemotherapy. The structure activity relationship (SAR) study of its analogues is still the subject of intensive research. Apoptosis cytotoxic mechanism has been identified as the major pathway. Research on the delineation of the in-depth mechanism of action is still on-going. Analogues of gambogic acid had been identified to be effective against a rare and special form of liver cancer, cholangiocarcinoma for which currently there is no chemotherapeutic treatment available.

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.

Computational Study on the Conformations of Gambogic Acid

The conformations of gambogic acid were studied using force fields, MM3*, AMBER*, MMFFs and OPLS2005, and B3LYP methods. In a model molecule, only the MM3* and AMBER* methods produced the same number of conformers as B3LYP, generating two conformations for rings 1 and 2, and a single conformation for rings 3 and 4. The preferred conformations of these rings are maintained in a conformer of the actual gambogic acid generated using the AMBER* and B3LYP methods. Although this calculated conformer matches well with the crystal structure, it shows that H43, C25=C26 and C30=C31 bonds may be misassigned in the crystal structure.

Cytotoxic and NF-κB inhibitory constituents of the stems of Cratoxylum cochinchinense and their semisynthetic analogues

A new caged xanthone (1), a new prenylxanthone (2), seven known xanthones, and a known sterol glucoside were isolated from the stems of Cratoxylum cochinchinense, collected in Vietnam. Compounds 1 and 2 were determined structurally by analysis of their spectroscopic data. In addition, five new (10 and 16-19) and eight known prenylated xanthone derivatives were synthesized from the known compounds α-mangostin (3) and cochinchinone A (6). Several of these substances were found to be cytotoxic toward HT-29 human colon cancer cells, with the most potent being 3,6-di-O-acetyl-α-mangostin (8, ED50, 1.0 μM), which was tested further in an in vivo hollow fiber assay, but found to be inactive at the highest dose used (20 mg/kg; ip). Of the substances evaluated in a NF-κB p65 inhibition assay, 1,3,7-trihydroxy-2,4-diisoprenylxanthone (5) exhibited the most potent activity (IC50, 2.9 μM). In a mitochondrial transmembrane potential assay, two new compounds, 1 (IC50, 3.3 μM) and 10 (IC50, 1.4 μM), and two known compounds, 3 (α-mangostin, IC50, 0.2 μM) and 11 (3,6-di-O-methyl-α-mangostin, IC50, 0.9 μM), were active. A preliminary analogue development study showed that 3,6-diacetylation and 6-benzoylation both slightly increased the cytotoxicity of α-mangostin (3), whereas methylation reduced such activity. In contrast, neither acetylation, benzoylation, nor methylation enhanced the cytotoxicity of cochinchinone A (6).

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.

Proteasome-inhibitory and cytotoxic constituents of Garcinia lateriflora: absolute configuration of caged xanthones

A new biflavonoid (1), a new xanthone enantiomer (2), five new caged xanthones (3-7), and several known compounds were isolated from the stem bark of Garcinia lateriflora, collected in Indonesia. The structures of the new compounds were determined by analysis of spectroscopic data, and the absolute configuration of the caged xanthones was shown for the first time at carbons 5, 7, 8, 8a, 10a, and 27, by analysis of COSY and NOESY NMR and ECD spectra. The biflavonoids exhibited proteasome inhibitory activity, and the known compound, morelloflavone (8) was found to have the greatest potency (IC(50) = 1.3 muM). The caged xanthones were cytotoxic towards HT-29 cells, with the known compound, morellic acid (10) being the most active (ED(50) = 0.36 muM). However, when tested in an in vivo hollow fiber assay, it was inactive at the highest dose tested (20 mg/kg).

Evaluation of the pharmacophoric motif of the caged Garcinia xanthones

The combination of unique structure and potent bioactivity exhibited by several family members of the caged Garcinia xanthones, led us to evaluate their pharmacophore. We have developed a Pd(0)-catalyzed method for the reverse prenylation of catechols that, together with a Claisen/Diels-Alder reaction cascade, provides rapid and efficient access to various caged analogues. Evaluation of the growth inhibitory activity of these compounds leads to the conclusion that the intact ABC ring system containing the C-ring caged structure is essential to the bioactivity. Studies with cluvenone (7) also showed that these compounds induce apoptosis and exhibit significant cytotoxicity in multidrug-resistant leukemia cells. As such, the caged Garcinia xanthone motif represents a new and potent pharmacophore.

Cytotoxic effects of hyperatomarin, a prenylated phloroglucinol from Hypericum annulatum Moris subsp. annulatum, in a panel of malignant cell lines

The cytotoxic effects of hyperatomarin - a prenylated phloroglucinol isolated from Hypericum annulatum Moris subsp. annulatum were assessed in a broad spectrum of tumor cell lines originating from leukemias, lymphomas and solid malignancies. The tested compound exerted strong concentration-dependent cytotoxic effects (IC50 values ranging 0.14-15.7 μM), comparable to and even outclassing in some cell lines those of the established anti-cancer drug daunorubicin. Exposure of different human tumor cell lines to hyperatomarin resulted in strong mono- and oligo-nucleosomal fragmentation of genomic DNA, as evidenced by 'Cell death detection' ELISA kit and by DNA-electrophoresis, which unambiguously indicates that the induction of apoptosis is implicated in the cytotoxic mode of action of the tested compound.