Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 12, Issue 8, 2012

Flavonoids are widely distributed in nature and a prevalent component of the human diet. Numerous biological activities have been reported. Some clinical trials or meta-analyses have suggested positive associations between flavonoid intake and human health, whereas others have not supported such a relationship. We currently highlight some responses that may be relevant to cancer chemoprevention, including antioxidation, anti-inflammation, and effects on NK cells. In addition, the prooxidant capacity of flavonoids may be relevant for the treatment of cancer. As is the case with other phytochemical constituents found in the diet, many questions over-shadow the results obtained with in vitro studies that do not take into account the ramifications of poor bioavailability, rapid and extensive metabolism, and physiologically relevant concentrations. To overcome some of these difficulties, greater emphasis has been placed on the study of methoxylated flavonoids, which may demonstrate more favorable pharmacokinetic properties. In terms of drug development, newer approaches such as nanotechnology could be useful. It is clear that flavonoids or flavonoid derivatives offer value for the chemoprevention of cancer. Many avenues of development are available and necessary for exploiting the impact on human health.

Cancers are the largest cause of mortality and morbidity in industrialized countries. Several new concepts have emerged in relation to mechanisms that contribute to the regulation of carcinogenesis processes and associated inflammatory effects such as the modulation of innate immune cells and adaptive immune cells that could infiltrate the tumor. In the tumor microenvironment, there is a delicate balance between antitumor immunity and tumor-originated proinflammatory activity, which weaken antitumor immunity. Consequently; modulation of immune cells and inflammatory processes represent attractive targets for therapeutic intervention in malignant diseases with the goal to restore the sensitivity of cancer cells to chemotherapies and to overcome resistance to current cytotoxic therapies. Numerous studies have reported interesting properties of dietary polyphenols in anticancer strategies notably by their pleiotropic properties on cancer cells, immune cells and inflammation. This review is dedicated to the current knowledge of the mechanisms of polyphenols (resveratrol, curcumin, genistein and epigallocatechin) against cancers through a modulation of the immune system and the pro-inflammatory mediator production. We describe the effects of polyphenols on the adaptative and innate immune cells that could infiltrate the tumor. Reduction of chronic inflammation or its downstream consequences may represent a key mechanism in the fight of cancer development and polyphenols could reduce various pro-inflammatory substance productions through targeting signal transduction or through antioxidant effects. Lastly, we analyze key molecular links between inflammation and tumor progression through nuclear factors such as NFκB or microRNAs.

Apoptosis is a critical regulatory mechanism to control tissue homeostasis. Accordingly, too little apoptosis can contribute to cancer. Natural compounds, e.g. polyphenols such as resveratrol, have emerged as promising agents for cancer chemoprevention and therapy, since they interfere with various major signaling cascades that are aberrantly regulated in cancers. For example, resveratrol can antagonize signaling events that prevent apoptosis or support cancer proliferation. Further elucidation of the signaling events that are regulated by resveratrol is anticipated to path the way for the transfer of resveratrol and its derivatives into clinical application for chemoprevention or treatment of human cancers.

One of the main reasons of cancer resistance to chemotherapeutic treatment is the presence of different ABC multidrug transporters in plasma membranes. The transporters extrude wide spectrum of anticancer agents out of cancer cells at the expense of energy derived from ATP-hydrolysis. Plant-origin polyphenolic compounds, mainly flavonoids and stilbenes or their synthetic derivatives, can modulate the main ABC transporters responsible for cancer drug resistance, including P-glycoprotein, MRP1 and BCRP. The recent studies on different resistant cancer cell lines enabled the discovery of a number of polyphenolic compounds able to reverse drug resistance in vitro and these compounds could be promising candidates for further clinical trials. The review summarizes the recent advances in the field of polyphenols interaction with ATP-binding cassette multidrug transporters. The mechanism of flavonoids interactions with the multidrug transporters and the structure-activity relationship are also discussed.

The development of tumor drug resistance is one of the biggest obstacles on the way to achieve a favorable outcome of chemotherapy. Among various strategies that have been explored to overcome drug resistance, the combination of current chemotherapy with plant polyphenols as a chemosensitizer has emerged as a promising one. Plant polyphenols are a group of phytochemicals characterized by the presence of more than one phenolic group. Mechanistic studies suggest that polyphenols have multiple intracellular targets, one of which is the proteasome complex. The proteasome is a proteolytic enzyme complex responsible for intracellular protein degradation and has been shown to play an important role in tumor growth and the development of drug resistance. Therefore, proteasome inhibition by plant polyphenols could be one of the mechanisms contributing to their chemosensitizing effect. Plant polyphenols that have been identified to possess proteasome-inhibitory activity include (-)-epigallocatechins-3-gallate (EGCG), genistein, luteolin, apigenin, chrysin, quercetin, curcumin and tannic acid. These polyphenols have exhibited an appreciable effect on overcoming resistance to various chemotherapeutic drugs as well as multidrug resistance in a broad spectrum of tumors ranging from carcinoma and sarcoma to hematological malignances. The in vitro and in vivo studies on polyphenols with proteasome-inhibitory activity have built a solid foundation to support the idea that they could serve as a chemosensitizer for the treatment of cancer. In-depth mechanistic studies and identification of optimal regimen are needed in order to eventually translate this laboratory concept into clinical trials to actually benefit current chemotherapy.

In this short review we report selected examples from recent literature to show the potential of natural-derived, low molecular weight polyphenols as antitumor agents. The two major groups of polyphenol analogues have been reviewed here, namely flavonoids and stilbenoids. Notwithstanding these limitations, we listed 75 compounds, many of them representing only the most potent member in a library. In addition, many studies afforded useful SARs which may be the basis for future optimization. In this regard, it is worth highlighting the close structural relationships connecting some families of tubulin inhibitors, namely analogues of chalcones, combretastatin A-4, and resveratrol. Some interesting hybrid molecules have already been obtained, such as chalcone-combretastatin and chalcone-resveratrol hybrids. The optimization of natural polyphenols reputed to be anticarcinogenic has also been addressed to improve their metabolic stability and a number of analogues, which are more stable to metabolic conversion and display comparable or higher antitumor activity than the parent compound, have been obtained. In some cases analogues with higher lipophilicity showed higher activity than the parent compound, in particular stilbenoids, flavanols, and flavone derivatives. Table 1 summarizes the main biological data on the natural-derived polyphenols cited within this review. As a whole, this survey of recently reported, natural-derived polyphenols, though not exhaustive, clearly indicates that intensive research is being carried out in the area of antitumor polyphenol analogues and suggests that in the near future some polyphenolic leads may become useful anticancer drugs or adjuvants in cancer therapy.

The interaction between dihydromyricetin (DMY) with human serum albumin (HSA) under the physiological conditions was investigated by fluorescence spectroscopy, circular dichroism (CD) spectra and UV-visible absorption spectroscopy. In the mechanism discussion it was proved that the fluorescence quenching of HSA by DMY is a result of the formation of DMY-HSA complex. Binding parameters calculated showed that DMY bind to HSA with the binding affinities of the order 105∼106 L·mol-1. The enthalpy change (ΔH) and entropy change (ΔS) were calculated to be -28.76 kJ·mol-1 and 18.21 J·mol-1·K-1, respectively, which implied that the hydrophobic and hydrogen bonds interactions play predominant roles in the binding process. The binding site of DMY on HSA may be located in hydrophobic cavity of subdomain IIA by the analysis data of fluorescence and synchronous fluorescence spectra. The specific binding distance r (3.37 nm) between donor (Trp-214) and acceptor (DMY) was obtained according to Forster non-radiative resonance energy transfer theory. CD spectral result demonstrates that DMY does not affect the secondary structure of HSA and can maintain protein stabilization. In addition, the effect of some common metal ions (e.g. Zn2+, Cu2+, Co2+, Ni2+, Fe3+) on the binding constant between DMY and HSA was examined.

The natural flavonoids as human digestive enzymes, such as α-glucosidase, α-amylase and aldose reductases inhibitors, have attracted great interest among researchers. The objective of this review is to overview the structures required of flavonoids for inhibiting these digestive enzymes. The hydroxylation on rings A and B of flavonoids improved the inhibition against these digestive enzymes. The hydroxylation on A-ring of flavones and isoflavones, especially at C-5 and C-7, significantly enhanced the inhibitory activities against digestive enzymes and the hydroxylation on positions C-3′ and C-4′ of B-ring of flavonoids remarkably improved the inhibition. The hydrogenation of the C2=C3 double bond on flavonoids decreased the inhibitory effects. The glycosylation of hyroxyl group on flavonoids weakened the inhibition against α-amylases and α-glucosidases. The glycosylation on 7-OH and 4'-OH of flavonoids significantly decreased the inhibition for aldose reductases. The glycosylation on 3-OH of flavonoids significantly increased or little affected the inhibition on aldose reductases. The methylation and methoxylation of flavonoids obviously weakened the inhibitory effects against α-amylase. The methylation and methoxylation of the hydroxyl group at C-3, C-3' and C-4' of flavonoids decreased or little affected the inhibitory potency against aldose reductases. And, the methylation and methoxylation of the hydroxyl groups at 5, 6, and 8 significantly increased the inhibitory capacity for aldose reductases. The methylation and methoxylation of flavonoids obviously affected the inhibitory effect for α-glucosidase in vitro depending on the replaced site.

Polyphenol-protein interaction (PPI) is reversible in that polyphenol-protein complex can be dissociated and release the free polyphenols. The aim of this study is to evaluate the contribution of polyphenol-protein interaction on improving the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging capacity of polyphenols. The DPPH radical scavenging potential of polyphenols was determined from 1 to 7 days under aerobic condition. The DPPH radical scavenging capacity of polyphenols depended on the structures of dietary polyphenols and gallic acids. The DPPH radical scavenging percentages of Group H polyphenols were weakened when kept in room temperature from 1 to 7 days. BSA rapid weakened the DPPH radical scavenging activity of polyphenols on the first day. However, the DPPH scavenging capacities of polyphenols in the presence of BSA overwhelmingly improved with increasing time. These results illustrated that BSA not only prolongs the effective time, but also improved the DPPH radical scavenging potential of polyphenols. The increasing DPPH scavenging percentages of polyphenols slightly decreased with increased affinities of BSA-polyphenol complexes.

We investigated the molecular events of the ruthenium complex NAMI-A (0.1 mM for 1 h) on cell cycle G2-M arrest in KB carcinoma cells. Flow cytometry analysis showed a progressive accumulation of cells in S phase at 16 h, and in G2-M phase at 20 h after the end of treatment. NAMI-A pre-mitotic stop to cell proliferation was due to the maintenance of the phosphorylated, inactive, form of Cdk1, caused by the activation of the ATM/ATR checkpoint, as confirmed by the up-regulation and phosphorylation of Chk1. All these events are related to intracellular ruthenium accumulation, as confirmed by the lack of similar effects in cell lines unable to take the ruthenium compound up. Considering the dependence of NAMI-A cell cycle arrest on the dose and on the length of cell challenge, and considering the prolonged NAMI-A t1/2 in vivo in the lungs, we proved an even greater perturbation of the cell cycle regulating pathways in lung metastases of NAMI-A treated mice. The ex-vivo data confirm the interaction of the ruthenium compound NAMI-A with the ATM/ATR pathway, leading to the modulation of cell cycle regulating proteins, that can break the metastases cell cycle progression off.

Melanoma and nonmelanoma skin cancers are among the most prevalent cancers in the human population. In the present work latex of Ficus carica cultivar Dottato from Italy collected from fruits and leaves was examined to assess its free radical-scavenging activity with 1,1-diphenyl-2 picrylhydrazyl (DPPH) and its phototoxicity on A375 human melanoma cells. The latex obtained from the fruits of Ficus carica cv. Dottato showed the best antiradical activity with an IC50 value of 0.05 mg/ml while the latex obtained from the leaves showed the best antiproliferative activity with an IC50 value of 1.5 μg/ml on the human tumor cell line A375 (melanoma) after irradiation at a specific UVA dose (1.08 J/cm2). Control experiments with UVA light or drugs alone were carried out without significant cytotoxic effects. Polyphenolic content of the samples was also evaluated. This is the first study comparing F. carica latex of leaves and fruits. Plant derived natural products have long been and will continue to be an important source for anticancer drug development.

The evasion of cancer cells from the induction of cell death pathways results in the resistance of tumor to current treatment modalities. Therefore, the resistance to cell death, one of the hallmarks of cancer, is a major target in the development of new approaches to selectively affect cancer cells. The complex interplay between individual members of Bcl-2 family regulates both cell survival and the mitochondrial pathway of apoptosis by maintaining mitochondrial membrane integrity (anti-apoptotic Bcl-2 subfamily) and by triggering its disruption in response to stress stimuli (Bax-like subfamily). BH3-only proteins, another Bcl-2 subfamily, act either by direct stimulation of pro-apoptotic proteins of the Bax subfamily or by interfering with anti-apoptotic proteins of the Bcl-2 subfamily. Thus, pro-apoptotic BH3 mimetics, thought to function as BH3-only proteins, are expected to improve the effectiveness of cancer treatment. BH3 mimetics could be either natural or synthetic, peptidic or only based on a helical peptide-like scaffold. Experimental and clinical evidence indicates that BH3 mimetics may not be sufficient to cure cancer patients when used as a single agent. BH3 profiling of cancer cells was introduced to better predict the in vivo responsiveness of tumor to BH3 mimetics combined with conventional therapies. In summary, targeting the Bcl-2 proteins is a promising tool with potential to generate new treatment modalities and to complement existing anti-cancer therapies. This review presents the current knowledge on BH3-only proteins and the spectrum of strategies employing BH3 mimetics in preclinical and clinical studies that aim at tumor targeting.

Epithelial ovarian cancer (EOC) accounts for approximately 80-90% of all ovarian cancers, and 75% of the patients are diagnosed with advanced disease (stage III and IV). Front-line systemic chemotherapy improves survival in women with advanced EOC; however, tumor recurrence occurs in almost all advanced EOC patients at a median of 15 months from diagnosis, and 5-year survival is estimated at 10 to 30%. Additionally, around 20% of patients do not respond to standard front-line therapy. Tumoral angiogenesis plays an important role in the pathogenesis of EOC, and its inhibition might improve survival in patients with advanced EOC. High-grade EOC is characterized by overexpression of vascular endothelial growth factor (VEGF), which drives dysfunctional tumor-associated angiogenesis, contributing to high interstitial pressure and increased vascular permeability. Diverse anti-angiogenic drugs are under investigation, and direct targeting of this pathway can be achieved by sequestration of VEGF protein using monoclonal antibodies (bevacizumab) or engineered binding site molecules (aflibercept), blockade of the VEGF receptor-2 with monoclonal antibodies or inhibition of receptor associated tyrosine kinase with low molecular weight inhibitors (cediranib, pazopanib, sorafenib or BIBF-1120).

DTA0100 is a new catalytic inhibitor of the human DNA topoisomerase IIα that induces G2/M phase cell cycle arrest in human solid tumor cells lines from various malignancies. In our study, we investigated the effectiveness of the combined treatment of ionizing radiation with DTA0100 on the survival of three representative human solid tumor cell lines: HeLa (cervix), WiDr (colon) and SW1573 (non-small cell lung cancer). The concomitant treatment of DTA0100 and irradiation showed a synergistic and antagonistic effect in the three cell lines tested. A synergistic cytotoxic effect of the combination of DTA0100 and radiation was confirmed by the median drug effect analysis method. It was found that in those protocols where the drug was administered after radiation the most synergistic effect was achieved. Our study constitutes the first in vitro evidence for synergistic effects between DTA0100 and radiation. This combination therapy might thus be expected to be more effective than either treatment alone in patients with cervical, colon and non-small cell lung cancer cells.

Gambogic acid (GA) is a caged xanthone that is derived from Garcinia hanburyi and functions as a strong apoptotic inducer in many types of cancer cells. The distinct effectiveness of GA has led to its characterization as a novel anti-cancer agent. There is an increasing number of research studies focused on elucidating the molecular mechanisms of GA-induced anti-cancer effects, and several critical signaling pathways have been reported to be influenced by GA treatment. In this review, we summarize the multiple functional effects of GA administration in cancer cells including the induction of apoptosis, the inhibition of proliferation and the prevention of cancer metastasis and tumor angiogenesis.