Discovery of natural products with metal-binding properties as promising antibacterial agents

INTRODUCTION

More than 50% of the clinically established antibiotics are either genuine natural products or derivatives thereof, featuring a mode of action decisively depending on their metal affinity and suitability as metal complex ligands. As their structural diversity and harvest from renewable sources is well-nigh inexhaustible, any future quest for affordable new antibiotics will have to concentrate on natural drugs with obvious metal ligating properties. Areas covered: The authors provide an overview of the promising developments in the field of antibiotic natural products with metal-binding properties with a specific focus on metal binders such as polyphenols, quinones, 3-acyltetramic and -tetronic acids. Works published by the authors are discussed in this manuscript as well as articles derived from PubMed and Scifinder. Expert opinion: Natural products with metal-binding properties possess a great potential for the development of drugs against various bacteria. There are many derivatives with great potential against multidrug-resistant bacteria as well. Synthetic approaches to structurally complex and/or rare natural products have added significantly to the cracking of synthetic problems. Thus, this field of scientific research appears attractive both to chemists and to clinicians.

Alkylating anticancer agents and their relations to microRNAs

Alkylating agents represent an important class of anticancer drugs. The occurrence and emergence of tumor resistance to the treatment with alkylating agents denotes a severe problem in the clinics. A detailed understanding of the mechanisms of activity of alkylating drugs is essential in order to overcome drug resistance. In particular, the role of non-coding microRNAs concerning alkylating drug activity and resistance in various cancers is highlighted in this review. Both synthetic and natural alkylating agents, which are approved for cancer therapy, are discussed concerning their interplay with microRNAs.

Interplay of non-coding RNAs and approved antimetabolites such as gemcitabine and pemetrexed in mesothelioma

Gemcitabine and pemetrexed are clinically approved antimetabolites for the therapy of mesothelioma diseases. These drugs are often applied in combination with platinum complexes and other drugs. The activity of antimetabolites depended on the expression levels of certain non-coding RNAs, in particular, of small microRNAs (miRNAs) and long non-coding RNAs (lncRNAs). The development of tumor resistance towards antimetabolites was regulated by non-coding RNAs. An overview of the interplay between gemcitabine/pemetrexed antimetabolites and non-coding RNAs in mesothelioma is provided. Further to this, various non-coding RNA-modulating agents are discussed which displayed positive effects on gemcitabine or pemetrexed treatment of mesothelioma diseases. A detailed knowledge of the connections of non-coding RNAs with antimetabolites will be constructive for the design of improved therapies in future.

Garcinol Sensitizes NSCLC Cells to Standard Therapies by Regulating EMT-Modulating miRNAs

Garcinol, a dietary factor obtained from Garcinia indica, modulates several key cellular signaling pathways as well as the expression of miRNAs. Acquired resistance to standard therapies, such as erlotinib and cisplatin, is a hallmark of non-small cell lung cancer (NSCLC) cells that often involves miRNA-regulated epithelial-to-mesenchymal transition (EMT). We used A549 cells that were exposed to transforming growth factor beta 1 (TGF-β1), resulting in A549M cells with mesenchymal and drug resistant phenotype, and report that garcinol sensitized resistant cells with mesenchymal phenotype to erlotinib as well as cisplatin with significant decrease in their IC₅₀ values. It also potentiated the apoptosis-inducing activity of erlotinib in A549M and the endogenously mesenchymal H1299 NSCLC cells. Further, garcinol significantly upregulated several key EMT-regulating miRNAs, such as miR-200b, miR-205, miR-218, and let-7c. Antagonizing miRNAs, through anti-miRNA transfections, attenuated the EMT-modulating activity of garcinol, as determined by mRNA expression of EMT markers, E-cadherin, vimentin, and Zinc Finger E-Box Binding Homeobox 1 (ZEB1). This further led to repression of erlotinib as well as cisplatin sensitization, thus establishing the mechanistic role of miRNAs, particularly miR-200c and let-7c, in garcinol-mediated reversal of EMT and the resulting sensitization of NSCLC cells to standard therapies.

Differential Methylation and Acetylation as the Epigenetic Basis of Resveratrol's Anticancer Activity

Numerous studies support the potent anticancer activity of resveratrol and its regulation of key oncogenic signaling pathways. Additionally, the activation of sirtuin 1, a deacetylase, by resveratrol has been known for many years, making resveratrol perhaps one of the earliest nutraceuticals with associated epigenetic activity. Such epigenetic regulation by resveratrol, and the mechanism thereof, has attracted much attention in the past decade. Focusing on methylation and acetylation, the two classical epigenetic regulations, we showcase the potential of resveratrol as an effective anticancer agent by virtue of its ability to induce differential epigenetic changes. We discuss the de-repression of tumor suppressors such as BRCA-1, nuclear factor erythroid 2-related factor 2 (NRF2) and Ras Associated Domain family-1α (RASSF-1α) by methylation, PAX1 by acetylation and the phosphatase and tensin homologue (PTEN) by both methylation and acetylation, in addition to the epigenetic regulation of oncogenic NF-κB and STAT3 signaling by resveratrol. Further, we evaluate the literature supporting the potentiation of HDAC inhibitors and the inhibition of DNMTs by resveratrol in different human cancers. This discussion underlines a robust epigenetic activity of resveratrol that warrants further evaluation, particularly in clinical settings.

Anticancer properties of a new non-oxido vanadium(IV) complex with a catechol-modified 3,3'-diindolylmethane ligand

In order to identify new active drug candidates against cancer diseases we investigated the tumor cell growth inhibition, formation of reactive oxygen species, mitochondrial membrane damage, cell cycle arrest and DNA binding activity of a new bis(triethylammonium) tris[1,1-bis(indol-3-yl)-1-(3,4-catecholate)methane]vanadate(IV) complex. It exhibited significant antiproliferative activity against various cancer cell lines, showed a stronger DNA binding than cisplatin and led to mitochondrial damage, a formation of reactive oxygen species, and a cell cycle arrest in the G2/M phase of cancer cells.

Pentafluorophenyl Substitution of Natural Di(indol-3-yl)methane Strongly Enhances Growth Inhibition and Apoptosis Induction in Various Cancer Cell Lines

Di(indol-3-yl)methane (=3,3'-methanediyldi(1H-indole), DIM, 1) is a known weakly antitumoral compound formed by digestion of indole-3-carbinol (=1H-indol-3-ylmethanol), an ingredient of various Brassica vegetables. Out of a series of nine fluoroaryl derivatives of 1, three pentafluorophenyl derivatives 2c, 2h, and 2i were identified that exhibited a two to five times greater anti-proliferative effect and an increased apoptosis induction when compared with 1 in the following carcinoma cell lines: BxPC-3 pancreas, LNCaP prostate, C4-2B prostate, PC3 prostate and the triple-negative MDA-MB-231 breast carcinoma. Compound 2h was particularly efficacious against androgen-refractory C4-2B prostate cancer cells (IC₅₀ =6.4 μm) and 2i against androgen-responsive LNCaP cells (IC₅₀ =6.2 μm). In addition, 2c and 2h exhibited distinct activity in three cancer cell lines resistant to 1.

Relations between approved platinum drugs and non-coding RNAs in mesothelioma

Malignant mesothelioma diseases feature an increasing risk due to their severe forms and their association with asbestos exposure. Platinum(II) complexes such as cisplatin and carboplatin are clinically approved for the therapy of mesothelioma often in combination with antimetabolites such as pemetrexed or gemcitabine. It was observed that pathogenic properties of mesothelioma cells and the response of mesothelioma tumors towards platinum-based drugs are strongly influenced by non-coding RNAs, in particular, by small microRNAs (miRNAs) and long non-coding RNAs (lncRNAs). These non-coding RNAs controlled drug sensitivity and the development of tumor resistance towards platinum drugs. An overview of the interactions between platinum drugs and non-coding RNAs is given and the influence of non-coding RNAs on platinum drug efficacy in mesothelioma is discussed. Suitable non-coding RNA-modulating agents with potentially beneficial effects on cisplatin treatment of mesothelioma diseases are mentioned. The understanding of mesothelioma diseases concerning the interactions of non-coding RNAs and platinum drugs will optimize existing therapy schemes and pave the way to new treatment options in future.

New naphthopyran analogues of LY290181 as potential tumor vascular-disrupting agents

A series of 19 analogues of the antiproliferative naphthopyran LY290181 were prepared for structure-activity relationship studies. We found the best activities for test compounds bearing small substituents at the meta position of the phenyl ring. The mode of action of LY290181 and eight new analogues was studied in detail. The compounds were highly anti-proliferative with IC₅₀ values in the sub-nanomolar to triple-digit nanomolar range. The new analogues led to G2/M arrest due to interruption of the microtubule dynamics. In 518A2 melanoma cells they caused a mitotic catastrophe which eventually led to apoptosis. The naphthopyrans also induced a disruption of the vasculature in the chorioallantoic membrane (CAM) of fertilized chicken eggs as well as in xenograft tumors in mice. In a preliminary therapy trial, the difluoro derivative 2b retarded the growth of resistant xenograft tumors in mice.

Recent developments concerning the application of the Mannich reaction for drug design

INTRODUCTION

The versatile multicomponent Mannich reaction occupies a salient position in organic chemistry and drug design. Sound knowledge of its scope and variations and of the biological activities of Mannich bases is crucial for the development and improvement of drugs for various diseases. Areas covered: The following article provides an overview of the latest developments in the field of drugs based on the Mannich reaction. Web-based literature searching tools such as PubMed and SciFinder were applied to obtain useful articles. In addition, pertinent literature that was recently published by the authors is discussed in this manuscript. The chemical structures of bioactive Mannich bases are also given. Expert opinion: The Mannich reaction represents a feasible and cost-effective procedure with great potential for drug development. Several newly discovered Mannich bases exhibit sound activities against various human diseases as well as favorable pharmacokinetics. Thus, scientific research about Mannich bases is prospering and appears very attractive both for chemists and for clinicians.

Chemical and Biological Aspects of the Natural 1,4-Benzoquinone Embelin and its (semi-)Synthetic Derivatives

BACKGROUND

Like the impressive biological properties of embelin, its chemical aspects have raised the interest of scientists in the field as well. A detailed understanding of the chemistry of embelin is necessary to fully exploit it medicinally.

METHODS

Search for embelin isolation and its chemical modifications was carried out using web-based literature searching tools such as Pubmed and Scifinder. Pertinent literature is covered up to 2016. Structures of bioactive embelin derivatives are provided.

RESULTS

Pure embelin, obtained from Embelia ribes berries extraction or by total synthesis, was applied for a number of biological assays. Semi-synthetic and total synthetic approaches led to new high affinity embelin-derived inhibitors of crucial protein targets and to new embelin derivatives with improved pharmacological properties (e.g., with better water-solubility or as applications for drug carrier systems).

CONCLUSION

This review provides a summary of the rich chemistry of embelin and the latest developments in the field of optimized (semi-)synthetic embelin derivatives including their biological activities.

Animacroxam, a Novel Dual-Mode Compound Targeting Histone Deacetylases and Cytoskeletal Integrity of Testicular Germ Cell Cancer Cells

Novel approaches for the medical treatment of advanced solid tumors, including testicular germ cell tumors (TGCT), are desperately needed. Especially, TGCT patients not responding to cisplatin-based therapy need therapeutic alternatives, as there is no effective medical treatment available for this particular subgroup. Here, we studied the suitability of the novel dual-mode compound animacroxam for TGCT treatment. Animacroxam consists of an HDAC-inhibitory hydroxamate moiety coupled to a 4,5-diarylimidazole with inherent cytoskeleton disrupting potency. Animacroxam revealed pronounced antiproliferative, cell-cycle arresting, and apoptosis-inducing effects in TGCT cell lines with different cisplatin sensitivities. The IC₅₀ values of animacroxam ranged from 0.22 to 0.42 μmol/L and were not correlated to the cisplatin sensitivity of the tumor cells. No unspecific cytotoxicity of animacroxam was observed in either cisplatin-sensitive or resistant TGCT cells, even at doses as high as 10 μmol/L. Furthermore, animacroxam induced the formation of actin stress fibers in cancer cells, thereby confirming the cytoskeleton-disrupting and antimigratory properties of its imidazole moiety. When compared with the clinically established HDAC inhibitor vorinostat, the novel dual-mode compound animacroxam exhibited superior antitumoral efficacy in vitro Animacroxam also reduced the tumor size of TGCT tumors in vivo, as evidenced by performing xenograft experiments on tumor bearing chorioallantoic membranes of fertilizes chicken eggs (CAM assay). The in vivo experiments also revealed a very good tolerability of the compound, and hence, animacroxam may be a promising candidate for innovative treatment of TGCT in general and the more so for platinum-insensitive or refractory TGCT. Mol Cancer Ther; 16(11); 2364-74. ©2017 AACR.

Interactions between anticancer active platinum complexes and non-coding RNAs/microRNAs

Platinum(II) complexes such as cisplatin, carboplatin and oxaliplatin are clinically approved for the therapy of various solid tumors. Challenging pathogenic properties of cancer cells and the response of cancers towards platinum-based drugs are strongly influenced by non-coding small RNA molecules, the microRNAs (miRNAs). Both increased platinum activity and formation of tumor resistance towards platinum drugs are controlled by miRNAs. This review gives an overview of the interactions between platinum-based drugs and miRNAs, and their influence on platinum activity in various cancer types is discussed.

Non-coding RNA/microRNA-modulatory dietary factors and natural products for improved cancer therapy and prevention: Alkaloids, organosulfur compounds, aliphatic carboxylic acids and water-soluble vitamins

Non-coding small RNA molecules, the microRNAs (miRNAs), contribute decisively to the epigenetic regulation processes in cancer cells. Problematic pathogenic properties of cancer cells and the response of cancers towards anticancer drugs are highly influenced by miRNAs. Both increased drug activity and formation of tumor resistance are regulated by miRNAs. Further to this, the survival and proliferation of cancer cells and the formation of metastases is based on the modulated expression of certain miRNAs. In particular, drug-resistant cancer stem-like cells (CSCs) depend on the presence and absence of specific miRNAs. Fortunately, several small molecule natural compounds were discovered that target miRNAs involved in the modulation of tumor aggressiveness and drug resistance. This review gives an overview of the effects of a selection of naturally occurring small molecules (alkaloids, organosulfur compounds, aliphatic carboxylic acids and water-soluble vitamins) on miRNAs that are closely tangled with cancer diseases.

Halogenated naphthochalcones and structurally related naphthopyrazolines with antitumor activity

Three 3-(3-halo-4,5-dimethoxyphenyl)-1-(2-naphthyl)prop-2-en-1-ones 1 and three structurally related 2-pyrazolines 2 were prepared and assessed in vitro for anticancer activity. The chalcones 1 were antiproliferative with low double-digit micromolar IC₅₀ values against six tumor cell lines whereas the pyrazolines 2 showed low single-digit micromolar IC₅₀ values against this panel. The pyrazolines inhibited ATP-binding cassette efflux transporters of types P-gp and BCRP while the chalcones inhibited selectively BCRP. All test compounds induced an accumulation of HT-29 colon carcinoma cells in the G2/M phase of the cell cycle and they interfered with the microtubule and F-actin dynamics, but only the chalcones induced apoptosis in 518A2 melanoma cells after 24h.

Current state of phenolic and terpenoidal dietary factors and natural products as non-coding RNA/microRNA modulators for improved cancer therapy and prevention

The epigenetic regulation of cancer cells by small non-coding RNA molecules, the microRNAs (miRNAs), has raised particular interest in the field of oncology. These miRNAs play crucial roles concerning pathogenic properties of cancer cells and the sensitivity of cancer cells towards anticancer drugs. Certain miRNAs are responsible for an enhanced activity of drugs, while others lead to the formation of tumor resistance. In addition, miRNAs regulate survival and proliferation of cancer cells, in particular of cancer stem-like cells (CSCs), that are especially drug-resistant and, thus, cause tumor relapse in many cases. Various small molecule compounds were discovered that target miRNAs that are known to modulate tumor aggressiveness and drug resistance. This review comprises the effects of naturally occurring small molecules (phenolic compounds and terpenoids) on miRNAs involved in cancer diseases.

Combretastatin A-4 derived 5-(1-methyl-4-phenyl-imidazol-5-yl)indoles with superior cytotoxic and anti-vascular effects on chemoresistant cancer cells and tumors

5-(1-Methyl-4-phenyl-imidazol-5-yl)indoles 5 were prepared and tested as analogs of the natural vascular-disrupting agent combretastatin A-4 (CA-4). The 3-bromo-4,5-dimethoxyphenyl derivative 5c was far more active than CA-4 with low nanomolar IC50 concentrations against multidrug-resistant KB-V1/Vbl cervix and MCF-7/Topo mamma carcinoma cells, and also against CA-4-resistant HT-29 colon carcinoma cells. While not interfering markedly with the polymerization of tubulin in vitro, indole 5c completely disrupted the microtubule cytoskeleton of cancer cells at low concentrations. It also destroyed real blood vessels, both in the chorioallantoic membrane (CAM) of fertilized chicken eggs and within tumor xenografts in mice, without harming embryo or mouse, respectively. Indole 5c was less toxic than CA-4 to endothelial cells, fibroblasts, and cardiomyocytes. In highly vascularized xenograft tumors 5c induced distinct discolorations and histological features typical of vascular-disrupting agents, such as disrupted vessel structures, hemorrhages, and extensive necrosis. In a first preliminary therapy trial, indole 5c retarded the growth of resistant xenograft tumors in mice. © 2016 Elsevier Science Ltd. All rights reserved.