Acridone Derivatives from Atalantia monophyla Inhibited Cancer Cell Proliferation through ERK Pathway.

The present study aimed to investigate the effect of acridone alkaloids on cancer cell lines and elucidate the underlying molecular mechanisms. The ten acridone alkaloids from Atalantia monophyla were screened for cytotoxicity against LNCaP cell lines by a WST-8 assay. Then, the most potential acridone, buxifoliadine E, was evaluated on four types of cancer cells, namely prostate cancer (LNCaP), neuroblastoma (SH SY5Y), hepatoblastoma (HepG2), and colorectal cancer (HT29). The results showed that buxifoliadine E was able to significantly inhibit the proliferation of all four types of cancer cells, having the most potent cytotoxicity against the HepG2 cell line. Western blotting analysis was performed to assess the expression of signaling proteins in the cancer cells. In HepG2 cells, buxifoliadine E induced changes in the levels of Bid as well as cleaved caspase-3 and Bax through MAPKs, including Erk and p38. Moreover, the binding interaction between buxifoliadine E and Erk was investigated by using the Autodock 4.2.6 and Discovery Studio programs. The result showed that buxifoliadine E bound at the ATP-binding site, located at the interface between the N- and C-terminal lobes of Erk2. The results of this study indicate that buxifoliadine E suppressed cancer cell proliferation by inhibiting the Erk pathway.

A comprehensive review on acridone based derivatives as future anti-cancer agents and their structure activity relationships.

The development of drug resistance and severe side-effects has reduced the clinical efficacy of the existing anti-cancer drugs available in the market. Thus, there is always a constant need to develop newer anti-cancer drugs with minimal adverse effects. Researchers all over the world have been focusing on various alternative strategies to discover novel, potent, and target specific molecules for cancer therapy. In this direction, several heterocyclic compounds are being explored but amongst them one promising heterocycle is acridone which has attracted the attention of medicinal chemists and gained huge biological importance as acridones are found to act on different therapeutically proven molecular targets, overcome ABC transporters mediated drug resistance and DNA intercalation in cancer cells. Some of these acridone derivatives have reached clinical studies as these heterocycles have shown huge potential in cancer therapeutics and imaging. Here, the authors have attempted to compile and make some recommendations of acridone based derivatives concerning their cancer biological targets and in vitro-cytotoxicity based on drug design and novelty to increase their therapeutic potential. This review also provides some important insights on the design, receptor targeting and future directions for the development of acridones as possible clinically effective anti-cancer agents.

Acridone alkaloids and flavones from the leaves of Citrus reticulata.

A new acridone alkaloid, reticarcidone A (1), decorated with an oxygenated isopentenyl group between C-1 and C-2, was isolated from the leaves of Citrus reticulata Blanco, together with nine known acridone alkaloids (2-10) and fifteen flavones compounds (11-25). The structure of those compounds were confirmed by analysis of comprehensive 1D and 2D NMR, and MS data. Reticarcidone A (1) was the first pyrano[2,3-a]acridone isolated from the genus Citrus. Some of these compounds showed moderated cytotoxicity against the five human tumor cell lines MCF-7, SMMC-7721, HL-60, A549 and SW480.

Structural optimizations and bioevaluation of N-substituted acridone derivatives as strong topoisomerase II inhibitors.

Previously, an array of N-substituted acridone derivatives have been reported as potent topoisomerase II (topo II) inhibitors, and preliminary structure-activity relationship (SAR) outcomes revealed that the linker between 1-NH and N-methyl piperazine motif of the tricyclic acridone scaffold significantly affected their anti-proliferative potencies. To further explore the SARs of acridone-derived topo II inhibitors, a wider range of novel acridone derivatives were herein synthesized via two rounds of structural optimizations on two validated hits, E17 and E24. Initially, the linker length was optimized, and then influences of N-methyl piperazinyl moiety and disposition of three N atoms on the bioactivity were investigated. As a result, a newly developed topo II inhibitor 6 h was found to be more potent than E17 and E24, thereby serving as a tool compound for the follow-up mechanistic study. Compound 6 h functioned as a strong topo IIα/ß inhibitor, caused obvious DNA damage, and induced apoptosis by triggering the loss of mitochondrial membrane potential (Δψm). Further molecular docking and MD study illustrated the favorable interactions of 6 h with both topo IIα and topo IIß subtypes.

Design and Synthesis of New Acridone-Based Nitric Oxide Fluorescent Probe.

Nitric oxide (NO) is an important signaling molecule involved in a wide range of physiological and pathological processes. Fluorescent imaging is a useful tool for monitoring NO concentration, which could be essential in various biological and biochemical studies. Here, we report the design of a novel small-molecule fluorescent probe based on 9(10H)acridone moiety for nitric oxide sensing. 7,8-Diamino-4-carboxy-10-methyl-9(10H)acridone reacts with NO in aqueous media in the presence of O2, yielding a corresponding triazole derivative with fivefold increased fluorescence intensity. The probe was shown to be capable of nitric oxide sensing in living Jurkat cells.

Room Temperature Phosphorescence vs Triplet-Triplet Annihilation in N-Substituted Acridone Solids.

Organic room temperature phosphorescent (ORTP) compounds have recently emerged as a promising class of emissive materials with a multitude of potential applications. However, the number of building blocks that give rise to efficient ORTP materials is still limited, and the rules for engineering phosphorescent properties in organic solids are not well understood. Here, we report ORTP in a series of N-substituted acridone derivatives with electron-donating, electron-withdrawing, and sterically bulky substituents. X-ray crystallography shows that the solid-state packing varies progressively between coparallel and antiparallel π-stacking and separated π-dimers, depending on the size of the substituent. The detailed photophysical studies supported by DFT calculations reveal complex dynamics of singlet and triplet excited states, depending on the electronic effects of substituents and solid-state packing. The programmable molecular packing provides a lever to control the triplet-triplet annihilation that is manifested as delayed fluorescence in acridone derivatives with continuous (both parallel and antiparallel) π-stacking.

Acridones: Strongly Emissive HIGHrISC Fluorophores.

An acridone derivative (N-methyl-difluoro-acridone, NMA-dF) is characterized with respect to its utility as an emitter in organic light emitting diodes (OLEDs). Using steady-state and time-resolved spectroscopy as well as quantum chemistry, its ability to convert singlet and triplet excitons into light was scrutinized. NMA-dF emits in the deep blue range of the visible spectrum. Its fluorescence emission occurs with quantum yields close to 1 and a radiative rate constant of ≈5 × 108 s-1. So, it processes singlet excitons very efficiently. Using 1,4-dichlorobenzene as a sensitizer, it is shown that NMA-dF also converts triplet excitons into light. With the aid of quantum chemistry, this is related to a reverse intersystem crossing starting from a higher triplet state (HIGHrISC).

A New Coumarin-Acridone Compound as a Fluorescence Probe for Fe3+ and Its Application in Living Cells and Zebrafish.

A new coumarin-acridone fluorescent probe S was designed and synthesized, and the structure was confirmed with 1H/13C NMR spectrometry, single-crystal X-ray diffraction, and high-resolution mass spectrometry. This probe has high sensitivity and selectivity for Fe3+ over other testing metal ions at 420 or 436 nm in acetonitrile-MOPS (3-Morpholinopropanesulfonic Acid) buffer solution (20.0 µM, pH = 6.9, 8:2 (v/v)). Under physiological conditions, the probe displayed satisfying time stability with a detection limit of 1.77 µM. In addition, probe S was successfully used to detect intracellular iron changes through a fluorescence-off mode, and the imaging results of cells and zebrafish confirmed their low cytotoxicity and satisfactory cell membrane permeability, as well as their potential biological applications.

Synthesis of Xanthones, Thioxanthones and Acridones by a Metal-Free Photocatalytic Oxidation Using Visible Light and Molecular Oxygen.

9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or quantitative.

Syntheses and evaluation of acridone-naphthalimide derivatives for regulating oncogene PDGFR-ß expression.

Upregulation of platelet-derived growth factor receptor ß (PDGFR-ß) has been found to be associated with development of various types of cancers, which has become an attractive target for anti-tumor treatment. Previously, we have synthesized and studied an acridone derivative B19, which can selectively bind to and stabilize oncogene c-myc promoter i-motif, resulting in down-regulation of c-myc transcription and translation, however its effect on tumor cells apoptosis requires improvement. In the present study, we synthesized a variety of B19 derivatives containing a known anti-cancer fluorescent chromophore naphthalimide for the purpose of enhancing anti-cancer activity. After screening, we found that acridone-naphthalimide derivative WZZ02 could selectively stabilize PDGFR-ß promoter G-quadruplex and destabilize its corresponding i-motif structure, without significant interaction to other oncogenes promoter G-quadruplex and i-motif. WZZ02 down-regulated PDGFR-ß gene transcription and translation in a dose-dependent manner, possibly due to above interactions. WZZ02 could significantly inhibit cancer cell proliferation, and induce cell apoptosis and cycle arrest. WZZ02 exhibited tumor growth inhibition activity in MCF-7 xenograft tumor model, which could be due to its binding interactions with PDGFR-ß promoter G-quadruplex and i-motif. Our results suggested that WZZ02 as a dual G-quadruplex/i-motif binder could be effective on both oncogene replication and transcription, which could become a promising lead compound for further development with improved potency and selectivity. The wide properties for the derivatives of 1,8-naphthalimide could facilitate further in-depth mechanistic studies of WZZ02 through various fluorescent physical and chemical methods, which could help to further understand the function of PDGFR-ß gene promoter G-quadruplex and i-motif.

Structure-activity relationship of novel acridone derivatives as antiproliferative agents.

Unlike other DNA topoisomerase II (topo II) inhibitors, our recently identified acridone derivative E17 exerted strong cytotoxic activity by inhibiting topo II without causing topo II degradation and DNA damage, which promoted us to explore more analogues of E17 by expanding its chemical diversification and enrich the structure-activity relationship (SAR) outcomes of acridone-oriented chemotypes. To achieve this goal, 42 novel acridone derivatives were synthesized and evaluated for their antiproliferative efficacies. SAR investigations revealed that orientation and spatial topology of R3 substituents make greater contributions to the bioactivity, exemplified by compounds E24, E25 and E27, which has provided valuable information for guiding further development of acridone derivatives as promising drug candidates.

Synthesis of novel cytotoxic tetracyclic acridone derivatives and study of their molecular docking, ADMET, QSAR, bioactivity and protein binding properties.

Acridone based synthetic and natural products with inherent anticancer activity advancing the research and generating a large number of structurally diversified compounds. In this sequence we have designed, synthesized a series of tetracyclic acridones with amide framework viz., 3-(alkyloyl/ aryloyl/ heteroaryloyl/ heteroaryl)-2,3-dihydropyrazino[3,2,1-de]acridin-7(1H)-ones and screened for their in vitro anti-cancer activity. The in vitro study revealed that compounds with cyclopropyl-acetyl, benzoyl, p-hydroxybenzoyl, p-(trifluoromethyl)benzoyl, p-fluorobenzoyl, m-fluorobenzoyl, picolinoyl, 6-methylpicolinoyl and 3-nicotinoyl groups are active against HT29, MDAMB231 and HEK293T cancer cell lines. The molecular docking studies performed for them against 4N5Y, HT29 and 2VWD revealed the potential ligand-protein binding interactions among the neutral aminoacid of the enzymes and carbonyl groups of the title compounds with a binding energy ranging from - 8.1394 to - 6.9915 kcal/mol. In addition, the BSA protein binding assay performed for them has confirmed their interaction with target proteins through strong binding to BSA macromolecule. The additional studies like ADMET, QSAR, bioactivity scores, drug properties and toxicity risks ascertained them as newer drug candidates. This study had added a new collection of piperazino fused acridone derivatives to the existing array of other nitrogen heterocyclic fused acridone derivatives as anticancer agents.

Application of Lanthanide Shift Reagent to the 1H-NMR Assignments of Acridone Alkaloids.

This study investigates the application of the paramagnetic shift reagent tris(dipivaloylmethanato)-europium(III) in NMR spectral studies of permethoxyacridone alkaloids (1-3) and pyranoacridone alkaloids (4-6). The induced chemical shifts (∆δ) of all protons were observed for the same molecule, and were compared to deduce the positions resulting from the distance nearby the Eu(dpm)3. Assignment of the H-2, H-4 and H-8 of polysubstituted acridones could be distinguished based on the least-squares method of lanthanide-induced shifts plotted against the mole ratios of Eu(dpm)3 to the substrate. The developed method is not only potentially useful for determining the planar structures of polysubstituted compounds, such as acridones, anthraquinones, xanthones, flavonoids, and phenanthrenes, but also applicable for their stereochemistry.

Lead Optimization of Second-Generation Acridones as Broad-Spectrum Antimalarials.

The global impact of malaria remains staggering despite extensive efforts to eradicate the disease. With increasing drug resistance and the absence of a clinically available vaccine, there is an urgent need for novel, affordable, and safe drugs for prevention and treatment of malaria. Previously, we described a novel antimalarial acridone chemotype that is potent against both blood-stage and liver-stage malaria parasites. Here, we describe an optimization process that has produced a second-generation acridone series with significant improvements in efficacy, metabolic stability, pharmacokinetics, and safety profiles. These findings highlight the therapeutic potential of dual-stage targeting acridones as novel drug candidates for further preclinical development.

Synthesis, biological evaluation and virtual screening of some acridone derivatives as potential anticancer agents.

Eleven novel acridone derivatives were synthesized and evaluated for their anticancer activity against 60 human cancer cell lines. Five compounds 8b, 8d, 8g, 8h, and 8k displayed very good in vitro antiproliferative activities well over 95% of the panels. The most active compound is 8k (5, 7-dibromo-3-phenyl-3,4-dihydroacridin-1 (2H)-one). In addition, 8k was the most sensitive agent in all 9 panels starting with prostate (0.075 µm), leukemia (0.116 µm), non-small cell lung cancer (0.164 µm), colon cancer (0.193 µm), CNS cancer (0.264 µm), melanoma (0.317 µm), renal cancer (0.403 µm), ovarian cancer (0.410 µm), and breast cancer (0.608 µm). Virtual screening studies also revealed that nine of the eleven compounds formed good binding interaction with the active site ATPase domain of human topoisomerase IIα (PDB: 1zxm). All nine derivatives exhibited binding affinities that ranged in values from -8.5 to -7.9 kcal/mol, indicating that they could be catalytic inhibitors of the nuclear enzyme, topoisomerase.

Design, synthesis and biological evaluation of acridone glycosides as selective BChE inhibitors.

Based on structure analyses of butyrylcholinesterase (BChE), a series of 21 acridone glycosides were designed, synthesized and evaluated in vitro for their BChE and acetylcholinesterase (AChE) inhibitory activities. d-ribose derivative 6f exhibited the greatest inhibitory activity on BChE (IC50 = 6.95 µM), and was the most selective inhibitor of BChE with the IC50 ratio of AChE/BChE was 20.59. d-glucose and d-galactose derivatives 6a and 6b showed inhibitory activities against both AChE and BChE. Moreover, compounds 6a, 6b, 6f and 5t were found nontoxic on SHSY5Y neuroblastoma and HepG2 cell and exhibited remarkable neuroprotective activity. Besides, compound 6f showed mixed-type inhibition against BChE (Ki = 1.76 µM), which renders 6f a potential agent for the treatment of Alzheimer's disease. These novel acridone hybrids might be used as efficient probes to reveal the relationship between ligands and BChE and pave the way for developing selective BChE inhibitors to further study the pathogenesis of alzheimer's disease.

Design, synthesis and biological evaluation of acridone analogues as novel STING receptor agonists.

STING (Stimulator of Interferon Genes) has become a focal point in immunology research and a target in drug discovery. The discovery of a potent human-STING agonist is expected to revolutionize current anti-virus or cancer immunotherapy. Inspired by the structure and function of murine STING-specific agonists (DMXAA and CMA), we rationally designed and synthesized four series of novel compounds for the enhancement of human sensitivity. In the cell-based assay, we identified six compounds from all the synthetic small molecules: 2g, 9g, and 12b are STING agonists that are efficacious across species, and all have the skeleton of acridone; 1b, 1c, and 12c just function in the murine STING pathway. Notably, 12b exhibits the best activity among the six agonists, and its inductions of both human and murine STING-dependent signalling are similar to that of 2'3'-cGAMP, which is a well-known STING inducer. While a protein assay indicated that 2 g, 9 g, and 12b could activate the pathway by directly binding human STING, 12b also displayed the strongest binding affinity. Additionally, our studies show that 12b can induce faster, more powerful, and more durable responses of assorted cytokines in a native system than 2'3'-cGAMP. Consequently, our team is the first to successfully modify murine STING agonists to obtain human sensitivity, and these results suggest that 12b is a potent direct-human-STING agonist. Additionally, the acridone analogues demonstrate tremendous potential in the treatment of tumours or viral infections.

Probing the Hydrogen Bond Involving Acridone Trapped in a Hydrophobic Biological Nanocavity: Integrated Spectroscopic and Docking Analyses.

Spectroscopic analyses reveal that acridone (AD) penetrates through the structure and enters the hydrophobic cavity of the protein ß-lactoglobulin (ßLG). Although the protein contains two tryptophan (Trp) residues, AD interacts with only one (Trp-19), which is authenticated by the appearance of a single isoemissive point in TRANES. Alteration in the secondary structure of the protein while AD pierces through ßLG is evident from the circular dichroism spectroscopic study. The ground-state interaction between AD and ßLG is proven from the UV-vis spectroscopic study and the static nature of quenching of intrinsic fluorescence of the protein by the ligand. The steady-state fluorescence study in varied temperatures indicates the involvement of hydrogen bonding in the ligand-protein interaction. Further, the time-resolved fluorescence anisotropy study gives a hint of the presence of a hydrogen bond in AD-ßLG interaction, which possibly involves the rotamers of Trp-19. In fact, the idea of involvement of rotamers of Trp-19 is obtained from the increase in fluorescence lifetime of ßLG in the presence of AD. The docking study agrees to the involvement of hydrogen bonding in AD-ßLG interaction. The direct evidence of hydrogen bonding between Trp and AD is obtained from the laser flash photolysis studies where the signature of formation of ADH• and Trp• through hydrogen abstraction between Trp and AD, loosely bound through hydrogen bonding, gets prominence. Thus, binding of AD to ßLG involves hydrogen bonding in a hydrophobic pocket of the protein.

A FRET-based ratiometric fluorescent probe for sulfide detection in actual samples and imaging in Daphnia magna.

Sulfide plays an important role in many important life processes, and abnormal levels of sulfide that may cause diseases. Sulfide is also essential in industrial production and food processing, and it is well concerned for environmental issues and food safety. In order to study the physiological and pathological effects of sulfide and the impact on the environment, it is still urgent to develop a convenient and effective sulfide detection technology. Here, we developed a ratiometric fluorescent probe 7-Nitro-1,2,3-benzoxoxadiazole-Acridoneacetylpiperazine (NBD-AAP) which is based on the fluorescence resonance energy transfer (FRET) between acridone and NBD fluorophores. The NBD-AAP probe could produce a ratiometric response to micromolar sulfide in buffer (pH = 7.4), exhibiting a significant enhancement in fluorescent emission ratio (F427/F552) and obvious visual phenomenon (orange to pink under visible light and yellow to blue under UV light). At the same time, this NBD-AAP probe also has excellent properties including high selectivity and low detection limit (0.19 µM). In addition, this probe has been successfully used for detecting the sulfide in actual samples (monosodium glutamate, beer, environmental water) and imaging in Daphnia magna. These results indicate that NBD-AAP has broad application prospects in sulfide detection and in vivo imaging studies.

Towards blue AIE/AIEE: Synthesis and Applications in OLEDs of Tetra-/Triphenylethenyl Substituted 9,9-Dimethylacridine Derivatives.

Aiming to design blue fluorescent emitters with high photoluminescence quantum yields in solid-state, nitrogen-containing heteroaromatic 9,9-dimethylacridine was refined by tetraphenylethene and triphenylethene. Six tetra-/triphenylethene-substituted 9,9-dimethylacridines were synthesized by the Buchwald-Hartwig method with relatively high yields. Showing effects of substitution patterns, all emitters demonstrated high fluorescence quantum yields of 26-53% in non-doped films and 52-88% in doped films due to the aggregation induced/enhanced emission (AIE/AIEE) phenomena. In solid-state, the emitters emitted blue (451-481 nm) without doping and deep-blue (438-445 nm) with doping while greenish-yellow emission was detected for two compounds with additionally attached cyano-groups. The ionization potentials of the derivatives were found to be in the relatively wide range of 5.43-5.81 eV since cyano-groups were used in their design. Possible applications of the emitters were demonstrated in non-doped and doped organic light-emitting diodes with up to 2.3 % external quantum efficiencies for simple fluorescent devices. In the best case, deep-blue electroluminescence with chromaticity coordinates of (0.16, 0.10) was close to blue color standard (0.14, 0.08) of the National Television System Committee.